MFI VS Weber on 911

[M491]

Hi all. I have a mechanic who is trying to convince me that MFI is more of a headache than anything (dumping fuel into the oil and getting all of 8 MPG), and that I should install my Weber carbs instead during my rebuild. He says Webers are better in terms of reliability, no issues with gas getting into the oil, and that fuel consumption would be lower. This sounds wrong to me. Is it true that MFI gets such poor gas mileage? Isn't there a really simple fix for the gas getting dumped into the oil?

The old mechanic said everyone wants MFI today because they either forgot or were not around to remember the huge problems MFI had. He said that was why they stopped using MFI and went to CIS and, later, Motronic. Is there any truth to what this long-time Porsche mechanic is telling me? Aren't Webers gas hogs and hard to keep in tune? Thanks in advance for any information and advice.

[neilk]

Sounds like if your pump is ready for a rebuild. I think you could probably rebuild the pump and throttle bodies for the cost of a new set of PMO (improved Webers).

Are the throttle bodies getting loose, or is the pump going south? I would definitely stay with MFI though. I have all the parts for my car to convert back to MFI, I just need to find the time to do it though.

[tomkirkcis]

Most engine can be made to run fine with webers or pmo's' but a non worn properly tuned mfi system is a joy. Great power and razor edge throttle response. the exception is a car with an engine modified to a non stock porsche configuration. getting exact space cams for the pump is impossible unless the factory built an engine with mfi for the exact configuation of the engine.. The value of the car with it's original mfi is also worth the investment. The mfi was replaced with cis for emmission reasons. Fuel economy is a consideration for a daily commuter, not your 911 IMHO. good luck tom

[M491]

Just to be clear, the car is currently basically runnng OK with MFI. I think the engine has a problem on cylinder 6, which I will have checked out and fixed soon. It seems to be running on five cylinders to me and it might be as simple as a bad plug wire, but could be a worse problem. Dunno. Anyway, the current, installed setup is a stock 911T MFI system working with a stock 2.4 911T engine. FYI, the currently installed engine is not the car's original engine. I have the original engine is on a pallet in my garage, just awaiting its rebuild. On the shelf, I have a good (checked out by a pro) 911E complete setup, which is what I would use on the rebuilt, original engine.

Most of my experience has been with CIS and Motronic, other than my 1972 911E with MFI, but that was a long time ago and I don't really remember the fuel economy figures. My other Porsche is a 1989 3.2 factory Turbo-Look Cabriolet (Motronic), which is a completely different beast.

Anyway, is it true that the use of Webers provides much better gas mileage over MFI? I did not think this was the case.

Further, regarding the MFI dumping fuel into the oil, isn't there some easy fix for this (like disconnecting the cold start solenoid or something easy like that)?

Thanks for the advice.

[EARLY911ZOO]

you dont remember because long ago gas mileage was a non issue.
A well tuned MFI is unmatched and unique and must be preserved.
you can adapt Webers to any 911 engine all day long however it is another story making a carb or CIS car into a MFI. unique heads, rare parts, fascinating fuel delivery system many have never seen.
I say keep it MFI btw MFI cars still sporting all their goodies are often worth $5K more.
As far as mileage, I believe the differences are not great, maybe a poorly tuned MFI will get poor mileage.
So albeit pricey to rebuild, once renewed it is more stable than carbs, it is well worth preserving and enjoying it driving.
nothing like an MFI howl.

[Flieger]

The sharp throttle response of MFI in tune is thrilling.

MFI mileage does not have to be any less than carbs. It depends on your driving style and the tune of the MFI pump. If it is too rich, it will use more gas. Properly set up, it gets better mileage.

Porsche switched to MFI from carbs to get more power but also found they could get better fuel mileage if they drove at a controllable pace, like at LeMans. For hillclimbs and the Targa and such they used more fuel because they were going faster than the old cars. The MFI has superior fuel atomization due to the high injection pressure. This is good for more powerful combustion. MFI shuts off fuel on the over-run if all the speed switches and such are working and the other barometric controls, speed controls, and throttle controls mean that it is very precise and efficient, more so than carbs.

