The V8 Owners Forum -

I'm contemplating installing 1.63/1.4" valves in my stage 2 heads and just pondering what the correct throat diameter would be.

I've seen 90% of valve size banded about a bit, but also that this 90% is far too large and it should be around 80 to 88% (all this info was not RV8 specific though).

Currently i'm running the stock 39.9/34.4mm valves and the throats are 36.5mm and 31.2mm.


Cheers
Ben

Ben,

The best results are from opening the throat to valve seat size and blending into the bowl, bullet the ends of the valve guides but leave the port runners standard size and only remove excess casting flash and this will maintain gas speed and avoid seperation of fuel air at lower RPMs.

You can polish the combustion chambers if you want to but never polish inlet ports.

If it's a 3.5 then these mods even with standard ( waisted stem and 3 angle valve job) valves and standard cam will yield 30+ BHP, for larger engines 3.9 - 4.6 + then bigger valves will improve flow.

For a road engine with a fast road cam look for around 60 BHP per litre to maintain good drivability.

kevin