BoostedMeOtter

So from the start ive planned for built motor with low CR to reduce detonation risk and help reach my goal of 300whp. I recently purchased a BP4W head that the previous owner did some cleaning of the head gasket surface. The machine shop had to take off .020 to clean it up

The .020 shave would put me at about 9.1:1 CR from what ive read. Am i fullish to just sale this head and start over with another one ? Or should i run it

borka

iTrader: (1)

I'm running 309hp on stock 94 pistons, which are I believe 8.8:1 and my block was decked with oem mazda MLS head gasket.

I think you'll be fine. depending on the actual turbo setup and how efficient it is.

Erat

iTrader: (5)

Should be fine. But depends on what he said ^. I'm closer to 10:1 and e85 is pretty much necessary with my hairdryer.

curly

iTrader: (25)

You can always order thicker head gasket...

technicalninja

.020 is a bunch on a head that has the camshafts riding directly in the head.
Was the head warped more than .005?
If it was then the only semi correct way to "fix" it is to cook the head in an oven to 400 degrees then reverse bend it in a press and let it cool in the press.
Machine shops will call this "heat treating" a head.
This can "correct" a badly warped head to the point the cam shaft bores are close to their original alignment.
It can also loosen valve seats and cause other issues.
It should only be done if a replacement head is not available or you don't care about longevity.
I would not use a badly warped head on a performance build.
Miata heads are not hard to get.
Often the "rebuilt" heads you can purchase (off e-bay) have been "heat treated" and assembled by morons.

If the head was badly warped and the machine shop just cut it flat you WILL have camshaft to head misalignment and problems down the road.
You can check this yourself. Bolt cams and cam caps into the bare head (no valves/springs) and see if cams turn with ease. If it's misaligned they will not.

The increase in compression is not a deal killer. Cam bore misalignment is

LeoNA

A lower CR or the correct CR for a given fuel will make more max HP because the larger unswept volume can fill during blow down. Small turbo's and higher compression in production cars is for better low end torque. Recently Porsche decreased the compression ratio on the 911T in an effort to increase max hp. 9:1 is a good ratio for gas and you should be able to hit 300 as long as everything else is sized correctly.

If the head warped more then .008" the cam will not spin freely out of the box not down the road. I usually do not heat them more then 300-350F when straightening. After the block is decked it can be bolted down with shims to straighten. It does not have to be perfect just better then .008". Usually these heads bend in the center around the 2-3 cylinders in a sad face shape. More bent on the exhaust side.

BoostedMeOtter

The previous owner used something too abrasive when removing the old gasket material. And left the surface rough it wasnt warped.

im in CA in an area were i dont have easy access to e85. So id be using our crap 91 gas and a garrett 2871r with a .86ar turbine housing

if someones got a good condition never been shaved BP4W head let me know.

cpierr03

As @curly mentioned thicker cometic head gasket is the easy solution if you're concerned. Honestly though with ~9:1 targeting 300 whp you're probably fine if your timing table is conservative to compensate for CA dino juice; that turbo should spin 300 within its efficiency curve

tehzack

I run STD bore CP forged 9:1 Pistons with a .025" Shaved BP4W head on North Carolina pump 93. I've run 20 psi @ 300+whp (car runs 11.7-11.8's @ 119mph) with no problem and I drive my car like a ********.


Be nice to your timing table and you'll be fine homie.

Newaza

Now that's the way to beat the snot out of a turbo miata LOL! Thanks for posting video..

borka

iTrader: (1)

Cool video. props

i'm too much of a ***** to drive my car in such fashion.