I bumped the compression up in the LMM to 17.5:1 and it’s like a free 50 horse, also I’ve burnt a half tank of fuel in 100 miles. What needs adjusted in the tuning so I know for my knowledge so it’s not back in the garage next week.
Raising LMM compression
- Thread starter WVRigrat05
- Start date
Not tuning related, but how did you bump compression? What pistons did you use? And did you change the rods?
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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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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 2-3° 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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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.
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?:rofl:
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.
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 didn’t, but it’s very responsive, gonna have to make some changes so the throttle isn’t so touchy, I’m curious how much I need to lower the timing now to keep things happier.
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.
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.
Slapped a set of LLY pistons in, grade C gaskets set valve lash at a tight .011 and let er eat on the 120 tune. Just changed the oil filter with 200 miles on it and it looks like strippers have been dancing in the motor, I’ll change the oil and filter at 500 miles and see what it looks like. Motor hasn’t been over 2500 rpm yet.