PMO 40 jetting for 3,2

[Alex911747]

Hi there,

I am rebuilding an engine for my 911 S/T Replica project and need some advice on jetting a pair of PMO 40 carbs I already own.

We have rebuilt many different stock and prepared engines, using all sorts of injections and many with Weber carbs, but we are totally new using PMO's.

I see that PMO's use Weber IDF jets, but I assume the jets' sizes that would go on a Weber carb are not automatically the same on a PMO?

Here are the specs of the engine:

- base is a 911 SC 1978 engine block slightly reworked, displacement increased to 3,2 liters, short stroke
- stroke 70,4 mm, bore 98,0 mm
- pistons JE98, compression ratio 10,5
- cylinder Head SC with enlarged ports
- lumenition ignition
- Garretson GE60 camshafts
- early style twin exhaust (65-74)

Single ignition planned, even though those cylinder heads already have double ignition slots. (maybe a future evolution... )

As a short stroke, the engine should easily take 7500RPM, and make about 280HP.

With Weber carbs, I would probably try those sizes to start with:

- emulsion tubes / F3
- idle jets / 65
- air jets / 185
- main jets / 170
- venturis / 38 mm.

What do you think would fit on those PMO 40s?

Thanks a lot to all the PMO's and engine rebuilding Gurus.

[mreid]

With that combination, you should really run 46's.

[Alex911747]

Yes, maybe you are right. But I still think it is worth a try with those 40's...
Any more help please?

[SoyRacer]

No. It's not worth it. You can't achieve a proper velocity limited to a 40mm horn. Your jetting will never match the engines needs and will always be starved for more air. 46mm or even 50mm for track use.

[Alex911747]

OK. I will probably go for Weber's 46 then...
Thanks a lot anyway for your inputs.

[Gordo2]

Alex,

PMO's aren't that much different than Webers... If you are familiar with Weber carbs, you will feel right at home with the PMO's.

If you are building the engine for track - then I agree, go with the 46's. Your 40mm vs 46mm decision partly depends on what kind of RPM's you plan to run... You mention 7.5k redline - that's what's primarily resulting in recommendations to go to the 46's. 7.5K RPM / high RPM redlines are only cool if your HP is still climbing north up until that point, otherwise there's no point in going there (safety margin is another thing - but ignition rev limiters help with that).

If you are building it for street and not planning on sustaining high RPM's, the 40's might work, but the engine's likely to fall short (run out of breath - not enough flow through the 40's) of 7.5k RPM (power would likely max out ~ 6.8k RPM if not lower). Meanwhile, the 40's would give you some good velocities, which if tuned right could produce a very torque happy motor (I actually swapped my 38 vents for 36's - provided better lower RPM response on a street engine; I didn't really care what kind of HP I made at ~ 6.3K RPM and up).

If you decide to try the 40's, I recommend monitoring the air fuel ratio (AFR), no matter what carb config you end up with - it really aids in dialing them in and ensures you aren't running dangerously lean.

I would also buy a jet reaming kit and a jet sizing tool. This will let you play with a combination of jet sizes to see what works - once you figure it out you can buy the size you need (they are flowed - so reaming gets you close, but not as good as a high quality flowed Weber jet).

Since you are somewhat non-standard (40's on a 3.2L) - jetting recommendations will be hit or miss. That said, here's my thoughts:

Idle Jets: Start with 60's - easier to open to 65 than to close them down to 55's (solder, drill, ream). The PMO's do have an idle air jet pre-built into the carbs. Gives you more stuff to play with to get the idle circuit right.

Your 40 PMO's have 38 vents - which is what was recommended for PMO 46's on my warmed up 3.2L. I would start with these.

F3 emulsion tube are pretty common - Mr. Parr seems to use those or the F16 in most applications.

Main Airs & Main Jets 185/170's - looks like a good starting point. AFM will tell you where you are at upper RPM - I'm guessing you will end up needing more fuel, requiring either larger jets, or smaller airs.

BTW - your early exhaust may limit your power at 7.5K RPM as well. The early exhausts (and SSI's) have a 1.5" OD primary - 1 5/8" to 1 3/4" OD is recommended for a high RPM 3.2. Again - depends how you plan to run the engine, and what you want to do with it.

Good luck,

Gordo

[Alex911747]

Hi Gordo, thanks very much for your comprehensive answer.

The car will be mostly for road use, even though it could be taken out to the track for fun 2-3 times a year, so it is not a track car.
As it will be very light as an S/T clone, maximum horsepower will not be that important. Something like 280HP would be very balanced with the car, even if the engine could probably deliver about 320HP.

It has more to do with the character and behaviour of the engine.
I am not especially into torquy engines. I find it much more fun revving it up and hear it singing when the power comes higher up, hence the short stroke config and high max RPM...

I wanted to try those PMO 40's, as we had them available in our shop on a shelf for a while...
I have never used PMO's as they are not really common in Europe. Always been using Weber's when it comes to 911 engines with carbs...

Anyway thank you for your very valuable inputs, I will let you know what I have decided to do and how it is going.