COMPRESSION

[YounGDeezY]

some 1 told me if i changed my compression of my 98 3.8L that i would gain hp but they also told me that it would require me to use 89 or higher octane. i only want to use 89 octane and as far as compression goes what is it that i'll be compressing. please give me some # for compression.

[99BlueSuperSix]

I think we only run 9.5:1 compression, which is why our engines are so reliable and low maintenance. If you run higher compression, which you can do with different pistons, longer rods or having your heads milled, you need to run higher octane gas because 88 doesn't compress as well, it will detonate/ping a lot because of the heat and compression.

[pgriffith]

Our stock compression ratio is 9.4. I have milled my heads and raised it to about 9.8. You will gain a few HP but will have to use Premium octane gas, like 91 or 93 highest you can get. I run 104 at the track.

[Roy]

i run 11.3:1 (ls1) and run 93 octane. i get no knock with 29 degrees of timing. raising the compression doesnt increase the power a whole lot. ive read its only about 3% for every full point of compression. check this out: http://www.ls1tech.com/ubb/cgi-bin/u...=004018#000000

[Arctc Wolf]

being able to run w/ aluminum heads allows a full point compression increase while being able to use the same octane fuel you were using before since aluminum dissipates heat much better than iron. We have iron heads at 9.4:1. To achieve the same thermal efficiencyw/ aluminum heads, you'd have to bump compression to 11.4:1. This is the beauty of aluminum.

[Roy]

i didnt realize 3.8s didnt have alum heads [img]redface.gif[/img] .

[Arctc Wolf]

no, we don't. we have iron. the ls1's have aluminum.

[Dominic]

<blockquote>quote:</font><hr>Originally posted by Arctc Wolf:
being able to run w/ aluminum heads allows a full point compression increase while being able to use the same octane fuel you were using before since aluminum dissipates heat much better than iron. We have iron heads at 9.4:1. To achieve the same thermal efficiencyw/ aluminum heads, you'd have to bump compression to 11.4:1. This is the beauty of aluminum.<hr></blockquote>

Not quite arctic. Under 6,000RPM and below 10:1 Iron heads are superior; above either of these conditions aluminum is. An aluminum head/block engine does require more compression to achieve the same VE, but its only like 0.5:1 more, not anything substantial like what you mentioned.

It is true we run 9.4:1 Compression. 10.5:1 is probably our maximum safe compression ratio on pump 91 (93 might allow a bit more...). If you change the compression much at all you will be required to run premium otherwise the knock sensors will be doing a lot of work retarding your timing.

Increasing your compression ratio from 9.4:1 to 10.5:1 on a bone stock 200hp producing 3800 will yield approx +25hp at the flywheel. Or +12.5% power. Its really beneficial if you're looking for all the power you can get n/a [img]smile.gif[/img] .

Personally I would rather lower the compression or leave it the same and go blown - its a much more efficient way of getting the power out of premium unleaded than simply raising the static compression.

[Again, I reiterate I am deriving these numbers with pscalc]

[ August 03, 2002: Message edited by: Dominic ]</p>

[Roy]

dont the holden 3.8s have alum. heads?

[Dominic]

They might, I am unsure. I have seen holden aluminum blocks and heads - but they were aftermarket. [img]graemlins/thumbsup.gif[/img]

[JAM]

If they are not alum, does anyone know where we can get alum blocks, and heads? Might be nice mod to shave off some weight too.

[Dominic]

<blockquote>quote:</font><hr>Originally posted by JAM:
If they are not alum, does anyone know where we can get alum blocks, and heads? Might be nice mod to shave off some weight too.<hr></blockquote>

Do some research on aluminum blocks/heads and you'll see why you're better off with the iron.

Your best bet however is a holden aluminum shortblock with alum heads - its not worth your time though.

[lando_pr]

Has anyone with a 3.4 milled their cylinder heads? Our 3.4 run with a 9.0:1 CR I thinks that's just too low. I would like to know what's our piston to valve clearance so I won't hurt my engine by milling too much.

[pgriffith]

Will the holden block work with our heads?

[Shadly]

<blockquote>quote:</font><hr>Originally posted by lando_pr:
Has anyone with a 3.4 milled their cylinder heads? Our 3.4 run with a 9.0:1 CR I thinks that's just too low. I would like to know what's our piston to valve clearance so I won't hurt my engine by milling too much.<hr></blockquote>
Don't do it. If you run the numbers, it's just not worth the cost and aggrivation of milling for very little extra horsepower. Plus you will need higher octane fuel all the time to prevent detonation. Try bolt-on parts first. They offer a better return on investment.