Race Engine Forum UK

Hello,
I was wondering if there is a rule of thumb regarding the optimum valve shape for a given engine. In other words what determines the shape of the valve? the combustionchamber, or the valve angle vs the port?
For example the penny on a stick valve seems to be the best choice for high performance A series engines. For TC engines the tullip is favoured. Both engines a rather diffenent engines. My engine is a Fulvia engine and the original valves are more flat than tullipshaped. The combustionchamber is hemispherical like the TC. Pistons are domed.

Guy will no doubt reply, but I will say his DVD gives some excellent examples of exactly what you are asking. Put in such a way that even I could understand what was being discuseed. !!!

Graham

On a head with little or no port horizontal offset from the cylinder centreline the main things that determine it (in my exp) appear to be:

1. Port downdraft angle - the angle of the port axis relative to the head face
2. The valve angle relative to the port axis.
3. The distance of the port floor from the valve throat and the shape of the short side radius.
4. The valve seat angles used.

The shape of the port, whether it's 'straight' or curved, whether there is a pronounced offset of the port axis (main chord axis) relative to the valve will have an influence but not so significant. The exit flow at low lift (up to about 4mm) can be heavily influenced by the angle just behind the 45 deg contact face and the top cut on the valve seat region and surrounding chamber area.

The combustion chamber shape per-se (leaving aside the issue of valve shrouding) has no bearing on the choice of valve design.

I have proved over and over there are no hard and fast rules - except to say that well-downdrafted heads seem to favour tulip type valves and downdrafted multi-valve heads are far less sensitive to seat shape than those with two-valves-pre-cylinder. On sidedraft heads like yours (and the TC Fiat behaves the same) I find that variations in valve head form (obviously on the non-firing side) will yield better low-lift flow or better high lift according to the form, and choosing the best shape is definitely a compromise and the decision must be made on the basis of detailed flowrig testing and take into account the actual cam lift.

Many examples can be found in 'cylinder head prep' my Virtual Workshop. I have not done any experiments on your type head yet though I have one to do this Spring; I imagine it will need something similar to the TC Fiat, a 12 deg head angle and sharp valve seat angles.

You should get the dvd, and yes, that's the first time I've said that!

GC