Running larger diameter intake runner vs head port size?

[timmay5]

So I have a 74’ base 911 that has a few faulty CIS components. Specifically the WUR (leaks, control pressure is off), fuel distributor (system pressure is low), and found a small crack in the airbox.

My 5 year plan with the engine was to rebuild as an RS clone and convert to carbs.

My conundrum now is I really don’t want to sink a bunch of money into the CIS system that I eventually was going to replace. I was considering just converting it to carbs now.

The issue I’m running into is it appears the base 74’ 911 came with 32mm intake ports. The PMO intakes come in 32mm but specify they are not for CIS cars. So the question is, can I for the time being run the 36 mm intakes on my 32mm heads?

I’m aware this is not ideal and that likely no performance will be gained over the CIS. I’m looking at this as a stop gap measure between now and when I rebuild the engine. I am really just concerned if it will run ok or cause any issues?

[Henry Schmidt]

Of course you can run that set up but if it was something I was doing, I would make a small spacer block with a taper from 36 to 32 that covered the CIS injection notch.
A one inch aluminum spacer should work well.
Raising the manifold one inch will have no effect what so ever, you might even call then "tall" manifolds.
Of course you would have to source some longer manifold bolts.
Belmetrics will have those.

https://belmetric.com/

[timmay5]

Originally Posted by Henry Schmidt

Of course you can run that set up but if it was something I was doing, I would make a small spacer block with a taper from 36 to 32 that covered the CIS injection notch.
A one inch aluminum spacer should work well.

Yes , I was thinking of doing the same but using the phenolic spacers from PK.

[Henry Schmidt]

Those would work but not nearly as well as a gradual transition with a 1" spacer.
If you make the spacers out of aluminum you can open them up when you get around to porting the heads. This plan allows you to get whatever benefit a longer runner might offer you. Depending on cam timing, the distance between the intake valve and throttle butterfly can effect engine performance.
Also keep in mind that "there is nothing more permanent than a temporary solution."

[timmay5]

Originally Posted by Henry Schmidt

Those would work but not nearly as well as a gradual transition with a 1" spacer.
If you make the spacers out of aluminum you can open them up when you get around to porting the heads. This plan allows you to get whatever benefit a longer runner might offer you. Depending on cam timing, the distance between the intake valve and throttle butterfly can effect engine performance.
Also keep in mind that "there is nothing more permanent than a temporary solution."

Yes, in my mind I was going to find a set of 32mm spacers and taper up to 36mm at the intake runners. But after some searching, I do not believe they exist for the CIS heads. So it looks like I will be doing as you recommended.

[Henry Schmidt]

Originally Posted by timmay5

Yes, in my mind I was going to find a set of 32mm spacers and taper up to 36mm at the intake runners. But after some searching, I do not believe they exist for the CIS heads. So it looks like I will be doing as you recommended.

We could fabricate a set for you "no matter how much it costs".