CAMS VS COMPRESSION ET AL.

[Guest Anonymous]

LET'S TALK CAMS SOME MORE!

 

SPIIRIT SAY'S I'M BAAACK!

 

On the intake stroke, some air/gas mixture starts to flow into the cylinder as the valve opens, but the greatest gulp comes when the pressure differential is the greatest. This occurs when the piston reaches its maximum velocity somewhere between 70 to 80 degrees ATDC.

 

On the Power stroke (In most instances), the gases are at a relatively low pressure by the time the crankshaft reaches 90 degrees After Top Dead Center (ATDC), so we can safely open the exhaust valve Before Bottom Dead Center (BBDC) to take advantage of "blow down" (The process by which pressure aids in the expulsion of burnt gases) .

 

On the exaust stroke, if the exhaust valve is not open a lot by the time the piston reaches maximum velocity, there will be resistance in the cylinder caused by excessive exhaust gas pressure. This produces conditions which are referred to as pumping losses (The work the pistons do pulling on (running ahead of) the vacuum in the intake manifold).

 

Near the end of the exaust stroke, you will see that while the exhaust valve is almost closed the intake valve is beginning to open! Here, (at TDC), both the intake and exhaust valves are partially open at the same time. For this reason, this part of the stroke is called the OVERLAP PERIOD, while the actual condition is referred to as "SPLIT OVERLAP".

 

On standard engines, the valves are only open together for 15-30 degrees of crankshaft rotation. In a race engine operating at 5-7000 RPM, you will find the overlap period to be in the neighborhood of 60- 100 degrees (which also translates into more total duration). With this much overlap the low speed running is very poor and a lot of the intake charge goes right out the exhaust pipe!

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GETTING DOWN TO BUSINESS

 

At the end of the exaust stroke, both valves are open because the effective intake stroke has already begun(for example) at 24 deg. BTDC of the exaust stroke. The piston now moves down the cylinder on it's intake stroke and this valve reaches full lift at 108 degrees ATDC (lobe center). The intake valve remains open until well after BDC to introduce a "Scavenging" effect (there is a phenomena occurring here that is caused by the inrushing charge being streched out like an extension spring. This action begins as soon as the piston passes TDC with the intake valve already open and continues until the intake valve is closed sometime after BDC. The piston is moving faster than the restricted incoming charge can follow, this streches the charge and it chases the piston. If we close the IV at BDC then we cut this streched charge into two parts and a noteable part of the charge necessary to fill the combustion chamber with the correct "air/Fuel" density is lost! However, if we keep the valve open past BDC a certain amount of time "in degrees", we can utilize the fact that the piston slows and stops and "slow starts" again at BDC. During this time frame the streched charge can be allowed to recover to it's normal state, and then we ideally close the valve on the full charge).

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We can start to add things up now. The crankshaft has rotated 180 degrees from TDC to BDC on the Intake stroke and the intake valve opened 26 degrees BTDC, so the total crankshaft rotation so far is 26 + 180 = 206 degrees. We started with a 268 degree duration camshaft, so that tells us when the intake valve will close: 268 - 206 = 62 degrees ABDC. (It has been my experience to find that many cam vendors do not give one critical cam function value so that the reader can evaluate things properly and that is the moment of "Intake Valve Closing". The omission

of this one item is causing me much distress)

!

Now that we have delayed the IV (intake valve) closing past BDC to get a full charge in the cylinder, we find that we have already sacrificed several degrees of piston travel in order to accomplish this. It is during this time that the charge riding on the piston sees the intake valve as still partially open and will begin to build up an "exit pressure" (Reversion) there as it begins to escape back into the induction system. The beginning of this effect can be tolerated as long as it dosen't upset the function of the air/fuel intake system as a whole.

 

There are two kinds of "Reversion" in the internal combustion engine and one is "Intake Reversion" (from cylinder pressure) and the other is "Exaust Reversion" (from the exaust system). The Intake Reversion is what we're seeing at this moment. The degrees of piston travel lost here (already done in order to get a full charge in the cylinder), cause the effective swept volume of the engine stroke to be reduced by that same amount. This means that the remaining charge volume (when compressed) is going to be less which in turn will reduce the Power realized from that Power Cycle; In short, we have lost some of our compression ratio.

