46 to 34mm manifolds

[Walt Fricke]

I have a set of Weber 46s, and am contemplating putting them on my '82 US SC engine, which is track only, but must retain the stock heads. Does anyone make a 46 to 34 mm intake manifold? I'm not immediately finding one to do this. Normally, 40mm carbs are what is recommended for a 3.0. But I have 46s, and am not asking or interested in some other heads/carbs.

[Eagledriver]

I don’t know if they are available or not. One acceptable way might be to get some thick insulators in 34mm size and taper the hole out to 39mm or whatever manifold you can find. These engines respond well to increased intake runner length so even taller spacers are helpful. Most SP911s with carbs have spacers over an inch tall. The limit is basically what will fit under your deck lid and the ability to still connect a throttle linkage. I know a guy who might be able to make more spacers.

-Andy

[Walt Fricke]

Thanks - Hadn't thought about the throttle linkage. I've seen some impressive HP numbers for SP911s.

My previous experience with carbs was mostly cook book, and included the advice that taller was better, though that just meant purchasing off the shelf taller vs short manifolds, and in a class not constrained by stock type rules. If I had a 78/79 engine with the big ports the manifold would not be an issue, but I don't. Buying a set of big port 3.0 heads would up the conversion cost quite a bit, and also call for the 8.5:1 pistons, and require dropping the engine (doable, but a carb conversion without having to do that is a big plus).

I wonder what the effects on flow would be of tapering from 39mm to 34mm in what, two or three inches? The whole idea here is based on the supposition that carbs will gain you enough HP so that the engine will be roughly as powerful as the late Euro SC motors, if the car can lose about 200 lbs more (current weight is 2702 w/driver, who has lost 30 lbs of himself and plans to keep it off).

[Trackrash]

I would get the tall PMO 40mm manifolds for the 34mm ports AND port them to match the 46 carbs.

[VFR750]

I think PMO will sell you 46-34 manifolds, brand new.

The aluminum castings certainly have enough stock on them for any combination.

[911 SLANT]

+1 on what Trackash said. That what I did with my 46mm PMOs. I bought the tall PMO 46/34 intakes from Richard then match ported to my heads

[Gordo2]

Originally Posted by 911 SLANT

+1 on what Trackash said. That what I did with my 46mm PMOs. I bought the tall PMO 46/34 intakes from Richard then match ported to my heads

Walt,

I'm pretty sure I have a like-new set of PMO 46/34 manifold on the shelf, in my shop.

Came with a set of PMO carbs that I bought, but I needed a set sized for 3.2L intake - so I bought a new set and shelved the ones that came with my 46mm carbs.

If interested - I can measure tomorrow and PM you a price.

[Gordo2]

Found this photo in one of my old threads that appears to show my manifold measuring 36mm at the intake:

PMO 46/36 manifold (???)



And the thread where I had posted the photo:Studs for B&B Exhaust and PMO Manifolds

In the post, Pelican toddu mentions PMO sells 36, 38 and 40's.

Gordo

[Dan J]

Walt,
I would sell those 46's for the big money they'll bring , Buy a set of new 40 PMO's and put the excess $$ in my jeans

[Walt Fricke]

Dan - the thought has occurred to me. I always lamented the spilled gas from changing main jets, which I often did a couple of times per weekend. Weber jets fit the PMOs, don't they?

Would I be giving up any race track HP with 40s? Anderson always urged guys to use 40s on 3.0s, but I don't think he was thinking of a track only car. On the other hand, I'd be keeping the CIS cams and pistons, which maybe would negate any more air passing at 7,000 RPM with the 40s vs. 46s?

[Eagledriver]

I don’t know if the small port heads need 46mm carbs. I do know that the big port heads definitely need the 46 carbs. I can’t quantify the difference.

[Gordo2]

Originally Posted by Walt Fricke

Weber jets fit the PMOs, don't they?

Yes, PMO carbs use Weber IDF idle, main and main air corrector jets as well as e-tubes.

Your other Q regarding carb sizing involves quite a bit of study in carb theory - and considerable experience.

As such, you can take Anderson's (Mr. Parr's or other experts') recommendations - or not.

Going outside of recommended parameters / known configurations (carb, chokes, jet sizing etc. for a 3.0L), will involve considerable trial and error - which is cool if you have the time, interest, patience and money to do so.

My general take (limited theory studies and experience...):
- If your objective is wide open power, with less concern / interest in responsiveness throughout the RPM range - then go with the 46's (i.e. engine envisioned to run at ~4k RPM's and higher).
- If your objective is to maximize power & responsiveness across the 2.5-6.5k RPM range, go with the 40's.

On my 46 PMO's I went from 38mm venturi's to 36's - which really made tuning and power across the RPM range much smoother / easier (no hesitation). Meanwhile, I"m fairly confident I gave up some higher RPM HP #'s doing so.

Gordo's PMO Carb Tuning

Good luck,

[911 SLANT]

Gordo how did you adjust the new hatchets on your 46mm PMOs.

[Gordo2]

Originally Posted by 911 SLANT

Gordo how did you adjust the new hatchets on your 46mm PMOs.

Been a while since I installed - memory is a little fuzzy.

I think installed them with the carbs on the engine - which made access a little difficult.

I recall setting them to a neutral position - pump lever lightly touching inside edge of the hatchet elbow / bend (hard to describe). Adjustment screws (nuts) set to equal positions on left/right banks.

Once basic install was done - I made a few runs over the same stretch of road, taking O2 sensor recordings, and equally adjusting (left/right) the amount of squirt as needed to smooth acceleration (small nut on bolt that reduces or lengthen's the pump travel).

Not too clear, but best I can describe / recall...

[911 SLANT]

I'm having a flat spot from 3100 to 3900 when a mash on throttle. If I slowly press pedal threw 3000 to 4000 no flat spot. Did dropping down to 36mm venturis help you with this?

[Gordo2]

Originally Posted by 911 SLANT

I'm having a flat spot from 3100 to 3900 when a mash on throttle. If I slowly press pedal threw 3000 to 4000 no flat spot. Did dropping down to 36mm venturis help you with this?

Slant - my typing has been impeded recently, just sent you a PM you are welcome to call to discuss.

But... measuring AFR is essential to understanding what's happening - especially in transition. If you don't have a wideband / O2 measuring and recording system, I highly recommend you get one.

[Eagledriver]

The “hatchets “ always supply too much fuel. Tighten the adjustment all the way so you get the minimum squirt. That will eliminate the flat spot.

[911 SLANT]

I thought flat spot threw transition from 3000 to 4000 was not enough fuel?