MFI heads with EFI

[Slanski62]

Is it possible to use MFI heads on a later motor with Motronic injection, if I plug the injector ports? Thanks.

Steve

[john walker's workshop]

What year heads and what year later engine?

[jpnovak]

No. The early MFI heads have a different head stud spacing and will not physically fit on anything larger than a 2.7.

[Slanski62]

The heads are from a 77 and have 86mm head stud spacing, per the ad.

[Walt Fricke]

JP's got the answer.

Porsche was not using MFI on any of its cars in 1977. Its top of the line street car (other than the 930) that year was the 3.0 Euro Carrera, and that car had the wider head stud spacing of all the subsequent 3.0 and 3.2s (86mm), and the Euro Carrera 3.0 was a CIS motor.

The 74 and 75 Carrera or RS had MFI, but it was a 2.7 with the regular 2.7 stud spacing of 80mm.

There was a 3 liter RSR back then which was MFI with slide valves, with an intermediate stud spacing of 83mm, between the spacing used from 1966 through the introduction of the 930 case, and the 930 case spacing for street models. But these would be rare, probably expensive, and you couldn't use them without the equally rare case. .

So the seller of these heads has some facts mixed up. Or maybe they were machined to take the MFI injectors? I don't know if that could be done. it would be, I think, a major undertaking to machine a 3.0 head, whether converted to MFI or not, to accept an EFI injector. They don't screw in.

[jpnovak]

Walt. Thank you for filling in the details.

Yes, the later heads can be modified for MFI. The best method is to weld up the CIS offset in the intake port and then drill and tap for the MFI injectors.

I am building a 3.0 motor like this right now. It will be EFI based on MFI stacks. I make adapters for the conversion.

@Slanski... Post a link to the ad for the heads and we can help further.

[panzerfaust]

how did the 954's do it? it was a MFI on a 3.0 motor.

[356RS]

The 954 SC RS had the heads modified/drilled for MFI injectors. No big deal. They also used the Kugelfischer MFI pump. Bosch had stopped making PED 6 KL... pumps buy then.

[Slanski62]

Here's the link. I hope it's okay to post it.

Porsche 911 RSR 3 2 3 4 MFI Cylinder Heads 49 41 5 Valves 40 38 Ports 3 5 3 7 | eBay

[356RS]

Yep, stock 78 - 79 SC heads modified for MFI and twin plug.

[panzerfaust]

If I'm not mistaken kugelfischer was absorbed by Bosch during 954 era but oddly enough they called it a kugelfischer instead of a Bosch?

954 was a single plug while the airplane motor was twin plug in 83/84.

[49willard]

Originally Posted by Slanski62

Here's the link. I hope it's okay to post it.

Porsche 911 RSR 3 2 3 4 MFI Cylinder Heads 49 41 5 Valves 40 38 Ports 3 5 3 7 | eBay

I sent you a PM on another subject

[Walt Fricke]

JP - a bit off topic, but I take it that welding up the CIS cutout (or the equivalent in the 3.2 heads)doesn't cause problems with the temper or whatever of the heads due to the heat? I've got heads for both which use EFI in the manifold, so this little relieved area isn't useful.

If I was clever enough I suppose I could mold a plastic insulator block which had a piece sticking off its bottom to fill the space. Or maybe mill that space larger to fit a standard shape, which could be held in with a set screw, and then milled flush with the bore. But paying a welder to fill it in might be less expensive overall.

Walt

[jpnovak]

Walt, It has not caused any issues welding up the port. You could consider it next time you have the heads off the engine.

Though I don't think this would make much difference in the flow characteristics - even on the track car at high rpm. In fact, it might help stabilize the MAP signal by creating a slight ledge to help disturb the reversion pulse at low rpm. Very much like a small step in the manifold would. Can't say I have ever really looked closely at it.

[jpnovak]

BTW, just to confirm with the OP. These heads will work on a 3.0 or 3.2. They have been modified for MFI. Though I question the asking price. I would think to modify stock heads like this would be cheaper - even including core price and machine work.

[Slanski62]

Great JP. That's what I needed to know!