Compression ratios and their effect on turbo'd engines
Turbo Mini Forums

I'm hoping someone on here will be able to help....

Now I know why everyone says you must lower compression on a turbo'd engine (carb'd anyway) - so pinking and knocking are eliminated or reduced. Am I correct in thinking that the contributing factors to knock are intake air heat and timing? and pinking the same?

So in effect if you keep the intake air cool enough, make sure the timing is spot on and don't go mad with the boost then you should be able to keep the standard compression ration???

yes that's the right idea.

the standard metro runs about 7psi on a 9.3:1 CR aith no inter cooler, if you stick a massive cooler on there you can probably run 10 psi safely,

or with no cooler you can probably run 10 psi and an 8:1CR.

the obvoius thing is to do both and run 15 psi on an 8.5;1 cr with an inter cooler.

thats just guesses on the figures though.

Detonation is not caused directly by boost but the associated increase in the intake air temperature due to the compression within the turbo/supercharger.

There is a certain temperature reached within the compression stroke at which the fuel will self ignite, either by pre-ignition or detonation.

The formula for adiabatic compression is as follows:



So if you can eliminate any increase in the cylinder intake air temperature, then there is no need to reduce the compression ratio.

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Might be worth adding that certain metals are also better at heat dissipation (think that's the right word) and allow you to run more boost at a certain CR.

or you could increase the RON of the fuel.

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Or, technically use water injection although I don't know if anyone here has used it with good results.

it tends to bring up alot of anti water injection comments but those using it are getting good results there is a couple of extreemely good threads on it

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I agree, i personally think that you should use a better intercooler or drop the CR to do it properly.

Saying that, i would consider using it as a safety feature, triggered only when IAT/ACT/whatever you want to call it go over a certain threshold in case of problems.

I'd rather have the cost of a few hundred £££ and a few extra kg's than a new engine.

http://www.rbracing-rsr.com/compression.htm
Use this calculator to work out what boost you can run before things start to go all wrong I work on the ratio of around 14:1 when useing max boost.
Useing that figure you can see why most run a mid 8 static compression ratio.

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i agree sturgie aswell
the saftey margine it can net you alone is worth the disadvantages and the extra boost head room cant be ignored though so for drag / sprint its perfect
but dont think i would run just wi. i would run with a properly setup engine and use the wi ontop

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Great! So either I take some meat out of the chambers and run 15psi - what power would inend up with? Or I run 8 psi and a massive intercooler and hope!

Thanks for the replies, all very interesting. Its all making sense now, I have a spare engine so I might experiment a little!

So 24 a 28 cc head chamber is 10 down to 9:1 CR?

I found this and thought you all might find it interesting

http://www.stealth316.com/2-turbotemp.htm

Or you could use this one http://www.not2fast.com/turbo/glossary/turbo_calc.shtml and do everything at once on one spread sheet