The key is setting up the MFI in proper tune and making sure the pump is in spec. If the pump is worn or out of adjustment, it will use more fuel and not have as good of response. It cannot sense airflow like carbs or CIS so it must be tuned to deliver the correct amount of fuel.

Regarding the fuel/oil mixing: the injection pistons rely on the extremely tight tolerance between them and their bores to keep the fuel separate from the oil inside the pump housing on the other side of the piston. Therefore, if your pump is worn, fuel and oil can mix. A pump will not do this if it is in spec.

MFI is better if it is tuned to spec. Carbs may be a little easier to get tuned but give up a little of the performance at the Nth degree.

[silverc4s]

Just to be clear, the car is currently basically runnng OK with MFI. I think the engine has a problem on cylinder 6, which I will have checked out and fixed soon. It seems to be running on five cylinders to me and it might be as simple as a bad plug wire, but could be a worse problem. Dunno. Anyway, the current, installed setup is a stock 911T MFI system working with a stock 2.4 911T engine. FYI, the currently installed engine is not the car's original engine. I have the original engine is on a pallet in my garage, just awaiting its rebuild. On the shelf, I have a good (checked out by a pro) 911E complete setup, which is what I would use on the rebuilt, original engine.

Most of my experience has been with CIS and Motronic, other than my 1972 911E with MFI, but that was a long time ago and I don't really remember the fuel economy figures. My other Porsche is a 1989 3.2 factory Turbo-Look Cabriolet (Motronic), which is a completely different beast.

Anyway, is it true that the use of Webers provides much better gas mileage over MFI? I did not think this was the case.

Further, regarding the MFI dumping fuel into the oil, isn't there some easy fix for this (like disconnecting the cold start solenoid or something easy like that)?

Thanks for the advice. M

Your cylinder #6 problem COULD be MFI related, but is more than likely a fouled plug, bad plug wire, or an intake valve related problem. You could swap injectors with cyl 6 and another cylinder to be sure that the injector is not plugged. Try a new spark plug and a known good plug wire first. Do a compression & leak down test on all cylinders.

Neither MFI or Webers (or PMOs) will win any fuel consumption awards. The MPG at cruise in top gear is okay, but in all other modes both carbs and MFI will not use fuel efficiently. I would say that MFI in proper CMA tune, freshly rebuilt should be able to outperform carbs for mileage.

Most old-line Porsche mechanics can work on Webers, while few have the skills and experience to get an MFI system to peak performance. So I would be concerned that he is leading you in the direction of his comfort factor.

[Chuck Miller]

As you know Mauro when everything is new, right, and tuned, MFI is a pleasure and a monster performer... However it's probably THE most uneconomical 'on demand' system EVER invented. Even on a hot rodded MFI motor the difference in mileage between low RPM 5th gear cruising and 10/10's track driving is mind boggling.... With mine that difference is 20 vs. 7...

... And YES, I hear that back in the day most old school Porsche wrenches thought they were at least finicky, and mostly a big PITA.... I also hear that in '69 the Bosch 'rebuild/new exchange program' was WAY beyond it's capacity.... But that was before mechanics were used to'm... then 'all' the MFI rebuilders were around..... now, there's about 3.

Now, with a ton of money, they can be made smooth and reliable...

.... the question is , what the hell happens when THOSE guys go away...

Cheers

[M491]

Yeah, I am wondering if the mechanic is guiding me straight into his comfort zone, or hoping I'll put on Webers and allow him to just keep the MFI. I dont really know. Thanks everyone for the advice and info. If I can drive calmly and get decent mileage, I don't mind poor economy on the track. It's not like I'll be running any endurance races. Cheers everyone!

[Flieger]

Let us know what you go with and how it works out.