Compression ratio question

DieselDemon18

Member
Aug 25, 2011
48
7
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Moreno Valley, Ca
Getting ready to do a mild build on my LB7 and just getting ideas, one thing i noticed is most of the retailers only stock 15.0, 15.5, 16.0, and 16.5 compression pistons now as we know the lb7 comes with 17.5 from the factory. Now i know i can order 17.5 from Mahle but im curious if there would be possibly an advantage to anythimg running 16.5 or even 17.0 truck has a 4094 turbo, stock fuel. Will Only pushing 5-600 on the regular but would like for it to be 8-900 capable just in case, but mostly want the reliability of being able to thrash on it and not having to worry too much, so whats everyones opinion? Stay stock CR or go reduced CR? Truck is also my tow pig for my relatively heavy RV
 

DAVe3283

Heavy & Slow
Sep 3, 2009
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I went with a reduced CR on my build and wish I kept the stock CR. If I was doing it again, especially with a 600-700 HP goal, I'd personally stick with the stock CR.

Not to say my engine runs badly or anything. But off idle throttle response and haze is easier to deal with on a stock CR motor, IMO. Just my $0.02.

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2004LB7

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Dec 15, 2010
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While I'm no expert in this area, I believe lower CR helps ease the stress on the pistons, rods and crank. As a side benefit, it also frees up a little more room in the cylinder to squeeze a little more air and fuel in it. It is easier to make a low compression high HP engine survive then a high compression one at the same HP. They do on the other hand suffer more from hazing, harder starts and reported less throttle response during low boost conditions

I would just ask your builder or supplier what they recommend for CR on your HP goals
 

Chevy1925

don't know sh!t about IFS
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Oct 21, 2009
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was that testing done after we started using large injectors?
One was adams truck, other was buddies back east. It was a lbz that got built. i dont know who the pistons were but they were 18.5:1. i might have the pics of the bowls on my phone somewhere. he put 80% overs in there and had steve tuning it. we were going back and forth from there as it rattled like hell under certain conditions and steve was asking me if it could be an engine issue. i didnt put that engine together so we asked things like protrusion, p to w clearance, headgasket used, and other parts used. it took a lot of pilot injection work and timing, then sounded like it was pretty good. his thought was throttle should be snappier, better mpg, more power less boost, etc. the typical tune he had was 1400uS so fairly decent. There was some tweaks here and there as time went on but it all kinda fell back to the same things adam saw on fingers pistons. didnt clean up any more haze the 80's might have had, throttle felt the same, still have a little rattle here and there under very specific conditions that were hard to track, and power wise was till on the same track as lower 17:1 or 16.5:1 compression. mpg negligible. he did say it fired off in the cold really easy and might have reved quicker/easier but was hard to say.

i thought about going higher in your truck but just went standard 16.5:1 everyone runs.

fingers has a pretty good write in adams thread about higher compression pistons and why they dont work like guys think. see if i can find it.
 

DieselDemon18

Member
Aug 25, 2011
48
7
8
Moreno Valley, Ca
One was adams truck, other was buddies back east. It was a lbz that got built. i dont know who the pistons were but they were 18.5:1. i might have the pics of the bowls on my phone somewhere. he put 80% overs in there and had steve tuning it. we were going back and forth from there as it rattled like hell under certain conditions and steve was asking me if it could be an engine issue. i didnt put that engine together so we asked things like protrusion, p to w clearance, headgasket used, and other parts used. it took a lot of pilot injection work and timing, then sounded like it was pretty good. his thought was throttle should be snappier, better mpg, more power less boost, etc. the typical tune he had was 1400uS so fairly decent. There was some tweaks here and there as time went on but it all kinda fell back to the same things adam saw on fingers pistons. didnt clean up any more haze the 80's might have had, throttle felt the same, still have a little rattle here and there under very specific conditions that were hard to track, and power wise was till on the same track as lower 17:1 or 16.5:1 compression. mpg negligible. he did say it fired off in the cold really easy and might have reved quicker/easier but was hard to say.

i thought about going higher in your truck but just went standard 16.5:1 everyone runs.

fingers has a pretty good write in adams thread about higher compression pistons and why they dont work like guys think. see if i can find it.
I appreciate the response i am just trying to get a general idea. Ive been around plenty of built motors with reduced compression but they were super high performance 900+ hp motors that did zero towing, only daily ish truck ive been around had 16.0 pistons in it and its a bit luggy off the line. Mine is going to be a mild street build for some fun when i want it but also hook up to my 15k pound rv and tow it without issue. I know lb7 has the highest CR of all generations of the duramax so i wanted to stay relatively close to it so i dont lose much bottom end but if i can find a middle ground with say a 16.8 or 17.0 ill run those. Just trying to gather as much information as i can before i pull the trigger on anything.
 

gmduramax

Shits broke
Jun 12, 2008
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I appreciate the response i am just trying to get a general idea. Ive been around plenty of built motors with reduced compression but they were super high performance 900+ hp motors that did zero towing, only daily ish truck ive been around had 16.0 pistons in it and its a bit luggy off the line. Mine is going to be a mild street build for some fun when i want it but also hook up to my 15k pound rv and tow it without issue. I know lb7 has the highest CR of all generations of the duramax so i wanted to stay relatively close to it so i dont lose much bottom end but if i can find a middle ground with say a 16.8 or 17.0 ill run those. Just trying to gather as much information as i can before i pull the trigger on anything.

