dynamic compression ratio

[Guest Anonymous]

KiD ViD;

 

COMPRESSION FORMULA PROBLEMS

 

There are two unknowns that appear as a result of combining the two Programs titled "Static Compression" and "Dynamic Compression" that I have been using in an attempt to resolve the problem of finding the optimim "Street Compression" value of 8.5:1 using Preium Pump Gas.

 

One is the fact that if you are building an engine from scratch, there is no way you can determine what the correct "Compression Height" of the piston should be (even when the degree of Intake Valve Closing ABDC is known). This is because there is no way for the student to prematurely know when the intake valve should close in order to achieve the magic number of 8.5:1 Dynamic compression. If you introduce an artbitrary number of degrees for this Intake Closing and juggle it (in the Dynamic Program) until the dynamic compression is 8.5:1, then things seem to fall in place all around but.....

 

If you look back at the Static Compression Calculator, you will see that the "so called" Deck Height is also an arbitrary value by reason of the fact that the Builder has no idea where the Piston Manufacturer is going to place the wrist pin center either. Now all bets are off as they go about the business of reducing size and weight, and relocate the piston on the wristpin for optimum performance according to real Deck Height and Rod Length (c-c). What this boils down to is that if you order the cam first, then with that info plus the completed head data and the fixed block deck height, the piston mfgr. can do a lot to keep you out of trouble and produce the correct pistons having the compression you need.

 

On the other hand, if you presume to randomly "Build as you go", you will find yourself jumping all over the place trying to appease the demands of the piston mfgr. (They may leave you with a call-out for milling several thou. off the block deck for example).

 

The piston manufacturer is the only one who has amassed enough Data and Experience over many years to be able to strike a triple match between Intake Closing, Piston Position and Dynamic Compression Ratio, and this is all because they know where, when, why, and how to ask all the right questions.

 

Now the second problem with combining these two compression programs is that again, the Dynamic one always gives you a result in "Chamber Volume" that is not what is expected simply because the volume in cc's of the head gasket has been added to the equasion (in spite of the fact that nothing is mentioned about this in either program). This in turn brings about our attention to the fact that this total value then must in some way also include any "Quench" area, +/- so called deck height, compression height, +/- Dome/Dish piston profiles and all their associated volumes; all these having a direct bearing on that ideal 8.5:1 DCR.

 

Nuff-Said I bet. So actually, although these programs are very enticing; because of their lack of detail (due to impossible circumstances), they remain "Toy's Of The Mind" when you get right down to it! But working with them does bring out some good things including the proper "Place for" and the "Value of" the piston manufacturer and where their very important role fits into proper engine building!

 

Did I say all that?.....LOL!

[KiD-ViD]

the program I have has inputs for head gasket bore and thickness so it has that part covered. also I am going to be using stock pistons because this will be an N/A motor and so what if shit breaks just go to the junk yard and pick up another block for $150. so that rules out the pit height as well. (the Lengine program has the deck height for all the possible setups.)

 

It has come before me that for every 0.005" removed from an aluminum head, you reduce the chamber volume by one (1) cc./cylinder. And, for the cast iron:, for every 0.004" removed, you reduce the chamber volume by one (1) cc./cylinder.

 

im not sure I understand this. I am under the impression that this figure would depend on the bore size of the combustion chamber. I wouldent think there would be a fixed number for all heads. also what is the difference between the alum head and the iron head. I see size as being size and I cant figure out how the material would matter. I have a feeling you were waiting for me to ask this though so please inlighten me if you can.

[Guest Anonymous]

KiD ViD;

 

This is what I get for repeating what I read.

 

I'm not sure but I was sorely tempted (from memory) to write that the cast iron head number was for quote "Older Heads", whatever that means. Anyway I am sure any race machine shop would be quite familiar with this kind of thinking about cc's per so many thousandsths removed. When you hear this kind of stuff I'm sure you always check it out with a reputable source before committing to it right? Because I havent. But it sounds good.....LOL And then there's the one about putting mothballs in the gas tank for more power.....how about that one ? Tried that?

