How narrow can you run an intake seat ?

[wriggley monkey]

I have an issue with intake valves hitting the bore wall. I would like to make the last race meeting of the year (no time to do a new head).

I was wondering whether I could turn down the valve diameter which would obviously narrow down the inlet seat. I figure that if I reduce the intake seat to .032" to .035" (probably .032") I should have enough clearance (it runs fine with .015" of clearance to the bore). Valves are Parallel to the bore.

The race is three hours long plus qualifying etc.

The application is a 4 cylinder inline engine cast iron cylinder head with cast iron seats (no inserts). Rev limit is 7000rpm with a fairly aggressive flat tappet cam and 1.6" intake valve (stainless). Seat spring pressure is 100lb.

Any views on running this narrow a valve seat ?

Thanks for any help.

[PRH]

.032” wide seating surface on an intake valve?? Sounds fine to me.

[David Redszus]

I have an issue with intake valves hitting the bore wall. I would like to make the last race meeting of the year (no time to do a new head).

I was wondering whether I could turn down the valve diameter which would obviously narrow down the inlet seat. I figure that if I reduce the intake seat to .032" to .035" (probably .032") I should have enough clearance (it runs fine with .015" of clearance to the bore). Valves are Parallel to the bore.

The race is three hours long plus qualifying etc.

The application is a 4 cylinder inline engine cast iron cylinder head with cast iron seats (no inserts). Rev limit is 7000rpm with a fairly aggressive flat tappet cam and 1.6" intake valve (stainless). Seat spring pressure is 100lb.

Any views on running this narrow a valve seat ?

Thanks for any help.

Based on a valve head diameter of 1.60", assuming a 45 deg seat angle, a seat width of 0.035" will result in a seat contact area of 156.6mm^2, or a seat contact to head area ratio of 12%.

Reducing the valve head diameter to 1.594", a seat width of 0.032", will result in a seat contact area of 144.2mm^2, or a seat contact to head area of 11.2%.

For an intake valve, BMW Motorsports recommends an area ratio of 9% for intake valves and 11% for exhaust valves.

[wriggley monkey]

Thank you both, Very helpful.

Brian.

[engineguyBill]

Make sure that the valve stem to guide clearance is within specs. Go with the high side of the specs if you are using positive valve seals.

[mag2555]

Question, does the amount of peak valve lift used place the valve into the travel / path of the top ring?

If not , then instead of cutting down the valve and likely loosing air flow if you notch the Bore that small .015" to ,018" you will likely gain air flow and drop the same amount of compression that you would have from cutting down the valve with no risk of burning out a valve seat during a 3 hour race.

A notch something like in my photo produces gains in Intake airflow that offset the loss in compression if you can't mill to make up for it.

I would mock thing up and look at this possible resolve!

[wriggley monkey]

The valve runs past the point where the top of the ring is at TDC.

Thank you for the suggestion though.