Carbureted LSX Forum - pump gas comp. ratio w carb




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ssinister550
03-03-2012, 02:36 PM
What's the most compression you carbed ls1 guys are getting away with? I know w fuel injection there's ppl running as much as 12:1 or slightly higher is there anyone getting away w that with a carb setup?


LS1MCSS
03-04-2012, 10:32 PM
I am running 12.7 and pump premium, but have my total timing set at 28 degrees until I fill up with race gas.

ssinister550
03-05-2012, 09:52 AM
Holy shit that's pretty stout. 28 deg total isn't very low for an ls motor either can you elaborate on your engine specs a bit more please?


NoShow
03-05-2012, 11:09 AM
Make sure you have lots of cam overlap if you run that high of comp

Prorac1
03-05-2012, 11:48 AM
Wow, 12.7 is about the highest ive ever heard of in a pump gas application. I have to give the OP credit, ive never thought about it before. Good question. Eric L

1badeagle
03-05-2012, 10:09 PM
My compression is 12:1 on my setup.

ssinister550
03-05-2012, 11:19 PM
good info guys, and i guess i didn't specify, but it'd be nice if we could get engine specs along w your c/r i.e heads, cam, power adder, shortblock specs, displacement, timing, carb size, etc

1badeagle
03-05-2012, 11:51 PM
good info guys, and i guess i didn't specify, but it'd be nice if we could get engine specs along w your c/r i.e heads, cam, power adder, shortblock specs, displacement, timing, carb size, etc

04 LQ4 6.0 370ci
4.030" bore
Stock stroke
Mahle pistons
Ls3 heads
Top secret custom cam
Victor jr
950 holley

Prorac1
03-06-2012, 01:23 AM
04 LQ4 6.0 370ci
4.030" bore
Stock stroke
Mahle pistons
Ls3 heads
Top secret custom cam
Victor jr
950 holley

Who specd the cam? And im sure a round about duration and lift wouldnt hurt, lol. It seems from what im finding that the LS3 heads seem to like some extra duration on the exhaust to help fight the large offset between intake and exhaust flow. And in carbed applications they seem to like a fairly low (numerically) intake centerline. Eric L

LS1MCSS
03-06-2012, 05:04 AM
Holy shit that's pretty stout. 28 deg total isn't very low for an ls motor either can you elaborate on your engine specs a bit more please?
It's the 84 in my sig.

LS1MCSS
03-06-2012, 05:11 AM
Wow, 12.7 is about the highest ive ever heard of in a pump gas application. I have to give the OP credit, ive never thought about it before. Good question. Eric L28 in the ragged edge with pump premium. I plan on having race gas next time I got to the track. I also have a set of 4.11 gears and a spool, so I can thrash my sons firehawk worse than I did the first time. :)

http://www.youtube.com/watch?v=hFbSHvj8VQQ

bitterman
03-06-2012, 06:18 AM
Its really hard to say a specific comp ratio will work.

These are just some of the things that play a role:
-Weight of vehicle
-Gear ratios (tranz and diff)
-Fuel quality in your area
-Cam overlap
-DCR
-Quench

Bruce

Prorac1
03-06-2012, 04:33 PM
JMHO, In theory you want to run as much compression as your fuel will allow. More compression equals more power. But, for instance, if you live in a state that only sells oxygenated 91 octane, that would be your limiting factor. But, on the inverse, on the 6.0 im building for my car, im only going to have around 9.9 or 10-1 comp, so I have a cam that keeps cranking compression up there and dosent bleed off what little compression I have. Eric L

bitterman
03-06-2012, 07:20 PM
Protrac that is what I did got a cam spec'd for the ly6 for my build as its 9.67:1.. Comes out to about 8.4:1 DCR but its still 222/234 @0.050
I might goto a 0.040 gasket to up compression and get a better quench at the same time.

Bruce

ssinister550
03-06-2012, 11:57 PM
Its really hard to say a specific comp ratio will work.

These are just some of the things that play a role:
-Weight of vehicle
-Gear ratios (tranz and diff)
-Fuel quality in your area
-Cam overlap
-DCR
-Quench

Bruce

I understand all of these things, but thanks for your input sir! And i just wanted to see what other people are getting away with, not asking what specific ratio will work for me.... As for "dynamic compression", I believe it is entertaining at best. It doesn't calculate inertial "supercharging" that takes place when the piston is coming back up before the intake valve closes, or the scavenging effect that the exhaust gas exiting the cylinder has on the intake tract when the intake valve cracks open. These, along with other airflow theory related phenomena, cannot be calculated simply by using physical specifications. Therefore, I'm not convinced that DCR can be definitively useful, but to each his own! :drive:

1badeagle
03-07-2012, 12:16 AM
Who specd the cam? And im sure a round about duration and lift wouldnt hurt, lol. It seems from what im finding that the LS3 heads seem to like some extra duration on the exhaust to help fight the large offset between intake and exhaust flow. And in carbed applications they seem to like a fairly low (numerically) intake centerline. Eric L

Patrick g sped'd it out to me. its not top secret, Im just not at home to look at the specs. I think its like 235/243 .621/.624 110+4 lsa

Prorac1
03-07-2012, 05:26 AM
Patrick g sped'd it out to me. its not top secret, Im just not at home to look at the specs. I think its like 235/243 .621/.624 110+4 lsa

Thats almost identical to mine. Eric L

bitterman
03-07-2012, 06:12 AM
I understand all of these things, but thanks for your input sir! And i just wanted to see what other people are getting away with, not asking what specific ratio will work for me.... As for "dynamic compression", I believe it is entertaining at best. It doesn't calculate inertial "supercharging" that takes place when the piston is coming back up before the intake valve closes, or the scavenging effect that the exhaust gas exiting the cylinder has on the intake tract when the intake valve cracks open. These, along with other airflow theory related phenomena, cannot be calculated simply by using physical specifications. Therefore, I'm not convinced that DCR can be definitively useful, but to each his own! :drive:

One thing you can look into is http://www.snowperformance.net/stage-2-boost-cooler-n-a.html It only works with high compression ratios or turbo/supercharger scenarios. I seen it on HP TV. They compared results on both EFI and carb setup. On the carb setup it did not make a huge increase but would allow much higher compression ratio.. They compared it to 116 Octane fuel. Something to think about.. Main downfall is you need to either purchase booster juice or get a source of methanol to mix with water.


I wonder if you could do a 13-14:1 engine using 93 octane and the water/methanol injection? I think it would work. Now we only need some sucker to step up and prove it :)

Bruce

johnyrrr
03-07-2012, 08:36 PM
Why cant you just mix in a little
e85 in your premium to up the octane and maybe change your jets to flow a touch more?

ssinister550
03-08-2012, 10:41 AM
Yea I've been wanting to do a high c/r e85 or 91/meth injection street motor for quite some time now. Was jus talkin to my buddy about it the other day. You could do like 14:1 on e, and if ur ever somewhere where E isn't available jus throw in some 91 and turn on the meth injection. I know it will work I might try it someday.