mark944turbo

Yes they look scary, but are they really that big of a deal? I have pulled apart several motors that had good compression only to find the walls pretty badly scored, like easily felt with a fingernail.

Also, what is your opinion about what causes them? Normal wear, heat, or foreign objects getting into the rings and scratching?

Finally, does anyone have a comparison of the effect of compression ratio on power with equal displacement? I think that would be interesting to look at before spending the money to get a motor sleeved, new pistons, etc.

Zero10

IMHO, if you're in that deep, clean up the scratches, bring things back to new. If that involves over-boring, then so be it. It's worth it in the end (or so I hope!)
Most deep scratches are caused by foreign objects, however taper and broad scratches are usually caused by piston skirt-wall contact, which I guess could be considered normal wear.

daniel951

i have a little higher compression on my 2.5L and i am making great power with bolt ons.

pk951

daniel951
Addict

Do you know what your'e compression ratio is? What kind of pistons did you use? How long have you been running with those pistons?

daniel951

the PO did the job. it looks like he is running 944na pistons. u can't run as high as boost like 951 pistons but i make more power down low. i plan on hitting 400rwhp at 20psi with no headwork or cam.

AL951

Daniel,how long have you have your engine running in this set-up?

Helstrm

I thought NA pistons would only handle around 6 psi of boost. They are not forged...

daniel951

the engine has been this way for over two years and i have no idea after that since ihave only owned it for two years. i have ran 16psi with no problems which so ever. when i dynoed my a/fratio weren't good so i have more power to be made at those levels. Danno is runnin the same setup with different pistons. i woul dhave to ask him to see exactly what ratio they are.

TurboTommy

Mark,
that's a very good question; I wondered about this as well
I myself have witnessed scratches like you describe and that particular engine showed no signs of decreased compression or even burning of oil.

I was told that with age it's a little carbon buildup on the rings/lands doing the scratching. So, I don't know.

Apparently, if the compression goes up from, let's say, 8:1 to 9:1, the thermal efficiency goes up from 28% to about 30% (give or take; but the gap is around 2% give or take a bit). This realization of two percent gain in thermal efficiency turns directly into power!
Therefore 30/28 or about 7% increase in power.
As the CR goes up the increase in thermal efficiency gets to be a little less, however.

mark944turbo

Thanks Tommy, thats just what I was looking for.

I have a block that is pretty badly scored, I think for the hell of it this summer I am going to build a 951 motor with NA pistons and lots of used parts, then just see how much power I can make before blowing it up. I have nothing to lose as sleeving is way too expensive and used na pistons should be almost free.