compression gurus, step on in

[clutchdust]

Trying to figure this out but I've never been one of those guys to break out the calculator when I really should.
Anyway, I'm picking up a set of AFR heads, on the cheap, that started life as 76cc chamber heads. They have since been milled to 70cc. I'm putting them on my .030+ 350 roller motor. The pistons I put in the short block were Speed Pro Hypers H618CP30, or .030 over hypereutectic with a .125" dome. According to Speed Pro, they should yield a CR of 9.34 with a 76cc head, or a 10.68 with a 64cc head. Now common sense would tell me that with these 70cc heads, I could safely assume that SCR would be just about exactly 10:1. But there are more factors at play than that. Unfortunately, I didn't bother writing it down but I do recall measuring when I did the initial build on the short block that my deck height was approximately -.010 to -.012" (just using a feeler guage). That and the Speed Pro specs state they factor advertised CR based on "commonly used head gaskets", whatever that means.
So here's the dilemma, I would like to run as high a compression ratio as I can safely get with pump gas, which is 91 octane out here on the wrong coast. For that, I'm assuming that somewhere around 10.2:1 is reasonably safe.
What would you guys say?
And how would you proceed?

 

[JAKE]

Wow, what a combination of numbers. How 'bout this - rather than have do all the number crunching:

Go to one of the sites that have a FREE CR calculaor and plug in all your numbers and see what it spits out. Then we'd have a firm starting point. From that, the recommendations can begin to flow.

Remember too, that dyno tests have shown that each point change in CR results in a 3-4percent change in power.

Jake

 

[mrvette]

Wow, what a combination of numbers. How 'bout this - rather than have do all the number crunching:

Go to one of the sites that have a FREE CR calculaor and plug in all your numbers and see what it spits out. Then we'd have a firm starting point. From that, the recommendations can begin to flow.

Remember too, that dyno tests have shown that each point change in CR results in a 3-4percent change in power.

Jake

When they say that POINT CHANGE, above, do they mean like say for instance 10.2 to one......do they mean the TENTHS, or one whole even integer ?? meaning to ME, I take it as point change means lets say an even 10.0 to 10-2 would supply 6-8% more power?? that seems like a lot to me, but with cam changes, that's easy...

:bonkers:

 

[BBShark]

This is a good article about the importance of calculating DCR and includes a downloadable calculator.

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

 

[Stroker-427]

I agree!

The most important number is the Dynamic Compressio Ratio..... not the geometric one!

With an aggressive camshaft (good overlap) the DCR at low RPM can be quite low..... but when the engine is revving at it's max efficiency RPM the DCR can be even higher than the geometric CR.

With a very good intake and good heads tuned with the right camshaft the volumetric efficiency can be over 100% in a range of RPM !!!!

 

[JAKE]

Wow, what a combination of numbers. How 'bout this - rather than have do all the number crunching:

Go to one of the sites that have a FREE CR calculaor and plug in all your numbers and see what it spits out. Then we'd have a firm starting point. From that, the recommendations can begin to flow.

Remember too, that dyno tests have shown that each point change in CR results in a 3-4percent change in power.

Jake

When they say that POINT CHANGE, above, do they mean like say for instance 10.2 to one......do they mean the TENTHS, or one whole even integer ?? meaning to ME, I take it as point change means lets say an even 10.0 to 10-2 would supply 6-8% more power?? that seems like a lot to me, but with cam changes, that's easy...

:bonkers:

The tests involved changing CR in one full ratio, 12.0:1 then down to 11.0:1, then 10.0:1, then 9.0:1 and finally 8.0:1.

Prior to each test, ignition timing and, IIRC, A/F ratios were adjusted to give the best results for each CR.

BTW, the 4% change occurred in the lower CR tests, like 8.0:1 to 9.0:1 and the 3% figure kicked in at the higher CRs.

The tests were run just to give the readers and idea of how much CR effects power.

Jake

 

[clutchdust]

Go to one of the sites that have a FREE CR calculaor and plug in all your numbers and see what it spits out. Then we'd have a firm starting point. From that, the recommendations can begin to flow.

Jake

OK, have any sites you know of? I sent in a request to Comp based on a cam recommendation for this configuration. I'm thinking of upgrading from my 276 to something in the 282-288 range, but want their opinion on that. Then again, they're cam peeps.