I haven't rode in many MFI 911s.

What I immediately zoomed in on is how snappy they are.

No matter how well you tune a carb's accelerator pump and ignition curve I'd say you just can't duplicate a MFI

as was just posted "High injection pressures give better fuel atomization and a better cylinder charge. "



max hp or torque etc is really another story

The operative phase for MFI is "properly set up". A lot of people say they can set them up, and I'm sure there a few who actually can. You may spend a fair amount of money finding them. As far as more power I would want to see a dyno sheet showing this, as there are other variables that would have an effect on the power. For ease of set up I would go with the PMO's.
'73S w/mfi
'75 Carrera RS look 3.4 w/50 pmo's

I shoulda let you drive my '72 S when the opportunity was there. Pretty sure you'd become an MFI fan.. I say this as a guy who has owned both carbs and MFI cars...when they were new.

The factory 2,0 S made 160HP with carbs, 170HP with MFI (and larger valves/ports). Same cam profile, same displacement, same exhaust.

Originally posted by:RWebb
There is also a cost: you've got that big heavy pump slung out there and it's noisy.

That's not noise. That heavy pump is singing a beautiful song at high RPM's.

I would want to see a dyno test where the motor was optimized for mfi & carbs, as just the exhaust could make a difference. My mfi/carb experience is different. My 2.4S mfi motor with fresh everything has a mid range surging/stumble at 2800-3500 rpm, compared to my 2.4E w/40 webers, 2.8 w/46 webers, and 3.4 w/50 pmo's, which don't.
Like I said, finding someone who can actually set an mfi up would be hard to find. I have nothing against mfi, just got tired of paying these P gurus who could really couldn't adjust it. Just based on my experience.

Lets go back over the posts. The thread is about PMO's, not just any carb like Webers or Solex. Please compare the attributes of the MFI as stock to the new PMO only. That was the original question.

Also, all this talk about MFI's in race cars. Atomization and position of injector. Remember that Porsche preferred the high butterfly version for flow and I imagine even more enhanced atomization.

I ran MFI at the track and it is a great system, especially compared to old carburetors. No float bowl problems in the corners, that's for sure. I also ran Webers on 44 cyl, so no direct comparison. I had problems with those. But, they were old and not properly rebuilt.

I'll stand my my opinion that the PMO's might have an edge if you consider all advantages to disadvantages.

gumba, the mid range MFI stumble is notorious. I wonder how much that has to do with the actual characteristics of the S cam vs. the profile of the space cam and other variables in the MFI pump. I've had SBC's that were cammed high lift with a lot of overlap and they seemed to have a flat spot too.

A lot of discussion in the past here has tried to establish the specifications of an early S motor camshaft. I don't know if anyone has the actual lobe separation angle, but it's possible that the cam is a bit of a compromise in design. I would have to say that I know modern day camshafts for these engines have changed the design slightly to eliminate some of the weaker characteristics while preserving the overall performance.

MY 69E had that notorious mid range stumble. The cure was a combination of ignition (return to Bosch CDI and correct coil, got rid of the silver one) and correct set up of the MFI. Again thanks to Ed Mayo. It can be done.

Part of the MFI issue is the transition from lean to rich or rich to lean. This can be cleaned up.

The car is strong no stumble and most important a joy to drive.

My 72 2.4T/S MFI runs beautiful. No stumble. A bit rich perhaps...

Religion
 

This debate is like religion . . . both are great when tuned properly and I don't believe one is better than the other. For the average 'Joe' like myself. My recent engine build/tune experience on my 2.5 SS with PMO 40's. I will say for a guy that has never worked on carbs like this before . . . PMO are very friendly, easy to work on and easy to tune . . . and Richard Parr is just a phone call away . . . with all the jets/tubes/etc. at my finger-tips (UPS delivery). I'm sure if I invested the time and expense I could DIY an MFI setup too. But . . .

So excluding cost . . . why did I go PMO's? Well, prior to my decision I rode shotgun in PWD72S's MFI p-car (on a hot run through Beaverton). And I've rode shotgun in Chuck Miller's MFI 73S, and Dave B's Weber 3.0 evil-cock-a-roach on the track. And my conclusion MFI vs. Carbs . . . they both do exactly what an intake system should do when properly tuned . . . haul ass!!!

MFI is just a more pricey alternative for DIY guy like me. And under these circumstances PMO's were the right choice.

Hope this helps.

