2 top cuts on a valve job

[202ci EH]

Might be more of a question for a stock eliminator racer
Had to start a topic as I couldn't find much in a search, of course it's application specific but does anyone use a second top cut after a 30-38° on a 45° seat?

Would a shallow 2nd top cut affect the way the air traverses the seat to the chamber for worse at high lift? Breaking away from the 30° versus following the chamber.

Cylinder head in question is heavily shrouded on one side and a flat chamber floor on the other side

My last effort on one of these heads had a raised port, tight chamber with the int valve sunk to get a .100 wide 40° top cut on a 50° seat, it flowed big cfm numbers (230cfm & 1.69 valve) but it wouldn't relate at the track so time for a reevaluation.

Cheers

[ProPower engines]

If I was starting with a head that had been machined 45 degree seats to start with and wanted to go to 50 degree I would either put a seat in or replace the existing seat to allow the valve to end up higher in the chamber.
I have also seen the area above the seats that resemble a pie crust edge in which case I use a counter bore cutter to clean up the edge above the top angle of the valve seat.

There is better flow and performance to be had by not having the valve sunk in the chamber in restricted class engines.
While it is more work to put seats in to me its worth the extra effort after all with all the plus's added up they equal more in the end result.

[jcisworthy]

I found flow bench gains with a multi angle back cut on the intake valves. You have to experiment with angles and widths to figure out what your valve job likes.

Just last week I found something and decided to change from a single .100 wide top cut on my seat cutters to two .050 wide top angles above my seat. I am having new cutters made up to experiment with this week.

Going from a 50* to a 45* will sink the valve and your throat diameter based on valve size will probably be larger than desired also. Sinking the valve can, and a large throat diameter will hurt air flow.

Seats can fix the valve job as mentioned then you can experiment with top cut angles on both the valves and above the seat for best results

You should see good improvements in flow and hopefully at the track as well.

[202ci EH]

Thank's guys, i'm starting a new head not modifying the old one, I couldn't keep the epoxy stuck in the raised port head. So no drama's getting the valve heights & angles.

The valvetrain architecture doesn't really support big valve lifts without $$ so need to work more on the area under the curve.

Funnily enough propower, the 50° head just flat would not work without the .100 40° where the valve face being flat on the bottom of the chamber. Spent a lot of time on that one.

Plenty of r&d to go, never having seen speier / darin morgan / larry meaux etc ever mention 2 top cuts made me wonder whether I was missing something.

Thanks fella's

[mag2555]

High velocity air can not make its way around anything tighter then a 15 degree bend without being slowed down!


At low overall port velocities like at low lift the 15 top cut will show some level of a flow gain, but your only making it more difficult to gain high lift flow with such.

The only time I have found a somewhat overall improvement in adding a 15 degree top cut is when working with a valve that has been sunk way more then it should be.

I have found that on Intake ports that are very close to going into tumble at high flow rates that a 15 top cut and the added total air mass that it brings with it WILL send the port off into tumble / sheer and then you have resigned yourself to putting seats into the head and or resorting to a much bigger valve to get things back into shape!

Another big thing to concider is that the more angles you use on in the valve job the more you cut back on the overall size of the throat you could have had for better mid and high lift flow, not to mention likely getting the head to the point where a clean up valve job down the road can't be done without installing seats!
Post script, that first paragraph has a big tip for ya on porting Heads!

[statsystems]

IMO, putting a top cut on Isnt sinking the valve.

[user-9274568]

All I'm going to say is a Stock Eliminator valve job isn't sealing on a 45 seat..

[202ci EH]

All I'm going to say is a Stock Eliminator valve job isn't sealing on a 45 seat..

Isn't that illegal? Guess it's how you read the rulebook.

Mag ~ yep fully aware we can only turn air 15° without detaching. A large part of the flow in this port flows accross the chamber and has to transition to the chamber floor from the top cut.

Gut feeling is a 15 or 20° top would probably flow better in reverse than it would forwards.
Sounds like im just gonna have to test and test.

[statsystems]

All I'm going to say is a Stock Eliminator valve job isn't sealing on a 45 seat..

And they have a lift rule........

[modok]

How about two cuts but reverse of what you'd expect?
37 on top a 27, you get it?