When does exhaust valve size become an issue?

 
Old 01-14-2019, 06:45 PM
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Default When does exhaust valve size become an issue?

There has always been oversized intake valves, but I don't remember talk of increasing the exhaust valves at just about any power level.

My question comes from playing with an old 12valve head and, well... for reference lets say an LS head had 25mm exhaust valves. Would that be a major restriction at stock power levels? at perhaps 150whp?
These exhaust valves are 35mm, which is less area/flow than a D16 2x26mm. Given that this is a 2 liter, would you make it a priority to increase that exhaust valve size?

If it is any consideration, stock intake valves are 30mm, although I ran across some 31mm cheap so those will be increasing.

Last edited by night; 01-14-2019 at 07:13 PM.
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Old 01-15-2019, 03:50 PM
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Default Re: When does exhaust valve size become an issue?

Originally Posted by night View Post
There has always been oversized intake valves, but I don't remember talk of increasing the exhaust valves at just about any power level.

My question comes from playing with an old 12valve head and, well... for reference lets say an LS head had 25mm exhaust valves. Would that be a major restriction at stock power levels? at perhaps 150whp?
These exhaust valves are 35mm, which is less area/flow than a D16 2x26mm. Given that this is a 2 liter, would you make it a priority to increase that exhaust valve size?

If it is any consideration, stock intake valves are 30mm, although I ran across some 31mm cheap so those will be increasing.
i asked this question before and everyone said keep stock size
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Old 01-16-2019, 05:59 AM
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Default Re: When does exhaust valve size become an issue?

Ya, that is the feeling I get everywhere, but it just doesn't seem sufficient to have D series size valves in a 2 liter engine. And the stems are 7mm so that is even more restriction.

Also, I've no idea why a performance question was moved to machine work :/
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Old 01-16-2019, 10:38 AM
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Default Re: When does exhaust valve size become an issue?

No clue why the move either.

I do recall reading a bit on valve size and the smaller size actually helps velocity. But I did get the impression they were speaking on the intake side. However, if it creates greater velocity flow on the intake, I am not sure why it would not do the same thing on the exhaust.

In the simplest terms, more power was seen with a smaller intake valve than a larger intake valve on the same motor. Higher velocity charges were seen. I would suspect you should also see higher velocity discharges.

I suspect there should be an optimum size. Too large everything drops, and too small everything drops.
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Old 02-11-2019, 07:25 PM
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Default Re: When does exhaust valve size become an issue?

in my experience, I've always calculated ideal exhaust valve size according to: maximum piston velocity, cylinder bore, and valve seat bore. Notice I said valve seat BORE, not valve head size. The key is to have the following equation equal the speed of sound (767mph):
X = area of cylinder bore
Y = total area of exhaust valve seat bore[s]
Z = maximum piston velocity

Z * (X / Y) = ~767mph

It's not an exact science, and there are lots of ways people can debate the above. But it has always worked out to be extremely accurate for me.

The above being said, also note again the measure of the exhaust valve seat bore and not the valve head size. It's a good possibility that your single exhaust valve seat bore area is noticeably larger than the total exhaust valve seat bore area on d series.
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