New head compression ratio question, How much is to much?
 

I am looking to replace my stock 76 CC heads with a set of Edelbrock Performer RPMs. My advertised CR is 8.5. I have been told that higher CR's are possible with aluminum heads. Edelbrock makes 64 CC heads and this would raise my CR to about 9.5. Smokemup.com's CR calculator puts my current CR at 8.5:1. This being said "How to rebuild your small-block CHEVY" by David Vizard says the 76 CC heads gave a 9:1 CR and going to 64 would yield 10.3 with flat top pistons. Big difference. He says only consider 8.5 - 9.0 max with today's fuels unless other considerations are taken. I know you guys with the high CR's and big cams claim not to have a problem but the valve overlap of performance cams reduces compression so your net compression is lower. I am planning on using a more conservative cam than may not impact overall compression as much. Any advice from you engine guru's out there?

Re: New head compression ratio question, How much is to much? (KJL)
 

Not an engine guru but:

CR is dependant on you pistons, gasket and combustion chamber volume. If you start with 8.5 and keep the same gasket thickness, .041 I think but may be .039, and switch to 64 cc heads then you will be about 9.5. With alum heads and prem gas, 93 oct, you can go above 10 to 1 so you should be fine. I run 87 oct with alum heads and about 9.3 to 1.

Alum heads are :cool:.

Re: New head compression ratio question, How much is to much? (KJL)
 

might need hi-test. is that OK?

Re: New head compression ratio question, How much is to much? (Matt Gruber)
 

According to DeskTop Dyno, going to 58cc heads from 76cc heads on my 76 L-48, raises compression ratio to 10.25 from 8.5......... :cheers:

Re: New head compression ratio question, How much is to much? (KJL)
 

Your right about cams and what your effective compression ratio is. It's refered to as dynamic compression ratio. I was able to run the Crane powermax H-flat 272 and later the 278 cam with @10.7 and iron heads.

Just keep your max piston quench to .038 - .044. So in a none decked block than means with typical pistons your @ .020 down in the hole at TDC. So to achieve max quench specs I used a .020 steel head gasket. On another engine combo I ran a surfaced block and a .030 copper head gasket.

right now I have JE pistons with a surfaced block which is .006 down in the hole at TDC and I use a .038 compressed gasket for 10.84 compression with aluminum heads.

Another thing to note is: Heads sold as 64 cc might be more like 66 cc if any chamber work has been done. so unless you really CC the heads it's more of a ruff estimate. Also the pistons vary in cc of valve pocket size. I have only ran two valve relief pistons for years to keep my compression up. :seeya

The first thing that ever really woke up my L-82 355 ci motor was going from 76cc to 64 cc heads. You have more power clear across the rpm range

Re: New head compression ratio question, How much is to much? (gkull)
 

Thanks for the info!! I am not sure if my block has been decked. I do know it was re-built at some point. Are steel head gaskets more difficult to use? i have been told that unless you have you block surfaced that steel gaskets may not seal well. Yes, after playing with DD2000 it became clear that one of the best things I could do to improve accross the board performance was to pick up the CR. And because this car will never see the strip, a good solid low end performing cam like the Summit 1101 will give me torque values near 400 ft-lbs in the 2000-4000 rpm range. Great for this traffic light to traffic light Atlanta driving. :yesnod:

Re: New head compression ratio question, How much is to much? (KJL)
 

I always just use the red gasket spray on the block side only of the head gasket. It makes for ease of swapping out the heads.

When you go to aluminum heads the only way to really go is for ARP stud kit so you won't damage or warp them. this brings up another problem. You really need to have a blue printed surfaced block. I learned the hard way years ago.

I installed ARP head studs in freshened motor. to cut corners and save money I didn't have the block threaded holes blue printed. So when i tried to intall the nice new aluminum head it wouldn't fit on because some of the studs were so far out of alignment.

I have never had a blown head gasket. but I also pay careful attention to TQ ing them down and after several heat cycles and then retorq them to spec. :chevy


[Modified by gkull, 12:44 PM 9/13/2002]

Re: New head compression ratio question, How much is to much? (gkull)
 

Stock engines also had dished pistons along with the 76cc heads which made for the 8.5 comp ratio. If the motor was rebuilt before than you will need to check to see if the pistons were replaced. If they were replaced with flat top pistons then the comp ratio would have increased to 9 or 9.5 to 1.

Re: New head compression ratio question, How much is to much? (gkull)
 

I spoke with Edelbrock and they strongly suggested NOT going with the 64 cc chamber size if using a conservative cam. They suggested going with their 70 cc head. I know AFR190's come in 68cc chambers, they may be a good compromise but they cost lots.... :confused: If I do have dished pistons the the compression calculator must be very wrong because with 76CC chambers and flat top pistons, it puts me at about 8.5. That is figuring 6 CC for valve reliefs. I was going to place my order today, but it looks like it is back to the drawing board. :rolleyes:

Re: New head compression ratio question, How much is to much? (KJL)
 

Also keep in mind that the 8.5:1 compression ratio is what you would have if everthing in the engine met blueprint specifications. The pistons are almost always a little further down the hole than the factory prints and the cylinder heads are almost never at 76cc. Your real compression ratio is probably much closer to 8-even than 8.5:1. Also, it takes a rise of about a point of compression to keep the same thermal characteristics of the combustion process when going from iron to aluminum heads. This means that everything else unchanged (port flow and efficiency), if you were to bolt on a set of aluminum heads with the same size combustion chamber, you would loose power.

Re: New head compression ratio question, How much is to much? (gerry72)
 

My engine is being builted at this time and it is going to be around 9.7 to 1.

Re: New head compression ratio question, How much is to much? (KJL)
 

What fuel? will you use hi-test?

Re: New head compression ratio question, How much is to much? (Matt Gruber)
 

My engine is being built now, but i am going w/ 64cc chambers, and 9.5 compression. I have heard that 9.5 works well w/ 92 octane fuel