TheSamba.com :: View topic - Static compression vs Dyanamic compression

Sun Mar 21, 2010 6:36 am

This was taken from Hot Rod magazine in June 2007... its a pretty good read and should help with all of the CR threads that are on here..... Yes I know its about a Chevy but the principle behind it are the exact same for every engine in the world... I hope this helps some of you guys out.

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Dynamic Compression
How to calculate dynamic compression on a small-block chevy
February, 2009
Dynamic Compression
-June 2007-

Question: I am building a 355 Chevy for the street using aluminum flat-top pistons with valve reliefs that at TDC sit 0.024 inch below the deck. The heads are 64cc iron Vortec 062s with screw-in studs and Crane PN 10309-1 springs and retainers for added lift. The cam is a Crane 282-H06. I am able to calculate the head gasket thickness necessary to achieve a static 10:1 compression ratio, assuming the valves are closed for the entire compression stroke. However, with this cam, the intake valve is open until 34 degrees ABDC. Should this be taken into consideration? If so, how do I calculate the actual working compression ratio in an effort to achieve a 10:1 ratio?
Peter Illemann
Goderich, Ontario, Canada

Answer: The compression ratio listed for any engine combo is always the static compression ratio, which is fixed via component selection and machining practice when the engine is built. As you've surmised, it assumes the cylinder is full of air and fuel at bottom dead center (BDC) and that all the mixture is compressed into the top dead center (TDC) combustion cavity. But an engine really doesn't begin making serious compression until the intake valve closes and seals the air/fuel mix in the cylinder. That's why engines with identical static compression ratios can have significantly different cranking pressures, as seen with a common compression gauge. Although part of this is due to displacement differences (a larger-cubic-inch engine is a bigger pump), the main influence is camshaft design. By changing certain cam parameters, it's possible to bleed off cylinder pressure on the bottom end, decreasing fuel octane sensitivity, even though its static compression ratio remains unchanged. The actual cylinder pressure an engine sees is often referred to as dynamic compression, because (unlike the static built-in compression ratio) it changes dynamically according to camshaft variations. The most important of these variations is the intake closing point, because it extends beyond BDC into the compression stroke. Closing the intake later aids top-end power at the expense of low-end torque. Down low, where the engine is most likely to detonate, the late intake closure bleeds off cylinder pressure, effectively dropping the dynamic compression ratio.

Rod length and crankshaft stroke also play into the dynamic compression equation because they alter the piston's position within the cylinder at the intake-valve closure point. How all this comes together is explained by KB Pistons' John Erb: "Consider a 10:1 Chevy with a 3.48-inch stroke, 6-inch center-to-center rods, and a very hot race cam that closes the intake at 90 degrees after bottom dead center (ABDC). This engine will think it is running with 6.17:1 compression and will be happy with 80-octane fuel. As a general rule, the best available pump gas will work with an 8.0:1 dynamic compression ratio. To get 8.0:1 with the preceding rod, stroke, and cam intake closing event, you would need about a 13.2:1 static ratio."

Erb's 8.0:1 dynamic guideline is for classic, old-school, all-iron engines. An advanced, high-tech engine can go a point or so higher-but even with the latest high-tech electronically managed engines, actually shooting for a 10:1 dynamic ratio is way too high. Calculating the relationship between piston position in the bore and intake closing point and then determining the effective dynamic compression ratio requires some pretty serious math, but KB Pistons has a Web-based calculator that makes it simple-just plug in the requested numbers.

KB Pistons (United Engine and Machine Co.)
Carson City, NV
800/648-7970
www.kb-silvolite.com

Sun Mar 21, 2010 7:36 am

03OrangeSVT wrote: This was taken from Hot Rod magazine in June 2007... its a pretty good read and should help with all of the CR threads that are on here.....

good thought, SVT, but since all the existing threads explaining dynamic vs. static compression didn't calm down the number of Compression ratio questions, this one likely won't either.. :lol: this one will be like all the other ones, good information lost in the pile that no one will search for... :wink:

Sun Mar 21, 2010 8:10 am

I still feel there is a difference between an aircooled engine and a water cooled engine.
You should write a letter to the editors of Hot Rod magazine and ask them if they have done any testing on aircooled engines.

Sun Mar 21, 2010 10:13 am

The article is explaining how to CALCULATE compression ratio. In terms of CALCULATING the ratio, static and dynamic and all variables involved, there is no difference between motors. Both have pistons, cam lobes, rods and valves. Therefore, there is NO difference in calculating what combination of parts will give a specific ratio. "I have this, this, this, this, and this with xxx cc in it. What is my dynamic CR?" That question can be answered by any How-To-CALCULATE-Compression-Ratio article.

Did you see the title? "How to Calculate..." Putting a fan shroud on the motor doesn't suddenly change the math.

03OrangeSVT wrote:

Erb's 8.0:1 dynamic guideline is for classic, old-school, all-iron engines.