 

Heh; if you're building out of the bone yard then I can understand that approach. Ah!.....the good old days!

 

Later.

[zxgts-4]

Well I just cant resist putting in my 2 cents worth.

All these web sites and links are just full of great information for any budding Automotive Engineer. Proving that to the average person it pays to seek out professionals to get good advice. If you cant get a grip on all the variables involved in engine design and dynamics, dont start building engines.

 

Most of us with street cars accept the fact if you want seriously more power use a bigger engine. For those into force feeding you get a bigger turbo or smaller blower pulley ( at the expense of engine life).

 

Thats my opinion. Neil

[KiD-ViD]

I understand all the variables but I was originally pretty much just wondering about volumectric pressure. I am pretty sure I get it now though. thanks you guys for the help. I will seek pro advise when it comes time but for now I am just trying to see what I can learn.

[zxgts-4]

Lets have another go! The magic No here is the one that comes after a dollar sign. The higher your VP the greater the quality of components is going have to be. You haven't stated what the motor actually is , brand and size. From that we can probably give better advice. Late model engines running high nominal and effective compression have knock sensors and computers to counter some of the variables.

Just picking out a camshaft is probably not the ideal place to start but we only learn from our mistakes.

Check out your nearest Tech book seller where you should be able to find books on street, strip or racing any type of engine.

Where you should be able to find the other engine specs to match your cam selection.

Theres plenty you can do to any engine it just takes time and money so good luck. Neil

[TimZ]

It has come before me that for every 0.005" removed from an aluminum head, you reduce the chamber volume by one (1) cc./cylinder. And, for the cast iron:, for every 0.004" removed, you reduce the chamber volume by one (1) cc./cylinder.

 

im not sure I understand this. I am under the impression that this figure would depend on the bore size of the combustion chamber. I wouldent think there would be a fixed number for all heads. also what is the difference between the alum head and the iron head. I see size as being size and I cant figure out how the material would matter. I have a feeling you were waiting for me to ask this though so please inlighten me if you can.

 

Head material has nothing to do with how much the chamber volume is reduced. As you suspected, it has everything to do with the shape of the combustion chamber.

 

For an open chamber head, the amount is fairly close to the area of the bore times the amount that was shaved. In the case of an 87mm bore, the number works out to about 0.75cc per 0.005".

 

For a closed chamber head the number will be considerably less - probably about half to two-thirds that amount.

 

For open chamber heads, the 1cc/0.005" rule of thumb is fairly close, simply because most piston engines are in the same ballpark (+/- 50%) as far as bore size goes.

 

 

Spiirit - What's the story with that avatar? It's making me all sad...

 

[KiD-ViD]

the motor would be a carbed l28...and probally stroked to 3.1. and thanks tim for the clarification.

[Guest Anonymous]

Thanks Tim; (I'm distraught because my dad won't front me for my $20k street rod).

 

Ya, the head shaving statemt was short on detail, not mentioning open or closed at all. I wondered about the material difference but assumed he knew something I didn't. From what else I have read I don't think we be shaving aluminum heads at all (ruins their trade value) but do the pistons instead. But, wise guys will do whatever it takes at the moment to fill their needs. But I know some will go fo a better pushrod angle that way.

 

The same guy was telling how you should leave your piston in the jig when removing material there and just weigh the aluminum shavings (at so much/gram or whatever). Never said if for balancing, valve clearance or what, maby all of the above. I never seen it fail, there's one in every neighborhood.....LOL. (unfortunately some of us still don't have a Bridgeport over in the corner for these piddling jobs because the're too big to carry off/steal by yourself).

 

And just look at the KiD! He's weakening. I think he's getting "Whileimatititus". Yep! Only a metter of time now and he'll be going SBC.....LOL.

[KiD-ViD]

ya tell me about it...I was on the sbc bandwagon when I first got into Zs then I got into going all original and now I am really leaning towards a turbo setup. I better just hurry up and do somthing and get stuck with it so I will be forced to make up my mind haha.

[Guest Anonymous]

KiD, are you a Libran by any chance? We are well known for our vascillating!.....LOL.

