Head Port size 2.7
 

I'm planning an engine build for a 2.7 and was wondering about the heads on my current set up 2.7 CIS.

I'm going to put 40mm webers (JE 9.5:1 GE60cams etc..), the port size for the CIS is 32mm.

Is this size going to be too resrictive do I need to go larger if so what are my options?

Matty74

Matty,
From Bruce Andersons "Porsche Performance Handbook", the '75 911 ports are 32mm.
The '75/77 911S heads are 35mm.
The '73/75 RS uses a 36mm intake port with a 35mm exhaust.
Based upon this info I would open 'em up.

Yes. The 32 mm intake ports will choke off the engine as the rev's climb past 5500 RPM with the HP maxing out at around 160 HP. GE60's are a pretty rev happy cam (peak torque around 5500 RPM and peak HP above 6800 RPM), so in absolute terms I'd recommend opening the ports out 37 to 39 mms depending on how much of the car's performance you want in the upper rev range versus low to mid-range. If the car is going to be primarily a street car, I'd use 37 mm's, if the car is going to be primarily a track car, I'd open the ports out to 39 mm's.

Given the fact that you're using Weber 40's though, there really isn't a lot to gain by making the ports bigger then the venturi in the carbs, 36 mm's will most likely work best since I suspect that you'll wind up losing quite a bit of low end flexibilty if you go with larger venturi then that. I almost wonder if you may not wind up a little over-cammed with the GE60's. GE40's might give you better performance in the 3000- 6000 RPM range, at the expense of the 6000-6500 RPM (before the venturi choke off the motor) where I would expect the GE60 will be stronger.

I've got the setup that you may want, 2.7 1976 911s with j&E 9.5:1
with weber 40s and a dc40 cam from John Dougerty. Great combo, from 3000 on up it pulls real strong. Very pleased.

Denny,

76 911s you have the 36mm ports I'm either going to have to ourchase new heads or machine mine as they are 32mm

I would myself open up the ports like Doug said, you still have the same size valves and a recon still has to be done

Any idea what this type of machining would cost?

One of our racers opened up his 34mm ports to 39mm on his SC motor. He did it himself with an air powered grinder. It was like a big dremmel. I thought it was a rather crude way to do it but it worked like a champ. He has at least as much power as the rest of us. He used a socket with an outside diameter of 39mm as a measuring device. Lots of aluminum shavings all over the work area.

-Andy

Yeah I think I might pay for a pro to do it any one in Sydney Australia know where I can get this done?

I have a 2.8L that was bored out from a 2.7L. I have 38mm intake with 46 Webers on it. the pistons have been bored out and cams replaced with high performance ones. Didnt get to document the valves, cams, and pistons last time I tore the engine apart. Also running a different distributor than stock.

A general rule for ports is 80% of the valve diameter

Regards