The discussion on lapping was excellent.

Any thoughts/tips on cutting the tri-angle on the valve seats? The S4 tech specs booklet shows angles of 30º, 45º and 60º from the inside out, with different 45º face widths for intake and exhaust.

This operation must be at least as critical as lapping.

 

The valve angles do seem to be very important. A. Graha, Bell in his book 'Four stroke performance tuning" has quite a few pages on it.

 

To some extent hand lapping the valves allows you to be SURE that the head work was done correctly and the valves actually are touching 360 degrees and correctly on the angle cut faces. The object is not to remove much material but to lightly kiss the surface where the valves hit the seats.

 

Don't they normally run a degree difference between the valve and the seat so that you get a relatively narrow seat where they mate. Then over time as the valves beat the seat this area widens?

 

Thats the way I remember it FB

 

FBIII wrote: "Don't they normally run a degree difference between the valve and the seat so that you get a relatively narrow seat where they mate. Then over time as the valves beat the seat this area widens?"

The only time I ever did a complete cutting and lapping job myself it was a disaster, so I freely admit I don't know nothing 'bout doin' no valve jobs, but ...

That Tech Specs booklet shows both mating surfaces, seat and valve, at 45º. The width of the surface is specified for the valve seat but not for the valve. (pp. 36 & 38)

 

You need to design the valve/seat interface for desired "margin", as that is what transfers the heat from the valve to the seat ... I think porsche has a minimum spec. I use different ones. Design your own and see the results.

Too big, and you lose flow, too small and you will tulip or drop the valve. And then there is the valve spring force. GT and GTS alwaus seem to pit the downside of the valve ... hmm ...knock, knock!

SC and turbo applications must design the margins very carefully, particularly on the exhaust side.

Seat geometry is critical to air flow ... somewhere I have a few heads each with different seat designs, and different flows. The factory seats are very good.

Valve guides are a similar situation and the specs must fit the application. Too lose and what Dr. Bob states is key. Too tight and valves stick.

Hmm, same is true for piston pins .... and just about every other dimension on an engine ..

damn, this can get long....

Oh, and lapping .. do it to check the margin with dykem. If you have to do it "alot" bring the heads back to the machinist.

 

Well, I was planning on making a more substantial contact area on valve-to-seat for the exhaust because the motor will be SCed. The pistons are Swain Ceramic coated, and the head chambers may even be coated as well.

I'll be buying some DyKem tomarrow.

Marc - many of us usually appreciate your info here, but can you just suggest or advize as much as you feel good about with dropping archaic hints? I know that GT cams pit - but they usually do so at the highest point. Maybe high lift cams don't have nearly as much "pointyness" to them as our GT cams do. I'm sticking with S4 cams and the original lifters, but may go with 944S2 valve springs as was discussed in my other thread. I want the valves closing properly and quickly with no bouncing at 6800rpm.

With the combustion temps and post-combustion exhaust temps I may see with 10psi and 9.5 compression I don't want any burned valves from weak closing rates or bouncing.