 

Now we have another problem. Hazarding a guess of a 92 octane gasoline rating with Preium Pump gas, we can go any direction with that as far as compression ratio (C/R) is concerned. It all depends on our engine build. Anywhere from 8.5-10.5:1 (or even more) might be considered for the street, much depends on your expertiese at matching parts and overall tuning. But how much compression did we have? How nuch did we lose? How to restore it or even get it up to the maximum the build can handle on 92 octane? Peace be with you my (uninformed) brothers, because now you must "crack the books" like I am doing!.....LOL!

 

By the way, there seems to be some confusion as to exactly what Static and Dynamic compression are and how they are measured. Logically my mind tells me, Static means just that, and is found by using a simple mathematical equasion (a formula). I say, Dynamic means just that, but at the same time there are apparrently two sorts to deal with; One is your typical garage type "Cranking Speed" variety which entails removing all sparkplugs, screwing in a guage, using a fresh battery, cranking the engine over about five times and noting the reading. The second is to simply replace one sparkplug with your guage, start the engine and whale away at selected RPM's while charting a performance curve or whatever (I really don't understand the need for this one)?. Any correction/comments by my peers would be appreciated here! No, I am not finished.....Compression Pressure? Left with what I have just said it would seem to be the same as "Cranking Pressure" with all plugs removed. How about it? Caution! If Static is being applied as a definition of cranking speed simply because the engine is only rotating but not running I am going to be upset at such a misnomer, but I suppose I will get over it.

 

 

Me?.....I want the perfect camshaft for my engine build. How to find it for sure? Well, you can let the manufacturer shove one at you (he goes by what you tell him you have and can get you in the ballpark if there has been no confusion during that simple correspondence, we hope) or, you could do what I am doing and force the issue until satisfied that a sensible grind can be ordered almost purely by your own knowledge on the subject. I call this "Static Learning" (written word) and it takes a lot of time. I am picky and I don't want to be swapping a bunch of cams and feeling like an idiot (simply because I don't knoiw what I'm doing).....he he! I want to orchestrate my own engine and I don't intend to spend big bucks only to lose hidden power potential to plain ignorance. I know that what I may think (in ignorance) may seem very good and even exact, only to discover later that I messed up again! She might run GOOD, but exactly what is GOOD relative to potential?

 

Well.....we are always going to be faced with this problem no matter how high a level of wisdom we can accrue as time passes, but why settle for less than what you know? And what you know comes in direct proportion to what you have studied and how hard you studied it, not to mention your percentage of comprehension of the material. Guys can tell you things about what you have studied that you never knew, and why is that? When I read an article in persuit of a goal I pen questions on the side about what I don't understand, and even other questions that come from between the lines. Answers to those things always raise other question and on it goes, but at some point in time you will raise your head up and say to yourself, "I KNOW KUNG-FOO"!

[Guest tt350]

Dang!

[jt1]

Spiirt, you're on the right track. Let me say that an engine is a complete system from air cleaner to the tip of the exhaust. There are many, many compromises made on each part of that system. Spending a lot of time optimizing one part of the system will pay small dividends if the rest of the system is not similarly designed. Carb, intake, head, head mods, static and dynamic compression, CID, headers & exhaust, fuel, rpm range and operating requirements all affect cam choice. Many changes can have so small an effect that the only way to know if it's an improvement is lots of expensive dyno time. So consider your entire system, remember that any engineering solution has many compromises, a lot of them economic, and that changing any part of the equation changes the results. It it also important to remember our V8 zcars are, to put it nicely, exhaust challenged, due to crappy header choice and exhaust packaging requirements.

 

So keep the whole package in mind, balancing all parts of the equation yield the most pleasing results.

 

John

[grumpyvette]

good! it looks like youve been paying attention, to my answers to previous posts. now youll need to look into port cross section/length, volumetric efficiency, scavageing effect, ram tuning,dwell time, and how they effect the cylinder filling effeciency and at what RPM ranges each has the greatest effect. (Im in BILOXI MISS. at the moment so I cant post to much info)

BUT youll find that the flame front takes about 30-40 thousands of a second to burn and build pressure, the piston moves away from TDC adout a 1/4" before the rapidly increaseing cylinder volume starts to drop the pressure ratio, and that you rapidly loose cylinder effective fill rates due to limited time after about 4500rpm-5000rpm.

youll also learn that as your dynamic compression drops what your really doing is tradeing a longer valve open time and more power strokes per minute for a lower efficiency PER stroke but a higher total of effectively used cylinder pressure per minute.

so what your left with is that the port size and length must match the engine displacement, the rod to stroke raio thats the longest that economically can fit will take best advantage of that critical first inch of piston travel away from TDC where almost all effective pressure is available due to the longer rods dwell time being longer.

the higher the cpr the more effective the burm can be and the greater the pressure on the piston can be up to the [point where detonation occures.

the faster the fuel burns the greater the percentage of that burn time in the cylinder can be PAST TDC where it does some good pushing the piston down the cylinder, but the longer it burns the longer the pressure peak can last!

the time available to fill the cylinders goes down with the number of strokes per minute, so youll want the max EFFECTIVE power strokes where the cylinders are still filling efficienctly, yet the lowest duration cam to maintain the highest dynamic compression ratio.

