Seat angle shoot-out

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David Vizard
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Seat angle shoot-out

Post by David Vizard »

We so often hear much controversy between various seat angles from as little as 30 to as much is 55°. However. I’ve yet to see anyone publish any flow figures showing the difference between these valve angles when nothing else is changed other than the valve angle and the approach to it.

So during one of my classes I thought I would get out the stuff to demonstrate such.

Sometime back I took an CNC ported 18° Chevy head and made provision for it to have inter-changeable intake valve seats. So with the help of my assistance Charlie Servidio, we ran the numbers and here they are shown below.

These tests involved valve seat inserts of 45, 50 and 55°.

The seat face width was held constant at 0.050 inches.

The angle below was 10° less seat so we had 45 with a 55 bottom cut, a 50 with a 60 bottom cut and a 55 with a 65° bottom cut. Each of these angles was followed by a lower cut of 10° more.

So as far as possible the seats were made in the same format. I also threw in one of my tricks seats which I demonstrate to the performance classes I hold.

I’ve included both graphs and charts so that so both trends and numbers are shown. (some folk don’t like my publication grade graphs but why anyone would want sub-standard artwork is beyond me.)

It seems to me the greatest avenue for success with 55° seat is not in how much extra it flows in the correct direction but how much less it flows in the reverse direction during the overlap period. It does appear that the success of high forward flow (at higher lifts) on a valve seat hinges on what it flows in reverse.

It’s worthwhile remembering that the cylinder doesn’t know how big the valve is it only knows how much air it will flow. This leads me to the conclusion that if we can get high flow off the seat and throughout much of the lift range the engine will benefit the same way as fitting larger valves with a more normal discharge coefficient. The problem is many engines don’t have room for any bigger valves so that’s already a maxed-out issue.

Conclusion - good low lift flow must be accompanied by moves which inhibit the reverse flow so that reverse and forward flow ratio’s remains unchanged or moves into a regime where it’s even better than normal.

Comments please.

DV
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Re: Seat angle shoot-out

Post by David Vizard »

Guys I am posting this just to move the subject to pole position.
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Re: Seat angle shoot-out

Post by GARY C »

Here are a few articles written on the subject, maybe others can post the ST threads in regards to this subject but Good Bad or Indifferent there has been a plethora of writings on the subject. All seem to be inconclusive.

https://www.enginelabs.com/features/val ... rk-engine/
https://www.hotrod.com/articles/1203phr ... valve-job/
http://www.superchevy.com/how-to/89678-valve-angles
https://www.eng-tips.com/viewthread.cfm?qid=84437

Those are just the few I have book marked over the past cpl of years.
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Re: Seat angle shoot-out

Post by GARY C »

This section will not allow me to edit my post so I will add this here.
It appears that the top cut, bottom cut and the short turn and throat size play a big part in the seat angle being used as well as the chamber shape blending well with the top cut. So I am not sure you could get a valid test without taking all of these into consideration.
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Re: Seat angle shoot-out

Post by skinny z »

David Vizard wrote:.
The added low lift flow effciency is about equal to installing a conventional valve with a 3 angle cut of 2.32 inches diameter.
DV

David
To assist with a conversation I'm having with a fellow enthusiast regarding a comparison of two cylinder heads, can you explain why you place such emphasis on low lift flow?
Case in point: Vortec vs AFR 195.
My opponent is stating outright power results.
I'm talking about efficiency in general.
AFR doesn't publish flow data less than .200".
The Vortec forte appears to be what happens between .050" and .100".
Who is the leader here in terms of low RPM torque production/VE?
Thanks in Advance.
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Re: Seat angle shoot-out

Post by BCjohnny »

[quote]Comments please[/quote]

I guess it's not much of a stretch, considering you've often been an advocate of them, that the 'DV Special' seat is 30degs or around that value which flow well at low lift ...... with other detail work to crutch the higher lift flow ?

Care to comment ?

TIA
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Re: Seat angle shoot-out

Post by RevTheory »

I'll throw a couple of things out for discussion.

I've seen CFDs showing how steeper seats tend to let the incoming charge get around the valve a little better at higher lift. There's also been some interesting discussion about how the reduction of low-lift flow builds a little more depression in the cylinder early on and that early depression pays dividends once the piston starts slowing down but the lift is still increasing and the steeper seats are now flowing more air. Think of pulling a slinky and letting one side go right about the time the piston is slowing down and nearing BDC but the valve is still within 10% of max lift and really flowing well.

Another point made was on the closing side. If you've got your intake tract sorted out well, you'll have some really good energy from BDC on up to IVC and 50s tend to have a higher velocity when the valve is close and don't flow well backwards. Apparently you can run a little later IVC before running into reversion issues.

Don't tar and feather me, David. I'm just regurgitating some things that do seem to make a lot of sense to further the discussion.
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Re: Seat angle shoot-out

Post by Stan Weiss »

Remember at lower lift the more you increase the valve seat angle the smaller the actual flow area between the valve and the seat gets.

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Re: Seat angle shoot-out

Post by David Vizard »

BCjohnny wrote: Thu Sep 03, 2020 7:43 am
Comments please
I guess it's not much of a stretch, considering you've often been an advocate of them, that the 'DV Special' seat is 30degs or around that value which flow well at low lift ...... with other detail work to crutch the higher lift flow ?

Care to comment ?

TIA
No the DV special valve seat is not 30 degrees, in fact it is hardley close to that.

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Re: Seat angle shoot-out

Post by David Vizard »

BCjohnny wrote: Thu Sep 03, 2020 7:43 am
Comments please
I guess it's not much of a stretch, considering you've often been an advocate of them, that the 'DV Special' seat is 30degs or around that value which flow well at low lift ...... with other detail work to crutch the higher lift flow ?

Care to comment ?

TIA
it is a little scary to give an absolute answer here but I would say that the Vortec head has a slight advantage at low rpm and possibly mid range. after that supior airflow of the AFR heads wins out.

All that said AFR's late boss, Ken Sperling, made the comment to me that low lift flow does not seem to be really important. However the AFR heads had, at that time, some of the best low lift flow seats. Also at that time AFR seemed to win more head shootouts than any one else!
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Re: Seat angle shoot-out

Post by skinny z »

David Vizard wrote:

>
> it is a little scary to give an absolute answer here but I would say that
> the Vortec head has a slight advantage at low rpm and possibly mid range.
> after that supior airflow of the AFR heads wins out.
>
> All that said AFR's late boss, Ken Sperling, made the comment to me that
> low lift flow does not seem to be really important. However the AFR heads
> had, at that time, some of the best low lift flow seats. Also at that time
> AFR seemed to win more head shootouts than any one else!
> DV

Thank you for that.
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Re: Seat angle shoot-out

Post by MadBill »

David Vizard wrote:...
> It seems to me the greatest avenue for success with 55° seat is not in how
> much extra it flows in the correct direction but how much less it flows in
> the reverse direction during the overlap period. It does appear that the
> success of high forward flow (at higher lifts) on a valve seat hinges on
> what it flows in reverse.
> ...
> Comments please.
>
> DV

David, have you done such a reverse flow comparison? Perhaps also with a 30° seat done up to the specs you published many years ago: https://www.google.com/search?source=un ... 20&bih=937
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