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post #1 of 10 Old 04-15-2010, 01:11 AM Thread Starter
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Best Compression for Boost

This topic has been covered until no end and I thought I had everything figured out until I read an article about a guy running 12.5 to 1 compression on a turbo mustang. He built two identical engines one with 8.5 to 1 and one with 12.5 to 1 compression. The 12.5 to 1 engine made roughly 300 more hp than the lower compression engine. Anyone on here have any insight on this topic? I would assume that you would have to run extremely high octane fuel to make this combo work.

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post #2 of 10 Old 04-15-2010, 02:00 AM
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The same difference is between a 12.5 302 vs 8.5 302.

Depends on what you mean by extremely high octane. E85, c16, q16 should handle that just fine. Was there a cam difference? heads? stroke/bore? tranny?

The higher cr will allow a faster spool more power at the same psi, but you also gain efficiency.

I'm sure there is more to it, but this is a quick answer and other will have more details.



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post #3 of 10 Old 04-15-2010, 07:55 AM
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On readily available fuels, you'll make more power by lowering the compression to 8.5 - 9 and upping the boost than by running a higher CR.

From what I understand though, the exact opposite is true of mod motors...
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post #4 of 10 Old 04-15-2010, 10:02 AM
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I'm going 9.1 on my 5.0 4V stroker......

I came, I saw, I gave her a towel......
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post #5 of 10 Old 04-15-2010, 05:56 PM
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^ This guy is slower than dirt and has no idea what he's talking about. ;-)
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post #6 of 10 Old 04-16-2010, 12:40 AM
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I'm running 9.7:1 on my 4.6 2V...it also depends on cams. Most cams will bleed off alittle dynamic compression. So you have to take that into affect as well...but the mod motors do like compression and boost. Has to do with better combustion chamber design...especially the 4V's. But I run around 15-16 degrees of total timing with 20lbs of boost and spray alittle meth to keep the 93 octane happy. Works great and is a blast to drive around the street. More so than a low compression dog that wakes up when the boost finally comes on.

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post #7 of 10 Old 04-17-2010, 10:34 AM
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More compression is always better with more boost....It just depends if your motor/fuel can handle it. With the fuels out now (e85/e100) there's no way I would build a low compression motor.


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Last edited by goatnipples2002; 04-17-2010 at 10:39 AM.
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post #8 of 10 Old 04-17-2010, 01:27 PM
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mine is 8:3 to 1

compound boost gt500 725/780 at the wheels

93 coupe 520/542 on 150 shot, currently installing a t76 hellion kit in search of 7xx............
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post #9 of 10 Old 04-17-2010, 02:06 PM
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8.5 to 1...


any higher and cylinder pressure's get to high and you lift head gaskets out, and blow the spark out.

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post #10 of 10 Old 04-17-2010, 08:18 PM
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Quote:
Originally Posted by Gearhead559 View Post
8.5 to 1...


any higher and cylinder pressure's get to high and you lift head gaskets out, and blow the spark out.
I suppose you shouls run studs, heads with a better deck and upgrade the ignition. Plenty have gone 11-12 on big boost, but it wasn't stock parts.



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