Low Compression + Pump Gas... How Much Boost?
Low Compression + Pump Gas... How Much Boost? Well, I did some searching and no one really wants to give a straight cut answer (which I understand), but everything I find either someone is spraying methanol or they’re mixing race gas. Then others are still in the “decision stage” of how much compression to run and everyone convinced them to go higher. I would have gone with more compression had I not been in such a hurry when I was getting the motor put together, but it’s too late now. 
So, I decided I’d just start another sort of redundant thread.
Motor setup is:
LQ9 block
stock crank
.030 over
-20cc pistons
LS9 head gaskets
Stock casting LS3 heads
8.7:1 Compression
EPP Blower cam 232/240 .595''/.608 115... yeah I know, it’s not a "turbo cam" (any comments from knowledgeable cam people about this cam in my setup are welcome btw since I'm curious)
Only spinning it to about 6500 +/- 100 rpm (if that matters)
Just your standard truck manifold turbo kit, 2.5” crossover, PT88 turbo, 4” down pipe, A2A intercooler
The fuel will run is 93 octane ethanol free.
There will be NO methanol injection on this car.
Obviously there are going to be variables like tuning/timing, but what do you think a safe amount of max boost would be given this setup?
I’m sure I’m forgetting some details that are relevant... just tell me and I’ll add it.
Thanks to anyone who has good info!
So, I decided I’d just start another sort of redundant thread.
Motor setup is:
LQ9 block
stock crank
.030 over
-20cc pistons
LS9 head gaskets
Stock casting LS3 heads
8.7:1 Compression
EPP Blower cam 232/240 .595''/.608 115... yeah I know, it’s not a "turbo cam" (any comments from knowledgeable cam people about this cam in my setup are welcome btw since I'm curious)
Only spinning it to about 6500 +/- 100 rpm (if that matters)
Just your standard truck manifold turbo kit, 2.5” crossover, PT88 turbo, 4” down pipe, A2A intercooler
The fuel will run is 93 octane ethanol free.
There will be NO methanol injection on this car.
Obviously there are going to be variables like tuning/timing, but what do you think a safe amount of max boost would be given this setup?
I’m sure I’m forgetting some details that are relevant... just tell me and I’ll add it.
Thanks to anyone who has good info!
We are both in the same boat with similar setups. My compression is around 8.4 though with a 78/75 turbo...I think we will be limited more so due to our thin deck heads but Idk what that number would be...
I'm not worried in the least bit about the heads lifting. So many people have proven that they are able to take 20+ psi without lifting, and I don't suspect I will be able to push the limits as far as the deck thickness of the heads go. Head studs and LS9 gaskets seem to go a long ways with these heads it appears.
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I agree with the boost figures just watch the timing. There is 6 degrees of overlap on that cam so its not the best for boost as it will bleed some off but its not gonan kill you either. The less overlap on a boosted car the better.
Well I think you can boost it to 15 psi on pump gas, but I'd compare the power you make at 12, 13, 15 psi to see what is most efficient.
I had a 348 cid, 8:1, 16 psi went 10.00@139 on a 1.5, GT 47-88. Had I pushed it that way I could have made it go 9.8@14xmph which I see a bunch of folks running on some pump gas combos. Every point of compression is reportedly worth 3% more horsepower. So my old engine needed more boost to make up for the low compression.
Was 867rwhp, 24 or 25 psi on race gas.
I had a 348 cid, 8:1, 16 psi went 10.00@139 on a 1.5, GT 47-88. Had I pushed it that way I could have made it go 9.8@14xmph which I see a bunch of folks running on some pump gas combos. Every point of compression is reportedly worth 3% more horsepower. So my old engine needed more boost to make up for the low compression.
Was 867rwhp, 24 or 25 psi on race gas.
What do you mean by "checking to see what is most efficient"? Are you saying if I pick up 30hp:1psi up to 13psi and then at 14psi only pick up 20hp I should leave it at 13? What will I get out of determining where it is most efficient? Maybe I am not correctly understanding what you are saying.
Say the tuner can take it up to 16-17psi without any detonation, a good afr, the plugs still look good, and it's still making more power... Is it still unsafe to be running that much pressure on pump gas or not? I guess what I'm asking is how do you decide where to draw the line?
I'm no tuner, and I do not have a great understanding of this subject (if you couldn't tell). So these questions are all at the risk of sounding like a complete idiot.
Say the tuner can take it up to 16-17psi without any detonation, a good afr, the plugs still look good, and it's still making more power... Is it still unsafe to be running that much pressure on pump gas or not? I guess what I'm asking is how do you decide where to draw the line?
I'm no tuner, and I do not have a great understanding of this subject (if you couldn't tell). So these questions are all at the risk of sounding like a complete idiot.
Last edited by Slow94Formula; Feb 27, 2013 at 07:26 PM.
I bought a boost gauge that only goes to 20 on purpose. 
Even if he did say he could take it to 20 I don't know if I'd let him. 17 (or even 18) sounds like plenty to me. To be honest I was quite happy with how the car ran on 11 pounds of boost, but if there's more in it...
Even if he did say he could take it to 20 I don't know if I'd let him. 17 (or even 18) sounds like plenty to me. To be honest I was quite happy with how the car ran on 11 pounds of boost, but if there's more in it... .
Say the tuner can take it up to 16-17psi without any detonation, a good afr, the plugs still look good, and it's still making more power... Is it still unsafe to be running that much pressure on pump gas or not? I guess what I'm asking is how do you decide where to draw the line?
Say the tuner can take it up to 16-17psi without any detonation, a good afr, the plugs still look good, and it's still making more power... Is it still unsafe to be running that much pressure on pump gas or not? I guess what I'm asking is how do you decide where to draw the line?