Valve Seat Contact Width

[DesertSasquatchXploration]

Was at my folks on Mothers day and started talking to my Old Man he worked in a engine rebuild shop for a while during the late 60's has since moved on to better things and going to retire this year. Talking about valve seats he was dead set on a thin seat 1mm/.040 max. I ran a floor sweep 1600 SP engine at 2.5mm for years was fine ran cool. I currently have my engine apart bought all new valves didn't cut the seats quick lap came out at 1.7MM guides are tight I'm going to run it.

[modok]

Hasn't really changed much since we invented gasoline.

too narrow wears fast and cools poorly
too wide.... too much carbon build up and it cools worse, flows a lot less too.

I like around .040 intake .060 exhaust

Manual says something similar, I betcha

But.... it is colored by...the times and experiences people had.
Softer valve seat materials wore a lot more rapidly, so you'd want to start narrower. That just makes sense.

Like how they'd build antique engines very tight because everything used to wear looser.
You built it that tight today, then it'll just always be too tight.
If you use good modern oil, air filter, gasoline and materials that is.

[Brian_e]

Big flow gains can be had narrowing the seat, and placing it correctly on the valve face. Adding a couple more angles will improve it even more. Probably the quickest, easiest for sure gains anywhere in the port are at the valve seat.

I prefer the .040, and the .060 seat widths also.

Having a really wide seat width won't automatically make the heads run cooler. It might help reduce the risk of burning a valve is all. That's why the hotter exhaust valve always has a wider seat width, and its placed more centered on the face. Its purpose is to pull the heat out of the valve head, and transfer it to the cylinder head.

Brian
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[SGKent]

Reinforcing what some others said -

the seat needs to be narrow enough to not allow carbon to build up. It is really something you have to work around to get a feel for it, but even the placement of the contact area is critical to air flow. Back cut the intake valves for additional flow. Don't back cut them if you are chasing fuel economy and not RPM and torque. Rough up the ports but get rid of any flashing and very rough areas.

Lots of shops make a wide contact area to get more cooling on the valve. The people who taught me built winning engines for the Unser family, as well as FI and Cosworth engines in the 1970's and early 1980's. It is hard to explain it to someone. Too thin and too far out and the valve can fail. Too wide and carbon builds up on a street car. Too far in and the air flow is reduced. Add things like spring pressure into it and the cam profile. They all interact.
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