Porting. How much is too much?

[jfjfjf2]

I have some after-market RV8 cylinder heads, good casting, to go on a high revving engine targetting 400+ bhp. Could someone tell me the minimum amount of material that should be left behind the valve seats between the inlet and exhaust ports?
Thanks, Julian

[jfjfjf2]

Thanks for reply, I didn't explain myself very well. I'm talking about the wall thickness between the inlet and exhaust runners, in the area between the valve and the valve guide. The wall thickness on the head I have measured varies from 3mm above one chamber to .6mm above one of the others at a point about 15 mm back from the valve seat. I'm suggesting that this has created an unacceptable area of weakness. I believe we could get away with 2-3mm since the casting is quite dense and strong but that .6mm is asking for trouble. The engine has a rally cam profile that is not gentle on the valve seats, with repeated heat cycles and alternating forces of intake and exhaust pressure on the wall I'm concerned about durability. In addition I'm worried about the consequences of the shock wave of the valve closing on the seat when perhaps 10% of the valve seat is not as well supported/braced as the rest.
Your thoughts please, the person who sub contracted the porting seems cross with me for suggesting his porter could have made any error or that I have anything to worry about. An experienced mechanic familiar with these engines thinks I have a point.

Regards,
Julian

[jfjfjf2]

Peter - In order to avoid any confusion over the measurements I sent the head to an automotive engineering firm. I was told by the person who had them ported that they must have measured them incorrectly. By touch they feel accurate but I shall jury rig some cheap digital calipers with some epoxy glue, two strips of metal to extend them and two at right angles pointing inwards to get around the valve seats. That should settle it.
Interesting to hear about the undercut valve seats, that's just what I feared. On an engine dyno the motor suffered a broken valve spring, exhaust valve, piston and considerable damage to the head which has been declared irreparable. Between the valves was broken away. I've had to buy a new head. I doubt I will ever pin down what caused the failure, it could after all be a faulty spring (all new) which started it. It has dual springs, just the outer spring was broken when it was on what was described as a gentle power run to about 6300 rpm. I'm pleased to hear that the scenario I suggested was not so far fetched or without precedent as I was told, but still fed up with being taken for a fool.
I have a horrible feeling the other head has been screwed as well.

Julian

Edited by jfjfjf2 on Monday 16th March 22:00

[jfjfjf2]

On balance I'm inclined to think it is spring failure, though points a&b do not apply in this case. I happened to notice the wall thickness in the area where the inlet and exhaust runners branch off. The engineers have measured them respectively as 4mm, 2mm, .0075" & .0075".
I'm more concerned with anticipating future failure than with apportioning blame for this one.

Julian