Largest bore diameter for MFI throttle bodies?

[stownsen914]

I would like to put MFI on an a 1980s era GTU 911. I know there are lots of options for throttle bodies, but I have some preference for using Porsche throttle bodies if possible. High butterfly stacks would be nice, but I'm not sure they're in the budget. I like the idea of using MFI throttle bodies, but the challenge is that I don't think I'll be able to bore them large enough for the intake ports on the 3.5L engine that's currently in the car. It has (I believe) 44mm intake ports. I'd like to use 46mm throttle plates and taper to the intake port, similar to how Porsche did it.

The problem is that 46 mm is the the OD of MFI throttle bodies. So clearly they can't be bored to an ID large enough. I'm curious what luck people have had boring these things out. I think I've heard 43mm.

Note that for the stacks I realize I'd have to do something custom. Possibly use some TWM spacers and air horns. I suspect that's not the bigger challenge though.

I did have one possibly crazy idea - what if the MFI throttle bodies were bored out large enough to press in a sleeve with a 46mm ID. I guess the OD would have to be at least 2 inches (~52mm). I realize this would be challenging because the throttle bodies would essentially fall apart if bored out like this. Would it be possible to weld in some pieces to hold the flanges and the throttle shaft bosses in their relative positions while the throttle bodies are bored out and the larger aluminum sleeves pressed in?

Scott

[HawgRyder]

AFAIK...the largest components are from the '69 S.
The throttle bores and stacks are all metal...not plastic...and can be bored slightly if necessary.
Bob

[stownsen914]

The later stacks are plastic, but the throttle bodies are metal (I assume magnesium). I'm not going to use the stacks - I only want to bore the throttle bodies.

[HawgRyder]

The throttle bodies on the '69 were aluminum.
I bored mine and got new plates from Holley Parts (the carb people) if I remember correctly...they were 1 3/4" which turns out to be 44.5mm.
Hope this helps.
Bob

[al lkosmal]

MFI throttle body dims

69-73E
top = 38mm
bottom = 32mm

69-73S
top = 38mm
bottom = 36mm


Note: Eurometrix offers them modded to 42mm top, 40mm bottom

quoted from above: "I did have one possibly crazy idea - what if the MFI throttle bodies were bored out large enough to press in a sleeve with a 46mm ID. I guess the OD would have to be at least 2 inches (~52mm). I realize this would be challenging because the throttle bodies would essentially fall apart if bored out like this. Would it be possible to weld in some pieces to hold the flanges and the throttle shaft bosses in their relative positions while the throttle bodies are bored out and the larger aluminum sleeves pressed in?"

You can't bore something out larger than the O.D........................but even if you could by doing what you suggest....you have to ask yourself why????? What possible advantage could doing this have over just using 46mm TWM ITB's or equiv.

[stownsen914]

I think I didn't explain myself very well ... boring them out larger than the OD will be challenging but not impossible. To be clear, I mean the OD of the narrowest portion of the throttle body. Certain portions like the flanges and throttle shaft bosses are somewhat larger. These areas could be supported with a jig or possibly by welding prior to machining so that relative positioning is not lost during the machining and while the aluminum sleeve is pressed into place.

My thinking in bothering to do this is to keep a modified, more period-specific part on a vintage racer, rather than new TWM or other more modern solutions.

Maybe not worth it though ...

Just thought of one other thing ... maybe find some motorcycle throttle bodies (a la Al's Triumph Speed Triple solution) ... but make sure they are 1980's era parts so they'd be "period correct" on a vintage GTU racer?

Scott

[al lkosmal]

Ah...now I see..............basically you would be left with the upper and lower flanges and the butterfly shaft bosses etc. and replacing the sacrificed throttle bore/tube with a new sleeve pressed on/welded. It does look to be doable...maybe using a 49mm O.D./46mm I.D. tube.....
A lot of work, but maybe worth it to be allowed into the race class.
regards,
al

[724doorE]

It almost seems like it would be easier to put together a wood core with a large enough OD dims, and work with a local foundry to have some cast, then machine. Once you get the cores figured out, the castings can be fairly cheap.... the machining...maybe not

[HawgRyder]

The limiting factor may be the heads.
The intake ports in the heads can only be as large as the stud spacing minus a sealing ledge will allow.
Bob

[BURN-BROS]

The 70/71 Stacks/T-bodies can yield the largest throttle plates @ 45mm with a 42 to 43mm intake port. Air horns are PMO 50's with a 52mm dia.

[BURN-BROS]

stack opened up to 52mm top and 41mm bottom. You can compare the size difference to the stock E port.

[chris_seven]

Does anyone know the throttle valve size used on the 906E?

The inlet and exhaust ports were 38mm and I would guess the throttle valve was larger than the 911S

[BURN-BROS]

They are 38mm

[chris_seven]

I found an article last night which was a review of a 906E with slide throttles.

I imagine that not all of them were fitted with this system?

[stownsen914]

Originally Posted by BURN-BROS

stack opened up to 52mm top and 41mm bottom. You can compare the size difference to the stock E port.

Thanks Aaron for the comments - very helpful.

Originally Posted by chris_seven

I found an article last night which was a review of a 906E with slide throttles.

I imagine that not all of them were fitted with this system?

Chris - Not sure if this was your question, but I believe most of the 906's were carbureted. Of the 906E's (I don't think Porsche made many injected 906's), I understood that they all had the slide throttles.

Scott

[chris_seven]

I realise most of the 906's were fitted with 46IDAs but I had hoped that some of the 906E's had conventional Throttle Bodies rather than slide throttles as I have a pair of 38mm TBs that need to be refurbished and bored as there is some marking in the throttle valve seat area.

I was hoping to be able to bore them to around 39mm and then fit new throttle shafts and bearings to complete the job.

I also have a Marelli S121 dizzy with 27mm shaft and 4 x sets of Contact Breakers to build a Twin Plug motor.

[BURN-BROS]

Hi Chris,

The 906e and the 910 ran slides. I have not seen a throttle body system for the competition cars until the hi butterfly for the ST's of the early 70,s(late 1970?). The Slides were fitted to the conventional 911R and the 916 engined R(1968?)

Here is run down of the years and the max throttle plate.

1969 (901 casting #) 41mm to 42mm. depends on how much bore is shifted from the casting.

1970-71 (911 casting #) 44mm to 45mm

1972-73 (pressure cast, no #) 40 to 41 mm. Thinwall casting makes for a very thin cross-section when bored.

Hope this helps.