Ported "small port" SC heads
 

The motor build is getting close as I got my heads back yesterday. The parts started out as small port SC heads that were twin plugged by Craig and then ported by my cousin here in Michigan. His shop normally works on American muscle, but has done head work on all my rally cars and my 944S2 track car... In the end we opened the heads to 40mm on the intake and 38mm on the exhaust...

In the end the heads flowed about 240 CFM @ .500" lift on the intake side... They also spent a bunch of time getting the velocity to where their calculations suggested which was around 300 FPS...

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Nice work! I have small port SC heads too.. I have convinced myself I should open mine up to 37mm or so to suit my higher compression 3.2 short stroke rebuild, but I don't have a lot of good math to back up that decision.. just basing it on dyno charts I have looked at and some very rough calculations of velocity based on info I have read on the site. I want to run a 993SS cam and I think the 34mm ports would probably shut things down a little prematurely... any advice on how you arrived at your 40mm figure? I recall your engine is oriented for rally racing.

How did it go opening the intakes? Just a plunge with a boring bar to the start of the valve guide area and then a little blending? Am I correct in thinking that the inside diameter actually opens up once you get past the face of the port? I don't have a good inside mic to get down in there and measure that theory but I think that is the case and I would be curious if you could verify. And any guidance on what I should expect to pay a machine shop to do this?

Hi,

Just another take on this,....

While small-port heads are limiting factors on a 3.2SS engine, simply opening them up with a milling machine or by hand doesn't assure the results you desire. In fact, its quite easy to ruin a good set of heads without doing this with a Flow Bench and a LOT of experience. Port velocity is easily compromised by simply making things larger. The throat, guide boss, & bowl must be carefully shaped to increase flow as well as maintain velocity and to prevent reversion issues.

Large-port SC heads work quite nicely with an engine configuration like yours and will easily support up to 300BHP (Carrera heads do even better, but cost more).

We've developed a CNC program to convert small-port SC heads into the big-port ones, as part of our PRC race engine development efforts that has worked very well. In this fashion, each & every head comes out exactly the same to ensure each cylinder makes the same BMEP.

Hope this helps a little and you can contact me directly if you have further questions.