Dynamic and Static Compression Ratio effects, recommendations?
 

I learned a good lesson lsat season about the effects of extreme compression in the marine environment. My small block checks in (after calculations) with a 10.8:1 SCR using 64cc heads and 8.8:1 DCR using my current Comp Xtreme Energy cam. I have fooled around with some different numbers for various cams in the Extreme Energy line and found that I could reduce DCR to levels that will tolerate high octane fuel with more timing (currently, my timing is limited in order to reduce detonation), though I'm not quite sure of what the effects would be in terms of low end performance (3.75" stroke small block) and high RPM capability. Larger cams off course need higher compression ratios to make power upstairs. For conversation's-sake, let say I move up two sizes in the line. I would go from the current XE274H to a XE254H. The jump would lower the DCR to 8.15:1 which would live well on pump gas, but would it still work in the application?

Bottom line is that I currently have too much compression. I'll lower it in one of two ways, a change to Sportsman II 72cc heads which are comparable to the Vortecs I have now (and should help with upper RPM capabilities to boot) or try some different cam choices to effectively "trick" the engine into thinking that the compression is lower than it actualy is. The head swap will get me into the 8.1:1 DCR range with the current cam which is more than liveable with mid-grade fuel.

Comments or suggestions?

Thanks!

TomZ, the dynamic compression increases as the volumetric efficiency of the engine increases-usually at peak torque rpm. The comprssion you seem to be talking about is cranking compression. If it is, you are not looking at the more revelent factors.

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How did you / do you calculate your dynamic compression ?

Blown1500,

Cranking compression for this engine has been between 190 and 200 PSI from what I remember over the summer.

In the end I need to be able to reduce cylinder pressure to a more usable level. Right now I have too much and the potential for detonation when tuned properly. Can you explain a little more indepth?

Ratchet,

I used a downloadable VB application that I found on a site that talks about dynamic compression. Here's the link:

http://cochise.uia.net/pkelley2/DynamicCR.html

I had read.. 8.5:1 dynamic compression ratio or less for service sation fuel.. (93 octane or less)...

my .02

A big cam (overlap) does bleed off cylinder pressure at low rpm which makes it helpful to raise your static CR to get usable torque out of the motor at low rpm.

At high rpm, the big overlap allows the intake charge to continue into the motor after BDC due to intake inertia which rams MORE charge in than it could otherwise. It is not uncommon for a cam with big overlap and a matched intake system (tunnel ram or similar) to far exceed the 100% volumetric efficiency used to figure static CR.

In a case like that, a big cam might bleed cranking compression down to a lowly 155psi or so, while providing a cylinder charge at high rpm comparable to a small cam PLUS 3psi of supercharging.

Do NOT fool yourself into thinking that a larger cam will eliminate the need for high octane fuel at high rpm. This is only the case if you use a horribly oversized cam that never achieves high volumetric efficiency (like the kid in high school who ran a .540 lift 322 degree solid lift cam in his 350 Camaro - sounded bad a$$ but couldn't run with a stocker).

The main thing a big cam will allow you to do on a given octane is to run more initial timing (some guys running big cams here lock out the advance and run 30 some-odd degrees all the time).

Hey TomZ, How's it going?

Very interesting topic. Now you've got me looking for ways to increase my DCR.

I think the problem that you're going to run into is the useful RPM range of the cam is also going to move up. You're going to get into the 7000 rpm range for peak horsepower, and you probably don't want to make the alpha scream like that.

Regards
Mike

Great thread
 

Tom Z,
Thanks for the info.
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Mcollinstn (& others)..........if the below is true, does it help out to spread the centerlines (like go to a 114) for top end power ?

Olysan, As for spinning an alpha, I've had them live (somewhat - I averaged an alpha a season with over 400 hp in front of them) by running them at a little more RPMS (around the 6000 mark) with a smaller prop, which usually gives you a hell of a mid range punch that actually wins a lot of the "pull up and stab it" races. The main thing with the alphas is keeping them cool if you take this approach. Doing this is pretty beneficial with small blocks, cause with a smaller prop, you keep pullin hard till the engine runs out of steam. 7000 may be a bit much, but I think 6000 is more reasonable.

