What compression ratio?

[James2]

What's safe for my engine?

The blue Bible tells us 7.3, regardless of other factors. Others say it depends on cam. Or the Dynamic Compression ratio.

What really determines safe CR?

BMEP. Brake Mean Effective Pressure. simple, it's the amount of pressure produced inside the cylinder. Things that affect BMEPt-

Static compression ratio
Cam
Cylinder displacement
Head flow
Induction flow
Exhaust flow
Spark advance

Also, affecting detonation and preignition is head temperature and design, piston squish.

So? Shut up and make a point.

the point is, a safe compression ratio choice can not be made using only the cam choice. Also Berg is wrong, because these other factors affect BMEP. What has to be taken in is the whole engine package to determine a safe CR.

For example, a 1915cc engine, running 10-1 CR with dual 48IDAs, large heads and a k-8 would have a BMEP of over 200 psi. Not safe for pump gas.
Take the same motor, same CR, but with a 86mm stroke and the BMEP drops to 188 psi.

Why? because the bigger motor makes more HP, but less hp per cubic inch. The specific output is less, so the BMEP is less. so it can run on pump gas, but the 1915 can not. Same cam, Same CR.

Or take the 1915, and remove the big heads and put stock heads on it. BMEP drops to to below 180 psi. again, it's safe to run pump gas. Again, same cam, same CR.

As a matter of fact, specific output is a better guide to picking a CR then just the cam. Higher specific output= higher BMEP, which needs more Octane.

This why I limited my own motor to 8.65 CR. I wanted to keep the BMEP below a certain point, and thus limited my self in HP.

IMHO, 1.4 hp per cubic inch is about the safe limit on 93 Octane pump gas.

What's this mean James?

It means larger motors can run more CR safely. It also means, no one can tell you what a safe CR for your motor is, unless they have ran the same combination as you before. Not just the same cam, but the same carbs, heads, and displacement.

[This message has been edited by James2 (edited 08-06-2002).]

[MagOO]

Air temp and rel humidity matt4er as well and i think if you put a stock head on the 1915 , it would sound like a box of castanets in a earthquake...big valves, big ports ,within reason should help the head stay cooler...

[James2]

The more air flow does cool the head, but the stock head limits air flow so much that the cylinder never completly fills, limiting the dynamic compression.

But , yes, that was my point also, there are lots of factors that limit Compression.

[jasonb]

BMEP is a power/displacement calculation that indicates the amount of stress that is being placed on your engine, so it affects the longevity of your internal parts. Cross a certain point and your longevity may drop to 0. That's why someone I know couldn't drive 10 blocks with his supercharged 1641 that used cast pistons. BMEP should be limited for any engine that wants to run 100,000 miles or 20,000miles or 10blocks.
But you'd be better off limiting BMEP through head selection, as you showed, rather than limiting CR.

Selecting the proper CR has to do with maximizing the efficiency of the combustion process. Many things effect this efficiency: Chamber shape, air density, charge temperature... Higher CR is one way to create a more efficient burn but you "max out" at the point where the compression itself creates detonation before the spark is applied.
But running CR too low can also have it's negative side effects, or so it has been reasonably argued. It would seem that CR decisions are best made based upon the dynamics of the combustion system.

BMEP, being a measure of the "result" of the combustion cycle, doesn't tell you where detonation is going to occur. In fact it gives you no insight into the dynamics of the combustion system at all. It gives you a measure of the forces acting on the system. BMEP is something that may be best dealt with the way we always have, by throwing more money at the engine. Forged crankshafts, forged pistons, aluminum cylinders, cryogenic treatment, turbo chargers (huh? Turbos allow you to run higher BMEP.)

But I agree with your basic premise that the issue of CR selection is much more complex than cam duration. We can maybe take one shortcut with less highly modified engines; we can estimate the dynamic compression of the stock, German engineered, system and then aim for that same dynamic compression using "bigger" cams by increasing the static compression.
In fact, every calculation we make is a shortcut. Take the method of calculating BMEP: HP X 13000/displacement X RPM. The real calculation would make you cry, there's all kinds of pi's, logs, gammas and torques.

[James2]

Thanks Jason,

what I'm looking for is a way to develope system to give someone a reasonable answer when the question is asked" what compression ratio?"