 

a rough idea of port airflow and potential hp can be found with this formula

(the formula for POTENTIAL HP FROM AIR FLOW is (.257 X flow X #of cylinders= POTENTIAL HP

now that means youll have a hard time getting over 420hp with a stock untouched TPI but could potentially use a STEATH RAM to feed over 550hp)

 

 

BTW FOR THOSE OF YOU WHO DON,T KNOW WHAT WE ARE TALKING ABOUT!

ok after looking into this for several years heres what Ive found <b> ITS NOT AN EXACT SCIENCE but..YOU CAN GET CLOSE ON YOUR EDUCATED ESTIMATES and YES IM LEAVING A BUNCH OF STUFF LIKE FLAME FRONTS,IGNITION TIMEING,PRESSURE PEAKS, ROD LENGTH,ETC> out of this discussion</b>

 

(1) volumetric efficiency filling the cylinders and the resulting pressure pulse pushing the piston down into the cylinder, if graphed follows the engines torque curve graph extremely closely, or put another way the efficiency of the cylinders filling and scavaging increases with the rpm level untill a point where theres just not enough <b>TIME to effectively fill the cylinders,</b once that rpm level is reached the TORQUE peaks and altho the total HP may continue to climb for awhile because the NUMBER OF LESS EFFICIENT power stroke increases per second the effectiveness of each individual power stroke effectively falls in power as the cylinder filling time and cylinder burn time gets lower

 

(2) the pressure produced in the cylinder that depends on the cam timing and RPM level exserts pressure for only about 20-24 degrees of the 720 degrees in the 4-stroke engines repeating cycle and the EFFECTIVE cam timing (VALVES FLOWING AIR)filling the cylinders is limited to about 250 degrees with even a hot cam (less with a stock cam)

 

(3) displacement and cam timeing plus compression ratio and rod to stroke ratio have an effect on where and when in the intake stroke the max flow rate happends in the port.

 

 

(4) now lets look at port size and engine displacement, lets say you have a 350 chevy with a cam that effective flow is in the 230 degree range, (that would be about a 250 degree cam) look here, we see the average 250 degree cam is most efficient at about 5200rpm

 

 

so taking a 350 displacement/8= 43.75cid per cylinder, at a max piston speed of 4000 fpm .at 5200 rpm thats 43.3 intake strokes per second, at the probable max engine speed of 6857 rpm thats 57 intake strokes a second thats 1899cid of air at 5200rpm and potential 2493 cid of air at 6857 rpm but remember the valves only effectively open 230 degrees or 32% of the time so the port needs to potentially flow between 5935cid and 7790 cid of air per second, thats between 5935cid and 7790cid flowing past the valve through a port, the calculator say a 2.6sq inch port is the correct size at that displacement and rpm level, now 5935 cid of air flowing past a port 2.6 sq inchs in size is moving at 190cfps (cubic feet per second), at 6875 its moveing at 325 cfps but theres not enough time to fill the cylinders

<b>so as a crude guide your looking too find a port size that keeps the air flow velocity at between 190fps and 325 fps</b>

 

now lets look at my 383, its about 10% larger so its quite logical to figure the engines port speeds will be 10% higher with same ports or youll need a port <b> that flows 10% more, NOT a 10% BIGGER PORT</b> at that cam timeing and torque peak.

