Yan

Hi!

Finally, yesterday I opened the head and my exhaust valve is blown. It doesn't look like the piston knocked on it, there are litterally parts missing on it. Also, the valve guide is a bit worn...

My question is: being given that the head is completly off with the valve cover off, will a typical head rebuilder accept to touch our heads? How much am I willing to spend in order to change all four exhaust valves?

Thanks!
Yan

BoostGuy951

The head is rebuildable and any competent shop should be able to handle it. The Exhaust valves are sodium filled and will run you from $80 to $100 Each. You can probly find them used for significantly less.

Yan

Nice, thanks!!!

I guess that the valves are the same on the 951 and the 944? Will the shop also be able to rebuild the seating and/or the valve guide is they are too worn?

brad-cam

The 951 has different exhaust and intake valves from the 944 - the 951 exhaust valves are sodium filled.

Yan

Ho! Ok.

I'll try to find some used ones.

CurlyDE

Forgive my ignorance, but what is the sodium for?

johne

Heat transfer, it pulls the heat away from you valve face. Atleast that's my understanding.

John

DAR951

One thing to remember about the turbo (951) head is that it has a ceramic lining in the exhaust port. As far as I know, this is NOT rebuildable/refinishable/replaceable... So check its condition before spending money on the rest of the head.

Yan

Which factor could make the ceramic lining unuseable?

Also, why did a part of my valve litterally desintegrate? A detonation?

IceShark

I have heard the ceramic lining can/will flake off in spots. Now if this totally makes the head unusable and what your options would be other than getting a different head, I don't know.

rage2

The sodium filled exhaust valves as well as the ceramic lining in the exhaust ports are used for heat transfer. It basically allows you to run a bit higher EGT's safely.

With the ceramic lining partially flaked off, the head is not unusable, but there would be hot spots if the car is run continuously on poor fuel (reduced timing, higher EGTs) or a bad tune. It's not the end of the world though if it's just a street car. I'd worry more if there was extensive track use.

BoostGuy951

You also have to consider bits of ceramic flaking off and going through the turbine blades. If its cracked that is.

eastendr

I'm modifying a spare head for my 951. New guides. The exhaust cermic ports are OK and new exhaust valves are in. The inlet has been ported / blended and port matched to the inlet manifold, which has also been smoothed out and matched to the throttle body etc ... I'm retaining as close as possible the original compression ratio.

Question. I want to increase the inlet valve size. So far, the most likely alternative are from the 944 2.7 which has 3mm bigger inlet valves, all other dimensions being the same. What are you guys using as larger valves ? I'm interested in alternatives since what Porsche charge for the new valves, they should be made of solid gold !

Secondly, I'm contemplating LR titanium retainers to reduce weight to allow a decent redline.

I'm heading to a T04E / map etc so the entire airflow should be pretty good (see other modifications in my sig) and allow power right up to the redline. What do you guys currently use as a redline with a high-flow conversion ?

The bottom end is currently standard / very good with just over 100,000 miles.

rage2

I say leave the head alone. The gains are fairly minimal for what you'd have to pay. It's not like the stock head can't make big power .

Laust Pedersen

The sodium in the exhaust valves is liquid at elevated temperatures and by “sloshing around” it transfers heat from the valve face backwards to the stem and then to the guide. As a rule of thumb, 70% of the heat in standard exhaust valve is transferred to the seat, so a good seating is essential for longevity of the valve. This also means that the cam follower has to function properly (at the very least, keep the oil clean).

Laust