Let's Talk Cam's (a little).

[Guest Anonymous]

Greetings Cohorts!

 

Just a little something I threw together. Your constructive comments will be appreciated!

 

 

"OVERLAP -VS- SCAVENGING"

 

OVERLAP: As a given engine cylinder rotates it's piston through the compression stroke, there is a time (in pre-determined crankshaft degrees) when both valves are open at once.

 

WHY?: The intake and exaust cam lobes are calculated and ground so as to accomplish a certain desired (and pre-determined) action in the combustion chamber and thus take best advantage of that area with respect to Power.

 

HOW?: By causing the trailing edges of the spent exhaust gasses from the previous cycle to mix with the front edge of the incoming charge, and as they leave (because of the rising piston), to actually help drag a fresh mixture in. In addition, the incoming mixture can be said to aid the spent exaust by effectivly pushing it out as the chamber fills with the fresh incoming charge.

 

IN TOTO: As the piston rises in the combustion chamber, the intake and exaust cam lobes are ground so as to cause the trailing edges of the spent exhaust gasses from the previous cycle to mix with the front edge of the incoming charge and actually help to drag the new mixture in. In addition, the incoming mixture can be said to aid the spent exaust in pushing it out as the chamber fills with the incoming charge. (This may seem contradictory without the rest of the story), read on...

 

SCAVENGING: (A drawing and/or forcing out of the spent exaust gasses through the entire exaust system by means of other previous spent gasses as they sweep through that same system). Where does this effect begin and end?

It begins with the cam grind and ends at the tailpipe outlet. So, the cam, intake runners, combustion chamber, headers, collectors and tailpipe are all involved. All these need to compliment one another for optimum performance.

 

GENERAL DATA

 

As a function of overall engine performance, the longer the overlap condition is (in crankshaft degrees), the less power the engine will develop at lower RPM and the more it will develop at the higher RPM. In other words; The torque/power range will be altered.

-------------------------------

Cams with lots of overlap generally give good engine power at higher RPM, but such an engine suffers from a condition called Reversion" at low RPM also described as as an "off-cam" condition. When such an engine is allowed to idle down into this range, the engine growls, spits and jumps, but snaps out of it when the throttle is opened.

 

This "snap out of it" condition is caused by opening the throttle which allows more atmospheric pressure to come into the cylinder and effectively push out the remaining exaust gasses in a more efficient manner and all Reversion is overcome (the bottom of the cam's operating range is approached/entered). Also, Cams are ground to work in consort with a "Scavenging Effect" caused by a properly tuned exaust system. The tighter this "tune" the less the undesireable Reversion effect.

 

AT LOW RPM, ENGINE IS "OFF CAM": Where normally the exhaust system gases draw (scavenge) the ones behind them along also, trailing pulses (if not kept moving along by some pressure at least equal to the forces in the exaust system itself and also the quickly waning forces of the piston as it continues to rise twoard TDC and expell the last vestiges of spent exaust gas in the cylinder), you should be able to envision the results of opening the intake valve too early. What the combustion chamber sees is that the piston is slowing down dramatically as it nears TDC and it's purging effect is dropping quickly while at the same time the exaust pulses in the exaust system are beginning to neutralize and preparing to expand (push back) into the chamber as soon as possible. This possibility becomes reality as soon as the "Backpressure" in the exaust system sees less pressure being exerted by the rising piston than it's own.

 

CONCLUSION: The whole idea behind "overlap" is to "CATCH" the combustion chamber at a certain point and for a certain number of crankshaft degrees so we can manipulate the Torue range to fit our particular car concerning what we are intending to use it for; but Reversion can get in the way bigtime if the cam grind is way off. This is a precise engineering art, even they have their trial and errors used to find what works in each case, a feat of expense far beyond me for sure! But anyway, Atmospherec Pressure enters the picture, pushing on the exaustpipr outlet at 14.7 (One atmosphere) while at the other (intake) end we have the same 14.7 with a normally aspirated engine, pushing to get past the intake valve (at WOT for example) and into the chamber.

So, the chamber sees those two pressures coming in along with the rising piston (near the top)and the possibility of Reversion. At what crankshaft degree shall we grind the cam overlap to begin to open the intake valve at just the right moment so as to maximize the overlap to compliment our desired "Torque Range" but give minimal Reversion at the same time? If we open the intake too soon then the essentially late rising piston (no lnger so near the top) and the reversion potential will surely overwhelm the incomong 14.7!

 

ECOMMENDATIONS: You can see there is seemingly no end to this situation and we are just looking at two things! I say, bone up on all cam facets possible to understand and then try to pick a good one for yourself. With some real knowledge you can ask three good sources for their recommendations and then using your own good judgment, pick YOUR cam!

========================================================

[Kevin Shasteen]

IMHO, I dont think you should pick a cam based on its overlap. You could have two different cams where every other spec being different while the overlap of each cam is identical. Overlap, just like any other cam spec, is not an island unto itself.