BTW, I'm recently built a 2.7RS Spec motor that was setup for MFI . . . however the more I work with these PMO's . . . I think I'm changing my plans.

No way - I tried to weigh a friend's MFI pump on a post office scale (snuck in after hours with a hat on so the security cam couldn't ID me and my greasy contraband! It maxed out the electronic scale - IIRC, they go up to 16 lbs. I should have used UPS scale. It was too heavy for my own (NBS-traceable) electronic scale also...

Remember, this thing was a diesel pump that was converted over for gas engines.

I would not use the plastic MFI stacks due to fire risk - the Mg ones are heavier.

Will be glad to see accurate wts. posted for mfi pump and stacks...

John & zotman - thx for filling in the details on your prior posts.

and...
- I'd love to visit you both, Grant & zotman!

I agree we should distinguish the the high butterfly version MFI from the regular, street car/low-level race version.

You can ask the question differently: do you want carbs or sequential fuel injection? MFI is the latter.

Gents

Thank you for the very interesting feedback :)

Certainly the theme that seems to echo through most of the comments is "Getting it all set up correctly"

The MFI pump on the car has been fully overhauled by a recognised UK expert, and reading the MFI set up guides in the Tech Articles section has made the system seem logical and easy to understand the set -up and adjustment mechanisms. However, you do seem to have to get THE right guy to do the final tuning.

This is my concern, as it may prove difficult to find an expert locally ( I know 2 "old hands" but whether they can be pursuaded to do the work is in question)

Carbs are potentially easier to get sorted and PMOs being a modern design should prove better than older period instruments.

I'll probably have a look at the MFI, to start with, but should it prove to be beyond my talents then the hefty investment in importing a set of PMOs may be the only solution.

One question regarding the electric pump that supplies the fuel from the tank at the front to the MFI pump - what sort of capacity/ fuel pressure is required from it. Does an "normal" comp spec carb pump have the capacity?

Regards

Michael

Michael,

Search here for extensive information on the fuel pumps, they are different.

Hey Randy....the MFI was used on the S and E engines strictly for the better performance and throttle response. Its just a better system.

It was ONLY put on the "T" engines in 72 and 73 for emissions. Other than emessions the enrty level T would of stuck with zeniths and webbers.

In my opinions even tho I love my MFI...I would go with PMO's for the track. Way easier to set up and maintain. Performance will still be incredible.

Oh on a side note.....my roommate is a race car mechanic, he has worked on Porsches, Lambos, DP cars and now works for Motec. His only words when playing around on my porsche was how great the throttle response was. I would assume he knows what he is talking about.

Lets not forget the two different versions of the 906, one carbed (901/20) and one with MFI (901/21) with yet again a 10HP difference in power. Or the '69E with 140hp vs the 130 hp of the Normal with the same displacement, ports and cams (bigger valves in '69).

Back to the key question: for "light competition" what will the sanctioning body let you get away with? What do the rules provide? What sanctioning body?

As counterpoint to a post a ways back...

Porsche put the injectors close to the combustion chamber with the slide-valve throttle because the high injector washed down the slides and could jam the mechanism due to lack of oil. The high butterfly was to smooth the airflow around the more restrictive butterfly plate.

From what I have heard from experienced enginer builders, there was no noticable decrease in performance when the injectors were placed lower. Now, many vintage racecars put the injector low, whatever throttle setup they had.

Michael - first, thought you were done or I would not hve diverged this thread so much...

I'd look thru the Check/Measure/Adjust document -- all of it. Imagine yourself doing that.

Once setup right MFI is said to last a looong time with no 'tuning.'

I hate to throw another option out... but - is aftermarket EFI an option for you? It optimizes all inputs; carbs do 1; MFI does 2.

It doesn't weigh a lot and the new intakes makes it look [sort of] periodish.

RW & John

The car will be used for Classic Road rallying, which involves road based regularities but also Special Tests, which are a mile or so long using tracks and lanes ( much like a mini special stage from stage rallying), with the occasional chicane or 360 pylon thrown in to keep the "Average speed down to the permitted level. Taking all that into account, the mid range and initial lift are really the important parts of the power curve to get right.

Basically you can use any "period" modification, the class I would be in is Pre '75 cars.

So carbs, MFI and I suppose you could argue that EFI could be used. However, a full modern EFI / management system would probably be stretching the rules, how sophisticated were the '74 injection systems for the RSR etc?