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Undercut vs straight stem valves

General engine tech -- Drag Racing to Circle Track

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travis
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Undercut vs straight stem valves

Post by travis » Mon Sep 09, 2019 11:10 pm

I was wondering what y’all’s opinions are between the 2. I’ve been reading that there are times when an undercut valve stem can cause turbulence in the bowl and cause reduced flow on a bench.

What kind of gains or losses have you guys seen between the 2?

GARY C
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Re: Undercut vs straight stem valves

Post by GARY C » Tue Sep 10, 2019 2:13 am

I always wondered if the throat was to big with an undercut would a straight stem help that?
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steve cowan
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Re: Undercut vs straight stem valves

Post by steve cowan » Tue Sep 10, 2019 3:15 am

When I done my cast iron 165cc dart heads I had no test valves at the time.
I had a set of undercut stem exhaust valves and had a hell of a time with severe turbulence, I am not saying it was the valves but after I purchased about 30 individual test valves I found that on the dart 180cc heads with a factory valve job flowed the best with a 11/32" stem tulip valve and a steeper angle on the tulip side works well on the bench.
I think there is a problem with under cut valves causing turbulence at the convergence point in the lift curve as the air doesn't know which way it wants to go, I will be testing at higher depressions to see what happens.
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MELWAY
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Re: Undercut vs straight stem valves

Post by MELWAY » Tue Sep 10, 2019 6:42 am

steve cowan wrote:
Tue Sep 10, 2019 3:15 am
When I done my cast iron 165cc dart heads I had no test valves at the time.
I had a set of undercut stem exhaust valves and had a hell of a time with severe turbulence, I am not saying it was the valves but after I purchased about 30 individual test valves I found that on the dart 180cc heads with a factory valve job flowed the best with a 11/32" stem tulip valve and a steeper angle on the tulip side works well on the bench.
I think there is a problem with under cut valves causing turbulence at the convergence point in the lift curve as the air doesn't know which way it wants to go, I will be testing at higher depressions to see what happens.


That’s interesting Steve
I have been porting some Dart 200cc SHP. And battling a turbulence issue in the ex port. Yet to try some different valves
3370lb Sedan 9.89@136MPH 358chevN/A

mag2555
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Re: Undercut vs straight stem valves

Post by mag2555 » Tue Sep 10, 2019 6:46 am

Under cut stems on the Intake side will always show a flow gain at certain valve lifts once the valve is beyond the .18D lift point.

If the port then does run into turbulence issues that means the port shape was right , and on the edge of having issues anyway ( and needed to be corrected ) and the added larger flow mass pushed it to the tipping point, or over the edge if you will?

And if your not testing thru the intended Intake Manifold then you've shot yourself in both feet right then & there!

On the Exh side a cut down stem can lead to turbulence and there are so many factors that can be involved in the why of it all that it's best in terms of time possible wasted on the dyno to just pay for a flow test along with good amount of velocity probing and see where the flow goes and how the numbers go!

In terms of Tuliped Exh valves they will always show a low lift flow gain , in fact some times as much as 30% more at .100" to .200", but there's no guarantee of added power from that.
In fact I have seen a power loss from running a Tulip valve and it had nothing to do with its added weight, it was do to the fact that even though the valve shape showed a big flow gain on the bench at one lift point , in use its added mass was choking off the Exh flow .

Perhaps when you run a Tulip Exh valve you need a very fast opening ramp to in effect get the valve out of the way.

Of course if your dealing with a hogged out Exh bowl and a beat up seat that you can only get 2 angles out of then a Tulip valve should be a overall help.

Walter R. Malik
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Re: Undercut vs straight stem valves

Post by Walter R. Malik » Tue Sep 10, 2019 11:18 am

You guys are humorous ... EVERY situation is different.
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