 

Also, I think its important for us all to remember that reversion has the probability of occurring twice in a performance engine: 1) during the overlap period where the engine is trasitioning from the Exhaust Stroke to the Intake Stroke, and 2) the time preceding the IVC on the Compression Stroke.

 

When you get into Induction Wave Tuning you are usually referencing a one of a kind custom intake manifold: which leaves most of us out in left field. If you are going to be efficient at Induction Wave Tuning & Exhaust Gas Scavenging it is best to have an even firing engine: this means a 180 crank. This too leaves most V8's out in the dark.

 

Overlap is a function of the cam lobes in relation to one another. The cam lobes are a function of the intended performance of an engine. Therefore, Compression...both Dynamic & Static are more important of a determining measure than simply looking at Overlap by itself. As the performance of your engine will only peak at one rpm it is essential the builder knows what they expect from their engine prior to building it. The intensity of that peak is simply a "CHOICE" you have to make: I dont think it is something one really has to ponder or deliberate over.

 

With a good 50 years of technical knowhow behind the belts of the modern (OHV Cam Tech) cam mfg's, it isnt too hard to come up w/a winning cam grind. I prefer to look at an engine build by first determining the Dynamic Compression Ratio-which leads one to the Prime Omptimum Static Compression Ratio. It is the duration of your cam that determines your Dynamic Compression Ratio and it is your Dynamic Compression Ratio that will determines where in the rpm range your peak power band will rise. It is the choice of your secondary engine components that compliment or hinder your cam choice.

 

Your secondary engine components, which either hinders or compliments, will either move the power band upwards or downwards by a couple hundred rpms to a thousand rpms. It all depends on your intenional needs of performance and supplementary component choices.

 

Good Rule of Thumb for Overlap & Duration:

 

Duration:

1) IVC @ Upper 50*'s ABDC = Typical Mild Street Performanc

2) IVC @ Upper 60*'s to Mid 70*'s = Hot Street & Mild Racing Cam

3) IVC @ 80*'s to low 100*'s ABDC = All Out Racing Cam's

 

Overlap

1) 10-35*'s Overlap = Acceptable Mileage or Good Towing Cam

2) 30-55*'s Overlap = Daily Driver & Good Low Rpm Torque

3) 50-75*'s Overlap = Hot Street Performance

4) 70-95*'s Overlap = Oval Track Performance & Auto Cross

5) 90-115* Overlap = Dragster/Comp.Eliminator Class

 

I agree with you in that once you have an understanding of cam speak then you can narrow down your choice to three or less cams. As for when is it best to close the IVC: I'ld say just at the point which precedes reversion. This is why I like to approach the cam choice first from an IVC standpoint-for it is the IVC relative to your displacement of your cylinder that determines the Dynamic Compression Ratio which in turn determines your Optimum Required Static Compression Ratio. The Dynamic Compression Ratio approach takes your displacement into consideration whereas the Overlap spec of the cam doesnt approach the displacement issue.

 

Dispalcement is the key into picking a complimentary cam for your power needs.

 

Anyway-that is my .02c's worth.

 

Kevin,

(Yea,Still an Inliner)

[grumpyvette]

SPIIRIT

you seem to have the basic idea BUT... the exhaust flowing away from the cylinder causes a negative cylinder pressure the incomeing charge tries to fill theres nothing pushing except the standard 14.7psi of sea level outside air pressure trying to fill the cylinders to normal air pressure untill a ram tune rpm is reached and even then it rarely exceeds 2 psi during the most efficient peak rpm range where volumetric efficiency peaks (about 300-700rpm)

 

I was asked about a previous post,my example of how do headers work? what I think your refering to was when I was trying to explain how headers help pull the exhaust out of the cylinders by useing the energy from the previously fired cylinder mass of exhaust rapidly moveing away from the exhaust valve causeing a negitive pressure at the exhaust valve that helps scavage the cylinder. what I said was you can demonstrate that negative presure caused by a rapidly moveing mass by getting a 2.5"x5' long section of pvc pipe hold it horizontally and insert a full coke can with some tape wrapped around it so that it just slides easily into one end of the 5' long pvc pipe, now put your hand over the end of the pipe you just installed the full coke can into and rapidly drop the other end strait down, as the coke can slides too the floor you will feel the negitive pressure on you hand caused by the falling mass. now a cylinders volume of exhaust moveing through a primary header tube works in much the same way, while it weights much less than the coke can its moveing far faster and carries enought energy that the same negitive pressure is formed at the exhaust valve by the slug of exhaust gas traveling in the header primary tube, [color:"blue"] now ideally if the header primary tube is long enought and of the correct dia. to still contain the mass of exhaust gas at the rpm that matches the cam timeing and cylinder volume,a peak of that negative pressure will be forming behind the exhaust valve as it opens during the rpm range where the engine spends the majority of its time, now temp. cylinder volume, compression ratios,cam timeing and interior pipe dia. all effect the rpm range that this effect peaks at effiency wise but in most v-8 engines a dia equal too the exhaust port and about 32"-39" will put that negative pressure wave caused by the previous fired cylinder at the exhaust port in the 4000-6000rpm range, look at this chart,