Now in your application with a 190fps-325fps air flow as a target plug your own engines info using the same math and see what port size you get, example lets say we want to build a 468bbc (468 bbc have a 4 "stroke so we have a 6000rpm max (STOCK)engine speed that,s 58.5cid x 8 cylinder displacement ,and with that cam timing about a 4000rpm torque peak, that,s 6088cid per second and 211 cfpm at 4000rpm and 317cfpm at 6000rpm , that suggests 2.66 sq inches-4.0 sq inches at 6000rpm for that 468 bbc

 

http://www.rbracing-rsr.com/runnertorquecalc.html

 

if youll look and compare youll see why the 468 engines tend to run better in the mid range rpms with oval port heads

 

http://www.newcovenant.com/speedcrafter/calculators/runnerarea.htm

 

<b>so the bottom line here is your looking for a port that flows about 2.0 sq inchs and 210cfm at 5000rpm and 2.68 sq inches 270cfm at 6857rpm average the two and youll be looking for a port of about 2.3-2.6 square inchs that flows between about 250cfm-270cfm at your cams peak lift because remember the port cant flow enough due to time restrictions at the peak rpm range</b>

 

 

 

BTW thats most closely matched by a 195cc AFR head below 5500rpm and ABOUT 235@.050 lift duration and a 210cc head above 5500rpm and 240@.050 lift duration on a 400cid engine

the short answer here is that its NOT PORT VOLUME ,ITS THE AVERAGE PORT CROSS SECIONAL AREA your looking for! a #1205 gasket size port is about correct for hot street and a 1206 port is about correct for an engine thats mostly used on the track

look at it this way a 195cc port thats a #1205 size cross section tends to flow about the same and have the same average air flow speeds as a 210cc port thats also a #1205 cross section. these about 16.38 CCs in a cubic inch, a port that measures about 2.5 sq inches like a #1205 port needs to be only about .67" longer to remain at the same cross sectional average, raiseing the port floor and roof and changing the entrance angles can easily account for a great percentage of that volume

 

http://www.chevytalk.org/threads/showflat.php?Cat=&Board=UBB64&Number=239801&Forum=UBB64&Words=LSA&Match=Entire%20Phrase&Searchpage=2&Limit=25&Old=1year&Main=239797&Search=true#Post239801

 

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

some good general info look closely at the duration used for each MATCHING rpm range. ALSO KEEP IN MIND THE DCR AND OVERLAP MUST MATCHlook herethese 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. 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 A> and keep in mind lower overlap duration and LSAs of 112-115 work better with EFI and I can tell you right now that intake durations not to exceed about 222 degs@.050 (intake duration)are what youll need for a close to stock TPI intakes rpm/tq range<BR><B> now just to make you crazy, 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

[Guest DaneL24]

I agree with SPIIRIT, I want to be able to understand the engine system well enough to predict how it will perform without some long trial and error process...so I too have been hitting the books.

 

I came up with a simple way of estimating "effective compression ratio" in relation to intake valve closure. To find the effective compression ratio, you use the equation:

 

{[3.1415...(Bore/2)^2][(Stroke/2)+(Stroke/2 X Cos Deg ABDC Int V Clos)]}/Com Cham Volume

 

Basically, you figure out the volume of one cylinder and use the Cosine of the degrees ABDC the intake valve closes to determine the actual colume of gases before compression, and then divide by combustion chamber volume to find the "effective compression ratio".

 

You can also use this method to figure out how much more static compression you will need to retain the same compression PSI with a bigger cam. This is of course an estimate, because it doesn't take into account port velocity, volumetric efficiency, etc...just compression ratio vs. valve timing.

 

Also, I always did wonder why heads were rated on runner volume as opposed to runner cross-sectional area, because cross-sectional area directly affects port velocity...not volume. Of course, volume is affected by runner length, and length can be affected by twists and turns in the runners...which do affect flow. It was nice to see some actual ideal port velocity numbers from a retired engineer too (190-325 FPS...thanks grumpyvette) Those stats will really help in determining the correct port size for a given engine displacement with a given powerband...

 

BTW, if you want to get into really weird cam profiles, look at cams for turbo motors. The positive manifold pressure scavenges the exhaust gases during the overlap period, and too much overlap will blow raw fuel into the exhaust, and can even ignite from exhaust heat and damage the turbine! A backfire in a turbo engine is more dangerous than in an NA engine. Hence, turbo cam grinds have very little overlap. I had an idea for a turbo cam grind that would open the exhaust valve earlier BBDC...and in theory would allow for a faster turbo spool...as well as more exhaust duration for high RPM breathing. Since most of the force from combustion has been applied to the piston before 90 ABDC, you could probably add a lot of timing before decreasing effective combustion stroke duration (spark plug firing to exhaust valve opening). Too early though and the turbine is directly exosed to combustion and can be damaged...as well as burning out exhaust valves. That cam profile has probably been done before...but I have never heard of it.