[Mk2] [Turbo] pistons and compression ratios

[TUBBY-DEUCE]

i may sound thick but can someone please explain why you would choose a higher compression piston over a lower compression.
ive read that higher compression engines suffer from detonation (knocking) more.

cheers

[diycinema]

i may sound thick but can someone please explain why you would choose a higher compression piston over a lower compression.
ive read that higher compression engines suffer from detonation (knocking) more.

cheers

Engine efficiency is directly related to compression ratio. Higher compression ratios allow more useful work to be extracted from the fuel. When you compress the fuel/air mixture it heats up. The more you compress it the hotter it will get. If it gets too hot it will pre-ignite. Diesel engines take advantage of this which is part of the reason why they're more efficient than petrol.

In a turbo charged engine you're compressing air before it's mixed with the fuel, then compressing it again in the engine. Because it's already been pre-compressed it will be hot so you're more likely to get pre-ignition. Turbo charged engines therefore run lower compression ratios than naturally aspirated making them generally less efficient.

For high boost applications you might want to run a little less than stock. Toyota slightly decreased the compression ratio in the rev3 3SGTE to help with the higher boost.

[TUBBY-DEUCE]

excellent info, thank you

[mattcambs]

In theory a higher compression ratio will improve low-end torque and off-boost driveability. The engine will rev higher more happily too (VTECs are very high C/R for example)

How this applies to tuning the 3S-GTE I don't know. It would be nice to think that you could select pistons to give a higher than stock compression ratio and reap the benefits.

If your charge cooling is up to scratch it might be possible to safely increase C/R. You might be sacrificing a little bit of max boost for a more tractable engine - I don't know?

Can any 3S-GTE engine builders/tuners comment on this?

[TUBBY-DEUCE]

i think standard is 8:5:1 compression.
ive seen packages that offer higher, would a slightly higher compression be better for a fast road car or track car running lower boost?
the higher compression wouldnt be suited to a drag type car i assume which would be running higher boost levels, which is not what im after anyway.
also im interested in how different it would drive, would ther be a noticeable difference?

[3S Service Centre]

Its vital to match your compression ratio to the (end state of tune).

(IMO)

Mildly tuned road going vehicles are ideal between 8.5 - 9.5 this allows plenty of compensation for the massive varient in fuel quality from the same station/pump/ and changes in conditions.

Highly modified cars/drag cars can take advantage higher compression ratios that will 1) give quicker spool 2) create a hotter exhaust gas at lower revs and make more drivable at lower rpms with crazy cams.

The above is based on the massive research I done for my car / info attained from 2 of our sub contracters who specialise drag / and our machine shop manager who runs the fastest road going rear wheel drive subaru in europe.

hope that helps

James

[TUBBY-DEUCE]

very helpfull thanks chaps, some good info there

[diycinema]

When i rebuilt my engine i used 8.8:1 pistons (same as rev1-2) in a rev 3 engine which usually has 8.5:1 Theoretically that gives me an extra 3.5% increase in efficiency, giving a very rough gain of ~8BHP for nothing; the downside being it would be more difficult to run higher than stock boost without upgrading the intercooler.

Compared to previously it spools much faster and feels like it has more torque. It's hard to compare directly as i put a new compressor wheel on the turbo during the rebuild so much of the effect is likely from that.