low boost/high compression
 

Hi Folks,

Sorry if this has been done before (feel free to provide the link ;))

Here in Australia, we have a class for basically stock sports cars that run on semi-slicks. Cams, pistons etc are free - stock inlet manifold, original head, block and crank - they can be modified but must remain basically what was in the car when it was sold by the manufacturer.

I currently have 11:1 pistons, larger valves, 276 Deg cams and a light flywheel with Exedy clutch. Oh and a Maruha head gasket.

Some of the other guys in the class run supercharged Lotus Elise/Exiges with high compression and they rock!

What is the general consensus of the forum about using an SE manifold/turbo on 6 - 7 lbs boost with the high comp pistons? Don't want to undo the work I have already done..

Slightly better response, less room for error, less overall torque. The topic has been discussed several times.

Add water injection or e85 and you have a winner.

Do you have E85 over there?

Yes, we do have E85 here! Forgot to mention that...

E85 you should have no problems.

This topic was done not very long ago and searching is free and a good way not to get bashed for a repost.

I will say I think it's not worth it on a 4cylinder engine now on a 6-12 cylinder engine yes. Also works better on an engine with a well designed head with go flow.

Since he already has an assembled, running engine which is set for 11:1 CR, the options as I see them are:

1. Do nothing.
   
2. Add turbo at low boost.
   
3. Add turbo at high boost, also completely rebuild engine for low compression.

I agree that option #3 would be "best" from a power standpoint, however the OP doesn't seem to be interested in building a new engine at this point. You seem to be suggesting that option 1 (do nothing) is the only worthwhile course of action. I'd suggest that option 2 has merit as a compromise between cost / labor and power gained, particularly if he has the ability to run E85 and utilize an aftermarket ECU (which I assume he already has) and larger-than-stock injectors.


This isn't a path often traveled by members on this forum, as most of us (myself included) are either too lazy to rebuild the engine with aftermarket pistons, or if not, have specifically built to lower CR, not to raise it. Still, it sounds like a potentially very interesting project.


colster3, pardon my ignorance (and perhaps it's a cultural-lingustic barrier at play) but what is "an SE manifold/turbo"?

More cylinders and well flowing heads are always a plus, but we're discussing a miata setup here, so who cares about other stuff.

I think it will be a GREAT setup for OP on e85. You got a ton of response and potential to make good power with little boost, and the e85 will definitely put a huge cushion on heat/det. I'd be worried about rods though. With high compression and even low boost stock rods will be on thin ice.

Joe you have a good point and so does 18psi. If you are running stock rods then I wouldn't temp a high compression motor even on low boost. Just to do rods you would be doing a rebuild so at that point you might as well try and drop to some thing a bit lower like 10:1 or 9.5:1. I did 9.5:1 and got great numbers out of it on a 16g with 20psi. Made 350whp and 305wtq. Had full boost by 3700-4000.

Thanks for the opinions guys,

Joe - the factory turbo Miata here in Australia was badged as an SE to differentiate from a locally produced SP - aftermarket kit fitted by the unofficial competition arm here.

The manifold is a Mazda part and the turbo (not familiar with size/ID numbers etc) is fairly small.

My concept was exactly as 18PSI had suggested. Great response, a bit more power and a couple of spots further up the grid. By the way, forgot to mention that the engine has Carillo rods, and ARP stud kit for mains and cylinder head...

So you have a mazdaspeed exhaust mani+turbo?

Since your engine has beefy rods this is a no brainer: go for it.

so people.... how about high compression/high boost?


think: 10:1 compression with 20-25psi

edit: with 100% meth injection.

someone here built a motor with high comp pistons (thinking they were 9:1 I believe?) and ran boost.

things didn't go well.

9:1 isn't high compression...did you mean something else?

Better throttle response - YAY!

Rotrex.

Hey mate where are you from?

Im from newcastle (nsw) running an e85 powered 10.5:1 NA, going boosted soon.

Dann

Hi Dann,
I'm in the Sutho Shire. I have built a 2F racer out of a 1999 NB.

Wheres that? lol

I figured 2f, whats your target weight?

Dann