Compression ratio

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Blackmax123

Member
Aug 2, 2014
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central Pennsylvania
malibu795 said:
DDP ran 19:1 in a 2.5 class...
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When I read this stuff it makes me wish I'd have went around 17:1. For my build. But at the time I was trying to leave room to grow.

With my built motor same fuel setup and same Danville stock appearing 2.5 vgt. I lost 30 rwhp on marks dyno but gained 20 ft lbs of torque. Needless to say switching to the s400 based 2.5 charger I'm currently running I didn't gain very much. Some of me wants to blame it on my compression ratio.
 
Frankin_truck

Frankin_truck

Member
Nov 22, 2013
147
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Blackmax123 said:
When I read this stuff it makes me wish I'd have went around 17:1. For my build. But at the time I was trying to leave room to grow.

With my built motor same fuel setup and same Danville stock appearing 2.5 vgt. I lost 30 rwhp on marks dyno but gained 20 ft lbs of torque. Needless to say switching to the s400 based 2.5 charger I'm currently running I didn't gain very much. Some of me wants to blame it on my compression ratio.
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Did your use the same cam?
 
malibu795

malibu795

misspeelleerr
Apr 28, 2007
7,885
304
83
42
in the buckeye state
I've ran stock 16.8 17 and currently 17.5 which is OEM for lb7/LLY.

There's days I wish I went 18-19:1 but it's really up in the air if stock rods would hold at 4-500hp.
 
Frankin_truck

Frankin_truck

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Nov 22, 2013
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Why do all the cummins guys want to run such low compression? I have heard of guys running 12:1 in cummins land
 
DangerousDuramax

DangerousDuramax

New member
Nov 3, 2006
124
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Houston, Tx
Frankin_truck said:
Why do all the cummins guys want to run such low compression? I have heard of guys running 12:1 in cummins land
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Because they have bigger slugs. More compression in a Cummins is way more force on the pistons than a smaller slug Duramax. A high compression Cummins will make a helluva lot more bang when it blows up. ;)
 
Frankin_truck

Frankin_truck

Member
Nov 22, 2013
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I could be looking at this the wrong way but cylinder pressure= power? To much can be hard on parts. I think the dynamic compression ratio would be lower at 12:1 if they were running same size turbo bringing down cylinder pressure. Must be some advantages to running low static compression ratio there are a few strong running cummins.
 
D

Digmax

Member
Jan 23, 2016
289
2
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Lower compression= room for more air and a larger combustion event= more power. Higher compression is more pressure in cylinder equals better off boost response???? Or what?
 
WVRigrat05

WVRigrat05

Wound for sound
Jan 1, 2011
3,081
4
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French Creek, West Virginia
My LLY ran like a whole new animal going from 17.5 to 16 and change, I did change injectors at the time of the build but I don't think injectors alone would have made that big a difference.
 
IOWA LLY

IOWA LLY

Yes, its really me
Feb 23, 2007
2,275
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WVRigrat05 said:
My LLY ran like a whole new animal going from 17.5 to 16 and change, I did change injectors at the time of the build but I don't think injectors alone would have made that big a difference.
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I'll bet they did.


Sent from my iPhone using Tapatalk Pro
 
Frankin_truck

Frankin_truck

Member
Nov 22, 2013
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I bet the injectors made the difference especially if you ran the same tune. Can't see how you would gain hp by dropping compression in that set up
 
catman3126

catman3126

Ehhh?.... You don't say?
Jul 24, 2012
2,636
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NE Oregon
Harbin_22 said:
I run 16.5:1 with around 85-90#'s boost from the triples. Blew headgaskets with standard H11 studs. Went to 625's and Oring heads, no problems for the last 10k miles
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Great just what I wanted to read. same comp, same boost levels, didn't do the 625's fml.
 
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