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Thread: From 8.5 to 9.0 compressionw while rebuilding?

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Posts: 11-20 of 20
2011-10-26 17:55:52
The difference will mainly be noticed out of boost and cruising, You will have a better response down low. Its definitely noticable. You will have a slightly faster spool time as well maybe 100-200rpm faster at most.

One thing about higher compression though is it loves bigger turbos w/ big turbine housings that flow really well. Helps spool time and makes more power everywhere without creating any problems. Which is why i chose to do 9:1 on my motor.

Id say go for it. No negatives really to it.
2011-10-26 18:08:53
2011-10-26 20:42:42
Thanx guys, you convinced me. Going to order a nice set 9:1.
Got a jwt tuned ecu right now, but going Nistune after the build.

Will post my experience here for future reference but I think 155 across the block instead of 118 psi will allready make a nicer ride

2011-10-26 21:54:12
How are the cylinder walls on the block? You going 86,86.5or 87?
2011-10-26 23:36:10
Didn't Dave Coleman state that lower compression spools turbo faster? Could have sworn he did. My 9.3 compression motor spooled slower on the dyno though. By 2-300rpm compared to the 8.5:1.

Has anyone actually tested it? I have enough power in 1st gear out of boost to chirp the tires when snapping the throttle open.

A higher compression ratio also means a higher expansion ratio. That is, on the power stroke, there is a greater difference in volume between top dead center and the point, somewhere near the bottom, when the exhaust valve opens and power production stops. This higher expansion ratio extracts more energy from your burning fuel (which is why high compression is actually more efficient), and actually leaves less energy in the exhaust to spool your turbo.

The only reason, then, that higher compression usually reduces lag, is that the engine makes more power off-boost, and can therefore accelerate to a boost-producing speed more quickly.
Ultimately, if you want good boost response, good power, and an engine that won't blow up, you're better off with a modern, efficient turbo sized for reasonable power goals, and low compression that's easy to tune.

For most, the rule of thumb is 3-4% more power for every full point of compression being run. But how much faster is a car with 5whp more in the rpm range of 1-3000rpm? I think going to 9:1 is the right step. But it depends on the amount of boost you want to run. Higher compression, less boost, more revs, and bigger turbo. I think that is the best way to get an efficient setup. If you want big boost, big torque, small turbo, using a lower compression will be more forgiving. Either way, at 8.5 to 9.5 you are really splitting hairs. The difference is hardly noticeable, at least in my experience.

I think the dynamic compression is overlooked many times over the static compression.
2011-10-27 07:30:12
Originally Posted by MR-4Door-SR20DET
How are the cylinder walls on the block? You going 86,86.5or 87?

3 months ago (new headgasket) they were perfect, so 86.5 its gonna be to keep a bit of beef left on the walls. But the engine is still in the car now so have to check when taken apart to make sure on bore size.

@Coheed: Makes sense that "less lag" is not a faster spoolup but indeed more off-boost power.

Friend of mine has an sc61 (.63 a/r) @ 1 bar/15 psi on a 10:1 DE and has fullboost just over 4100 rpm. I was realy impressed by it compared to my 2871 .86 spoolup.
2011-10-27 13:02:02
A journal style S-Cover sc6262 with a .63a/r is what I'll be running. From all the searching I have done, it should hit full boost somewhere around 4500rpms.
2011-10-27 17:13:38
Got brand new JE pistons (87mm, 8,5:1) for sale now btw haha, anyone interrested?
2011-10-27 18:48:59
2011-10-27 19:16:16
New is US$525,- so can let them go for US$425,- ex. shipping.

PM for more info etc. otherwise this turns in a for sale section haha.
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