I asked the tech department at proracingsim.com, if their software (dynosim, etc) computes dynamic compression ratio. His response is below, in which he says DCR is contrived. I've seen threads about how the DCR is more important than the normal CR. Thoughts?

Dynamic Compression Ratio is not a real thing. It is contrived to illustrate the VE of the engine. As the induction system drives more charge into the engine than it would consume at low speed, some say the Dynamic Compression Ratio goes up. There is no change in the compression ratio of the engine. It’s mechanically fixed.

However, using Dynomation you can easily see the increase in mass that enters the cylinder and evaluate the VE of the engine.

You don’t the illusion of Dynamic Compression Ratio with Dynomation since it shows you everything you need to determine engine efficiency, trapped mass, reversion, cylinder pressure, and much more.

 

DCR is way more important than SCR...

I learned this when I built my motor.

 

DCR is kinda mis-named, because it is not dynamic, and does not change.
The only difference between it, and static compression, is DCR is a calculation of compression ratio using the stroke distance after the intake valve closes in the engine cycle.

When that valve event occurs, is controlled by the camshaft. The "larger" the cam, typically the later (after BDC) the intake valve closes, thus lowering the effective, or dynamic compression. If the piston is traveling up, but the valve isn't yet closed, you are not compressing anything, you are pushing air back into the intake. This is why DCR is always a much lower value than static CR.

This is also explains why a huge cammed car can get away with 12:1 compression (or more) and still run fine on pump gas, but the same engine with a mild cam cannot do the same at that compression ratio. Unlike what's stated in your quote, DCR is a mechanically-fixed ratio too. He is obviously using a different definition of it in his reply.


The note you received, is someone more focused on calculating actual cylinder pressure and the volumetric efficiency (VE) of the engine, both of which are highly variable, and much MUCH more complex and involve many more components, such as intake manifold design, head port volume and design, header sizing and design, etc, etc. to accurately calculate and model in software.

What we use for DCR is a very simple calculation that is useful for one thing only...to quickly/easily determine how much compression can/should be run for a specific cam in an engine, while keeping it within the limits of the fuel octane you intend to use. The kicker is...what is the optimum DCR?
For an LS engine, popular opinion seems to put optimum n/a DCR at about 8.7:1 for 93 octane, and 8.5:1 for 91 octane. Can you get away with more? Perhaps. Should you run less? Maybe. There is not a hard and fast rule here, these values tend to be shared, compared, and from what I've seen, this seems to be about the accepted "ideal" to shoot for on a n/a engine using typical "street quality" fuel.

This is why you got the reply you did. It's not really a scientific way to approach things, but yet it's an easily calculated number that explains why one car can run all day on pump gas with 12:1 compression, but another running 10.8:1 cannot do the same thing. The DCR difference between the two will tell the story.

 

Y2Kvert4me
do you know what compression ratio i would have if i milled my stock heads .04 and used stock gasket size.
thanks

 

Well, that leaves an awful lot out, because it doesn't account for volumetric efficiency. A restriction in the inlet tract (even if it's only the throttle blade) will reduce dynamic compression ratio. A blower can raise it to almost any level.

Mostly, dynamic compression ratio is a somewhat feeble attempt to relate build specs to the quality of the fuel which will be used. The reason I say it's feeble is that it varies widely according to combustion chamber design, heat transfer capabilities of the materials used, capabilities of the cooling system, and intake air temperature.

 

Static compression leaves even more out...but people still build engines around that value.

Everything I stated in my quick reply referred to a n/a engine...boost obviously changes everything.


And no, a throttle blade, intake restriction, or even boost won't change DCR in the definition I explained. It is a mechanical calculation. Those items will change VE no doubt.

 

It's been so long since I've been in the NA world...
In the forced induction world, DCR always factors in air density.

 

You are thinking of a very different definition then, something more along the lines of VE.


Static compression = Need to know cyl bore, stroke, gasket thickness, gasket bore dia, deck height, chamber cc, piston dome/dish cc.

Dynamic compression = Need to know static CR + rod length and IVC°.

 

Y2K, great info!

Do you know the formula on how to calculate DCR based off CR, rod length, IVC°?

My setup is: 10.93CR, stock rod length (6.098), IVC 46° (I'll calculate it, if you can post the math)

I'm very curious to know what a stock LS1's DCR is.

 

Too much. (from looking at your profile).

Assuming stock LS6 heads, milling .040" would land you around 56cc chamber size. With stock MLS gasket that would put you around 11.85:1 static compression.

I don't have Excel on this comp and can't calculate your DCR, but with an X3 cam, I can tell you it will likely be too much (for pump gas), and you will also definitely have to cut notches in the pistons if you want to mill the 243 heads that much.

You'd be better off milling less, and running a thinner gasket IMO. (but you'd still have to flycut the pistons)

 

well its alreadys been done. i flycut the piston .1. ive been running around like this for the past 500 miles. i hope i didnt mess it up

yes ls6 heads with bigger valves and pp job and stock mls gasket

 

It's not the end of the world by any means...pull some timing in the tune if you see KR. If you don't get any knock or ping, even more power to ya

But from a clean-sheet design question, I would have done it a bit differently.

 

thanks