Oddjob

Found this after the headwork was done. Not visble prior to the cleaning. All 4 have some showing, this is the worst one. Nothing loose or flaking yet. Guy at the shop said he has seen worse reinstalled and used. Not sure how long its been like this or how long it will hold together.

lart951

Did someone hacked in to Jim's account? usually people ask questions and you respond. I have seen worst from running cars no damage to turbine wheel.

Oddjob

Nah, I have been working on these cars for a long time, but I think I learn something new or see something Ive never seen before, everytime I turn a wrench.

Droops83

I haven't dealt with this on a 951 head, but I work at a Porsche shop and we have encountered this on 964 heads a few times. Once we deemed it bad enough that we recommended new heads . . . those aren't cheap!

My worry besides possible turbine damage is that a perfect storm of events could also cause a chunk to get caught in between the exhaust valve and seat. Haven't actually seen this happen but just throwing it out there.

I suppose my answer is, with my shop manager's hat on, I'd say that 951 heads are cheap enough that you should try another, though that is too bad that it wasn't apparent till AFTER the headwork. On my own car I'd run it for a bit and recheck.

TonyG

I don't see why you can't just remove all of the ceramic and blend the ports and run the head.

TonyG

Dougs951S

I agree Tony. Is there any word to what extent the ceramic actually retains heat energy and hastens spool? Or in other words, what would you actually lose running an n/a head or a turbo head with the ceramics gone? My guess is its only a few hundred rpm at most.

Voith

I think this ceramic is there to isolate head from toomuch heat and steam pockets.

TonyG

It has nothing to so with heat or steam pockets.

The ceramic liners were used to keep the heat in the exhaust to help spool the turbine faster.

Which is also the same reason the turbo head has the hump on the exhaust port floor (which is to increase the velocity of the exhaust gas).

On the race engines we used the N/A heads which don't have the ceramic nor do they have the hump on the exhaust port floor. This because we could get better exhaust port numbers from the N/A head vs the turbo head. Excess heat was never an issue.

That said, I see no reason why he couldn't just remove all the ceramic and have the exhaust port ridges (where the ceramic meets the aluminum) blended and call it a day.

TonyG

Dougs951S

So are you saying that the higher flow achieved from a worked N/A head vs a worked turbo head is more advantageous than keeping the extra heat in with the liners? I'm preparing to build a big valve head soon, am I better off using an N/A head?

TonyG

On a stock car, the port flow numbers are fine. So they focused on gas velocity and keeping the heat in the exhaust gas rather than transferring the heat to the cooling system. Which is what you try to do to reduce lag.

But you can't get the exhaust ports to flow big numbers with the design of the 951 exhaust port. That's why we use the N/A head.

So the answer is yes.

TonyG

hosrom_951

Got a couple of exhaust ports with missing ceramic pieces here and there, runs fine. Turbo is just fine (or was).

Scott H

How many companies have installed ceramic port liners since the early 90s? The only two cars I know with ceramic port liners are the 951 and late model aircooled cars.

harveyf

I have also read that when Porsche put in the plumbing for the turbo they had trouble meeting the regs with regard to getting the cat up to temp quickly enough, with a cold start. The liners helped get more heat down to the cat. So any way you cut it, putting in a N/A head might be the way to go. Transfer those sodium filled exhaust valves from your turbo head to your replacement head. They do represent a heat rejection item that you want to maintain.

TonyG

Agreed.

And yes... definitely reuse your turbo exhaust valves.

TonyG

Dougs951S

Thanks for the input guys, thats exactly what I was planning on doing. I'll be porting the head and going to +2mm intake valves along with a general rebuild and decking, and an offset cam key to get my cam advance back.