skiddus_markus

How do the CR and boost pressure relate when tuning an engine.Obviously you can potentially run more boost on a lower compression engine but at what point does the extra boost outweigh the benefits of a higher CR engine?

How much does the extra compression affect engine life and what woould most people prefer?

bruce

iTrader: (5)

I'd be interested to know as well, as I had to have my heads skimmed after the head gaskets went. My car has the pe phase 1 conversion which was running well before at 1.2 bar, but now I get det and limp home mode when wot in 4th and 5th at 4500 rpm. A dose of NF stops the limp home mode cutting in but doesnt stop the red flash on the knock link.

ALi-B

iTrader: (1)

A lower CR, allows higher boost and more ignition advance as there is less squish so there is less chance of auto-ignition upon the compression stroke (det), also you can use lower octane fuel, or use a smaller intercooler. Ultimately you can get more top end power and more overall power when compared to a high CR (until the spark gets blown out ). Obviously to get more power with a low CR you need a bigger turbo to get more pressure/volume, big intercooler and fuel mapping to suit etc. Which is where the sacrifice comes in.....
Off-boost power, as off-boost you are more or less running a very inefficent n/a engine. So round town and low rpms it will be gutless. Along with the bigger turbo mods you've done you'll have more lag to contend with.

Now a higher CR is much more tractable off-boost, the engine running without the turbo is more efficient and more powerful. But that tractability is at the cost of limited maximum boost, the need for more retarded ignition, high octane fuel, bigger intercoolers and fuel mixture enrichning to help reduce the det.

If your into big power numbers - go for the low CR. If you want driveability - go for the high CR

It is a balancing act to get it right, as ideally you want to run as much boost as possible with the highest CR as possible, along with as much ignition advance, and as high octane fuel as you can find.

Engine life is down to how well the modification have been done to limit any det. If you up the CR, expect to lower the boost, retard the ignition, enrich the fuel, up the octane and uprate the intercooler to compensate...lots of factors to play with!

skiddus_markus

I've already gone for the higher compression route on my GTiR.It's been raised from 8.3:1 to 8.7:1 by using a 1mm steel HG.It might not be a lot but I know a few people with 2mm HG's(this reduces it to around 7.5:1) and I didn't want to go down the low CR/massive boost/big turbo route.I don't like lag and will be happy with 400ish bhp/lbft at 1.4-1.6 bar on a relatively small turbo.I'm not worried about det as I have an aftermarket ECU,large fmic and water injection.

I understand what you've said Ali but is there any kind of formula where you can work out at which point(ie boost/power)the low CR engine surpasses the high CR one?Or is it going to differ from engine to engine?

ALi-B

iTrader: (1)

It will differ from engine to engine, there are just so many factors to take into account. I don't think there is the perfect number, certainly it ties in with other mods to dictate if needs to be higher or not.

The general ideal is around 8.5 on most cars. Scoobs run 8.0:1 some tuners run lower but that is as low as I'd like to take it, and maybe 9.5 as an absolute max limit, but at that level you'd struggle to run any higher than 1.0bar boost safely.

If there is a magic formula for working it out, I haven't seen it yet. Certainly there is a point where a higher CR will be detrimental to overall performance as you keep knocking back the ignition and richen to control the det.

Fangoria

iTrader: (1)

I run higher compression ratio - 8.5:1

But I also run very high boost and advance - doesnt seem to impact on peak power on mine, or Bob Rawles car

Then again it would probably be a different matter on much higher i.e. over 9:1 compression engine