"Back angle" on 911 valves

[stownsen914]

I'm having Ferrea make some valves for a 911 race engine (long story). I'm looking for any information on the right back angle on the head of 911 valve. Intake and exhaust, assuming they are not the same. Apparently the back angle is important, and different engines require different angles.

As an example, pic below showing a couple different back angles, steeper one on the left, and a flatter valve head / shallower angle on the right.

Thanks for any input!

[Neil Harvey]

I'll jump in.

Typically the left valve would be exhaust and the right an Intake. However, every port can be different. They don't always follow the same convention. To be sure, you need to decide on the seat angle, seat width, port shape, then flow.

A 45° seat angle will flow different to a 52°, as will a 55° and a 60°. The top angle will change all of these as well.

Welcome to my world.

For what I expect you are doing, using a 45° seat with widths of 1.0mm Intake and 1.3mm- 1.5mm exhaust and a top angle of approx 30°, go with a fuller radius exhaust and a flatter Intake. Typically angles of 20° and 12° are common.

[Zuffenwerker]

Originally Posted by Neil Harvey

go with a fuller radius exhaust and a flatter Intake. Typically angles of 20° and 12° are common.

Neil your saying a 20° back cut on the E and a 12° back cut on the I might work well for his application?

[Neil Harvey]

I'm confused by your choice of wording. Back cuts on valves typically means a second cut above the main seat angle, usually on exhaust valves.

A back "angle" is the actual shape (angle) of the valve between the seat face and the radius onto the stem. I took what you were looking for was this, based on having valves made.

Depending on the valve diameters you are using, the angles I have suggested are typical for ports and seat angles used on these heads.

Often, you are limited to what blanks are available too. Lately, valve suppliers are limited to what they can supply. Best to ask.

[stownsen914]

Thanks very much Neil for sharing your thoughts on this. Your assumptions were correct, and the 12 and 20 degree back angles are in line with what I am hearing from Ferrea.

And yes it's the back angle i.e. shape of the valve head I'm asking about, not a back cut near the sealing surface.

[trond]

curious about this and how important it is ? Will it make a significant difference to how the engine drives or not. I just got my heads back and they said not to worry about the valve angles. That got me worried

[stownsen914]

@trond were they talking to you about back angle, or perhaps back cut on the valve or something else? Back angle is something that is usually inherent to the valve design and not so much a procedure that is done to your heads.