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Big Boyz Compression Calculator

6.3K views 6 replies 7 participants last post by  route66paul  
#1 ·
Just was on the site, at 95" and SE Pro heads at 72cc cylinder head volume, corrected for 3500 feet above sea level.

I have done a cold compression test of 190psi per cylinder.

Reverse calculations taking elevation and compression test as bench marks, I get a Static compression of 10.72:1 on the Big Boyz Site.

Is this accurate, meaning if I was looking at a cam with 10.2:1 or 10.5:1 compression, am I too high?

What Cam do I want....current cam is Andrews 37b with Heavy Pinging.

Thanks for reading
 
#2 ·
Sounds right to me, based on a friend's build with 72cc heads that wass set for 1.75 with the cam he was using.

As for which cam, well that's a whole other question but if you are running sbout 190 ccr, it doesn't sound bad. If your performance is otherwise good, you may want to take away some timing at the problem throttle positions to get rid of your ping...
 
#4 ·
you got a lot of good answers in your last post.
I see you definitely know now you cant use the woods 5 that you were asking about before.
you are going to get the same answers again. because it just a matter of internal combustion physics.
Your problem started when you put the SE heads on! because the lower cc's increased your compression.so now you have basically four choices.

#1 e-bay the SE heads and go back to a stock head that has more cc's. this will lower your compression back down and allow you to run the 37 cams . you can get them ported and cc'ed to match the 37 if you choose. whatever your wallet will allow. But in stock form they will work with the 37 this is a tried and true build done by many .It is probley the most used combo around. You didnt have problems with detonation when you had you stock heads on did you?

#2 Have your SE heads done to increase the cc's to match the 37's profile. sink the valves increase the chamber whatever the machinist (porter recommends).

#3 Go to a cam set that likes the compression you are running. something that the intake closes at 42-46 deg. I thought this is what you decided from your last post?

#4 Take some timing out in the areas that you are experiencing detonation (pinging) this would require you to change your timing settings on the T-max . That would hurt hp and tq If..... it was running correct now but....it will feel like a increase in power because you will not have the detonation and will be able to crank on the throttle now and not have to let up when it stars pinging!

#4 opinion is based on the actual 190 cranking pressure you say you have. From you previous post and this post I dont think you really know what your compression is.But if in fact you are at 190 and 188 ccc then it could be tuned to run without detonation. Your just not going to be able to plug in the T-MAX and expect it to compensate. You are going to have to do some timing tuning adjustments to get it there. JMHO
 
#5 ·
<<SE Pro heads at 72cc cylinder head volume>>

Did you actually cc the heads? The printed 72cc spec was a typo. I have never seen a set less than 76cc and most were 77/78cc.

<<I have done a cold compression test of 190psi per cylinder.>>

What pistons? What head gasket? Did you check actual cylinder deck height?

Might want to borrow anotherr gauge to double check the reading.
 
#7 ·
Many times the lifters will give a low CCP, because they do bleed down. The way to be sure is to use solid lifters for this test, which is impractical. Not knowing your true comp can have you speculating all kinds of scenerios. The only real way it to measure the heads, know which gasket you have, and to be precise, know the deck height of the pistons, and the displacement of the pistons you have.