What size Webers to use 40 or 46s
 

for a 79 911SC CIS, short stroke 3.2 (9.3:1), 964 cam profile, SSI, + 2 in 2 out M&K muffler, and light weight flywheel.

I was at a get together at the local brewery the other day and one of the DE guys told me I had to go Webers. He was running a stock 78 SC and had 40 Webers. Since I am a 3.2 should I be going 46s? I do autocross and hope to do some DEs once the car is fully sorted. 40s seem to be more available can they be made to support my engine?

Thanks
dog

I would do a tbitz EFI kit instead. similar throttle response, equal power and better gas mileage.

In general, 40s should work just fine. They will have better drive-ablilty off-idle and more torque at low rpms. 46s will provide higher rpm power as they flow better.

40

My understanding is that PMO solved Webers low rpm drivability problem with their revised 46s. If you go with used webers, then 40s are the way to go. Jamies TBitz is is a great recmmendation.

Jamie, when are we going to get your 3.2 ss on the dyno?

Not and expert but 40's are on the edge as you could make up to 270fwhp with such a build.

Not sure I would use 964 cams thought on a 3.2SS that can hold up to higher rpms.

Such a build would love a much more aggressive cam.

Still a great motor. If low end is your total goal, keep the ports at about 34-36 and run the 40's. If buying PMO's call them and let the real experts help you.

I think port size has a lot to do with powerband, my 46mm Webers are mated to 38mm ports and running (old style) S cams on my 3.2SS.

Consider 42's from Performance Oriented.

I met a very good SCCA 914-6 2.8 single plug racer.

Everyone else was running huge ports up to 40mm +/-.

He started with small port heads and only opened them up to 35mm. He felt it gave him an advantage out of the corners. He still made as much HP as anyone. He ran 46's and made over 280hp.

Quality over quantity. Cam duration probably has something to do with it.

Not an expert.

Thats REALLY a very good alternative,.....:) :)

Steve, is this conversion available on a set of PMOs?

I have 40s sitting atop a 3.0 SC and am getting ready for an increase to 98mm with some mod s cams and port work.

Go with the 46's you have the large port intakes. You will flow better with the 46's.

I'm not aware of anyone performing that modification to the PMO's. IMHO, your 3.2 with mod-S cams needs 46's if you wish to have power to 7K.

Joe, not trying to hi jack but if you decide to sell the 40's let me know.

I'm set up with Paul Abbott (Performance Oriented) to get my 42s, seems to be a real nice guy who does some real nice work.

now for the configuration, venturis 36 or 38 ?
I'm not looking to make a screamer out of this motor, I'd like more a mid - range engine with some torque. I do PCA autocross and hope to do an occasional DE.

The engine is currently 79 911SC CIS, short stroke 3.2 (9.3:1), 964 cam profile, SSI, + 2 in 2 out M&K muffler, and light weight flywheel.

Looking forward to any input anyone has.
dog

36's for torque and throttle response,....:)

wow, i was doing some searches and found this thread. very similar to what i am thinking about building. except i have the smaller intake port heads from an 81 SC. not looking to hijack, but i'm wondering what i could expect using the 81 heads as opposed to the larger port heads from the earlier 3 liter motors, or should i consider getting different heads. the performance handbook (3rd ed. at pg 174) shows a dyno run on a build like this (and it looks pretty good), but it did not seem clear (unless i'm missing something) whether the small or large intake port heads were used for that motor. what sort of difference would it make. i would expect the top end to be lower, but would it still pull well at least to 6500?

-Winterfarm

The small-port heads work just fine with stock or 964 cams. If you change pistons to allow more aggressive camshafts, I'd recommend the large port versions, or have yours CNC'ed to the early specs so they flow properly.

ive got an email address and phone number for a guy who's selling both 40's and 46mm webers. if anyone wants his name and numbers, send me a PM.