Now here is a question for all you turbo junkys......

From my understand the compression on a stock turbo with a P90 head and dished pistons is 7.8:1 correct ( sorry if im a little off thats from memory )

When you mix flat top pistons with a p79 head produces an estimated compression ratio of 8.8:1.....

now here is the question...

What if you were to take flat top pistons and a P79 and installed them on a turbo for the compression ratio of 8.8 : 1....this would increase your torque over what you used to have because of the raised CR ( right? ) but still be a low enough compression that with an intercooler could produced the high end power of a turbo motor? now i dont know if this has been done and its just a theory on my part. Let me know what you think....

So basically im saying could you use a P79 cycl head and flat-top pistons for a turbo and produce MORE power ( torque and maybe HP ) than a stock turbo?

Matt

P.S sorry if i post in 2 spots, this forum doesn't get visited as often...

yes that CR is about right. most modern turbo charged apps run a CR somewhere in the 8s. but they do that with modern fuel injection and intercooling.

if you where to try and raise the CR of a stock 280ZX turbo into that range without intercooling or better fuel managment you will run into some problems. add an intercooler to the mix and you will be better off..but still limited to around 10psi (this is a very bad guess on my part. considering that all turbo motors will react differently depending on timing/load/temp/ICeff./fuel rating etc. take what i say with a grain of salt.)

add progrmable FI, forged pistons, and a good FMIC. and a 8.8 ratio should be no problem. and then you have a major rebuild....and in that case..you might as well build a 3.1:ermm: :classic:

It may seem backwards, but, within limits, a lower compression ratio has a greater power potential on boost than a higher compression ratio engine. With the higher compression engine you will have a better throttle response, but a higher tendency towards detonation at full boost. You then must intercool to a greater extent, or back off on your ignition timing. All other things being equal, you make more power with the stock 7.4:1 engine over the 8.8:1 flat-top engine.

Sleeper78Z your correct on the high compression motor. Anything that is 9: and higher would not be an optimal turbo engine.

The optimal compression on a turbo motor would be 8:5:1. Lower compression allows you to run higher boost but doesn’t mean you’ll have more power then the higher compression motor. On a lower compression motor…more boost also means more heat. Big turbo would have less heat but more lag. On a 8:5:1 turbo setup – less boost means less heat and you would have less lag on a higher compression motor using a big turbo. There would be less stress on the motor on a higher compression motor (8:5:1) to achieve 300hp then a lower compression motor (7:4:1)… This is considering both are tuned correctly with identical setup. Things start breaking when your compression is to high.

You will see a noticeable difference with your 8:8:1 compression turbo setup but also your chances of detonation is high if you turn up the boost. You can get away with 11 psi if you have a intercooler, run colder plugs, and a healthy motor. 8:8:1 is a little high if you planning to build a high performance engine. You should hit the 13’s easily.

You can turn up the boost more if you can retard the timing under boost. The MSD 6BTM allows you to do this. 12’s is possible with this device. http://store.summitracing.com/partdetail.asp?part=730 I’m not mentioning this just because I have one for sale. :stupid:

Retarding the timing under boost is one of the small features with a programmable FI engine management.

Ron's reply says it all - if you want your L Series motor to last with high HP outputs, lower the compression below 8:1. 8.8:1 will give you a nice responsive motor with crisp performance at low boost, but steer well clear of high boost with high compressions ratios.....its technically possible to do it - but I wouldn't recommend it at all unless you have a very well tuned, very well configured EFI setup with a computer ready to do all the funky stuff (retarding ignition et al) when sensing detonation.....and at 8.8:1 and over 10psi - you *will* have detonation!

I blew my 240z turbo a couple of times before figuring out some of these nasty things....expensive way to learn.

Hi all,

I agree with Ron, try and get the CR to 8.5:1 and have a good real time programmable EFI computer plus a good intercooler.

Make sure that the tuner knows what he/she is doing and the tuning is done on a dyno, not off the seat style tuning!

However I wouldn't personally drop CR into the 7s, I would stay between 8 and 8.5:1

Good luck with your turbo zed project (you have seen the light)