Race Engine Forum UK

and valve sizes?

Just measured. Took the measurment of the cap overall and the diameter where the cut ends. (don't know how to call it)
Standard valves intake 27,6/32,1, exhaust 24/28,1
Bigger ones that I have Intake 30/33,4, exhaust 25,2/28,5

The standard valve size is a bit modest.

I would go maybe 33.4mm inlet, 28.5mm. That gives a good valve-valve head area ratio (73% ex/in). You need to make a careful assessment of the space for the new seat inserts. Cannot remember if the standard ones will accept either of those bigger valves but you must have the insert at least 1mm bigger on diameter than the valve).



G

I know that even bigger valves are being fited without changind the seats, but I will measure the inserts too.
By the way is it OK to use exhaust valves instead of the inlet valves? As the ones that I'm about to fit were made of some renault exhaust valves.

Swapping inlet/ex valves: the problem may be the head shape is not right, remember that the medium is flowing the 'wrong' way.

G

I looked at the shape it's identical to my original inlet valves. Maybe the seller forgot what they were made of.
P.S. what is the medium?

Medium is the mixture of air and gasoline.

Hello Guy, now I'm studying the information you gave me. Please confirm if I understood correct how to cut the seat. Here is a drawing 15 degrees 45 where the valve contacts (1,5 mm wide) and 75 degrees.

Yes that is right. On the inlet side the throat needs to be blended to get rid of the sharp edge where the 75 deg meets the parallel lower section. The top cut/angle is not critical but it is important to blend the chamber which you should do before the seat work. You can superimpose any top angle on the insert and if you get it right it will 'marry' nicely ot the chamber relief work. The top angle can be 40-35-30deg or less but the shallower it is (ie: less than 30 deg) the more it will cut into your nice new shiny combustion chamber.

For max flow the short radius on the inlet side needs to be enlarged and blended right up to the seat insert like this XE shown. I've sketched it too. The outer port floor should be dead straight and the new SSR will start earlier - some 2cm from the seat insert and give a much more progressive approach to the insert than before. Don't shave the alloy right off - the last little bit will be a smaller radius superimposed to connect to the insert just below the contact face. That in my exp is the only way to get high flow. If you don't modify it you just get turbulence at the turn.

Hope this helps,

G

I'm not sure if I uderstand what is ment by short radius, SSR and outher port floor.

try this.....

You should really try and buy my new book which has ALL of this in it.

And please don't just say 'I can't afford it' - I like to give advice but it doesn't pay my bills..

G

now it is clear :) and I was thinking right the first time. The day will come when I'll buy your book ;) Thanks Guy.

Good evening,
was thinking to modify the manifold I'm about to use. Is it worth the effort?
I want to change the angle (red lines on the drawing is the way I want it to be)

you are quite right - the inlet manifold axis should ideally line up with the port axis. Any variation in angle leads to a flow loss.

G

Guy Croft wrote: On inlet I went for 27.5mm vertical and 26.3mm horizontal thru. ...
G

Some more things to clarify... what do you mean by this?

M.