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Old 11-24-2013, 09:48 AM   #1
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Higher CR with PD blower?

11:1 static CR vs. 10:1 static CR What would make more power in an LS engine with a Positive Displacement Blower on top? E85 for fuel.

I've read different thought processes all over the interwebz with different conclusions. I've also seen engine builders chime in and report more compression would help with a PD blower. Other thought process of low CR+more boost=higher HP vs. high CR+same boost=less HP. I assume this is having the octane/fuel be the limiting factor on timing advance? I am running E85.

Would like to hear and/or see some real world examples.

My car has 10.5:1 static CR 6.0 liter with a 2.3 Whipple. This is with 317 heads and close to stock sized Combustion Chambers. I removed the 243 heads due to guidance given by a reputable tuner who indicated the 11:1+ static CR would be too much.
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Old 11-24-2013, 10:31 AM   #2
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More compression means more horsepower. Always. With that said, 9:1 to 10.5:1 is the general to keep compression at for boost.
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Old 11-24-2013, 11:20 AM   #3
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Assuming you have the octane, higher the better.
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Old 11-24-2013, 01:34 PM   #4
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Quote:
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Assuming you have the octane, higher the better.
This. Adjust boost & timing for maximum power, within safe air/fuel ratios, given octane used. E85 allows more timing assuming you have a robust fuel system to deliver enough of it.
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Old 11-24-2013, 03:54 PM   #5
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I think fuel is the key here, the more octane the more compression you can run. On a 93 octane tune is where I think compression should be lowered.
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Old 11-25-2013, 06:41 AM   #6
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Quote:
Originally Posted by 10aSSee D View Post
I think fuel is the key here, the more octane the more compression you can run. On a 93 octane tune is where I think compression should be lowered.
Or keep the compression and modulate boost & timing accordingly.
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Old 11-25-2013, 08:08 AM   #7
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Quote:
Originally Posted by 2SS45th View Post
More compression means more horsepower. Always. With that said, 9:1 to 10.5:1 is the general to keep compression at for boost.
9:1 to 10.5:1 CR...is this the range that is standard based on pump gas (93 octane)?

I understand that there is a point of diminishing returns with CR. HP does not follow a linear line, increasing as you increase CR. But there is a noticeable difference between a 9:1 motor and a 11:1 motor.
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Old 11-25-2013, 10:40 AM   #8
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Yes, pump gas. Lower compression will also be less prone to detonation.
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Old 11-25-2013, 03:52 PM   #9
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11:1 and boost with E85 would be awesome, you can always fiddle with compression ratio to a certain extent by swapping out a cam if you think its too much/little in the future.

With that said centrifugals are the blowers that usually work better with high compression ratio because they sort of take-off where peak volumetric efficiency (peak torque) would otherwise fall meaning you can run more peak boost or more compression on a given fuel octane with a centrifugal vs a PD blower.
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