http://www.engr.colostate.edu/~allan/fluids/page7/PipeLength/pipe.html

http://www.engr.colostate.edu/~allan/fluids/page7/PipeLength/pipe.html

now figure that exhaust is 1300f or 977k so thats about 25" at 6000rpm and about 39" at 4000rpm to get that pressure wave to work for you scavageing the cylinder(helping to pull exhaust out and the new intake charge into the cylinder.)hers stuff to read,

 

http://www.burnsstainless.com/TechArticles/Theory/theory.html

 

http://www.engr.colostate.edu/~allan/fluids/page5/page5f.html

 

http://www.engr.colostate.edu/~allan/fluids/page5/page5f.html

 

 

 

BTW it should be obvious why shorty headers don,t make as much power,....the primary tubes being much shorter are also much less effective at controling that negative pressure wave timeing and strength over anywhere near the same rpm range.

 

 

read this

http://www.mercurycapri.com/technical/engine/cam/lca.html keep in mind the same thing is happening on the intake side of the engine in that properly designed intake ports use the inertia of the coluum of air moveing into the intake/port/cylinder to INERTIALLY RAM the the coluum of air into the cylinders durring some rpm ranges, if the two effects can be matched to the correct cam timeing the cylinders can be packed with more that 100% of the air normally filling the cylinders voluum/space at outside air pressure[/color] there bye allowing the engine to build exceptional torque by burning greater amounts of fuel/air mix for its size at that rpm range

 

http://www.mercurycapri.com/technical/engine/cam/lca.html http://www.mercurycapri.com/technical/engine/cam/lca.html [/url]

some good general info here look closely at the duration used for each MATCHING rpm range. ALSO KEEP IN MIND THE DCR AND OVERLAP MUST MATCHlook here these are the valve timeing overlap ranges that are most likely to work correctly

trucks/good mileage towing 10-35 degs overlap

daily driven low rpm performance 30-55degs overlap

hot street performance 50-75 degs overlap

oval track racing 70-95degs overlap

dragster/comp eliminator engines 90-115 degs overlap

but all engines will need the correct matching dcr for those overlap figures to correctly scavage the cylinders in the rpm ranges that apply to each engines use range.[/b]

http://cochise.uia.net/pkelley2/Overlap.html http://cochise.uia.net/pkelley2/Overlap.html http://cochise.uia.net/pkelley2/DynamicCR.html http://cochise.uia.net/pkelley2/DynamicCR.html

, don,t forget that the longer your rods are (closer to the ideal 2:1 rod to stroke ratio) the wider the lsa should be and the longer the stroke the wider the lda (lobe displacement angle) should be[/b]http://www.compcams.com/Base/Images/Articles/CC_CC0101-001-2.jpg"[/img] http://www.compcams.com/Base/Images/Articles/CC_CC0101-001-2.jpg

if cams are a mystery please take the time to read these, it will get you a good start

 

http://www.newcovenant.com/speedcrafter/tech/camshaft/1.htm

(read LESSONs 1-8)

 

http://www.mercurycapri.com/technical/engine/cam/lca.html

 

http://www.wighat.com/fcr3/confusion.htm

 

http://www.cranecams.com/instructions/valvetrain/camfail.htm

 

http://www.mercurycapri.com/technical/engine/cam/index.html

 

http://www.idavette.net/hib/camcon.htm

 

http://www.cranecams.com/master/adjustvt.htm

 

http://www.centuryperformance.com/valveadjustment.htm

 

http://www.totalengineairflow.com/tech/valvelashing.htm

 

http://www.chevytalk.com/tech/engine/Cam_Selection.html

 

http://www.chevytalk.com/tech/101/Cam_Theory.html

 

http://www.babcox.com/editorial/ar/ar119736.htm

 

http://www.mercurycapri.com/technical/engine/cam/vtg.html

 

http://www.n2performance.com/lecture1.shtml

 

http://www.symuli.com/vw/camp1.html

 

http://www.symuli.com/vw/camp2.html

 

http://home.wxs.nl/~meine119/tech/camqa.html

[Guest Anonymous]

Thanx Kevin;

 

Looks like I really flushed out a couple of "Good Ol' Boys" and I'm sure glad of that!

 

First let me say that I hope I haven't steered any inexperienced readers wrong too badly. I must admit that I am just a new student to the art of camshaft functions but, I am a tenacious student with an open mind when I lock in on the objective.

