Check my math, static CR.

enkeivette

Well-known member
Joined
Mar 30, 2008
Messages
990
I have 74 cc heads,
5cc valve reliefs in my flat top pistons,
a 4.03 SBC bore,
a 3.75 stroke,
my HG is .061 compressed,
with a 4.2 bore.

I got 9:1 with an online calculator.
 
Missing the deck height, assuming it's right around .030 (9.025 deck height, piston @ .005 down of 9.000) you'll end up w/ 9:1 yes.

however WHY do you have a .061 compressed gasket? You are running a non intercooled supercharger right? You would want at least some quench left to help prevent detonation, you now have a quench height of .061+.030 if you have a stock deck height, otherwise less!!! In other words, NONE because just the compressed gasket thickness reduces the quench because .060 is on the verge of doing nothing at all. Why not get a thinner gasket? A LOT thinner.
 
Missing the deck height, assuming it's right around .030 (9.025 deck height, piston @ .005 down of 9.000) you'll end up w/ 9:1 yes.

however WHY do you have a .061 compressed gasket? You are running a non intercooled supercharger right? You would want at elast some quench left to help prevent detonation, you now have a quench height of .061+.030 if you have a stock deck height, otherwise less!!! In other words, NONE because just the compressed gasket thickness reduces the quench because .060 is on the verge of doing nothing at all. Why not get a thinner gasket? A LOT thinner.

Woops, the pistons are .025 down, deck has never been surfaced as far as I know. The one that I'm taking off now is .041 compressed, 4.166 bore, puts me around 9.4:1.

I started a thread on this over on speedtalk, and a bunch of engine builders told me that with boosted apps, quench does not matter:

http://speedtalk.com/forum/viewtopic.php?p=110967&highlight=#110967
 
Yes but read what that one guy wrote:
The purpose of a quench zone is to produce squish velocity to assist flame velocity and reduce burn angle.

If a combustion chamber is so designed as to closely depend on squish velocity to prevent detonation, you are running at the bleeding edge. An increase in clearance will reduce squish velocity. So will higher engine speed. Since cycle to cycle burn properties vary considerably, depending on squish velocity is inviting disaster.

If the compression ratio is increased, so to will be the compression pressure (and compression temperature). The squish jet speed will be reduced because it must push against a higher chamber pressure.

Engine destruction is the result of a race between flame speed and autoignition time.

Better be safe than sorry, having at least some quench will not hurt at all., however if you don't have it it may come back to bite you. There's no sense in using that thick a head gasket if you can get away with a thinner one. Or are you purposely using it to reduce the CR?
What heads do you have?
 
Yes but read what that one guy wrote:
The purpose of a quench zone is to produce squish velocity to assist flame velocity and reduce burn angle.

If a combustion chamber is so designed as to closely depend on squish velocity to prevent detonation, you are running at the bleeding edge. An increase in clearance will reduce squish velocity. So will higher engine speed. Since cycle to cycle burn properties vary considerably, depending on squish velocity is inviting disaster.

If the compression ratio is increased, so to will be the compression pressure (and compression temperature). The squish jet speed will be reduced because it must push against a higher chamber pressure.

Engine destruction is the result of a race between flame speed and autoignition time.

Better be safe than sorry, having at least some quench will not hurt at all., however if you don't have it it may come back to bite you. There's no sense in using that thick a head gasket if you can get away with a thinner one. Or are you purposely using it to reduce the CR?
What heads do you have?

I am, I can run 19lbs at 9:1 but only 15lbs at 9.4:1.
 
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