911 T & S Head Volume Question
 

I have a set of 73.5 T CIS heads that I want to convert to 73 S specs to run with PMO carbs.

My limited research indicates that:

T Valves are 46mm Intake/40mm Exhaust
T Ports are 30mm Intake/33mm Exhaust

S Valves are 46mm Intake/40mm Exhaust
S Ports are 36mm Intake/35mm Exhaust

I would obviously need the T heads ported to S specs and have the CIS port blocked. I will be running stock OE Mahle 2.4 S pistons (84P11+ (oversize group 1)) with a stock 73/74 2.4 crank and rods. Is the volume of the combustion chamber the same between the T and S heads?

Thanks for your help.

Greg

Hi Greg,

Basic chamber volumes are the same between all of the 2.2-2.4 heads. 2.7 ones are slightly larger. I would strongly recommend measuring yours so you know precisely what your CR will be as thats important with today's gasolines.

Its possible to have your heads CNC'ed to match "S" heads, but its not cheap to do. Plugging the CIS injector holes is easy. :)

Hi Steve,

Thanks for the feedback. I'm a newbie to this so it's nice to get professional feedback. I'd hate to go down this road and have someone tell me it can't be done. I will call you sometime and discuss my options.

Greg

Send them to Steve. I've got the exact same heads and very similar plans. He's getting mine next week.

Hi Matt. What cams are you going to use? E, S, combo E/S, other?

After talking to Steve yesterday I think I’ve settled on Mod S versus Mod solex. My engine is a little different because it’s going to be a 2.5 short stroke. But the heads will be near identical to yours.

Thanks for the info, Matt. It's greatly appreciated. I bought my small spigot 7R case from Tom Butler and he suggested going the 2.5 route also but then I found the 2.4 S pistons which may change my direction a bit. He suggested a grind somewhere between E and S. Not sure if that's still applicable. Your short stroke 2.5 will be a hoot to drive.

Thanks again,
Greg

That would be my choice as well. My 2,5ss uses E cams, due to the old PCA class restrictions. More cam would be ideal. Depending on your motors configuration and intended use the S cams could also be a consideration.

Single plug or twin?

Yes.

I’m having Steve prep the heads and lower covers for it. But on the first passby I will set it up single plug. I am undecided on injection versus carbs. So I bought both and will try them both out. I’ve built a couple dozen Subaru engines but I’ve never done a 911 one. I am intending for this to be a fun process. My car is my toy.

73.5 CIS heads easily accept EFI injectors. Get the long tip DEKA common BMW 3-, and 5- series applications. They will fit nicely into the intake port. Then run MFI stacks for an ITB EFI setup.

Not interested. I'm building a period correct look into my car.