 

What I wrote was just from reading recently and trying to get things in an order where I could actually fully understand and apply them to the engine I am still building in my mind. So far, I have realized that the cam is the backbone of any engine and must be treated as such for max. results overall, plus the fact that cam tentacles seem to reach into every aspect of engine building, gearing and even the tires.

 

About the Overlap thing; I was using the Overlap as the root of any cam design effort because of "Lobe Centerline" considerations which I had read "Cannot be changed" once set? My mind is now open about this because of my lack of any solid "industry" reference point so far. I mean there are juxtaposed opinions out there and you can't just grab anything and hold it as Bible without some solid facts.

 

I pass on the induction wave tuning right now.....LOL

 

I see overlap as providing the root for getting the

biggest bite of fuel possible into the cylinder by utilizing both duratduraion and lift. In other words, the function of duration and lift are both riding on the overlap centerline positions of the cam lobes, and once the two are set, their overall function with respect to chamber filling depends strictly on the overlap. (I am a student here).

 

Dynamic/Static Compression: I need your eyes. We

seem to have Dynamic and Static ratios as opposed to rotating and non-rotating conditions. For the Dynamic I assume a cranking speed, for the Static I assume a due calculation. Is this correct? Now where does the compression "Pressure" that I have read about come in? And what is this "Prime Optimum" static ratio you are finding with the Static Ratio when that is a matter of fixed volume calculations? (Straighten me out please).

 

I appreciate your Duration and Overlap list. I want your formula for anticipating the "Just at the Point which preceeds Reversion" statement, (I hold you to it)! And please try to keep it to no more than three pages...LOL.

 

Your parting statement makes several things you said more clear: You speak of matching cam to engine while I have been matching engine to cam. (Very interesting).

[Guest Anonymous]

Grumpy;

 

Old man, I am going to get you for all this homework (URL'S) you laid on me! I should respond in a few days.....LOL!

[Kevin Shasteen]

Spiirit,

 

When looking for a cam dont look at the Lobe Separation Angle (Overlap) first as the overlap as one of the cam's functions is third to fourth on the list. First and foremost you should be looking at IVC (Intake Valve Closing) prior to anything else. After all, Duration & Lift continue well after Overlap has ran its gamut.

 

As far as my comment, "The Point Before Reversion Occurs": well that is basically an Oxymoron-just checking to see if you were truly listening. The real question you have to ask is, "When does too much Reversion become too much Reversion?"

 

The answer to that question is something the engine builder must answer because in any Normally Aspirated mild, moderate, or wild Performance Engine there is going to be Reversion. Understanding the Dynamic Compression [Ratio] will shed light on exactly how to determine your engine's powerband.

 

Without going into a lot of detail, simply know that an 8.5 DCR is perfect for a street engine on pump gas while a 9.0 is considered a perfect DCR for a race engine. Once you begin working the math you will find that engiens are being built, in each category (both the street & race engines) well below their 8.5 & 9.0 DCR's.

 

The difference in a perfect DCR and an engine that falls short of the perfect DCR is by many considered a Compromise because your engine is bleeding too much Cylinder Pressure prior to the IVC relative to SCR (Static Compression Ratio). What the perfect DCR does for an engine is it allows the engine to idle at a 600-650rpm's. Any DCR that falls short of the perfect DCR will decrease your idle quality (higher idle rpm's) and likewise so to will your Engine Vaccum decrease at the same time: thus the rumpity rump that many enthusiasts enjoy: the problem is that most enthusiast who like that rumpity rump dont know to what extent they are truly hurting their performance.

 

By understanding DCR you will have a much deeper understanding in key knowing what idle you want prior to building your engine: this is much better than simply guessing...and guessing wrong. Once you understand the DCR issues you can choose that rumpity rump and exactly how much of it you want prior to ever touching a wrench.

 

Before you can calculate DCR you must have a Crank Angle Chart. This chart maps the piston's location in the cylinder w/each degree of crankshaft rotation.

 

If you wish to check out the Crank Angle Chart, I would suggest you go to your nearest Barnes & Noble or Hastings bookstore & find the book:

 

"How To Build Max Performance Chevy Rat Motors: Hotrodding Big Block Chevy's 90's Style" by Ed Staffel (Its a Grey Book w/red print on its Binder) published by Cartech w/a Copyright of 1996.

 

Once you have found this book go to the top of page 108 for an example of a Crank Angle Chart. Then skip over to pages 112-113 for instructions on how to use that Chart.

 

The only problem w/this book is that it doesnt explain how to get your own Crank Angle Chart for your own engine. Each Chart will differ slightly depending on the Offset of the Crankshaft, Connecting Rod Length, Piston Compression Height: all three of these issues are then calculated w/in your Block Deck Height to determine actual piston position.