I would just run the standard 16.5:1 and call it a day. Run higher rail pressure if you need to. The difference between 16.5, 16.8, 17 you won’t notice a difference
 
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Chevy1925

don't know sh!t about IFS
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Oct 21, 2009
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I appreciate the response i am just trying to get a general idea. Ive been around plenty of built motors with reduced compression but they were super high performance 900+ hp motors that did zero towing, only daily ish truck ive been around had 16.0 pistons in it and its a bit luggy off the line. Mine is going to be a mild street build for some fun when i want it but also hook up to my 15k pound rv and tow it without issue. I know lb7 has the highest CR of all generations of the duramax so i wanted to stay relatively close to it so i dont lose much bottom end but if i can find a middle ground with say a 16.8 or 17.0 ill run those. Just trying to gather as much information as i can before i pull the trigger on anything.
Anthony now owns my old truck that did all you want your truck to do. i will tell you that the "lug" you feel has far more to do with converter choice, gearing, tires, tuning, and turbo/s than compression ratio by any means.
 

Bdsankey

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I appreciate the response i am just trying to get a general idea. Ive been around plenty of built motors with reduced compression but they were super high performance 900+ hp motors that did zero towing, only daily ish truck ive been around had 16.0 pistons in it and its a bit luggy off the line. Mine is going to be a mild street build for some fun when i want it but also hook up to my 15k pound rv and tow it without issue. I know lb7 has the highest CR of all generations of the duramax so i wanted to stay relatively close to it so i dont lose much bottom end but if i can find a middle ground with say a 16.8 or 17.0 ill run those. Just trying to gather as much information as i can before i pull the trigger on anything.
Mahle Motorsport pistons without reliefs are advertised at 16.8:1, with reliefs they're advertised at 16.5:1.



I personally went with non-reliefed pistons for my DD/tow pig build.
 
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Woody35

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Jan 3, 2013
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I wonder if higher compression could reduce soot on emission motors? Ignoring NOx of course
 

S Phinney

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They aren’t running the emissions motor with higher compression so I would think it has some drawbacks or the OEM guys would be doing it.


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Woody35

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Jan 3, 2013
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NOx would go up quite a bit I would assume. Probably have to pump some serious quantity of DEF to combat it. Plus a lot of EGR. In the aftermarket though with a stronger bottom end might be something to look into if your trying to push the envelope with all the EPA goodies. I dont think most states are putting sniffers in exhausts anymore
 

2004LB7

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You could combat the nox to some degree with water injection by cooling down the combustion some. I would also think reducing time should help too. But if you're doing this then what's the point of higher compression if you are defeating the benefits. Manufacturers probably don't do it because it is harder on lower end parts and head gaskets, etc. And harder to meet nox requirements. Soot is pretty easy to capture but not nox
 
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Woody35

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You could combat the nox to some degree with water injection by cooling down the combustion some. I would also think reducing time should help too. But if you're doing this then what's the point of higher compression if you are defeating the benefits. Manufacturers probably don't do it because it is harder on lower end parts and head gaskets, etc. And harder to meet nox requirements. Soot is pretty easy to capture but not nox
I 100% agree with you. I was talking about the potential benefits about soot reduction in a modified truck (turbo, injectors, etc...) that still has all the emissions intact. I surmise NOx output could still be controlled with DEF. Might be refilling the tank a lot more though than the average truck though.
 

MarkBroviak

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Great article on the subject and I've ran both ways with it. My last ProMod engine had 17.5:1 Oval race cast in it and I had zero issues with it but there is a lot to it making them happy. Street truck motors really should stay close to factory ratios to make them perform the best without a lot of back and forth trying to get them happy. The piston design itself plays a big part in the way it acts also.

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DieselDemon18

Member
Aug 25, 2011
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Moreno Valley, Ca
IMO there is no reason to go higher than 16.5 on any build. Especially on a Daily Driver. Regardless of year.
Jon,
On one of my many visits to Mike L's shop i was talking to him about my engine build and he told me to call you for pistons, i really want to run a set of your pistons as they've always fascinated bu them since they came out, however the coin they cost is a bit above my price range at the moment. And i didnt want to call for free advice and then take my business elsewhere thats just not my style. I do appreciate the chime in. If i do find room for growth in my budget i still may contact you for a set of pistons
 
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