Who is running a 8.9 or below CR??
09-01-2006, 02:11 PM
#24
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8.7:1 here, stock bore size Edelbrock 67cc heads and custom Ross pistons.
When the belt is working properly, I'm hitting a steady 19psi at a little over 6500rpm. No problems here with pump gas, timing circa 18-19 deg.
Goes even better with more timing and meth, but works great on normal pump fuel too.
If anything I almost wish I had went for around 9.0:1
For the guy who wanted pistons for 7.8:1 with stock heads, why not get them custom made ??
When the belt is working properly, I'm hitting a steady 19psi at a little over 6500rpm. No problems here with pump gas, timing circa 18-19 deg.
Goes even better with more timing and meth, but works great on normal pump fuel too.
If anything I almost wish I had went for around 9.0:1
For the guy who wanted pistons for 7.8:1 with stock heads, why not get them custom made ??
Last edited by stevieturbo; 09-01-2006 at 02:18 PM.
09-01-2006, 02:37 PM
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Originally Posted by longrange4u
I am going to go with my 408 w/ Wiesco -32cc dish pistons.... and some 6.0L heads (72cc chambers) that will put me right at 8.483 Static CR
09-01-2006, 04:31 PM
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Originally Posted by ty_ty13
i think itll be lower than that after its all put together and filled with alky to test the exact compression.
09-01-2006, 09:53 PM
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Originally Posted by longrange4u
Why is that? What do you think it will be at?
09-01-2006, 10:19 PM
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Search is my friend.... ok GM MLS gasket is .055 thick.... bringing my Static CR down to 8.1-8-2 (depending on the calculator)
Last edited by longrange4u; 09-03-2006 at 02:58 PM.
09-02-2006, 01:12 AM
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Originally Posted by longrange4u
Why is that? What do you think it will be at?
09-02-2006, 04:03 AM
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Im going to run 8.5 C/R on a LS2 364cid or 9.0 C/R on a LS2 402cid. The reason im going with .5 lower on the smaller engine is because im using a D-1SC and it will produce at least 2psi lower on the 402cid than the 364cid. So to maximize my gains the .5 C/R is added to the 402cid setup. If im running a Turbo setup i will most likely go with 8.0 C/R on the 346-370cid and 8.5 C/R on the 392-427cid.
09-02-2006, 02:34 PM
#36
Originally Posted by Zombie
My motor is 8.7:1, wish it was 7.8:1 though. I can only run 12psi without meth on 91 octane with 11 degrees peak timing ramping up to 15 by 6k. Well, thats my summer tune where intake air temps can be 125+ degrees.... PRE TURBO
If I would have had a choice in comp ratio it would have been as low as I could have gotten it so I could run more boost on pump. Got my pistons for $200 though so I can't complain that much
Hoping to run 15psi in the cooler months.
If I would have had a choice in comp ratio it would have been as low as I could have gotten it so I could run more boost on pump. Got my pistons for $200 though so I can't complain that much
Hoping to run 15psi in the cooler months.
Is that right? Did I miss something? Just wondnering...shouldn't that thing have 350+rwhp N/A?
09-03-2006, 02:01 PM
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mine is 8:1
"What's better, low compression and more boost or high compression and less boost?"
There are certainly reasons to try to raise compression ratio, namely when off-boost performance matters, like on a street car, or when using a very small displacement motor. but when talking purely about on-boost power potential, compression just doesn't make any sense.
People have tested the power effects of raising compression for decades, and the most optimistic results are about 3% more power with an additional point of compression (going from 9:1 to 10:1, for example). All combinations will be limited by detonation at some boost and timing threshold, regardless of the fuel used. The decrease in compression allows you to run more boost, which introduces more oxygen into the cylinder. Raising the boost from 14psi to 15psi (just a 1psi increase) adds an additional 3.4% of oxygen. So right there, you are already past the break even mark of losing a point of compression. And obviously, lowering the compression a full point allows you to run much more than 1 additional psi of boost. In other words, you always pick up more power by adding boost and lowering compression, because power potential is based primarily on your ability to burn fuel, and that is directly proportional to the amount of oxygen that you have in the cylinder. Raising compression doesn't change the amount of oxygen/fuel in the cylinder, it just squeezes it a bit more.
