Valve seat cutting

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I ran into several heads lately that had been rebuilt and the seats had been cut using a Serdi type seat cutter. The top cut on the seats were at a 15 degree angle which is the standard cutter angle used for automotive applications. Using this angle on a Norton creates a notch in the seat. I always figured this was not good for the flow so I tested it this weekend.


 
A flow bench probably does not accurately replicate what happens in a port in a running motor. The gas column in the port on a running motor is sonic. The difference yo have detected might be much greater in practice then it is on the flow bench.
 
@acotrel

Just because one repeats it zillion times doesn't make it correct.
As mentioned several times previously, there is no super Sonic flow in intake ports and highly pitched whistling intake ports are a sign that something is not correct.
Just in certain circumstances and locations do exhaust ports go beyond mach1.

Kind regards Christian
 
I did not know this, Thx for taking the time to put it up here, good info for sure. Cj
 
Could differences between the two heads' porting be contributing to the difference? Ideally, you'd need to show the flow before/after seat cutting differences on the same port I'd guess.
 
Could differences between the two heads' porting be contributing to the difference? Ideally, you'd need to show the flow before/after seat cutting differences on the same port I'd guess.
Well that would be a perfect way to do it but actually the low lift flow is not going to be affected much by small differences that might exist between two rh10 heads so I think the results are pretty accurate.
But I am installing new seats in the head with the 15 degree top cut so I can do a comparison here later this week.
 
Why would these 15 degree seat cuts be used on other engines if they perform poorly? Does it depend a lot on engine head design/flow characteristics?
 
I once had the valve seats cut in a Triumph head using the three cutters. The motor performed significantly better. But it might have performed better, if the valve seats had been cut the usual way. According to that flow bench, an improvement was achieved at low flow rates. How does that low flow rate relate to revs in a running engine"? Is there an anemometer which would measure the flow rate on a running motor at various revs and throttle openings ?
 
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Why would these 15 degree seat cuts be used on other engines if they perform poorly? Does it depend a lot on engine head design/flow characteristics?
Most hemispherical chambered heads use a higher angle top cut on the seat to blend the seat into the shape of the chamber around the seat.
Most auto engines are nearly flat around the valve seat so a 15 degree cut is used.

High performance automotive cylinder head builders will often use a 30 degree cut between the 45 degree seat and the 15 degree top cut to blend the seat into the surrounding surface. Add to that a 60 to 75 degree bottom cut [depending on the bowl shape] and you have the 4 angles commonly used on performance auto heads.
 
I once had the valve seats cut in a Triumph head using the three cutters. The motor performed significantly better. But it might have performed better, if the valve seats had been cut the usual way. According to that flow bench, an improvement was achieved at low flow rates. How does that low flow rate relate to revs in a running engine"? Is there an anemometer which would measure the flow rate on a running motor at various revs and throttle openings ?

The flow at lower lifts are important at any RPM. Minimum cylinder pressure on the intake stroke occurs well before the valve is at full lift.
 
I've always thought when you freshen up a race motor on the cheap cutting seats further than you should and powers not as sharp as the first time that other than compression is down slightly the low lift flow might be the issue. You always read that a good valve job is one of the best things a motor can have
 
I once had a piece of cast iron valve guide go through my motor while I was racing. I did not know it had happened until I pulled the motor apart and found a mark on the piston and a ding in the exhaust valve seat. I'd noticed the motor was a bit slower, but I thought it was my tuning which caused that. I was always adjusting things to make the motor faster. I rode at about three race meetings with the bike off-tune. The same thing happens with jetting. You sometimes think the bike is going as fast as it can, but when you get the jetting better, it goes much faster.
 
The train is leaving the station already.

Long ago someone was getting a Ducati 900 SS (belt drive) ready for some racing duties (the series) and did not like the sunken valves.
I said they will be like that due to the camshaft specs and the included valve angle. (Valve clash)

They duly got the heads back with all new valve seats at some expense and when they did a test fit the valves clashed. ?

I went like this (shrug)
Then they sunk the new valves back to where they had been previously but in the new seats.
Ohh well. :D:rolleyes:
 
When the three cutter are used, the actual valve seat can end up about 2mm wide. So wear on the valve seat is probably greater due to the concentration of the load from the valve spring. But the shallower angle beyond the actual seat is probably much better for performance. Near and at top dead centre when the valves are open , there is flow across the top of the piston into the exhaust as well as into the cylinder . In many bikes there is probably a very strong Kadency effect, depending on the exhaust system. Control of noise levels is a limiting factor. If you get your exhaust system working well, your bike is probably too noisey. It is probably the reason many GP bikes use Exup.
 
@acotrel

Just because one repeats it zillion times doesn't make it correct.
As mentioned several times previously, there is no super Sonic flow in intake ports and highly pitched whistling intake ports are a sign that something is not correct.
Just in certain circumstances and locations do exhaust ports go beyond mach1.

Kind regards Christian
I do not think I said supersonic flow occurs in exhaust pipes. Do you think fat exhaust pipes are used on two-strokes because they get the gas out quicker ? If you cut the exhaust port up on a two stroke to shift the power band up the rev range, if the port is not angled down away from the piston, the noise level when the motor runs can be horrendous. With a cam, the exhaust opening rate in a four stroke engine is slower, but the effect is stiill there, I do not use fat pipes on my bike, and I have stubs in my exhaust ports. Their IDs exactly matches the IDs of my skinny header pipes which connect by spring loaded slip joints - NO steps or quick angles. If there is a reflected sound wave there is likely to be a bump in the gas flow at the step
 
@acotrel

Just because one repeats it zillion times doesn't make it correct.
As mentioned several times previously, there is no super Sonic flow in intake ports and highly pitched whistling intake ports are a sign that something is not correct.
Just in certain circumstances and locations do exhaust ports go beyond mach1.

Kind regards Christian
I do not think I said supersonic flow occurs in exhaust pipes. Do you think fat exhaust pipes are used on two-strokes because they get the gas out quicker ? If you cut the exhaust port up on a two stroke to shift the power band up the rev range, if the port is not angled down away from the piston, the noise level when the motor runs can be horrendous. With a cam, the exhaust opening rate in a four stroke engine is slower, but the effect is stiill there, I do not use fat pipes on my bike, and I have stubs in my exhaust ports. Their IDs exactly matches the IDs of my skinny header pipes which connect by spring loaded slip joints - NO steps or quick angles. If there is a reflected sound wave there is likely to be a bump in the gas flow at the step
 
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