Compression Ratio and piston Question
Thread Starter
Melting Slicks
Joined: Jul 2000
Posts: 2,848
Likes: 0
From: Beacon NY
Compression Ratio and piston Question
If it makes any difference in the questions I have posted below, I'm in the process of building a 383. My block has a bore of 4.04 and I have alumminum cylinder heads with 72cc combustion chambers. I was originally planning on using -7cc pistons, but I bought them with the wrong bore in mind and I need to re-purchase.
1) what is the highest compression ratio that I can run in my engine and still be able to use pump gas (91 or 92 octane around here). I have aluminum cylinder heads. I was under the understanding that a compression ratio of 10:1 in an engine with aluminum heads would still allow me to us 91 octane gas without any issues.
2) Can someone point me to a reliable compression ratio calculator? I have found several, but they all seem to give me a small variance in compression ratio even when I enter the same numbers.
3) What is the difference between cast pistons, forged pistons, and hypereutectic pistons? Which makes for the best in terms of reliability?
Thanks in advance
1) what is the highest compression ratio that I can run in my engine and still be able to use pump gas (91 or 92 octane around here). I have aluminum cylinder heads. I was under the understanding that a compression ratio of 10:1 in an engine with aluminum heads would still allow me to us 91 octane gas without any issues.
2) Can someone point me to a reliable compression ratio calculator? I have found several, but they all seem to give me a small variance in compression ratio even when I enter the same numbers.
3) What is the difference between cast pistons, forged pistons, and hypereutectic pistons? Which makes for the best in terms of reliability?
Thanks in advance
NCM Grand Opening Veteran
Joined: Sep 2005
Posts: 20,979
Likes: 384
From: South of giving a damn
St. Jude Donor '11, '17
I'll give it a shot.
3. The difference in pistons is the way they are made (excluding the different metal formulations) Cast & hypers are both cast pistons (poured into a mold), with the hypers having a higher silica content. Forged pistons are, well forged. The piston is created by forming the metal under tremendous pressure. Forged is the strongest, by far. From everything I've seen, forging is the way to go in high perf engines.
2. Can't help here, I use my DynoSym to calc CR.
1. 10:1 is probably the limit for 91 octane, without using boosters.
3. The difference in pistons is the way they are made (excluding the different metal formulations) Cast & hypers are both cast pistons (poured into a mold), with the hypers having a higher silica content. Forged pistons are, well forged. The piston is created by forming the metal under tremendous pressure. Forged is the strongest, by far. From everything I've seen, forging is the way to go in high perf engines.
2. Can't help here, I use my DynoSym to calc CR.
1. 10:1 is probably the limit for 91 octane, without using boosters.
Safety Car
Joined: Jul 2001
Posts: 3,921
Likes: 2
From: York PA
Originally Posted by Victor
1) what is the highest compression ratio that I can run in my engine and still be able to use pump gas (91 or 92 octane around here). I have aluminum cylinder heads. I was under the understanding that a compression ratio of 10:1 in an engine with aluminum heads would still allow me to us 91 octane gas without any issues.
2) Can someone point me to a reliable compression ratio calculator? I have found several, but they all seem to give me a small variance in compression ratio even when I enter the same numbers.
2) Can someone point me to a reliable compression ratio calculator? I have found several, but they all seem to give me a small variance in compression ratio even when I enter the same numbers.
http://cochise.uia.net/pkelley2/DynamicCR.html
Le Mans Master
Joined: Sep 2001
Posts: 7,739
Likes: 630
From: Unreconstructed, South Carolina
Thread Starter
Melting Slicks
Joined: Jul 2000
Posts: 2,848
Likes: 0
From: Beacon NY
Originally Posted by stickboy
to figure out what fuel you can run you need to figure out your dynamic compression which is determined by static compression and cam. Check out this site for more info and a calculator.
http://cochise.uia.net/pkelley2/DynamicCR.html
http://cochise.uia.net/pkelley2/DynamicCR.html
Wow, good information. Being a complete novice, I never heard of that before. Thanks.
Le Mans Master
Joined: Sep 2001
Posts: 7,739
Likes: 630
From: Unreconstructed, South Carolina
We don't have sufficient info to know what your deck height actually is ... but your aluminum head 72cc 385" motor with -7cc pistons can be set to produce 10:1 static CR & good quench ... via head gasket choice:
Felpro P/N 7733 composition 0.041"
Napa/Victor Reinz P/N 5746 composition 0.026"
Felpro P/N 1094 steel shim 0.015"
I dunno just how practical dynamic CR calculations are for typical street/hi-po motors ... there're plenty off-the-shelf cams that'll work just fine with typical street/hi-po combos.
Felpro P/N 7733 composition 0.041"
Napa/Victor Reinz P/N 5746 composition 0.026"
Felpro P/N 1094 steel shim 0.015"
I dunno just how practical dynamic CR calculations are for typical street/hi-po motors ... there're plenty off-the-shelf cams that'll work just fine with typical street/hi-po combos.
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Burning Brakes
Joined: Feb 2006
Posts: 1,024
Likes: 1
From: AL
agree with jackson. . . i'd say get the quench dimension right, somewhere around 0.040-0.045 for steel rods if memory serves, and select parts to work together. cam mfg is much help; use them; they know something about pistions too! you can do the math to check combinations that are offered, run them on simulators, etc., to use for comparison more than for absolute values.
Melting Slicks
Joined: Apr 1999
Posts: 2,010
Likes: 3
From: Moon Twp. PA USA
Originally Posted by Victor
If it makes any difference in the questions I have posted below, I'm in the process of building a 383. My block has a bore of 4.04 and I have alumminum cylinder heads with 72cc combustion chambers. I was originally planning on using -7cc pistons, but I bought them with the wrong bore in mind and I need to re-purchase.
1) what is the highest compression ratio that I can run in my engine and still be able to use pump gas (91 or 92 octane around here). I have aluminum cylinder heads. I was under the understanding that a compression ratio of 10:1 in an engine with aluminum heads would still allow me to us 91 octane gas without any issues.
2) Can someone point me to a reliable compression ratio calculator? I have found several, but they all seem to give me a small variance in compression ratio even when I enter the same numbers.
3) What is the difference between cast pistons, forged pistons, and hypereutectic pistons? Which makes for the best in terms of reliability?
Thanks in advance
1) what is the highest compression ratio that I can run in my engine and still be able to use pump gas (91 or 92 octane around here). I have aluminum cylinder heads. I was under the understanding that a compression ratio of 10:1 in an engine with aluminum heads would still allow me to us 91 octane gas without any issues.
2) Can someone point me to a reliable compression ratio calculator? I have found several, but they all seem to give me a small variance in compression ratio even when I enter the same numbers.
3) What is the difference between cast pistons, forged pistons, and hypereutectic pistons? Which makes for the best in terms of reliability?
Thanks in advance
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