Weber 40s or 46s

[Kremer#2]

Should I use weber 40s or 46s 0n my 82 911SC? Am I correct in thinking that my 82 has the small intake ports? 30mm???

[haycait911]

if US spec ports are 34mm. use weber 40's.

[Dodge Man]

In Polytechnic we built 175cc racers and our team had a slightly smaller carb with some racing mods. Our racer was way faster out of the corners than others running the larger ID carbs. It was about usable power rather than peak HP. It is probably worth some research on running the 40's with the US "high velocity" heads. Carbs tend to flow a lot better CFM than CIS anyway. IMHO Bigger may not be necessarily better in your situation. Back in the day, big (over) carburation was good conversation at the Drive-In Burger Stand on Saturday night. Best of Luck with tuning the carbs. You should be very happy with well tuned carbs.

[haycait911]

my 3.0 has 40 webers with 34mm venturis into 34mm ports. and dynoed 250hp and a very fat torque curve.

as dodgeman said. bigger is not better. unless you're camming for hp after 7k, stay small.

my dim understanding is that for a fixed displacement a smaller diameter intake runner gives higher intake velocity which is better for torque. FWIW my 3.4 on the stand is getting 40 webers with 36mm venturis into 36mm ports.

3.0 dyno day - a pleasant surprise

[TT Oversteer]

I had a 78SC with the big port heads and 46mm PMO's. That car ripped!

[al lkosmal]

Originally Posted by haycait911

my 3.0 has 40 webers with 34mm venturis into 34mm ports. and dynoed 250hp and a very fat torque curve.

as dodgeman said. bigger is not better. unless you're camming for hp after 7k, stay small.

my dim understanding is that for a fixed displacement a smaller diameter intake runner gives higher intake velocity which is better for torque. FWIW my 3.4 on the stand is getting 40 webers with 36mm venturis into 36mm ports.

3.0 dyno day - a pleasant surprise

+1....torque is where the fun is at for street/daily driving.

regards,
al

[sundevil64]

Can 40's be opened to 42 or a little more for a 3.2? or can 46's be choked down?

[1QuickS]

Yes, 40's are easily bored to 42mm but larger bores require extra work since the wall where the progression holes are located is lost during the machine work. The wall must be plugged and after the boring is completed then the progression holes relocated.

If non-standard bores are selected then the taper of the main venturis must also be machined from the waist diameter of the venturi to the new, larger exit diameter of the throttle bore.

Sleeving down from 46 can be done but better to sell the 46's and get the 40's to open up.

I provide these services: Performance Oriented Services

[Dodge Man]

Originally Posted by TT Oversteer

I had a 78SC with the big port heads and 46mm PMO's. That car ripped!

"Big intake ports" & big carbs is usually a good match with proper tuning. It is all about matching the components to work in "harmony" to accomplish your objective. I run factory stock big port 83 Euro CIS 3.0 with SSI and it is nice for spirited weekend drives. It is way different from my old 80 US 3.0. Back in the day, my college buddy beat a built 454 Chevy that shook the ground with a rather mild Ford 351-C at the strip because the 351-C components & gears were very well matched. IMHO. Assembly the motor components & gears to work in harmony to accomplish your objective. Matching flow ID/transitions is an old established practice. Look up displacement to carb ID charts with footnotes about RPM, Head Configuration, CAM Profile, etc.. guidelines. M2CW It is your $$$$ so make it count.

[sundevil64]

Originally Posted by 1QuickS

Yes, 40's are easily bored to 42mm but larger bores require extra work since the wall where the progression holes are located is lost during the machine work. The wall must be plugged and after the boring is completed then the progression holes relocated.

If non-standard bores are selected then the taper of the main venturis must also be machined from the waist diameter of the venturi to the new, larger exit diameter of the throttle bore.

Sleeving down from 46 can be done but better to sell the 46's and get the 40's to open up.

I provide these services: Performance Oriented Services

Thanks Paul. I just bookmarked your site.

John