 

If you want to create your own Crank Angle Chart then you will have to use basic Trigonometry to do so. Why Trig? Because the interrelated parts Crankshaft Offset, Connecting Rod & Piston Wrist Pin all create angles relative to the centerline of those components: specifically a Triangle-so basic Trig fits in there nicely.

 

If you want to understand Camshaft Technology and have not purchased this book yet-I would suggest you do so:

 

"High Performance Camshafts": the Best of Hotrod Magazine Volume 11, published by Cartech w/a Copywrite of 2000. It is a Black Book w/white lettering on its Binder.

 

I would suggest that once you understand both Camshaft Vernacular & their functions as well as DCR's, that from now on whenever you read about an engine build up in a magazine, you calculate that engines DCR. Once you get in to this habit you will quickly realize that magazine articles rarely give you [all] the info needed to make a reasonable decision on their engine build. You will find that they rarely speak about Vaccum & Idle Quality. When they do speak about Vaccum & Idle Quality they will leave out some other issue: cant blame them too much-as there are a lot of issues to remember when not only building an engine but putting together a magazine every month. Yet if you get in the habit of Calculating DCR then you will eventually begin to recognize patterns. Understanding the Compression Formula is also invaluable as it is this forumula that allows you to work the math forwards or backwards depending on what elements your have or which elements the magazines articles have left out.

 

Lastly, Just keep in mind that DCR, Vaccum, & Idle Quality will rise and fall based on the Duration: and that it is the Duration of your Cam relative to Cylinder Displacement that acts as the revolving door in determining if and when your DCR, Vaccum & Idle Quality moves up or down the rpm scale.

 

Knowing when and how DCR, Vaccum, and Idle Quality are effected leads you into realizing which Cylinder Heads are best for your needs-and this leads you into knowing which Intake Manifold best suits your enigne: but as far as I'm concerned-it all begins w/DCR.

 

GRUMPY-YOU OUT THERE? Grumpy posted not too long ago a site where someone offered a free download on a computer program for a Crank Angle Chart...perhaps he can repost that free download.

 

Kevin,

(Yea,Still an Inliner)

[grumpyvette]

[grumpyvette]

http://www.iskycams.com/ART/techinfo/ncrank1.pdf

[Guest Anonymous]

Kevin;

 

Ya Kevin, well I'm playing it safe and trying to get all the innards and the bolt on's matched up before I spend the moola! The problem with that is, it is a real task even tho' I lean twoard that sort of thing.

 

As to the cam, I just want to be able to follow what the vendor has in mind for the build. Not having experienced a lot of this stuff it is beginning to weigh me down. Rod ratios, piston height, deck height, quench, compression ratio and pressure, all seem more mixed than even the cam. And then the whole thing is mixed together by duration, overlap, lift etcetra. When I read that the compression ratio will decrease with more overlap I begin to wonder how an average guy can manage to put together even a halfway decent engine (which they probably don't without some good help).

 

I understand all this stuff so far but keeping it in my mind long enough to begin arranging my thoughts in a productive manner is not yet. I will just have to keep plugging away until it happens.

 

I know that your help and Grump's will be of great value to me when things start to jell. I am not going to "run ahead" and buy any engine stuff until I have the whole picture in my mind which is something you guys can do 10-20 times faster than I can right now. It is now that I wish I hadn't dropped out of the scene when the '53 Olds came out and discouraged me and my flathead. I have a engine picture in my mind right now but it is probably mostly full of disasterous holes.

 

I have been all thru the standard trans. thing, looking for the right gears for my purposes and after I spent two+ months on that, I read up on powerglides and have been working on that; which means True stall, flash stall, intense converter studies and so forth. This is another bag of worms that I will also let the vendors have the final word but, I want to be a able to box with them a little just to make sure they don't fluff me off with a quickly selected "Shelf item". You know what I mean.

 

You guys gave me so much cam data etc. I was getting a headache and switched to the tranny's for a while, then I will go back again. All this will take time and I have plenty of that (could you find it in your heart to loan me $10,000.00 perhaps)?.....LOL. At first I thought it would take me 2 yrs. to do all, then it became 2.5, then 3. Looks to me like I bit off one hell of a chew!

 

I guess my problem is that I want to build a winning engine (I should say "A hot contender") the first time out, no if's, and's or but's, so time spent figuring will give me the best shot using a mindset like that. At least I should come up way ahead of just slapping stuff together expecting big time results.

 

To answer your statements in order given:

 

Para. 1: I see your point about overlap and am looking at duration (what happened to "Dwell"? I see how stromgly the IVC is involved with DCR. I must study this relationship further.

 

Para 2: Oh, I am listning alright! Right now I would say that too much reversion is when torque loss in the lower rpm range (caused by too much duration and/or overlap) is so great that it interferes (bogs) the engine coming off the line etc. and cannot be corrected with gearing and/or tires because then, peak rpm would be reached too soon and trap time suffer.