 

Sereiyusly though, what you need is two jobs!.....Har, Har!

[Kevin Shasteen]

I hate these duplicat entrys --ignore this one obviously

[Kevin Shasteen]

Kid,

 

The majority of data on DCR's available to the commoners, such as you and I, are mostly on American V8's as the specialty teams that work on other engine types are not so quick to release torque/hp figures w/any of their other engine specs. What I am saying is that most of us can talk V8 stuff all day long, but when it comes to a non-V8: well, the data is somewhat minimal in regards to DCR/SCR after effects of component changes. So, the data you request may actually be in the mind of an expert, and they are not always willing to release their secrets. So if you are going to apply this DCR/SCR tool, then you will have to familiarize yourself w/the "Airflow Patterns" you find in whatever engine family you choose to play around with.

 

I believe what you have to know, or understand, is that your modifications are dealing w/airflow pressures...as John stated earlier. In encountering Pressure you will always be looking at airlfow's: 1) Velocity, 2) Density, 3) Volume.

 

When using the DCR/SCR formula's as tools; you must understand that any variance from the "Factory Setting" will result in your moving the power band up or down in the RPM range. Most folks attempt to increase the volume of the ports both in the intake & in the cyl.heads: this usually moves the power band substantially higher in the RPM range. In our inliners the peak power occurs in the +6000 rpm range...exactly how much futher up the rpm range do you want to move(?). This is where recognizing airflow patterns in an engine family is paramount: such as the limited cyl.head breathing we find in our Z inliners, as this is something you must address.

 

If all you are going to do is throw in a cam & not touch anything else then your approach, "What can I get away with" is a valid question...but still you must be able to recognize airflow patterns. Now, what would be the result if you replaced your inliner's cam w/a monster cam, keeping in mind this minimizes your DCR-which moves your power band even higher than the already high +6000 RPM range; then you adjust your SCR accordingly, yet did nothing to the intake manifold nor the cyl.head to improve Airflow Velocity at lower rpm's? Probably detonation/pre-ignition simply due to the inliner's inability to breath at lower rpms. This is what I mean when I say "Recognizing Airflow Patterns". This is also why the DCR/SCR can not be looked upon as an absolute by themselves.

 

If it were my inline that was getting modified, I would want to know how to create more torque down low...and not be so concerned w/the addt'l power up high. This is what I mean about "recognizing airflow patterns" whereas in a SBC there is plenty of torque down low due to its better breathing capability. This is also why you could get away w/a DCR of 7.0 on a mild SBC & still possibly skate by w/out Detonation...yet this is also still a huge compromise-so why compromise when you dont have to?

 

In other words, the DCR/SCR tool is not an absolulte-rather it is a gauge to aid the builder in helping them get to a much closer level of expected performance than they otherwise would not have had if the tool never existed in the first place. Did that make any since? Yet what good is it as a tool if you dont first know how the rest of your engine breathes?

 

As far as "What can you get away with": that all depends on how much you alter. Let us say you had a completely stock engine and all you wanted to do was upgrade your cam. If this were the case you could probably get away w/a DCR all the way down to 7.5 and your engine would still run on lower octane. I would only want to know why you would want an extreme compromise such as that. If you took this approach your SCR would not be sufficient & your performance gain would be minimal in comparison to your performance loss. Now consider a hipo build w/monster cam, max'd out DCR/SCR's...you surely could not get away w/lower octane fuel in this latter example.

 

Now, if you were completely upgrading all the engine components in a total hipo rebuild-you would build it correctly to begin with, so why would you want any compromise at all? After all-our inliner's peak power is in the +6000 RPM range...why would you want to move that peak power any higher; rather you should want to build in lower rpm torque. If you are totally rebuilding then start w/the 8.5 DCR, to begin with, & build into your engine whatever SCR the math tells you is required: just remember the inherent "Airflow Patterns" such as the Datsun Inliners inability to breath down low in its NA set up. This is why that engine reacts so well to a turbo...it needs the addt'l lung capacity in the lower rpms (Remember-recognizing airflow patterns) and adjusting your build accordingly = success.

