Boost vs. Compression
Thread Starter
Racer
Joined: Oct 2009
Posts: 251
Likes: 3
From: Tahlequah Oklahoma
Boost vs. Compression
Ok say you are going to build a high HP lsx 427 for a Reliable Street car and you had 93 octane and methanol injuection for fuel.
Would you:
1. Build a low compression motor and boost the crap out of it or
2. Build a high compression motor and use lower boost?
I know that intercooled boost makes about 20-25rwhp/lb of boost.
How much does a point of copression make?
My Mustang was a 9:1 331ci and it made 795rwhp at 24psi with 91 octane and meth.
Would you:
1. Build a low compression motor and boost the crap out of it or
2. Build a high compression motor and use lower boost?
I know that intercooled boost makes about 20-25rwhp/lb of boost.
How much does a point of copression make?
My Mustang was a 9:1 331ci and it made 795rwhp at 24psi with 91 octane and meth.
Last edited by chevpwr; Jun 17, 2010 at 10:00 PM.
Burning Brakes
Joined: May 2007
Posts: 887
Likes: 1
From: Dallas TX
I personally would drop the compression down to around 10:1 so you dont lose all low end tq when you are out of boost. With the motor at 10:1 and 93 and meth only you should be able to run 15-16psi easily for 800-900rw.
Drifting
Joined: Aug 2009
Posts: 1,412
Likes: 0
From: Texas 1,002rwhp/928rwtq
St. Jude Donor '09-'10
If the car is set up properly, you will make a lot more power with boost than compression. Case in point; H/C LS7's are barley cracking 600rwhp on pump gas and FI builds are making over 1,000 fairly regularly.
Safety Car
Joined: Aug 2004
Posts: 4,067
Likes: 62
From: Houston Texas
At the end of the day it all comes down to DCR (Dynamic Compression Ratio) you can get the exact DCR with low CR and High PSI or high CR and low boost... BUT why loose drivability with low CR and why add more boost when boost makes heat. Five more psi is 55 degrees more in the cylinder.
Go at least 10.5:1 with low boost and meth never hurts.
Go at least 10.5:1 with low boost and meth never hurts.
Le Mans Master
Joined: Oct 2007
Posts: 6,149
Likes: 11
From: Greater Detroit Metro MI, when I'm not travelling.
Ok say you are going to build a high HP lsx 427 for a Reliable Street car and you had 93 octane and methanol injuection for fuel.
Would you:
1. Build a low compression motor and boost the crap out of it or
2. Build a high compression motor and use lower boost?
I know that intercooled boost makes about 20-25rwhp/lb of boost.
How much does a point of copression make?
My Mustang was a 9:1 331ci and it made 795rwhp at 24psi with 91 octane and meth.
Would you:
1. Build a low compression motor and boost the crap out of it or
2. Build a high compression motor and use lower boost?
I know that intercooled boost makes about 20-25rwhp/lb of boost.
How much does a point of copression make?
My Mustang was a 9:1 331ci and it made 795rwhp at 24psi with 91 octane and meth.
Ultimately you have decide how much power you want to make, figure out how much boost it will take to get there, and run the highest compression ratio you can at that boost level while still having a margin of safety...
Drifting
Joined: Jan 2009
Posts: 1,646
Likes: 2
just look at GM's new LSX LS3 Long block 9:1 compression, make more HP stock that and LS3 and is completely forged 450hp and 444tq... Think you will lose torque, just look at there dyno graph.. I just ordered one today btw 
Going to spin the **** out of my D1 with it.. Or I may just sell my D1 and P1 and go TT with it.. Thank goodness I get below GM employee pricing!
Ohh yeah, 12month 12K mile warranty on a blower motor from GM is nice too..
Going to spin the **** out of my D1 with it.. Or I may just sell my D1 and P1 and go TT with it.. Thank goodness I get below GM employee pricing!Ohh yeah, 12month 12K mile warranty on a blower motor from GM is nice too..
Last edited by TrenAman; Jun 18, 2010 at 11:51 PM.
Pro
Joined: May 2007
Posts: 511
Likes: 12
From: Parkville Maryland
Lower compression actually increases the size of the combustion chamber at the top of the stroke doesnt it?
So if you have more area to fill with the same PSI, you are exponentially filling cylinder with more air/fuel.... right?
I dont really know, just gussin...
So if you have more area to fill with the same PSI, you are exponentially filling cylinder with more air/fuel.... right?
I dont really know, just gussin...
Last edited by brokenparts; Jun 20, 2010 at 03:52 PM.
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Safety Car
Joined: Mar 2006
Posts: 4,448
Likes: 4
From: Sarasota Florida
St. Jude Donor '06
Depends on your goals. If you're going to run a boosted 427 on the street you don't need the absolute max power unless you're going to run slicks and a cage at the strip, or visit Mexico regularly. On streetable tires is there a difference between 900 and 950 rwhp? How much power do you plan/need/want to make?
More compression will yield better throttle response and fuel economy off-boost, and that's where you'll be most of the time. How much "slack" or safety margin do you want for a bad tank of gas, etc? Cam choice will also enter into this.
So again, depends on your goals and what "street" means.
I'm conservative, maybe paranoid. I ended up at 9.82:1 and a 232 cam for my new setup (which isn't quite done yet), targeting 10-11 psi on a water/91 octane tune, and 16-17 psi on a 93/meth tune. If I had it to do over I might have done 2 or 3 tenths more, but I decided it wasn't worth fussing over, so I took it as-is.
How much power do you plan/need/want to make?More compression will yield better throttle response and fuel economy off-boost, and that's where you'll be most of the time. How much "slack" or safety margin do you want for a bad tank of gas, etc? Cam choice will also enter into this.
So again, depends on your goals and what "street" means.
I'm conservative, maybe paranoid. I ended up at 9.82:1 and a 232 cam for my new setup (which isn't quite done yet), targeting 10-11 psi on a water/91 octane tune, and 16-17 psi on a 93/meth tune. If I had it to do over I might have done 2 or 3 tenths more, but I decided it wasn't worth fussing over, so I took it as-is.
Last edited by Gannet; Jun 20, 2010 at 05:45 PM.
Melting Slicks
Joined: Jul 2006
Posts: 3,356
Likes: 1
From: Chesapeake Va
The key here is the fuel. 93 octane, combined with meth, for a street friendly setup, the compression needs to be kinda middle of the road. Too many people want to run too low of compression in the name of boost. Most of the time this causes low power, and tip in issues.
For a pump gas application, even a higher boost, high hp motor, I would recommend high 9:1 compression.
The FI alcohol motors run NA type compression. 11:1, 12:1, on straight Meth, big boost, big HP.
I tuned an alcohol funny car for a while. We ran almost 13:1 and a sh*t load of boost. Guess that's not in the realm of this discussion though. LOL
For a pump gas application, even a higher boost, high hp motor, I would recommend high 9:1 compression.
The FI alcohol motors run NA type compression. 11:1, 12:1, on straight Meth, big boost, big HP.
I tuned an alcohol funny car for a while. We ran almost 13:1 and a sh*t load of boost. Guess that's not in the realm of this discussion though. LOL
Le Mans Master
Joined: Oct 2007
Posts: 6,149
Likes: 11
From: Greater Detroit Metro MI, when I'm not travelling.
No, not at all. How much you fill the cylinder with air and fuel depends entirely on how large the cylinder is, how much boost you are running, and how long the intake valve is open for. The compression ratio does affect the final combustion chamber area, but that does not have a bearing on engine breathing; a 6L engine is still only taking in 6L of air regardless of how much it compresses it in the end.
