A question for the MFI Evangelists

Hi Folks,

I have recently undertaken the task of rebuilding my motor with the assistance of a retired Porsche tech. He's very knowledgeable, but also very firm in a few of his convictions. The one I'm wrestling with is his undying devotion to MFI. The engine we're building at this point looks like it's going to be a 3.0SC with 100mm P/C's and "S" cams so approx 3.3L. Maybe 3.2 heads, but he apparently has adapters for the heads to run MFI on 3.2's, or 3.0 heads if I go that route as well.

I have found an MFI setup for sale, that was originally designed for a 3.4. It's basically all been rebuild pump/throttle bodies, by reputable businesses, and the price is fair (competitive with my other option 46IDA's). I'm sold on the attributes of MFI, the reliability, throttle response over my 40IDT
s, excellent atomization, and I think I've heard Warren mention something about how they sound?

Anyway, the unit was built on 2.2 cores. So the TB's are 2.2 (which he says is a good thing), but the pump is also a 2.2 (which he classifies as being a bad thing). He doesn't seem to know why, but just seems to remember that while 2.2 pumps are sometimes used in these applications it's mostly by the racers. This car will be seeing 70-80% street driving.
Also if I can remember the discussion he also took issue with the TB size vs. Butterfly size. The seller states them to be 46 TB with 42mm butterflies. My mechanic says they should be the opposite say 42mm TB with 44/45mm butterflies.

So I guess my questions are:

Is there a reason why 2.2 pumps are normally not used in modified form on street buildups?

Which is right regarding TB to Butterfly size?

Finally, is something like this even going to work? My mechanic is a huge advocate of MFI in this instance, but last night I did some bedtime reading in and issue of "Up-Fixin'", and B.A. said that he didn't think that MFI was a good idea for even a 2.7 /w T cams. I of course will be using "S" cams on a 3.3. Bruce finishes by saying that "Your problem is that ther just wasn't a mechanically injected 911T engine, so there isn't a pump that will match" Do I have the same problem (no MFI 3.3 "S" motor), or has there been changes since B.A. penned this in 1988?

Thanks,

Adam

I can only comment on the part dealing with 2.7 and t cams. I have a 72 911. Installed 2.7 rs p&c's. Had the T cams ground for what was called a slalom grind. Had pacific fuel injection calibrate the pump for me with the specs of the cam. Had the local shop guru setup the pump for me the first time. This car runs great. No probs. what so ever. The pump was from a 2.4 motor. I now adjust it myself after 4 yrs of tinkering. Oh yea, i run the stock throtle bodies and heads. Yours sounds way more complex

Hello

The Bosch MFI is usable up to 100 - 110 HP/L and in my eyes far better then a carb set up.

Porsche had a 3,0 RS engine with street/rallye setup and therfore you can use that "Meisternocken". Most dificult is seting partial throttle situations to work. Most people overrich the system giving up one of the advantages over the carbs.


Total Fuel delivery from the MFI is limitet and will not support more then 300-330 HP.
Porsche used the Kugelfischer set up for higher output rates.

The optimal runner is a large diameter wedging down the way to the valveport. ( lika a trumpet horn ) Upsteping is better then downsteping. But there is no sense to use i.e. 48 mm Throttlebodys with 45 mm Butterflys with 40 mm Runners. Especialy when you have a standard 40mm intake plenum and 36 mm Valve

This is generatet from the american muscle scene where nothing matches perfect and so you use steping to avoid streaming break ins caused by missmatching.

This all is so basically engine building and simple to callculate that everyone can see the facts. No myhts.

Can´t remeber the reson why not to use the early pumps. WUR & Stop magnet is retrofittable. Early pumps have a slightly lighter housing and maybe have a weaker bottom end.

Grüsse

I am far from an expert on MFI but I'll throw my 2 cents in anyway. MFI seems superior to carbs in general when the MFI setup is matched to the engine for the reasons you mentioned. However, with carbs you can always change jets because carbs are somewhat generic. You setup a carb using adjustable jets once you pick the carb of the appropriate bore for the engine. With MFI however the whole thing is driven off a space cam, and unless you know how to regrind the cam you are going to be stuck with whatever cams are available. Now Porsche engine builders know which cams work well in specific applications, but once you start setting up a non-standard engine you are in an experimental area. Is that where you want to be?

Let me know if I'm off-base with this...

How funny this is almost the same post I did a few days ago. I've been planning on a 3.0 MFI coversion for my 72 Targa project. I'm planning on contacting Pacific on Monday to try and answer the same questions.
That's good news about the throttle body adapters. I didn't know they existed. Where can I get them?
Bobby

Hi Bobby,

My mechanic told me to contact Gus at Pacific on Monday too to ask about the 2.2L pump, I just figured I'd give the bbs first whack at it, so I wouldn't have to bother him. As far as adapters, I get the feeling that these are a custom piece my mechanic has fabbed up. I'll check with him, and see what the deal is with obtaining these.

Thanks,

Adam

Originally posted by Bobboloo:
How funny this is almost the same post I did a few days ago. I've been planning on a 3.0 MFI coversion for my 72 Targa project. I'm planning on contacting Pacific on Monday to try and answer the same questions.
That's good news about the throttle body adapters. I didn't know they existed. Where can I get them?
Bobby

Hello

A big volume Mastercam will work better then the low displacement units. Hotest and best choice is maybe the said 3.0 RS. But the Pump rebuilders also have several Mercedes cams or other tricks.

The bad thing is that you only have 7 setings to play and the screw to turn is hard to reach.


Basically you can overbore the existing ones. The later plastic runners can not be bored up so everyone runs for the old "S" Runners and Velocity stacks.

Boring up is limitet if you would like to go larger diameters then you need race stuff and thats more expensive. Mostly you go for a Motec upgrade then too.

Best way for good performance is to flow the complete assembly on the bench.

Grüsse

Hi Bobby,

I checked with my mechanic, and he said that while adapters are avaliable for the 3.2 heads but the 3.0 heads are too tall for a spacer/adapter. What he does is have Ollie's modify the heads to accept the MFI manifolds in cases where 3.0 heads are used. I haven't decided whether I'm going to pony up the extra cash for the 3.2's or have the 3.0's modified, and they're going to ollie's anyway so not too much of an issue.

Good Luck,

Adam

Originally posted by Bobboloo:
How funny this is almost the same post I did a few days ago. I've been planning on a 3.0 MFI coversion for my 72 Targa project. I'm planning on contacting Pacific on Monday to try and answer the same questions.
That's good news about the throttle body adapters. I didn't know they existed. Where can I get them?
Bobby