 

The DCR approach is merely a tool. Its purpose is to allow the end user, by way of math ratio's, an appropriate gauge for making SCR corrections. This correction is an attempt to build an engine that has adequate airflow velocity leading up to the cyl.head(s) intake valves...but this should not be looked upon as an absolute-it is close to an "almost absolute" but in and of itself is not an absolute.

 

Kevin,

(Yea,Still an Inliner)

[pparaska]

I'm not sure if this has been mentioned yet, but DCR is just another way of expressing the cylinder pressure (due to compressing the mixture after closing the intake valve).

 

For instance, Look at the equations at:

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

and the spreadsheet in "Note 1"

 

Notice the "Effective Compression Ratio" (CRE). This is the same as Pat Kelley's "Dynamic Compression ratio".

I've run the same parameters through both to be sure.

 

Also, notice that "Cranking Pressure" is the absolute pressure

in the chamber, directly related to the Effective Compression ratio

by: CP = CRE^1.2 * AP. AP is atmospheric pressure, 14.7 psi at sea level.

 

Gage pressure is just CP - AP, since a gages show pressure relative to the atmospheric pressure present around the gage workings.

 

So DCR, CRE are directly related to "Cylinder Pressure".

 

No mystery.

 

Just as people have recommendations for max cylinder pressure on 93 octane gas (usually 200 psi), they have recommendations for max DCR.

 

The one thing about DCR (CRE) that is different is that it not tied to the prevailing atmospheric pressure. I'd think that cylinder pressure that a certain octane gas would detonate at would be a function of absolute cylnder pressure (CP), not DCR.

 

So wouldn't cylinder pressure (CP) be the thing to shoot for?

[Guest Anonymous]

You know what I think? I think you guys had better start discriminating amongst the articles and their termenology that you're reading Consider the author.....Is he using the proper accepted american language to describe the subject or is he introducing his own fabricated terminology as he goes along and confusing the issues?

 

If we're going to help others we can't go around doing like this and expect not to utterly confuse the very ones we want to help! It's bad enough to see the errors we ourselves make in even the most fundimental areas without having to observe a total disreguard for the fully established and long time standard accepted when writing about engines in america.

 

A lot of the stuff that keeps popping up in this website concerning engines is strictly "one man's opinion" and no attempt is made to follow any established rules, while the mind is left to wander as the writer sees fit as he expresses his own opinion in a 'Everything has to be different" attitude. You come into my classroom with that stuff and I straighten you out real quick! Pick up any well written book on engines and you won't find any of the screwy terminology I am talking about here, so stop it!

And if the shoe fits, WEAR IT!

[pparaska]

did i say something i shouldn't have? i was just trying to tie things together.

[Guest Anonymous]

Pete;

 

Just a passing comment as food for thought to all Pete, I'm not calling any names.....LOL

[Kevin Shasteen]

.., "Effective Compression Ratio" (CRE). This is the same as Pat Kelley's "Dynamic Compression ratio".., Also, notice that "Cranking Pressure" is the absolute pressure in the chamber, directly related to the Effective Compression ratio by: CP = CRE^1.2 * AP. AP is atmospheric pressure, 14.7 psi at sea level.., DCR(CRE) is not tied in to atmospheric pressure, wouldnt Cyl.Pressure (CP) be the thing to shoot for?

 

Pete,

 

Let me ask you this. When you are looking at a Dyno run & you notice where your peak power plumits-is this not a result of the airflow velocity inside the Cyl.Head Intake Ports going supersonic? This moment where your airflow goes supersonic hinges not only on atmospheric pressure...but is also acting upon your SCR, and your SCR is a function of your DCR. Your SCR can only be determijned once you have calculated your DCR and your DCR combines the atmospheric pressure in determining your Cylinder Pressure (CP) upon the compression stroke. My suggestion is this, I dont think you can dismiss the DCR/SCR relationship any more than you can dismiss atmospheric pressure in determining Cylinder Pressure: they are all relative to one another. Building an engine isnt done by one formula alone. There are a multitude of issues to nail down: atmospheric pressure isnt acting alone-it dances w/all issues hand in hand.