Look, he's already added the disclaimer. We're doing math, not building V8s. :roll:

Sun Mar 21, 2010 10:33 am

^^^^ I agree, a fan shroud doesnt magically change the laws of physics.

Sun Mar 21, 2010 10:49 am

Regardless of how you calculate the compression ratio, the detonation point of the fuel remains the same. It is determined by the amount of heat that is imparted on it due to either heat gain from mechanical means and heat gain from sources such as piston temp, head temp, exhaust back pressure and F/A mix. While the others may be the same between a Air cooled and a water cooled motor, Piston temp, head temp and exhaust back pressure will all be higher and add more heat to the F/A mix and causing the fuel to detonate at a lower C/R. ALWAYS.
Physics always rules... :shock:

Sun Mar 21, 2010 10:55 am

That's not what I was referring to.

Btw, I did a little math using that awful Chevy calculator :roll: at kb-silvolite and it turns out that an FK-98 running at 6.6:1 has a dynamic CR of 4.6:1. FOUR to one. My father's Model A runs at 4.22:1. Does it ever occur that maybe that motor would be happier at a DCR of 7 or 8? And that maybe the SCR has to be at 12 for that to happen? Should we take everything into account or should we just use blanket statements?

Sun Mar 21, 2010 11:29 am

Sigurd wrote: that awful Chevy calculator at kb-silvolite

There's nothing wrong with the kb calculator. If you take the time to go through the posts here on TS and gather data on good engine builds then enter that data in the kb calculator you will see a trend develop with respects to DCR. By "good engine builds" I mean, for example, engines like Glenn Ring's 2180 built by Art Thraen or Gerico's 2276 by DRD, among other good builds. If you enter these guys engine specs in the calculator you'll find that in just about all cases the DCR is very close to 7:1. Depending on the cam used, static CR can be all over the place from 8:1 to almost 10:1. For a motor basically intended for reliable street use on premium gas shoot for a DCR of 7:1.

Sun Mar 21, 2010 11:38 am

I was being sarcastic. I forgot the rolling-eyes face.

Sun Mar 21, 2010 11:55 am

Sigurd wrote: That's not what I was referring to.

Btw, I did a little math using that awful Chevy calculator :roll: at kb-silvolite and it turns out that an FK-98 running at 6.6:1 has a dynamic CR of 4.6:1. FOUR to one. My father's Model A runs at 4.22:1. Does it ever occur that maybe that motor would be happier at a DCR of 7 or 8? And that maybe the SCR has to be at 12 for that to happen? Should we take everything into account or should we just use blanket statements?

Your fathers Model T was designed to run on Full Range Naptha. Not gasoline as we know it today.

The motor would not be happier due to the combustion chamber design. I dont think it is possible to get that type of CR in a model T.

The DCR is also not Clear. One thing to remember is that the CR is lowered due to Valve overlap. On a NA engine you get a shot of hot 1100F gasses back into the combustion chamber and even intake depending on the duration of the overlap. This can add conciderably to the heat content ot the F/A mix. So basicly the more overlap, the lower DCR. But this leads to higher F/A temps due to the reversion and less CR can be used. Damn Physics... :D

Sun Mar 21, 2010 1:20 pm

DarthWeber wrote: If you enter these guys engine specs in the calculator you'll find that in just about all cases the DCR is very close to 7:1. Depending on the cam used, static CR can be all over the place from 8:1 to almost 10:1. For a motor basically intended for reliable street use on premium gas shoot for a DCR of 7:1.

Thats exactly why I said you cant just say that you cant build a 10:1 motor for the street in a blanket statement.

And I agree with the DCR of around 7:1

Sun Mar 21, 2010 2:24 pm

Jimmy111 wrote: Sigurd wrote: That's not what I was referring to.

Btw, I did a little math using that awful Chevy calculator :roll: at kb-silvolite and it turns out that an FK-98 running at 6.6:1 has a dynamic CR of 4.6:1. FOUR to one. My father's Model A runs at 4.22:1. Does it ever occur that maybe that motor would be happier at a DCR of 7 or 8? And that maybe the SCR has to be at 12 for that to happen? Should we take everything into account or should we just use blanket statements?

Your fathers Model T was designed to run on Full Range Naptha. Not gasoline as we know it today.

The motor would not be happier due to the combustion chamber design. I dont think it is possible to get that type of CR in a model T.

The DCR is also not Clear. One thing to remember is that the CR is lowered due to Valve overlap. On a NA engine you get a shot of hot 1100F gasses back into the combustion chamber and even intake depending on the duration of the overlap. This can add conciderably to the heat content ot the F/A mix. So basicly the more overlap, the lower DCR. But this leads to higher F/A temps due to the reversion and less CR can be used. Damn Physics... :D

I jumbled my thoughts. I was not justifying 7:1 in the A; I was referring to my hypothetical FK98 motor. We get detonation in the A at 5.5 as it is. The spark advance mechanism is manual and the range of acceptable gets smaller as CR goes up. Not to mention the babbitted bearings don't like it so much.