113 heads and compression
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Instructor
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113 heads and compression
About two days ago I asked about adding 113 heads to an '85 or early '86. Some of you wrote that the compression would jump from 9.0 to about 10.2. I thought that the compression would only rise to 9.5:1. Is that true, or did GM also switch from flat top pistons to dished pistons when the aluminum heads were added in order to limit the compression increase?
Team Owner
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From: Athens AL
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The pistons on 85 and 86 are different but thats not your major problem.
The problem is the combustion chamber size, 76cc head on 85-86E vs the 58cc head on the 113 (there is some variance), thats going to bump CR well above 9.5.
The problem is the combustion chamber size, 76cc head on 85-86E vs the 58cc head on the 113 (there is some variance), thats going to bump CR well above 9.5.
Race Director
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From: The Top of Utah
YOU are not changing pistons. What you are doing is subbing 58 cc chambers for 76cc chambers. You can do the math to figure the result. Last time I did, I recall it was in the 10.2 to 10.3 range. Remember too, that an aluminum head will tolerate a half, to a full point, of compression over an iron head.
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Thread Starter
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Thanks for the answers. So, if 113 heads were installed on an '85-'85E, and 9.5 compression is desired, new pistons are required to compensate for the 58 cc chambers on the aluminum heads. Who makes a top quality aftermarket head to which the original intake manifold and other parts would fit and that would maintain 9.5 compression without a piston change?
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There's nothing wrong with the higher compression, it's just that I'd be happy with 9.5. Despite my time at road racing school, I drive pretty slowly, so the original iron heads are probably fine for me. I just got curious about the ramifications of switching to 113 heads. I would consider the switch during an engine rebuild.
Drifting
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From: Thomson Georgia
I believe you are going to end up with 10.5 or better compression. If I remember, on a 4" bore, a 10cc change in chamber size gives you about 1 point change in compression. This is a premium engine already.
This is really a good swap. You end up with great quench, a better flowing, lighter head, that doesn't aggravate detonation.
More torque, more horsies, better throttle response, better gas mileage - assuming the same driving habits, and I would think a cooler running engine.
This is really a good swap. You end up with great quench, a better flowing, lighter head, that doesn't aggravate detonation.
More torque, more horsies, better throttle response, better gas mileage - assuming the same driving habits, and I would think a cooler running engine.
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From: The Top of Utah
Originally Posted by DSKRALL
Who makes a top quality aftermarket head to which the original intake manifold and other parts would fit and that would maintain 9.5 compression without a piston change?
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From: CT
>>did GM also switch from flat top pistons to dished pistons when the aluminum heads were added in order to limit the compression increase?
Does anyone know this answer? I have an 86E and am considering the aluminum heads if I ever have to take off the stock iron ones, but am concerned about too much compression for pump gas. Another variable along with combustion chamber size is gasket thickness. For example, GMPP lists gaskets with 0.41" compressed thickness as having 9.1cc volume. 0.28" Steel gaskets (~6.2cc) are available and GMPP lists 0.51" composition gaskets (~11.3cc) for production aluminum heads. Anyone know what's the thickest gasket that can be safely used (in the event iron and aluminum head motors have different pistons)?
Does anyone know this answer? I have an 86E and am considering the aluminum heads if I ever have to take off the stock iron ones, but am concerned about too much compression for pump gas. Another variable along with combustion chamber size is gasket thickness. For example, GMPP lists gaskets with 0.41" compressed thickness as having 9.1cc volume. 0.28" Steel gaskets (~6.2cc) are available and GMPP lists 0.51" composition gaskets (~11.3cc) for production aluminum heads. Anyone know what's the thickest gasket that can be safely used (in the event iron and aluminum head motors have different pistons)?
Instructor
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From: CT
I'd be interested in doing the same... I'm assuming you're running the stock cam -- did you notice improvement in performance or mileage and did you change the PROM? I believe the Al head '86 had a different chip since Hypertech info has different P/Ns for an 86 with and w/o iron heads. 
Burning Brakes
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From: Manhattan
Originally Posted by 1986Z51
>>did GM also switch from flat top pistons to dished pistons when the aluminum heads were added in order to limit the compression increase?
Does anyone know this answer?
Does anyone know this answer?
I believe they were both stock engines. I've not seen pics of the iron headed L98 pistons.
Drifting
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If you want to boost your compression here is what you need to figure out.
Do you have flat top pistons or dish???
What is the displacement of the engine???
Is it bored or stock???
Let's figure out a few common situations and you can go from there.
FIRST MOTOR IS A 350 76cc-----64cc
350 stock bore .080 dish piston 7.89 8.79
350 stock bore flat top piston 8.24 9.24
350 .030 bore .080 dish piston 7.99 8.90
350 .030 bore flat top piston 8.34 9.35
383 stroker .030 bore flat top 9.22 10.41
400 stock bore flat top 9.50 10.71
These are figured out using Speed Pro pistons. You may wish to check out your compression ratio yourself since I can only give you a standard piston #.
As a rule of thumb they will raise a 350 gm up .75 to 1.25 points over a 76cc head
On a 400 they average a raise of 1.25 to 1.75 over a 76cc head.
Do you have flat top pistons or dish???
What is the displacement of the engine???
Is it bored or stock???
Let's figure out a few common situations and you can go from there.
FIRST MOTOR IS A 350 76cc-----64cc
350 stock bore .080 dish piston 7.89 8.79
350 stock bore flat top piston 8.24 9.24
350 .030 bore .080 dish piston 7.99 8.90
350 .030 bore flat top piston 8.34 9.35
383 stroker .030 bore flat top 9.22 10.41
400 stock bore flat top 9.50 10.71
These are figured out using Speed Pro pistons. You may wish to check out your compression ratio yourself since I can only give you a standard piston #.
As a rule of thumb they will raise a 350 gm up .75 to 1.25 points over a 76cc head
On a 400 they average a raise of 1.25 to 1.75 over a 76cc head.
Thread Starter
Instructor
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113 heads and compression
Raistlin and nobodyunknown,
Thanks for the interesting replies. Raistlin, your comments reminded me to check Mike Antonick's book "Corvette Specs." It lists 10:1 compression for 1991 only. The Chevy press release for '91 says that power and torque remained the same as that for the '90 engine with 9.5 compression.
At any rate, after reading all of this information, I finally realized that as long as detonation is no problem, adding 113 heads without a piston change is the logical thing to do.
Thanks for the interesting replies. Raistlin, your comments reminded me to check Mike Antonick's book "Corvette Specs." It lists 10:1 compression for 1991 only. The Chevy press release for '91 says that power and torque remained the same as that for the '90 engine with 9.5 compression.
At any rate, after reading all of this information, I finally realized that as long as detonation is no problem, adding 113 heads without a piston change is the logical thing to do.
Last edited by DSKRALL; Sep 6, 2004 at 06:36 PM. Reason: better writing
