PMO 46mm V PMO 50 mm on 3.4 Race motor?

[slow car]

What is your experience of running 50mm(45 chokes) PMOs V 46mm(42 chokes) PMOs on a 3.4. Dyno Results ? Comparisons
I have a pair of both.
The set up I am running is ( race use only car 2200lb loaded) NEW BUILD
12.5:1 JE (98mm ie 3.4)
Twin plugged MSD
Gapless rings
3.2 Heads(match ported)
GE80 Cams
1,3/4 headers- straight pipes.

What will be the pros and cons .
Thank for your help

[roblav]

Check out your handy dandy Weber Manual illustration to figure out venturi size given cc/cylinder and RPM at max HP. Or use the equation. Or ask Paul Abbot or the PMO guy.

[stownsen914]

I don't have a dyno sheet to share, but a 3.4L track motor with specs like yours begs for the 50's.

[Bigtoe32067]

50’s for sure. 46’s would go on a 3.0l race motor.

[MBruns]

In my testing over the years and my opinion for a race 3.0 and larger you will make more power with 50mm & 45 vent. hands down, for a DE car and part time street hot rod its still a toss up on a 3.4 as to whether you would be better off with 50's over 46's jetted properly.
Mike Bruns

[slow car]

Thank For all the feed back.
I am dynoing the car with the 46s and 1/58 headers then change them to the 50s.and 1 3/4 headers
Will be interesting to see the difference on the dyno

[MBruns]

PM the jetting for both if you like and I will see if I can save you some time, what do you have for exhaust ?
Mike Bruns

[slow car]

For today's test 1 5/8" headers into straight through muffler
I will post results.

[icarp]

Please post the Dyno data , thanks

[silver911rdb]

I'll be very interested in this as well. I have a 3.4L with 50's and started out with 12.5:1 compression. The heads have gotten reworked with bigger valves. It runs a high lift cam, dual plug, MSD ignition system. I've always had really poor driveability at low RPM. It doesn't really clear up until I get into the 3500 rpm plus RPM range. I've rebuilt the carbs, replaced all the ignition bits, had it dyno tuned and adjusted. It's always stumbled really badly while accelerating from a standstill until I get to the higher RPM's. I recently rebuilt the engine and brought the compression down a bit for longevity. I have to say with this latest rebuild it's run better than I've ever felt it go before. But the low rpm problem persists. I really am thinking of either trying to go down to a 46 or maybe an EFI system. Please keep us posted on this thread as I'm really interested in your results.

[stownsen914]

Silver911 - It sounds like a tuning challenge. I have a 3.5 race motor with similar specs including a full race cam and PMO 50s, and it runs surprisingly smoothly straight from idle.

[john@poolcovers]

As i am running 105 Octane fuel with a Stoichiometric A/F of 13.5:1 i have needed to go to 2.5 mm mains from 1.8mm on pump 14.7:1 non ethanol fuel.46 PMOs
i will be running the 50 mm PMO,s and 1 3/4 headers up on the dyno soon.
SlowCar