It's one that is asked often and debated hotly. There is no simple answer, but as you point out, a short cut answer may be developed, maybe here.

And I know BMEP is not the only factor, because some new cars develope BMEP way over 200, and run fine pump gas.

[Marc]

Regarding dynamic compression ratio....

Why do we put in larger-than-stock cams, anyway? Right, to allow the engine to turn higher speeds and make more power by allowing the cylinders to fill better than the factory cam does. By altering the timing of the valve opening/closing events we can take advantage of the momentum of the incoming charge to continue to fill the cylinder even though the piston is already well on its way up, thereby getting more air & fuel in than a stock cam would allow. At low speeds power is lost, though, because there is insufficient charge momentum to capitalize on this effect and the cylinder will fill LESS than it would with a stock cam - some of the incoming charge will be pushed back out of the still-open intake valve, disrupting the operation of the carburetor and causing that rumpity-rump sound that we associate with a "hot" engine. This decrease in volumetric efficiency at low speeds can't be avoided without having some form of variable valve timing like modern engines are starting to use. If the cylinder only fills 80% of what it would have at the same RPM with a stock cam, it takes a 25% higher compression ratio to achieve the same cylinder pressure that you'd have had with the stocker - necessary to recapture the power lost by the big cam at low speeds. By this reasoning, a 9.375:1 static C.R. gives a "dynamic" C.R. of 7.5:1 at 80% V.E. and would lead one to believe that Regular, or at most Mid-grade, gas would be all that this engine would need to avoid detonation, overheating and engine damage.
But let's not forget the reason we put that big cam in there in the first place. It certainly isn't done to reduce the V.E. of the engine - on the contrary it's done to INCREASE the V.E. at higher RPM. In some cases the V.E. can actually exceed 100% at the speeds for which the cam was designed. I won't belabor that point, but it should be easy to see how the V.E. could easily be, say, 90% at higher RPM. At 90%, our 9.375:1 engine has a dynamic C.R. of over 8.4:1 and will need a steady diet of Premium (or better) fuel if it is ever going to be run at elevated RPM.
And if you don't intend to run it at 7,000 RPM, why put a 7,000 RPM cam in it in the first place? It's generally accepted that you'll lose more usable RPM at the low end than you'll gain at the top end when you cam-up an engine. Big cams and high C.R.s are for racing. For street use and longevity, use them in moderation. But most of all realize that the concept of "dynamic compression ratio" has been around longer than the internal combustion engine - nobody just discovered it, although some apparently would like us to think they did - and the laws of physics haven't been rewritten.

[jasonb]

Since recommended CR is such a common question it probably deserves an online calculator.

How about a calculator that allowed you to input your Stroke, Bore, Chamber Volume, Fuel type, Altitude, Boost Pressure and Cam from a drop down list and output a Static CR recommendation?
The program could use an equation to work backward from a safe Dynamic CR to arrive at Static CR and required Deck Height.

The ultimate ACVW resource would be a program that would allow you to input certain details about your desired engine combination and then fill in the other details. The shortcut would be that since your dealing with certain know quantities (IDA manifolds, IDF manifolds, dual Kadrons, Bugpack 1-3/8" exhaust, Web 218 cam) you can get away with using empirical data and simpler equations than your typical desktop engine program. The program wouldn't create a simulation of the engine but would use equations to output results or parts combinations or, alternatively, output engine parts for desired results.

That's jumping ahead though. Does anyone believe that an ACVW Recommended Compression calculator would be a valuable resource? If so, what recommendations do you have for how it should work?

[MASSIVE TYPE IV]

I know my margins, I know my standards, and I know they work. I prove it every day.....Ever heard a member here complain about one of my engines running hot or crappy????how about one exploding?? It will happen, just as everything will, but obviously what I do, and have done works, all else aside.....

Main thing is that no one thinks about Dynamic CR, it has been around since the first engine fired up, but a certain group of individuals never used it in their testing, so I guess that makes a VW engine different??? Nope.

------------------
Jake Raby
Raby's Aircooled Technology
www.aircooledtechnology.com

[Deathbug74]

there probably isn't an online calculator because someone would sue the maker when they blow their engine up

------------------
you can't squash this bug!