So the big question becomes, how much boost do we gain for X amount of compression? The best method we have found is to calculate the effective compression ratio (ECR) with boost. The problem is that most people use an incorrect formula that says that 14.7psi of boost on a 8.5:1 motor is a 17:1 ECR. So how in the world do people get away with this combination on pump gas? You can't even idle down the street on pump gas on a true 17:1 compression motor. Here's the real formula to use:
sqrt((boost+14.7)/14.7) * CR = ECR
sqrt = square root
boost = psi of boost
CR = static compression ratio of the motor
ECR = effective compression ratio
So our above example gives an ECR of 12.0:1. This makes perfect sense, because 12:1 is considered to be the max safe limit with aluminum heads on pump gas, and 15psi is about as much boost as you can safely run before you at least start losing a significant amount of timing to knock. Of course every motor is different, and no formula is going to be perfect for all combinations, but this one is vastly better than the standard formula (which leaves out the square root).
So now we can target a certain ECR, say 12.0:1. We see that at 8.5:1 CR we can run 14.7psi of boost. But at 7.5:1 we can run 23psi of boost (and still maintain the 12.0:1 ECR). We only gave up 1 point of compression (3% max power) and yet we gained 28% more oxygen (28% more power potential). Suddenly it's quite obvious why top fuel is running 5:1 compression, that's where all the power is!!
8.5:1 turns out to be a real good all around number for on and off boost performance. Many "performance" NA motors are only 9.0:1 so we're not far off of that, and yet we're low enough to run 30+ psi without problems (provided that a proper fuel is used).
EXCERPT TAKEN FROM INDUCTIONMOTORSPORTS.COM
"What's better, low compression and more boost or high compression and less boost?"
There are certainly reasons to try to raise compression ratio, namely when off-boost performance matters, like on a street car, or when using a very small displacement motor. but when talking purely about on-boost power potential, compression just doesn't make any sense.
People have tested the power effects of raising compression for decades, and the most optimistic results are about 3% more power with an additional point of compression (going from 9:1 to 10:1, for example). All combinations will be limited by detonation at some boost and timing threshold, regardless of the fuel used. The decrease in compression allows you to run more boost, which introduces more oxygen into the cylinder. Raising the boost from 14psi to 15psi (just a 1psi increase) adds an additional 3.4% of oxygen. So right there, you are already past the break even mark of losing a point of compression. And obviously, lowering the compression a full point allows you to run much more than 1 additional psi of boost. In other words, you always pick up more power by adding boost and lowering compression, because power potential is based primarily on your ability to burn fuel, and that is directly proportional to the amount of oxygen that you have in the cylinder. Raising compression doesn't change the amount of oxygen/fuel in the cylinder, it just squeezes it a bit more.
So the big question becomes, how much boost do we gain for X amount of compression? The best method we have found is to calculate the effective compression ratio (ECR) with boost. The problem is that most people use an incorrect formula that says that 14.7psi of boost on a 8.5:1 motor is a 17:1 ECR. So how in the world do people get away with this combination on pump gas? You can't even idle down the street on pump gas on a true 17:1 compression motor. Here's the real formula to use:
sqrt((boost+14.7)/14.7) * CR = ECR
sqrt = square root
boost = psi of boost
CR = static compression ratio of the motor
ECR = effective compression ratio
So our above example gives an ECR of 12.0:1. This makes perfect sense, because 12:1 is considered to be the max safe limit with aluminum heads on pump gas, and 15psi is about as much boost as you can safely run before you at least start losing a significant amount of timing to knock. Of course every motor is different, and no formula is going to be perfect for all combinations, but this one is vastly better than the standard formula (which leaves out the square root).
So now we can target a certain ECR, say 12.0:1. We see that at 8.5:1 CR we can run 14.7psi of boost. But at 7.5:1 we can run 23psi of boost (and still maintain the 12.0:1 ECR). We only gave up 1 point of compression (3% max power) and yet we gained 28% more oxygen (28% more power potential). Suddenly it's quite obvious why top fuel is running 5:1 compression, that's where all the power is!!
8.5:1 turns out to be a real good all around number for on and off boost performance. Many "performance" NA motors are only 9.0:1 so we're not far off of that, and yet we're low enough to run 30+ psi without problems (provided that a proper fuel is used).
EXCERPT TAKEN FROM INDUCTIONMOTORSPORTS.COM
