Duramax Diesels Forum Truck of the Week
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Old 04-08-2018, 01:54 PM
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Default Raising LMM compression

I bumped the compression up in the LMM to 17.5:1 and itís like a free 50 horse, also Iíve burnt a half tank of fuel in 100 miles. What needs adjusted in the tuning so I know for my knowledge so itís not back in the garage next week.
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Old 04-08-2018, 03:25 PM
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Originally Posted by WVRigrat05 View Post
I bumped the compression up in the LMM to 17.5:1 and itís like a free 50 horse, also Iíve burnt a half tank of fuel in 100 miles. What needs adjusted in the tuning so I know for my knowledge so itís not back in the garage next week.
Not tuning related, but how did you bump compression? What pistons did you use? And did you change the rods?

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Old 04-08-2018, 03:31 PM
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Not tuning related, but how did you bump compression? What pistons did you use? And did you change the rods?

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pistons...
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Old 04-08-2018, 03:35 PM
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pistons...
Okay..... Lol. Maybe I should of been more specific as I'm interested in what he used to rebuild this engine as I know he's not too awfully nice on things and like watching the longevity of the build. So.


Did you use lb7/lly pistons with a thinner gasket?

Or an actual aftermarket piston for the achieved 17.5 ratio?

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Old 04-08-2018, 03:48 PM
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he'll have to answer as to what his specs are
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Old 04-08-2018, 04:18 PM
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Quote:
Originally Posted by WVRigrat05 View Post
I bumped the compression up in the LMM to 17.5:1 and itís like a free 50 horse, also Iíve burnt a half tank of fuel in 100 miles. What needs adjusted in the tuning so I know for my knowledge so itís not back in the garage next week.
Increase CR is free HP increase, throttle response will go up as well

Usually playing with timing map to get the most out of it... If it's sharp rattling need to pull timing.. usually 2-3į will make it stop..

I've found more adjustment was need in part throttle area vs WOT
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Old 04-08-2018, 04:33 PM
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Increase CR is free HP increase, throttle response will go up as well

Usually playing with timing map to get the most out of it... If it's sharp rattling need to pull timing.. usually 2-3į will make it stop..

I've found more adjustment was need in part throttle area vs WOT
Mines just snappy around 2200 and drinks fuel, a lot of it. Definitely runs better, it’ll blow the tires off over half throttle on tune 3, don’t like that.

And no rattle, it’s quieter than it was before, just likes fuel and at 2200 rpms it’s like a power band on a dirt bike, something needs cut back for sure.
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Last edited by WVRigrat05; 04-08-2018 at 04:40 PM.
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Old 04-08-2018, 06:16 PM
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Scratch the fuel, just filled up again and at 1/4 tank it only took 12 gallons, gauge is off worse than before, so I averaged 14.6 mpg on a 7 month old tank of fuel.
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Rig ride/new DD-02' Black Eddie Bauer explorer 4.6, -180k, still sporting original trans, outlasted two Duramaxs.
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Old 04-09-2018, 03:18 AM
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Well that worked out...
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Old 04-09-2018, 06:23 PM
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Quote:
Originally Posted by WVRigrat05 View Post
Mines just snappy around 2200 and drinks fuel, a lot of it. Definitely runs better, itíll blow the tires off over half throttle on tune 3, donít like that.

And no rattle, itís quieter than it was before, just likes fuel and at 2200 rpms itís like a power band on a dirt bike, something needs cut back for sure.
What's tune 3 supposed to be at the re tires... If you picked up ~"50hp" on base tune it will be more on the higher tunes

Maybe you're not used to the extra HP and need to recalibrate your right foot?
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Old 04-09-2018, 08:51 PM
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I seriously doubt it bumped your HP much. It will change the character of your engine's response.

At 17.5 :1 , Mine runs like a raped ape above 2200. Loves to spin. Pulls hard all the way to the 4500 limiter.

But, if I ever have cause to do it over again, I'll go with 16.5 : 1.
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Old 04-10-2018, 03:31 AM
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Why John?
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Old 04-10-2018, 06:27 AM
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Originally Posted by Fingers View Post
I seriously doubt it bumped your HP much. It will change the character of your engine's response.

At 17.5 :1 , Mine runs like a raped ape above 2200. Loves to spin. Pulls hard all the way to the 4500 limiter.

But, if I ever have cause to do it over again, I'll go with 16.5 : 1.
Quote:
Originally Posted by NC-smokinlmm View Post
Why John?
Curious as well. My new motor is 16.5:1
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Old 04-10-2018, 08:25 AM
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Quote:
Originally Posted by Fingers View Post
I seriously doubt it bumped your HP much. It will change the character of your engine's response.

At 17.5 :1 , Mine runs like a raped ape above 2200. Loves to spin. Pulls hard all the way to the 4500 limiter.

But, if I ever have cause to do it over again, I'll go with 16.5 : 1.
No, I know it didnít, but itís very responsive, gonna have to make some changes so the throttle isnít so touchy, Iím curious how much I need to lower the timing now to keep things happier.
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Rig ride/new DD-02' Black Eddie Bauer explorer 4.6, -180k, still sporting original trans, outlasted two Duramaxs.
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Old 04-10-2018, 09:37 AM
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Several personal reasons for wanting lower CR. I've had the chance to compare them side by side.

First, and most important to me, the lower CR engines are much quieter and smoother power.

The "bump" in response at 2200 is a pain in the butt with the higher CR engine. I've tuned most of it out, but it has been a struggle. I think the cam is playing a role here on my engine, but have seen the same thing on stock cam engines with higher CR.

I personally have not seen any increase in MPG. Actually the opposite. That is comparing ovals to ovals. But the pistons were not the only mod. So the comparison is not worth much.

I don't think the ring package on the Duramax is up to the task of higher CR. I have not looked into it much other than some chamber pressure observations. Whereas a 16.5 CR piston would show an effective 16.4-16.5 ratio on the pressure graphs, The 17:5 pistons would be more like 17.3-17.4 effective CR. I attribute the drop in effective CR to blow by and have not chased it beyond that.

Lastly, a Higher CR is harder on parts. A jump from 16.5 to 17.5 is about 7% more strain on all the parts just in increased pressure at any load.

Just my opinions. Each to their own. When customers ask, I recommend 16.5 for a Daily Driver, 16.0 if you are running a lot of boost such as with twins.
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'04.5 EC/LB 4x4.
Howard Rods w/ oiler hole.
Oval Chambers and valve reliefs cut in Mahle Cast Pistons. 17.5:1
SoCal Stage 2 heads, 3388 Cam, Billet Flywheel, and harmonic balancer.
Danville 68mm VVT Turbo
100 over injectors.
PPE Dual CP3s
ProFab Castflow headers and uppipes.
100 PSIA MAP Sensor
Kennedy dual lift pump.

Dynoed 690 HP / 1200 TQ
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