 

Para. 3: Again I ask your definition of DCR (cranking/running?), SCR (cranking/calculated?) and dyno readings because I have become confused by reading several explanations of all these. Is it safe and sure to calculate DCR and act on it, or are you assuming dyno data when you say DCR? I have to get this straight in my mind so we can really banter.

 

Para. 4: Now this one I understand better. I have been reading where the '60's muscle cars were using 11:1 ratios (Based on what measure?) and using cam duration to lower that to 8.5:1 DCR, so you come in accord with that part. And, hmmmm. Now I am seeing where these very high ratios of 12-15:1 are entering the picture. It is because of the excessive combination of duration and overlap moving the power band higher (or is overlap not in the picture here)?.

 

Para. 5: I understand that a "compromise" must be accepted with duration and ovrlap in order to move your strongest torque band higher in the rpm range. I will assume there is a "Chart" available for IVC -vs- SCR? I understand the Idle performance part ok but a modified engine is always going to have that problem (if you want to call it a problem) isn't it?.....And this is compensated for with stall Speed or clutch slipping tire size/gearing, as the case may be? (I am loving this chat).....LOL. At the end of this para. you seem to be indicating that "Most" of those using a heavy cam are not properly compensating for it's use. Is this so?

 

Para. 6: (Self explanatory comment).

 

Para. 7,8,9, 10, 11, 12: Thanx for the book tip.

 

Para. 13, 14, 15, 16: Thanx for the book and tips'

 

Para.17: This is food for thought and a carrot on a stick for me, Har!

 

By the way, I think Grump has responded to your query but unfortunately, all I see is the old red x in a box.

[Guest Anonymous]

Grumpy;

 

I got your Isky chart. I been all over that "Tech" article and never have seen a link to what you have there.......What? You a member or something?

 

Anyway, thanks a lot. I love to study/analize these charts and graphs. This looks real good to raise me up some. I am still cogitating all those URL's ol' buddy!

[Guest Anonymous]

Mr Grumpy;

 

I have read all the URL's and your dissertations once so far, and with fair dilligence.

 

First I want to thank you for your trouble. Second I weant you to know that all I needed was provided and then some (those URL,s must be the result of long time searching etc. and I appreciate that also). Third, there was one particular URL about compression that I particularly enjoyed . It was the best laid out and clearest documentation I have read on any complicated subject in a lifetime! This man is an exellent teacher!

 

I hope to get together with you again on the subject of the intake aspiration part of the picture at a later date.

 

Thanks again

[Kevin Shasteen]

Spiirit,

 

Glad to see you making progress: if you want reading materials-just ask Grumpy, he is an excellant researcher (thumbs up Grumpy).

 

As far as "my definition for DCR". Yes it is a calculated figure. Picture yourself in a lab room in front of a chalkboard with a lab coat on. I raise my hand and ask you to show me on the chalkboard how you determined the Static Compression Ratio of the engine you are building.

 

You begin by drawing one cylinder of your engine: lets say it is from a 350ci SBC which has 8 cylinders. How do you go about knowing how much volume is in one cylinder? You divide 8 into 350, like so:

 

350/8 = 43.75ci

 

If you wish to convert ci's to cc's then multiply your ci's by 16.387.

 

So, your one cylinder has the volume of 43.75ci. Now you draw your piston at Bottom Dead Center and indicate the 43.75ci created after you stroke has pushed your piston to its Top Dead Center location. Now you draw your Total Combution Chamber of 4.61ci. Now you can use your "Compression Formula" to determine your SCR, like so:

 

(43.75 + 4.61) / 4.61 = 10.49:1...or 10.5:1 SCR

 

Now, you have an idea of what SCR is. The definition of DCR (Dynamic Compression Ratio) is: "The Volume remaining above the Piston at the moment your IVC (Intake Valve Closing) occurred. So once again-you are at the chalkboard. Now begin drawing your piston & cylinder.

 

Picture if you can your camshaft's intake lobe rotating to the point your IVC takes place: this moment will occur on the Compression Stroke just after the piston has passed BDC as the piston begins rising towards TDC.

 

As the piston rises-the IVC event ocurrs. Now, STOP RIGHT HERE! Now measure the amount of Volume above that cylinder relative to your Total Combustion Chamber volume...put these volumes into your Compression Forumula and you will have calculated your DCR.

 

Let us say your IVC occurred whereby 80% of your cylinder Volume remains. Take your cylinder volume & calculate what 80% is, like so:

 

43.75ci x .80 = 35ci

 

So, now you have 35ci Cylinder Volume after the IVC. Now insert this into your Compression formula along w/your Total Combustion Chamber of 4.61ci, like so:

 

(35 = 4.61) / 4.61 = 8.59:1....or 8.6:1 DCR

 

Hoooray, you just figured out your DCR on a cam that closes its Intake Valve in such a manner that leaves enough volume in the cylinder relative to your Total Combustion Chamber Volume that allows an 8.6:1 DCR. This engine will idle at 600-650rpm's and its torque power band will peak in the 2800-3500rpm level...a perfect street engine!

