Compression and boost

[Syed Shah]

In the 'piston' thread, Nathan mentioned TODA pistons having a very low compression ratio of 7:1. Hence being ideal for a mega-boost project only.

This got me thinking, what is better: Compression or boost.

On a turboed engine,

More compression gives better spool and more power. The downside is that at a given level of boost, better fuel must be run to avoid detonation. Yes?

More boost means more power, but high boost cannot be run without low compression, otherwise det will occur easily. Yes?

My question is, would it be better to run say (random figs for example) 35psi and 8.5:1 compression or 30psi and 9.5:1 compression?

Thanks

[Nathan]

You generally run as much compression as you can for the horsepower you require. Your limits are det and EGT. If they go up too far, you need to lower the compression. It can turn into a law of dimishing returns though. Running more and more boost enables more power but reduces the low down power as you have no off-boost cylinder compression left.

Your examaple figures would be far different from that IMO. A petrol engine on 9.5:1 would be quite high for a turbo, and you'd be lucky to run much more than about .5 bar. Go to 8.5:1 (Mk4) and shazaam you're already capable of 1.4 bar. It doesn't take a lot of compression drop. I would think that a Mk4 on 7:1 would be good for 2.5+bar no probs. Similar to what the big cossy boys run as it goes. They run about 7.2:1 IIRC. Obviously all figures are thrown up there as an example and theres a lot more to this than I have posted.

[Syed Shah]

Thanks for the reply Nathan.

So run as much compression as you can for the amount of boost you need to run to achieve you desired power goals.

So, that brings me to my next question:

What does this mean when applied to our engines?

What boost can be run on stock 8.5:1 compression, on C16/similiar hi-spec fuel? With reasonable timing setup etc.

Thanks

[Nathan]

I really couldn't hazard a guess when using fancy fuels- we've never been there. We do useable, every day road cars in general and our customers are big on longevity so we don't usually push the envelope, although we ALWAYS reccomend 97+ fuel. Personally I would say that the limiting factor with the Mk4 engine is the integrity of the pistons rather than the actual C/R. For example, we don't like running more than about 1.4/1.5 bar on the stock pistons with say a big single/twin. Put some decent pistons in, even at 8.5:1 and I reckon you could see 1.7 bar with some fancy fuel, such is the good design of the engine. Some engines would be detting their nuts off under those conditions and getting REAL high on EGT's, the Mk4 seems to cope with relatively high boost/compression ratios very well. So do the Lancer EVO engines as it happens. 1.4 bar no probs on them either. Christ, they are even coming out of the crate with that now.

Thinking about your question, that would actually make a very good engine....keep the comp up, but use decent pistons so they don't fall apart, run say 100+ RON fuel with a small single @ 1.6~1.7 bar with a decent bit of mapping. Could make a strong road car that...would need shares in '76' though.

Would really like to know whether it could be pulled off though. We had a JE- pistoned big twin on Abbeys dyno at 1.6 bar which was still pinking even after a fair bit of retard on the ignition, so it's fair to say you are getting near the edge at 1.7 bar on stock comp even with fancy fuel. No doubt the yanks do it though....;-)

[Syed Shah]

Thanks again for the detailed reply.

That is pretty much what I was thinking, stock compression (mainly because the best pistons are in this compression, and I would prefer off-the-shelf Cosworth to custom JEs and none of the aftermarket head gaskets are proven to be as good as the stock one), with some good fuel and lots of boost.

I think, judging by the US guys, that it is now possible, given that we have proper means with which to control everything now with proper standalones. I know your a HKS man, but would you consider an AEM or Motec ECU?

How about high boost + lots of timing retard vs lower boost and more advanced timing? Big subject? Should it be evaluated in another thread?

[Chris Wilson]

High static compression is used to give good or reasonable off boost response. If you were building a very powerful and responsive N/A engine you would run as much compression as possible, say 12 to 1, maybe more sometimes. A turbo engine, off boost, is just an N/A engine, so with 12 to 1 squashy it would be very good, but adding boost is adding DYNAMIC compression, so very soon that 12 to 1 is 14 to 1 and the engine will be pinking like crazy. Build it as a 7.5 to 1 compression and off boost performance will fall substantially, but you can add a fair amount of boost, which adds dynamic compression, before you hit det. You might ask why does a turbo engine with a CR of say 7 to 1 under 1 bar of boost, give much more power than an N/A at 12 to 1 CR? The answer of course is not only does boost add compression, it also crams in a much greater density of air, to which the ecu adds fuel, than the N/A one, which is relying on atmospheric pressure and maybe, in a really good design, some tuned pipe ram effect. The turbo will always win through though, given a reasonable amount of boost.

If you want reasonable off boost performance, with stock or near to stock cams and maybe a small single turbo, stick to about 7.5 to 1 minimum CR. If you are sacrificing off boost performance for ultimate top end, maybe 7 to 1 or 6.75 CR. Off boost it will be VERY soggy though. Some low pressure turbo engines run close to N/A compression ratios and cleverly map boost to just add torque and avoid det. They work well, but the brutal top end is always absent, too much compression to run much boost at all...

HTH

[Syed Shah]

Thanks for the reply Chris

In repect to our engines, what would 7:1 compression allow us to run before det occurs, on some good fuel?

Also, what to you think about high boost/timing retard v low boost/timing advance?

Thanks

[Chris Wilson]

To save me a lot of typing let me refer you HERE

[Syed Shah]

Thanks for the link Chris. Lots of reading for me to do. I'll post up any questions I still have afterwards.

[Chris Wilson]

Originally Posted by [b

Quote[/b] (Chris Wilson @ Jan. 17 2004,19:42)]To save me a lot of typing let me refer you HERE

In fact, although the EFI system they sell is basic, the site is a mine of really useful, well presented info. It should be made compulsary reading for people on the "other" Supra UK forum, but i fear some of the more recent members may have literacy problems

Site root is HERE