944S pistons in a 8V 2.5L???

[TNT25169]

Has anyone ever tried using the 10.9CR 944S pistons in a standard 8v 2.5L I recently took apart an S engine and I have the pistons so the thought crossed my mind. Would the valves clear the pistons? Any thoughts would be appreciated. Thanks.

[HondaDustR]

It seems like they would as long as the valves clear them, being the same bore/stroke size and having the same tolerance groupings. Nothing a little custom machining couldn't fix if the valves don't miss by too much. I've been wondering the same thing, and it would make an awesome modification if it did work.

[callmethewander]

What's the benefit?

[TNT25169]

Increased compression ratio...torque. I have a feeling the valves would clear though.

[HondaDustR]

Yeah, then shave the head to hit 11.0:1 (or more)!

[ERAU944]

according to my understanding, i think what you want is the 88 non-s pistons. they are higher CR pistons - in 88 the regular 944 got the same pistons as the euro pistons (these were higher CR from the beginning). the 944S pistons are machined for smaller valves, and more of them.

if you use the 944S pistons, i think you may find interference issues. also, this will change the combustion chamber shape in a less than beneficial way.

good luck!

[TNT25169]

I already have me an '88 but I was just throwing the idea out there. I think you're right though about the smaller valves. The grooves in the tops of the pistons look like they were designed for a smaller valve and the large valves of the 8V heads just wouldn't clear. Thanks for the input.

[ERAU944]

oh, duh. it would help if i read a signature now and then wouldn't it?

yeah, i'd do some careful research before trying it, but remember CR takes into account combustion chamber design. the head has a different shape, so will affect it - just how i don't know, but you may find it to be more trouble than it's worth since the CR change may be negligible, and the swap can be very costly if you factor in machining and labor.

keep in mind also that the valves are larger on the 8v, and are small on the 16v

can of worms!

[ERAU944]

or... maybe i'm really not observant today... is your avatar a can of worms? looks open to me

[TNT25169]

C'mon, It's St. Patty's Day and my avatar is Chief Wiggum from the Simpsons dressed as a beer to pull over drunk drivers. Is there anything more fitting for the day? I think you're right about the chamber, valve location and clearance. In the end, I'm probably better off finding another '88 for my race car project. Thanks for your 2 cents. Anyone want to buy some 944S pistons?

[HondaDustR]

What does the crown on 944S pistons look like anyway? I couldn't find any pictures.

Heck, if I had disposable cash, I'd buy them and try it.

[TNT25169]

I will take a picture tomorrow.

[TNT25169]

Here's a picture, sorry it's a little blurry.

[ERAU944]

after looking at that, it looks like the 8v pistons have the valves closer to the center of the piston itself, and the S pistons have the reliefs very close to the edges (they'd need to to fit all the valves in the head).

all in all, a good thought, but the benefits would be questionable.

i wonder what the VE (volumetric efficiency) difference on the 944S vs the 944 is...

[HondaDustR]

It's hard to find pictures of 944 parts!

But I think you'd be better off using euro 10:6:1 CR pistons if you can find them, or are they what you already have? Compared to a 10.2:1 piston, the S piston might actually have a lower CR in an 8v head. The S piston has a very pronounced concave surface versus the 8v 10.2 CR's nearly flat crown.

10.2:1 pistons. The euro pistons are exactly the same only with a dime-sized circle.


9.7:1 piston


The 8v head seems to have much more space because of it's wedge shaped chamber.



The 16v chamber seems to be much more compact and needs much more room taken out of the top of the piston.




Seems like the best mod would be to use the euro pistons and cut down the head for more compression, possibly milling the valve reliefs slightly if needed. Of course, then you would be retarding the cam timing ever so slightly, possibly calling for an adjustable cam gear, which probably would be a good idea anyway, since I've heard the stock gear can vary +/- a couple degrees anyway just from machining error tolerances.

Or just get one of the 968 16v turbo conversion kits here. I came across this looking for pictures and it is unbelievable!
http://www.powerhaus.com/968t.htm

[tkddave951]

Do what what I did. Run the euro or 88 pistons and shave the head to get the compression ratio you want.
I would pre-assemble the engine and measure the piston to valve clearance before doing any work to the head.

[HondaDustR]

Originally Posted by tkddave951

Do what what I did. Run the euro or 88 pistons and shave the head to get the compression ratio you want.
I would pre-assemble the engine and measure the piston to valve clearance before doing any work to the head.

Just out of curiosity, what compression ratio were you able to get by shaving the head, and how much did you take off?

[racerstev]

Does anyone know the chamber volume of both of these heads and the volume
of the pistons? I'm looking into turboing my 87 S engine and need a lower CR to
pull it off..

Steve

[v2rocket_aka944]

bumping an old thread...
by my math, sticking an 8v head onto a 944S short block (after flycutting the pistons to suit the 8v arrangement of course) works out to just under 9:1 cr...

anyone actually done this?
i have a 944S shortblock in storage i need to figure what to do with.