Bringin up CR
09-28-2004, 01:36 PM
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Bringin up CR Hi, I have a 9.4CR motor, stock heads, stock cam, dished pistons (-8.6cc).
This was for a blower motor but thinking of going NA again. How high could I safely get my CR up with new heads? Does a lower cc head provide less air flow, bc I know that is how to bring it up some...
Will also probably spray a big shot through this.
Is it worth going back to NA with these pistons or is my CR too low?
Thanks
This was for a blower motor but thinking of going NA again. How high could I safely get my CR up with new heads? Does a lower cc head provide less air flow, bc I know that is how to bring it up some...
Will also probably spray a big shot through this.
Is it worth going back to NA with these pistons or is my CR too low?
Thanks
09-28-2004, 04:39 PM
#2
Assuming when you say "stock heads" you are referring to stock LS1 heads I would say yes you can get the compression up with a set of heads most definately. I think a good set of milled 5.3 heads would do the trick. Plus have room for a nice sized cam. You could even run a thinner head gasket. Lots of possibilities.
10-12-2004, 06:04 PM
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Originally Posted by Lafngas98z
Assuming when you say "stock heads" you are referring to stock LS1 heads I would say yes you can get the compression up with a set of heads most definately. I think a good set of milled 5.3 heads would do the trick. Plus have room for a nice sized cam. You could even run a thinner head gasket. Lots of possibilities.
Another question: With more compression on an NA motor, do you have to decrease the timing more? For example, could a 11.5cr motor run the same timing as a 10.5cr motor with max advance?
Thanks
10-13-2004, 01:08 AM
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Originally Posted by SSkeet
Are smaller cc heads going to flow any less than larger cc heads? What I am saying is, is there any performance loss with going smaller cc? I figure with 58cc heads I could probably bring it back up to about 10.8+ which would be pretty good.
Another question: With more compression on an NA motor, do you have to decrease the timing more? For example, could a 11.5cr motor run the same timing as a 10.5cr motor with max advance?
Thanks
Another question: With more compression on an NA motor, do you have to decrease the timing more? For example, could a 11.5cr motor run the same timing as a 10.5cr motor with max advance?
Thanks
As for your other question, yes, as the CR for a given motor goes up, how aggressively you can advance the timing for a given fuel octane and avoid detonation will go down. So, you can either take out some timing (not so good) or start using a high octane fuel (much better). Also, keep in mind that you cannot rely on an engine's "static" CR as a bottom line figure. The "dynamic" CR (or effective CR while the engine is running) is the real issue here, and dynamic CR is also largely determined by camshaft selection. A cam with a large amount of overlap is going to bleed off cylinder pressure, lowering the dynamic CR. Hence, a 10.5 CR engine with a stock type cam and an 11.5 CR engine with a more radical cam can end up having the same dynamic CR, which is really the key factor in determining how much timing and what quality fuel the motor will require to avoid detonation.
10-13-2004, 08:11 AM
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Originally Posted by Silverhawk_02TA
Head flow is much more a factor of your intake/exhaust runners, so no, a smaller chamber will not in and of itself limit flow.
As for your other question, yes, as the CR for a given motor goes up, how aggressively you can advance the timing for a given fuel octane and avoid detonation will go down. So, you can either take out some timing (not so good) or start using a high octane fuel (much better). Also, keep in mind that you cannot rely on an engine's "static" CR as a bottom line figure. The "dynamic" CR (or effective CR while the engine is running) is the real issue here, and dynamic CR is also largely determined by camshaft selection. A cam with a large amount of overlap is going to bleed off cylinder pressure, lowering the dynamic CR. Hence, a 10.5 CR engine with a stock type cam and an 11.5 CR engine with a more radical cam can end up having the same dynamic CR, which is really the key factor in determining how much timing and what quality fuel the motor will require to avoid detonation.
As for your other question, yes, as the CR for a given motor goes up, how aggressively you can advance the timing for a given fuel octane and avoid detonation will go down. So, you can either take out some timing (not so good) or start using a high octane fuel (much better). Also, keep in mind that you cannot rely on an engine's "static" CR as a bottom line figure. The "dynamic" CR (or effective CR while the engine is running) is the real issue here, and dynamic CR is also largely determined by camshaft selection. A cam with a large amount of overlap is going to bleed off cylinder pressure, lowering the dynamic CR. Hence, a 10.5 CR engine with a stock type cam and an 11.5 CR engine with a more radical cam can end up having the same dynamic CR, which is really the key factor in determining how much timing and what quality fuel the motor will require to avoid detonation.
My last concern is, if I go NA, will the dished pistons have any effect on performance? Would flat be better for NA or does it not matter? If it will effect it, is it enough to really consider pulling the pistons and changing them?
Thanks again