Intake manifold from 66-73 911

[neilca]

I have a crack in one of my intake manifolds. Are these manifolds made of aluminum or magnesium?

[Flieger]

From 1969 to 1971 the MFI stacks and throttle bodies were Magnesium. In 1972, with the 2.4 liter engines, the stacks alone changed to plastic, with integrated cold start injectors. They cannot be bored out as much as the Magnesium stacks, nor do they lend themselves to modification for Webber style air cleaners.

[neilca]

My intakes have carbs on top of them.

[Flat6pac]

Manifolds are magnesium for the carbs, I have at least one..
Bruce

[Grady Clay]

neilca,

What year/model is your 911?
’66-’73 is too wide range to give you a proper answer.

Best,
Grady

[neilca]

Grady,

I don't know exactly. I bought them some time ago with some Zenith's. I can weld magnesium, it just isn't as easy as aluminum.

[Henry Schmidt]

I believe all of the early 911 manifolds that came on street cars were magnesium.

If your manifolds came attached to a set a Zenith cards they are probably original equipment 70-71 manifolds and are inappropriate for use on most engines.
The runner on both ends are badly restrictive and cause inconsistent flow across the heads. Porting is possible but not very cost effective.
Middle runner

End runners

[Bob Ashlock]

So after reading Henry Schmidt's post, what is the alternative? For example, I've been playing with my 70 911T engine that has 40 Webers sitting on those stock magnesium manifolds that I assume originally had Zeniths on them. Do those skinny ports contribute significantly to the low-performance of the T engine and if so, given no other changes (cams) would porting them be of any benefit? A few extra horsepower here and there is always nice, especially if it comes at a relatively low cost!

[Henry Schmidt]

PMO makes a nice alternative to the stock Porsche manifolds.

I don't see how you gain much by porting or new PMOs if your running a stock 2.2 T engine.
If you were running higher compression, more aggressive cams and larger ports in the head then changing the manifolds would seem imperative. Without these changes I would guess you would see little or no difference.

[Bob Ashlock]

Thanks Henry, yes that is what I suspected. In my deep dark past I used to play around with Chevy V8's and found the same thing ... each item by itself (larger valves, bigger ports, hotter cam, bigger carburetor, headers, etc.) didn't seem to make much difference. But when they all play together, then you see the benefits.