How much dynamic compression is to much....
How much dynamic compression is to much.... I think I may be a bit too high...
Shooting for a 93 octane on this engine.
The Static Compression is 12.14:1
The Dynamic is 9.5:1
Ive seen a few trucks on here running 12:1 with boost on top of that but they had a lower DCR.
Any opinions/ first hand experience? Im getting mixed opinions searching/ asking around. e85 is available in my area but id rather not be limited to certain fuel stations....
Engine is a LQ9, flat tops, decked 241s @57cc, heavily ported, comp cam 219/227 .607 .614 112
Shooting for a 93 octane on this engine.
The Static Compression is 12.14:1
The Dynamic is 9.5:1
Ive seen a few trucks on here running 12:1 with boost on top of that but they had a lower DCR.
Any opinions/ first hand experience? Im getting mixed opinions searching/ asking around. e85 is available in my area but id rather not be limited to certain fuel stations....
Engine is a LQ9, flat tops, decked 241s @57cc, heavily ported, comp cam 219/227 .607 .614 112
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Staging Lane
Joined: Feb 2013
Posts: 62
Likes: 0
From: Tampa, Fl
I've always been told 9.0 dcr is max with 8.5 being ideal. A later intake closing will lower your dcr which is how those truck probably run lower dcr than you. And on the timing thing there's always been the dispute of will low compression high timing make more power than high compression low timing but these motors are so efficient on the combustion process that it doesn't take much ignition lead. But neither has ever been proven correct. Its just which ever you want to believe.
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Let's double check your static CR. Are the pistons true flattops or do they have valve reliefs? Even -4cc valve reliefs will make this engine 11.5 CR.
I would change heads or go bigger on the cam (that is a rather small cam).
I would change heads or go bigger on the cam (that is a rather small cam).
Yeah, the calculator I'm using puts you at about 12:1 static CR. Seriously, sell those heads and get something better. I was never a fan of 241 heads no matter how well they were ported. Or get a cam with more valve overlap.
You're probably going to have to flycut with the heads decked down to 57cc. That'll kill a little compression. But I've always shot for 8.7:1 as "ideal" if your tune is spot on. 9:1 if you are willing to really push it and drop some timing and 8.5:1 for 93 safely.
My setup is at 8.5:1 due to being in the hot humid swamp of Florida (on 93 of course).
My setup is at 8.5:1 due to being in the hot humid swamp of Florida (on 93 of course).
Don't know what else to tell ya. Yes, 9.5 dynamic CR is too much on pump gas. Looks like you have two choices: lower the static CR or go with a different cam. A bigger cam will probably have clearance issues with those heads. I vote for different heads.
FormerVendor
Joined: Nov 2012
Posts: 390
Likes: 0
From: Dyno Tuning in KY
I don't know about you guy's but I'm getting slightly higher numbers than whats listed here.... Using the below info I am getting 12.26 static compression using LS3 head gaskets and with that cam straight up with it on its 107 ICL I am getting 9.97 dynamic compression.
Using this info:
Bore 4.000 in
Stroke 3.622 in
Rod Length 6.100 in
Piston Comp. Height 1.328 in
Piston Dome/Dish 0.00 cc
Head Comb. Chamber 57.00 cc
Head Gasket Thickness 0.052 in
Block Deck Height 9.230 in
Cylinders 8
Head Gasket Bore 4.070 in
Boost Pressure (0 if NA) 0 psi
This puts the piston poking out .009" if everything is standard size.
Using this info:
Bore 4.000 in
Stroke 3.622 in
Rod Length 6.100 in
Piston Comp. Height 1.328 in
Piston Dome/Dish 0.00 cc
Head Comb. Chamber 57.00 cc
Head Gasket Thickness 0.052 in
Block Deck Height 9.230 in
Cylinders 8
Head Gasket Bore 4.070 in
Boost Pressure (0 if NA) 0 psi
This puts the piston poking out .009" if everything is standard size.
Staging Lane
Joined: Feb 2013
Posts: 62
Likes: 0
From: Tampa, Fl
I got different numbers too but I used slightly different ones than you
Rod length: 6.098
Deck height: 9.24
Piston volume: +1.2
He's running lsr 269 so intake closes at 62°
I found somewhere that its 1.2, idk if that's creditable or not but with stock gaskets I got 12.18 and dcr I got 9.8 and ls3 gaskets got me down to 9.4xx
Rod length: 6.098
Deck height: 9.24
Piston volume: +1.2
He's running lsr 269 so intake closes at 62°
I found somewhere that its 1.2, idk if that's creditable or not but with stock gaskets I got 12.18 and dcr I got 9.8 and ls3 gaskets got me down to 9.4xx
In my post I was talking Dynamic. One thing to remember about dynamic is it's not a real number that dynamically changes - it's simply a calculation. I don't put too much into the number except to make sure my cam isn't going to give me 7.2:1 DCR for example.
You can run in excess of 9:1 DCR. A lot of small cams with milled heads are up in he 9.5 range when calculated. Just check your cranking compression. If it's over 200psi you'll have problems with 93 octane. Shoot for it around 180ish.
You can run in excess of 9:1 DCR. A lot of small cams with milled heads are up in he 9.5 range when calculated. Just check your cranking compression. If it's over 200psi you'll have problems with 93 octane. Shoot for it around 180ish.