 

As far as Duration & Overlap being a compromise-this is backwards: which most magazines imply (due to limited funds & lack of understanding of DCR). Due to lack of funds-they imply you should build your enigne and then try and pick a cam. When in reality you should build your engine based on its power needs. It is the power needs and your desired peak power rpm range that determines what cam your engine should get. Once you understand DCR then picking a cam is not a [Compromise], rather it is a Decision...and that is what it should be: a decision not a compromise.

 

As far as a Chart existing for IVC -vs- SCR...your question should be IVC, which leads to DCR which determines your SCR. and No, there is no chart: the chart generated is the data you collect once you understand how to calculate DCR. You can then take an article in a magazine/internet and it's dyno run (which indicates the peak torque & hp) and calculate that engine's DCR. Each time you perform these calculations you will become a little more familiar with the math. You will see how the numbers begin talking to you-they really bring on a whole new meaning.

 

Remember this: you can underbuild your engine and it will rev to 5000rpm's, you can build your engine just right & it will rev to 5000rpm's, or you can overbuild your engine and it too will rev to 5000rpm's. So, which build do you want-do you want to under build, build just right, or over build your engine?

 

How you determine if/when you have under or over built your engine is if your peak power band occurs inappropriately to when you need it most. DCR accompanied w/appropriate secondary components will determine when your peak power manifests in the rpm range.

 

Learn the math calculations and how to work them backwards.

 

You need a Crank Angle Chart to determine the percentage of Stroke remaining above the piston after IVC. Dont forget to check out the books I suggested as complimentary to the URL's Grumpy gave you.

 

Good luck & have fun. Dont press yourself too hard on your engine build-everyone makes mistakes somewhere along the line. Just try to minimise those mistakes & you will do just fine.

 

Kevin,

(Yea,Still an Inliner)

[grumpyvette]

let me add to that info above looked at in another way.

 

lets look at the example Kevin Shasteen gave you above, lets assume (to keep things simple) that you can make 1 ft lb per effectively burn cubic inch of fuel air mix!

now that would mean that at 1500rpm with a 350 displacement youll have 750 x 350 cubic inches burning (every other stroke is a power stroke remember) 350 ft lbs at 1500rpm or 99.96hp

the formula for hp is (tq x rpm /5252=hp) with a cam that closes the intake valve at bdc.

now look at this chart and this cam,

http://dab7.cranecams.com/SpecCard/DisplayCatalogCard.asp?PN=114681&B1=Display+Card

the intake valve closes at about 77 degrees past bdc and according to the chart runs at a torque peak near 4500rpm. now look at this chart

http://www.iskycams.com/ART/techinfo/ncrank1.pdf

that means that you needed to delay closeing the intake valve untill 77 degrees past bdc to leave enought time to fill the cylinder but by doing so you have effectively lowered the VOLUME COMPRESSED BY THE PISTON to an effective stroke of about 2.4" or reduced the engines effective volume to 241cid but effectively raised the NUMBER of power strokes to 2250 or 241ft lbs x 4500/5252=206hp thats a 206% increase in hp!

but theres other factors at work here! remember this diagram

well at 4500 rpm if you have a tuned intake and well designed header scavageing the cylinders a effective cylinder filling boost in air pressure filling those cylinder due to ram tuning could effectively raise the pressure filling the cylinders from 14.7 at sea level by 10% or more which could very easily boost that 241 displacement to an EFFECTIVE displacement of 265 cid boosting that 206 potential hp to 227hp, or a a 227% increase in hp

 

YEAH I KNOW YOUR THINKING IM NUTS!

LOOK AT THIS DYNO!

 

IVE REALLY SERIOUSLY UNDERSTATED MY CASE!......... INCREASES OF over 300% due to a combination of increased RPM,a cam change and improved cylinder scavaging are comon

btw read this

http://victorylibrary.com/mopar/cam-tech.htm

[pparaska]

Grumpyvette, that cam (114681) actually closes the intake valve at 71 deg ABDC. I think you were looking at the exhaust valve opening, at 77 BBDC. Anyway, that means that the the effective stroke is more like 2.55". Since you convinced me to use that cam, I know the IVC by heart .

 

Anyway, that works out to a 260 cubic inches of effective engine volume (on a .030" over 350 with 5.7" rods).

 

I THINK I understand the rest of your post. Let me see...

 

The cam Kevin used closes the Intake at a point where 80% of the volume of the cylinder is left. On a 350 with 5.7" rods, that's a IVC of about 61-62 deg ABDC. That's a pretty mild cam. One like this one:

http://dab7.cranecams.com/SpecCard/DisplayCatalogCard.asp?PN=100042

It has a .050" duration of 210 degrees, a torque peak at about 2700 rpm, buy that bar chart in your post.