 

Yes we should shoot for Cyl.Pressure..but we shouldnt throw out the DCR/SCR baby with the Atmoshperic Bath Water.

 

Spiirit,

 

Dont let the math overcome you. This "Cylinder Pressure" that the site Pete posted talks about & that John mentioned is the next step in a long line of connecting the dots. If you want to understand & apply book smart math to your engine then you will need to grapple w/these issues.

 

Look at the formula: CP = CRE^1.2 * AP

 

It is using the DCR that you have been familiarizing yourself with, only it refers to it as CRE: he says Tomato' I say Toma'to...same thing. Only now you have the next step in using your DCR tool, and that would be to use your DCR obtained, combine that DCR with your Atmos.Press. to calculate your Cylinder Pressure with the above given formula.

 

Unfortunately, when using this formula you now need to understand how to go about calculating Atmospheric pressure.

 

No one is pulling a fast one over you. There are performance enthusiasts everywhere-in every country. They post their thoughts just like you and I do. Sometimes they are smarter than you & I and in posting their thoughts they will use vernacular that is foreign: this doesnt mean that what you have been reading prior to that is invalid-it just means that sometimes you just have to read thru what some people write-and recognize it for what it is.

 

If you were not already familiar w/Cyl. Pressures then you should be aware that a low cranking cyl.pressure equals low rpm torque while a high cranking pressure equals excellant low rpm torque. To obtain your cranking pressure simply remove all your spark plugs and do a compression test in each cyl. to obtain their own "psi" reading.

 

The books you read always quote three classes of psi readings, these are the BMEP=Brake Mean Effective Pressure, which usually approximates three classifications or levels of engine performance possibilities, and they are:

 

1) 130-145psi = Standard Passenger Car

2) 165-185psi = HiPo/Sports Car

3) 185-210psi = Dedicated Racer

 

Remember-these are simply tools to achieving an end. The end would be the best edjumacated guess possible...then put in the R&D & have fun: and make changes along the way: that is called experience.

 

Dont let it beat you or confuse you. Be patient and keep reading: when you least expect it the brain will cease in being strained and the light bulbs will begin coming on & staying on.

 

SPIIRIT, FWIW: This V/P index site has been posted many times in the past years I have visited this board. I have even written those formula's down from that site, yet, this is the first time I have understood them. I understand them now only because in the past three years I have read eng.perf.articles till my eyes & brain both hurt. In fact I forgot all about the formula Pete posted: even tho I have already written in down...now that I understand it I will remember to add it to my DCR/SCR tools. Now my edjumacated guess will have further depth: this is what it is all about.

 

Thanks Pete for posting that site again. 8)

 

Kevin,

(Yea,Still an Inliner)

[Guest Anonymous]

Well thank you Kevin!

 

Now I better understand where you are coming from!

 

I have no further questions or comments on the matter and consider it closed!

 

By the way, I was raised in San Antonio, Dallas and Wichita Falls. Left there for California in 1957. Never heard of Ravenna, Texas. Whereabouts is it located.....in the Valley?

[Kevin Shasteen]

.., I have no further questions or comments on the matter and consider it closed! Never heard of Ravenna, Texas. Whereabouts is it located..,

 

SPIIRIT,

 

You should know better than to think anything regarding engines is a closed subject. Ravenna is about 90 miles NE of N.Dallas.

 

Regarding BMEP, Brake Mean Effective Pressure, they will be at their highest when the engine is at its most efficient: and that is during peak torque. So when you obtain your BMEP figures from Cranking Pressure then you should know that those numbers will only reflect your low rpm torque and should not expect those numbers to reflect your BMEP at peak Torque nor at peak HP.

 

BMEP mirrors BSFC, Brake Specific Fuel Consumption, in that it is rising or falling depending on the rpm range the engine finds itself in at any one point: and both BMEP & BSFC will be at their most efficient during peak torque. BSFC, while at its most efficient, will be a lower number while BMEP, is at its highest rating, during peak torque.

 

Just a reminder-so that we are all reading from the same page.

 

Kevin,

(Yea,Still an Inliner)