Choking down 46 PMOs

[Number1_dog]

I have a 1979 short stroke 3.2 with 9.3 compression, 964 cam grind and SSIs.
With the stock CIS it used to be a real torque beast. I put on 46 PMOs with 36 Venturi’s
I lost the low end torque but it revs great. I was thinking if I could get some of the low end torque back if I choked the carb down a little. My local PMO expert said I should have put on 40mm PMOs. The smallest 46 Venturi PMO makes is the 36.
Question do Weber 46 IDA 34 (33, 32) Venturi fit PMOs, can a talented machinest make new venturi’s, how about 3D printing.

[targa72e]

Hi,
I cant answer your venturi question but have a couple of thoughts for you.
Do you know you lost torque from dyno testing or you think you lost torque because of how the car feels?
If you do not have back to back dyno comparisons I can say from experience that a engine with more power up top will "feel" like it has less torque than a engine with less top end. This is quite evident with early cars where the T engines are descripted as torquey and S engines are described as peaky even though the make very similar torque below 4000RPM.

The exageration about peaky S engines.

That said I don't believe changing the venturi will make a significant difference in the low end torque. A smaller venturi will give the carb a better vacuum signal that can help with fuel metering and drivability.
Port size and length, intake runner size and length and cam choice and compression, ignition timing all effect where the engine makes peak torque. If the intake runner is too larger and favors higher rpm power putting a restriction (smaller venturi) in the intake will not make the low end better (unless it helps a bad fueling problem) but may hurt the top end.
Before and after dyno results will be the best way to see if you are making progress. butt dyno often wrong.

john

[Number1_dog]

I Autocross the car, and before the carbs with the CIS, when taking off the car would squat and the rear would go down. It doesn’t do that now.

[Flat Six]

I read here a few years back about someone with (I believe) the same setup, 3.2SS. Bought 40mm PMOs and had the bores & butterflies bored to either 42mm or 44mm. Don't recall if these were PMO carbs or their ITB setup though. Might be worth considering; I'd guess you might be able to get decent $$ for very lightly used 46 PMOs.

Good luck

[snbush67]

I’ve got a similar motor and the torque is greatly improved over CIS.

3.2 95mm Cylinders : Bored to 98mm (3.8582")from EBS
98mm J&E Pistons/pins/clips : Compression Ratio 9.5:1: Dome Volume 35.2cc
Gotze Rings : Top, 2nd and Oil : End gaps: .0015mm
GT2-108 Dougherty Racing Cams : Timing set at 3.2mm
High Lift Racing Springs and Retainers : Spring Height : 39.5mm, 110 lbs

PMO Carburetors and Manifolds : 46mm
Venturies: 38
Main: 160
Air: 190
Idle: 55
Idle Air: 130
Emul: F11

Custom headers and M&K 2/2.

[kent olsen]

My 40 mm webers were opened up to 42 mm by Paul Abbott many years ago when he rebuilt them. 3.0L twin plug with special cam with lower torque peak. 245 hp @ 6800 rpm 225 ft/lbs @ 4500 rpm at the crank.

[Number1_dog]

Gentlemen, my engine is milder than what you guys are describing. My pistons are max moritz, so they limit the cam I can use. The heads are stock SC and the SC cam was reground to 964 specks. I assume I am putting out 210 hp, up from the stock 180.
I thought about getting new 40 PMOs and selling my 46’s, but I felt I could try choking it down a little first for less $

[snbush67]

Your first post leads me to believe that you ran the motor as a 3.2 SS with CIS, and then changed to the 46 PMO carburetors and lost low end torque. Did I get that right?

[Number1_dog]

Yes. My local PMO expert tried to explain the problem indicating the Venturi effect on the CIS being smaller than the 45 PMO had greater velocity? I know I am not stating what he said, but it was akin to not having some back pressure from your exhaust system when you have it too large.

He has a 2.7 in his 1974 911 with 10.5 to one compression and wild cam and he runs 40 Webers he thinks the 40 PMOs would be more streetable. I am not running on the track so I don’t need high speed power. I Autocross.

[Eagledriver]

Smaller venturies will not help with your low end torque. What will help is taller intake manifolds. A one inch longer intake path will move your torque curve 500 rpm or so lower. Many race cars with carbs have spacers below the manifolds to accomplish this.

[stlrj]

You answered your own question: the CIS was a torque beast! What part do you not understand?

[Number1_dog]

Quicker throttle response and extra Hp!

[snbush67]

My car has phenolic spacers between the manifolds and the intake ports, I think they are 1.5 cm, plus two gaskets.