Back to the cam thing..........interesting topic !

The only way 10.8:1 compression with iron or aluminum heads is gonna work is by using some good fuel. With that said you may as well cam the motor for the RPM you intend to run and run good fuel or octane boost or lower compression. I know what you are trying to do and it will make the motor survive on pump gas by lowering the cylinder pressures, but it will run like a pig. Big cams need high compression to make up for the bled down pressures at low RPM, as has already been said. If you put a cam in to make the motor detonation free at say 5000 RPM it will have to be something so large it may not even idle and power and throttle response will be soggy in marine RPMS. Just my opinion. Good luck.

Within a given range, wider lobe centers will make for a flatter torque curve while narrower lobe centers will make for a "less flat" torque curve.

An alcohol burning Outlaw dirt sprint car may work well with a 102 degree lobe center. This, in an optimized engine combination, will create a hard-accelerating beast of a motor that is designed to run only within a narrow RPM range.

School bus cams usually have wide lobe centers.

The sprint cam, however, will have dramatically different lift specs, duration specs, and ramp speeds than either the school bus cam OR a cam you would otherwise choose for your marine motor.




What all this DOES NOT mean is that putting a 102 lobe center cam in your 502 will make it a sprint car motor. A given engine combination will only work "so well". If you've got an oval port 9:1 motor, then you've got a range of torque curves you can choose from. You gotta pick a cam that matches up to one of those curves. To go outside that range (on either end) will minimize your horsepower potential.

Now that we got that out of the way, there are arguments that a BBC marine motor makes best power on either 114 degree LC or 109 degree LC. The pitchmen for either spec are equally convinced of the superiority of their cam. What's the answer? You got me.

Bottom line is that the torque curves are affected by the lobe centers on an otherwise identical cam. Flatter for wider. Less flat for narrower. When you go "less flat", the general consensus is that the peak rises and the ends fall.

Lot's of good information here. Thanks for all the input. I kept trying to leave the effects of a more top-end-ish cam out of the equation. I have strong compression now (more than I thought I did, which really ticks me off since the builder "claimed" that it wouldn't be more than 10:1... the only one who'll do it right is you... ) so it could definitely support a larger cam, but would it support the environment that it has to operate in? Based on RPM range of the aforementioned cam though power would be made from 2800 through 7000 RPM, too much I think. The drive couldn't handle the RPM at the top, the current heads wouldn't support it anyway, and the longevity of the engine would be killed altogether.

It looks like for my situation a change to the Sportsman IIs will help me meet my goals of having a hard running and reliable combination. The flow numbers are comparable, and since the Vortecs are a limiting factor on larger displacement engines, I'll be able to gain some top end in the process. Comp believes that the cam is pretty good for the combination though they would have spec's a 112* LSA over the 110* that the auto grind uses.

One last thing you could try.
 

Most modern camshafts have the intake advanced to 106 or less. This closes the intake valve too early for an application like yours. Closing the intake valve early extends the compression cycle building more compression. Retard the camshaft centerline back to 110 or 111 degrees ATDC. This holds the intake open longer, shortening the compression cycle.
It is a cheap solution and might help a little.
Sincerely
Dennis Moore

That might work...
 

According to my calculator, retarding the ICL to 111* would drop DCR to 8.54 which might very well work with the Vortec heads as far as reliability is concerned.

How would that effect performance though? Retarding the cam (basically installed with -1* of total advance) would raise peak power RPM and, wouldn't it? I guess the con would be decreased power output on the bottom end (which will make the drive happier). The pros would out weigh that (increased power upstairs, less prone to detonation, etc).

Better to go this way or would the heads be the best choice?

TomZ, the heads are by far the better choice. Cams, especially na, don't like to be retarded unless they are very large, very high rpm. The fuel economy and low end power will suffer a lot at -1*.

Well, that's pretty much what I thought. The heads will make it work better in the end.

Time to start preparing the wife! ;)