 

I think what you're saying is that for a cam that closes the intake valve at BDC (per your last post), the hp will be around 100 hp, topping out near 1500 rpm. By leaving the valve opened until 77 (or 71) deg ABDC, you move the torque peak up to around 4500 rpm, and increase the max HP of the engine by 206% or 227% (with ram tuning of intake and exhaust) or over 300% according to the dyno plot.

 

Is that last part what you are getting at?

 

If so, all that's needed is to put up with less low end torque, use some gears, etc. and enjoy the 300+% hp peak.

[Guest Anonymous]

OBSERVE THIS STATEMENT I FOUND ONLINE

http://www.streetrodstuff.com/Tech_Stuff/Tech_Terms_II.php

--------------------

Indicated horsepower:

 

The method of rating an engine that is based on the actual power developed by the engine in an indicator diagram. This reading includes the power necessary to overcome engine friction. The indicator diagram is produced by an oscilloscope that makes a drawing of the events occurring within the cylinder. It records the pressure existing at each instant of a complete engine cycle from the time the combustible mixture is drawn into the cylinder until the completion of the exhaust stroke. The area of the diagram is proportional to the power generated.

 

IHP = (PxSxAxC)/33,000 IHP = indicated horsepower, where:

 

P = Mean effective pressure (in psi)

S = Stroke (in feet)

A = Area of the cylinder (in sq. ft.)

N = Number of power strokes per minute

C = Number of cylinders

------------------------

If 'N' is a factor in this formula then why dosen't it appear as a function in the progression?

[pparaska]

Looks like they got that wrong. I found this:

 

http://www.aednet.org/aed_foundation/engines.cfm

"IHP =  R2 x pi xL/12xNxPxn" 
          ------------------------             
                33000 

R2 x pi = area of cylinders, inches2

(note - R2 is R*R or R squared, not R*2)

 

L/12 = length of stroke, inches

 

N = engine speed in RPM

 

P = mean effective pressure, PSI

 

n = number of cylinders

 

R = radius

 

I put our paper 350 in there, used 4500 rpm, 2.015 bore radius, 3.48" stroke, and 112 psimean effective Pressure (average of the 85 and 140 they say an NA engine sees) and got 452 IHP.

[grumpyvette]

pparaska

your correct but remember thats a 300%increase is at 4500rpm over the same engine with a milder cam at 1500rpm

 

 

SPIIRIT

If 'N' is a factor in this formula then why dosen't it appear as a function in the progression?

 

the formula for hp is (torque x rpm /5252=hp)

if you had 400ft lbs of tq at 1000rpm

1000 x 400/5252=76hp

move that same 400ft lbs to 5000rpm

5000 x 400/5252=380hp

[Guest Anonymous]

Thank you both gentlemen!

[Kevin Shasteen]

Correct me if I'm wrong here, but are the HP formula's being bantered around just simply a spin off, and maybe a simplification, of the classic HP formula initially used to determine the old steam engines power output?

 

From what I little I understand about Steam Engines (very, very little) accept from what I have read: is that pressure not only acted on top of the piston but also underneath it. As long as you had steam-the pressure was kept at a constant reading. The gasonline engine is an animal entirely different. not changing. It is an internal spark combustion engine. It utilizes a spark to ignite its air/fuel charge: as a result the PSI will rise/fall instantaneously preceding and following the combustion cycle.

 

When I say classic HP formula I'm referring to the:

 

HP = (P x L x A x N) / 33.000

 

P = Working Pressure measured in psi

L = Length of stroke in feet

A = Total Area of piston in square inches

N = Number of Power Strokes per minute

 

Because the steam engines (For the Railroads) used huge strokes & an even larger than life connecting rod L was a reference to length of stroke in feet.

 

To make use of the formula for a stroke in inches you would use:

 

HP = ((P x L x A x N) / (12 x 33,000))..., or

 

HP = ((P x L x A x N) / 396,000

 

The problem I see w/this, as Grumpy pointed out, is that it doesnt take into account the different pressures the engine sees thru the rpm range of idle to maximum rpms. In other words it isnt a progressive formula. Instead it appears to be good for peak power & peak hp numbers only.

 

Using the previous formula, everyone has already given, it is understood that in order to obtain HP you must know Torque & its rpm from which the measure was taken. To obtain Torque you must know HP & its rpm the measruement was taken. However, if you dont know either torque nor hp but you do know both the high psi readings and the low psi readings then you can use the classic HP formula in calculating hp & torque.

 

I personally dont think the PLAN/396,000 formula is for anything other than peak power output: so it is kind of one sided but if you dont have anything other than psi readings then you can figure torque & hp.

 

...so it is not a total loss in my most humble opinion. 8)

 

Kevin,

(Yea,Still an Inliner)