Quench distance '68 390

A little wordy and a rant from an Engineer but I liked this post when I was doing my rebuild and cut my own valves, FYI I did three angles:

Having spent 20 years studying the effect of cylinder head modifications on my flowbench all I can tell you is it's far too complicated to summarise into simple rules like "more cut angles = better".

A completely smooth radiused seat will not necessarily flow more (usually less) than one with discrete seat angles and sharp edges between them. Sharks have rough skin (in fact most fish have scales) because the discontinuities actually reduce surface friction and drag. Golf balls have dimples because that lets them fly further than a ball with a smooth surface. "Smooth" does not always equate to friction free or higher flow.

The choice of seat width and bottom cut angle are far more important than just the number of cut angles which tell you bugger all about the knowledge of the person doing the job. Most places just have a small range of standard cutters which can't possibly be right for every valve size and nearly everyone cuts the main seat too narrow. A nice wide seat conducts heat away much better. The OE seat width is usually spot on because the person who designed the engine knew a lot more than the idiot with the Serdi who thinks 1mm wide valve seats are "race seats" or somehow better than what was there to start with.

Seat concentricity is more important than just about everything else combined and the commonly used Serdi machines are awful at achieving that unless the guides are in perfect condition. In fact most places with Serdis will try and tell you they need to either replace the guides first or at least hone them out (which of course buggers them) until they fit their next larger seat cutting pilot just to get the machine to work properly (they won't actually admit it's to get the crappy machine design to work properly of course). Far better are machines with fixed pilots which lock into the guides like the Sunnen system than rotating pilots like the Serdi where any play between guide and pilot translates straight into seats with poor concentricity.

Seats that flow well at high valve lift don't always flow well at low valve lift and vice versa. Seats that flow the absolute most might not last very long. Everything is a compromise and every seat I've ever cut has been tailored to the exact engine spec, cam lift, its intended use and the valve sizes.

If you want some simple rules for general use the 45 degree seat should be about 4.5% of the inlet valve diameter wide and use the same width on the exhaust seats which will usually translate to about 5.5% of their diameter seeing as they are smaller. So a 50mm inlet valve will want a 2.25mm wide seat and so on.

A 70 degree bottom cut always outflows the normally used 60 degree cut because it better splits the transition from 90 degree throat to 45 degree seat. A 60 degree bottom cut means a 30 degree transition from the throat and only 15 degrees from the seat angle which is a stupid way of trying to minimise the change of flow direction. Obviously the ideal would be to split the 45 degree difference and use a 67.5 degree bottom cut.

So why does everyone use cutters with 60 degree bottom angles? Because that's what the seat machine manufacturers list as stock items and hardly anyone out there is smart enough to question it. Actually OE heads usually have the correct 70 degree or similar bottom cut angle which your local engine reconditioner will happily and cluelessly bugger up when he refurbishes the head for you.

Similarly the top cut should split the difference between seat angle and chamber roof angle, i.e. with a flat chamber roof use a 22.5 degree top cut. With hemi heads or other heads with angled roofs use a bigger angle like 30 or 35 degrees.

So phone up a few people and ask them some searching questions because you now already know more than they do. Ask them what seat width they suggest and why. When they say they use 1.5mm on everything because that's the cutter they bought with the machine or that wide seats are good for road heads and narrow seats are good for race heads you can put the phone down. Ask them what bottom cut angle they use and why. When they say they've never given it much thought or they use 60 degrees because that's how the cutters are made you can put the phone down. Same for top cut angle.

Ask them how much time they've spent testing different seat widths and angles for flow on a flowbench. When they say "what's a flowbench?" you can put the phone down.

Ask them what concentricity level they strive for. They'll probably say their seat machine has a vacuum tester built in and they test every seat for leakage - at which point you can put the phone down. The trouble is a 45 degree valve and a 47 degree (or any other angle) seat will still touch at one point all the way round so it'll seal against vacuum but be worse than useless and burn straight out in service. I've watched a Serdi machine test every seat as perfect for leakage but then none of them would actually lap in and it all had to be redone. Only the fact that I was standing there glowering at people meant it got redone properly! I glower very well when the occasion requires it.

You have about as much chance of getting a set of valve seats cut perfectly (well what in my view is perfect) as of winning the lottery. In fact it is a lottery. If you can find someone who can answer all the above questions without you having to put the phone down first please let us know.