MFI or Webers pros and cons?

[Tommy]

Hi all, first time post here, I have a 70 911E and some MFI problems (pump needs rebuild, linkage worn out, etc.) which has me considering converting to webers. My mechanic thinks webers will get much better gas milage than the MFI (which goes against conventional wisdom, but maybe he's right?) and carbs will be more reliable. How about performance, will I lose power with webers? Will the throttle response change? Will the value of the car go down with webers on it instead of the original MFI? Is there any advantage of keeping the MFI system?

Tom
70 911E

[scargo70]

Try keeping the MFI but if you were to convert to webers at least keep the system so the next owner or you can re-convert.

[HalM]

For sure keep the old MFI components for the next owner.

You need to run the numbers accurately. Call Gus at Pacific Injection. Look at Eurometrix web page (http://www.eurometrix.ws/) and price out getting Matt to do the works.

Then compare that to new PMO's and their complete install package. My guess is it will be close to a wash.

That said, I have PMO's on my 2.7 RS spec motor. Throttle response is great and I get 22 MPG cruising on the highway. Finding a wrench within 200 miles that really understands MFI was my major consideration for going with carb's.

Unless you are racing or concouring, I don't think you will miss MFI. Carb's are period correct but not original.

[JCR]

My 73S came with Webers installed and the MFI in a cardboard box. Great performance with the carbs - nice sound, no sport muffler backpressure worries, and I've outrun many an MFI car, as well as SCs, Carreras, 996s... .

They've been reliable - I've only had to blow out the idle jets periodically (easy task). One downside is slightly less throttle response than MFI.

That said, I do intent to reinstall the MFI someday for originality - VERY glad I have the parts 'cause BOY are they $$$. Also glad I'll have a pair of Webers I can sell to help fund the job!!

[kenikh]

I am just getting ready to convert my Weber carbed car back to MFI. I thought long and hard about it but carbs just don't float my boat. I have the good fortune of having very able MFI tuners in my area (Seattle, Wes Ingram) so that played some into my decision. If you do go with carbs, KEEP THE MFI components and leave everything you can on the car (pulleys, return lines). It will make your life easier when you decide to finally fix and reinstall the MFI (and trust me, you will one day).

I understand your hesitance in fixing the MFI as it is very expensive to fix. Still if it is in your budget (I also suggest calling Eurometrix) you'll thank yourself for keeping the MFI. You won't recognize your car once the MFI is fixed and tuned.

[Shuie]

Reconditioning a pump & and a set of throttle bodies is probably going to cost more than a nice 40mm weber setup for your car. If you and your wrench are comfortable with a set of carbs then go for it. Whatever you do, DO NOT GET RID OF THE MFI STUFF! These parts are getting impossible to find. Once you sell it, its gone. BTDT.

I really dont think you are going to get better fuel economy from a set of carbs than you would from a sorted MFI system. If all of the parts of the MFI are in good working order and tuned according to CMA your engine will get reasonable fuel economy and be pretty reliable. Your problem is that its going to take some time, money, and someone who knows MFI to sort it out. If your wrench does not know MFI or the process that is followed when sorting and MFI car out, do not let him touch it. I dont care how good of a mechanic he is, if he doesnt follow the logical process the car is going to wind up worse off than it is now.

[RussianBlue]

Great responses here....

I for one absolutely love MFI, I love:
1) How it looks, love that fuel dist and lines etc...
2) How it sounds - especially on de-accelaration ;p
3) The fact that it is Porsche history and racing history at that!
4) I somewhat like the fact that it is hard to tune - very much like dealing with a beautiful woman ... bah nevermind won't go there.
5) Obviously, originality blah blah blah

Absolutely keep the MFI stuff or ahem sell it to me?

I vote for keeping the MFI w/ the condition that you find a viable tuner/wrench

2cents

[JimR]

MFI is great and not that hard to tune. Use the procedure "Check, Measure, Adjust" and it'll rock. If your MFI components are worn, fix them and then get the system readjusted. It'll be better for the value of your car and give you more HP than carbs (~10HP). Carbs will require work if you buy used, and whether new or used, carbs require regular fiddling to keep running right.

[rgunson]

I bought my 1971 2.2S with webers on it and the original MFI in a box

I was determined to return the car to original spec so had the MFI rebuilt (it didn't cost that much & the guy in the UK who did it said it had no internal wear - he simply de-coked the internal with a solvent then replaced all seals & reassembled

But my car wouldnt run properly when I put the MFI on (yet it seemed to run perfectly with the carbs)

This is when I learnt, as previous posts have also mentioned, don't blame the MFI! MFI is fantastic - I am reliably informed that it will run for 100K miles without adjustment. Why? - because it is built to AIRCRAFT, not automative specification. How? - I understand it originates from a ME-109 (German WWII aircraft), then modified by Mercedes to serve for many years as a diesel pump on their vehicles, before being further adapted by Porsche for the 917 race program in '68/9, then onto the 911's

The problem was my engine. Again as previous posts have mentioned your engine must be in tip-top condition as the MFI is very sensitive. At this point I learnt that I had +30% compression loss in 2 cylinders, that the inlet butterflies were not snapping shut (the bushes were worn), etc, etc.

With carbs you can hide all of these faults (as the guy who sold me my car did!) because you are able to adjust each choke to compensate for differences between cylinders, like compression loss and other factors arising from wear & tear

With MFI you can't because the adjustment affects all 6 plungers on the pump at once - you can not adjust them separately

What are the advantages of MFI over carbs?

1. MFI produces an induction howl at high revs that carbs do not (which I personally think is fantastic - others may not)
2. MFI contains a compensator for atmospheric pressure - eg to maintain an optimum air/fuel mix when travelling over ranges/mountains which carbs don't have
3. MFI can produce additional horsepower - for your 'E' you can replace the cam in the MFI pump with an 'S' cam & get the guy who reconditions your MFI to reset it to 'race' settings instead of 'road' - yes it will be snotty when cold and not pull cleanly at low revs, but all tuning for top-end will have this effect (incl carbs)
4. As already mentioned MFI basically nevers needs to be adjusted. Yes it is fiddly to get right, but once achieved it stays in tune (other than the points)
5. Originality

It would be a crime not to retain the MFI. These original parts are becoming rarer & rarer. My advice would be to check the compression on all cylinders first. If you have more than 10% difference between the lowest and highest cylinder then the MFI willl not work properly (as a rule-of-thumb).

Finally, if webers are really superior to MFI why did porsche invest so much time and money in race development of the system for the 917s, and then transfer it to their road cars if it was an inferior system. Just to be clear the CIS system that followed the MFI system in late '73/74 was a bit of a dog as it was a common chamber system, and whilst a lot cheaper to supply was simply not in the same league as the 6 chamber MFI system (one dedicated to each cylinder)

Kind regards





Reiss.

[HalM]

Finally, if webers are really superior to MFI why did porsche invest so much time and money in race development of the system for the 917s. . . .

With all due respect, no one has suggested that Webers are superior to MFI. They are, however, a less expensive, viable alternative that many shade tree mechanics and professional wrenches understand. And, FWIW, carb's make some nice noises that neither MFI or CIS/EFI can produce.