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

[Stan Weiss]

The port CSA should be sized based on a number of engine parameter which would include engine size, rpm, VE. CFM shown on a flow bench can be misleading if the port is no also probed to check and see that it also has the correct velocity thought out. If the CSA is correct but the shape is not right then this can cause some areas of very high or low velocity.

Stan

[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

[PeterBurgess]

How are you measuring the wall thickness to be 0.6mm?

We have seen valve seat inserts collapse into the ports on SD1 derivative V8 heads when they are undercut dramatically.

Peter

[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

[PeterBurgess]

The engine that had the faulty heads dropped power on every run on our rollers, v low compression on one cylinder and not very even on the others. On inspection the inlet valve seat had recessed in head and valve throat/bowl in danger of imminent compressive failure. All other inlet throats exhibited a bulge/bulges at the junction of hard insert and soft aluminium which could only have been caused by the insert compressing the too thin ally below the insert and bulging into the valve throat. The problem is lack of support immediately under the insert not necessarily beyond the limits of the insert (at right angles to the insert).
On the track the problem we encountered would probably have progressed to kill the engine.

Peter

[Pumaracing]

I'm unable to envisage from the description which port measurement you're referring to here. However it sounds more as though you have a valve spring problem.

Valve springs can fail due to either:

A) Not being used with correctly fitting caps or spring seats. A common problem I used to see from the idiot fraternity was using double springs on stock caps so the inner spring was just wedged on the cone of the cap and eventually burst apart.

B) Spring going coil bound due to insufficient fitted length for the cam lift. There should be a minimum of 1mm free travel on any spring at full lift.

C) Poor heat treatment. In this case the spring will usually lose some of its free length very rapidly, that drastically reduces the fitted load, the valve starts to bounce at high rpm and eventually the spring breaks.

[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

[Pumaracing]

Check the spring free lengths vs nominal spec and report back to us.