Originally posted by QWICSVT:
As far as the "ports" go in the heads. We are using 2.5L SVT heads for the higher-comp ratio. I do not deny that 3L heads have larger ports than a 2.5L head but they do not have the compression ratio that we desired. My comment was directed at installing 3L valves into 2.5L heads.
As to the engine management question...that solution will be posted shortly.
Thanks for the interest gents!
QWIK,
I'm in complete disagreement about the size of the valves seats and the ports. I have measured almost everything you can think of and that is an area that I paid special attention to. The valve seat inside diameter is the important part of the measurements. The 3L heads use the SAME valve seat as the 2.5L heads...Same outside diameter, but the inside is cut out more and they are over 1mm LARGER on the inside diameter. Not to mention that the intake port/runner in the head is wider overall by several milimeters.
If you look closely at the 3L heads and the 2.5L heads side by side, you can tell that they use the same geometry regarding valve placement, and that they use the same basic valve seat. As I already said, the 3L is cut wider on the inside to compensate for the larger port. The valve seat is actually thinner than the 2.5L because of it.
Doing a simple calculation will show that even a 1-1.5 mm increase in that diameter results in about 6 - 10 % increase in cross sectional area. THAT is a significant improvement.
A good port/polish up to the seat will help, but the seat is still the restrictor.
When I did mine, I had widened the whole intake port/runner in the heads including the 'bowl' under the valve seat. I decreased the angle that the air would have to flow into the chamber past the seat as well. THEN I took the heads to the shop and had them re-cut the seats to fit the 3L valves. I also told them what inside diameter that I desired the seats to be and they cut them to that new diameter. THEN I took them home again and matched the intake passage to the newly cut seats and polished everything.
All said and done I put a lot of time into this step and it was definitely worth it.
warmonger
