High compression or low compression... can make up my mind?...Need pros help

STE92

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Mar 3, 2017
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Just recently decided to built my engine, power goal is 800 hp with possibility of maybe going for more down the road. I'm looking at all the current forged piston offerings and all the different compression ratios from 9:5.1/10:0.1/10:2.1/10:5.1. The car is currently on E85 w/stage 3 fuel pumps and Doc race IM w/750cc inj. Had a talk to a reputable engine builder and he recommend go with the higher compression piston cause of the fueling and my power goals. Wondering what are you thoughts on what compression ration go with and is there anything else I should be taking in consideration. Really appreciated your help.
 

Traf

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Aug 3, 2017
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Increasing CR on a DI engine running E85 is a better idea than reducing it. That being said, i don't know how the N54 would react to that or if it has been done already.
 

STE92

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Mar 3, 2017
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But why? Most engine I see being built for this platform are low comp.
 

Traf

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Aug 3, 2017
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When a lot of people are wrong doesn't mean they are right.
Usually its a good indication to look at the manufacturer, and you see a clear trend on DI engines, CR is going UP. I know they are doing it mostly to comply with emission regulations, but still.

Add to that E85, and you have absolutly no reason to lower CR. Altho you might want a closed deck if you increase CR and go for big numbers.

I'm no pro, but this is my opinion.
 

colo_evo

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tisdrew

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Jun 27, 2017
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A good portion of the people I know going big power in the evo and supra community go high CR 11-13 coming from 8 stock. Everyone is running e85 or c16 anyway so you don't need your engine to be 91-friendly.
 

STE92

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I'm not planning go that high more like 10:5.1, good to know, just haven't heard too many ppl in our platform running high compression engines like that.
 

Panzerfaust

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Jul 3, 2018
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I think the majority of built N54 engines in the last few years have gone with 10.5:1, actually. I know at least most of the vendor cars and big builds I've seen have. You're running e85, have a reasonable power goal and the engine builder even said to go up. You've got nothing to lose and itll help you hit 800whp with less work.

If you were going to try to break hp records with it or turn it into a tube frame chasis trying to run 7s and planning to run a gigantic turbo at 40psi with spray, then I'd say go lower. But you'll be hitting a nice whp number that people have run stock blocks at for shorter periods of time, so go up!
 

langsbr

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Apr 5, 2017
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Oems go higher cr for fuel economy and emissions, not power.

I vote for lower cr than stock, but not too much lower. Why would you make an engine that is already sensitive to knock and octane even more so? Seems counter intuitive. I see plant of pros for decreasing, but none for increasing.
 
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Raise it for ethanol. If you want to make bigger numbers on lower octane fuel (like pump gas), drop it. If it's a mix, keep stock or slightly lower. I'm out in AZ so if I wanted a built motor (not yet....) I'd probably drop it so the car is still worth driving on our lousy ACN91.
 

Traf

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Aug 3, 2017
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It has been clearly highlighted recently that the N54 is greatly limited by the oem head. Maybe do headwork or N53 head before thinking about increasing, or lowering, the CR.
 

STE92

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The head flows more than enough for me, car made close to 800 already all stock. Just want build bottom end ad some safety, so I can got beat on the track without throwing a rod through the block lol
 

MoreBoost

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Jul 27, 2017
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Lower compression (slightly) and raise boost. That's the safest recipe for big power.
N54s are very knock limited. Get the head ported as well.
 
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