Recreate high butterfly injection stacks

[stownsen914]

Today I noticed that early MFI injection stacks look remarkably like factor high butterfly stacks, at least dimensionally. See pics below. I wonder if one could take some of the early mag MFI stacks and machine them to the desired ID and make bosses forthe throttle shafts. Hmmmm ...

High butterfly:



MFI stacks:




I'm thinking a lot about fuel injection stack options for an ex-IMSA GTU 911 that raced in the 1980s. I'm planning to put MFI on it (it currently has PMO 50s). The car is currently pretty much as last raced in 1989, so I'd like to keep any mods period correct. There are a few considerations:
1. Convert an MFI pump to work on a large race engine. Not trivial, but there are several rebuilders of pumps who can do this.
2. Where to put the injectors. I could modify the head to put the MFI injector there like the factory did, or use some MFI injector adapter blocks (I have a set actually).
3. What throttle bodies and stacks to use? This is where it gets trickier if you don't want to spend piles of money on high butterfly or slide throttles. This is the trickiest part of the conversion in my case, I think.

It would be easy to bolt on some TWM or similar throttle bodies, but they wouldn't be period correct. I've considered using standard MFI throttle bodies, but I'd be limited by the size of the throttle plates for my application. I understand the largest you can put into MFI throttle bodies is 44mm. The intake ports on the engine in the car now are 45mm. Ideally throttle plates should be a couple to several mm larger than that.

So why not make a high butterfly setup similar to the factory ones out of standard early (metal) MFI stacks? I have no illusions ... it would not trivial. What would need to be done?
1. Bore the ID for 45 mm at the base of the stacks and for 48 mm (or maybe a little larger) at the top. The throttle plates would go in the top of the stacks. I measured the standard MFI stacks, and they appear to have enough meat on them to allow for this IDs. For reference, factory RSR high butterfly stacks are 43mm at the base and 50 mm at the top.
Edit - taper boring the stacks could be challenging, perhaps more so on the two angled intake runners on the ends of each end.
2. Machine for throttle shafts. I think this shouldn't be too tricky. The shafts themselves and throttle plates can be sourced from a throttle body supplier like Kinsler or TWM. I think the trickier part here would be making some bosses to have a good place to put thottle shaft bushings, and welding these to the stacks.
3. The throttle shafts need to be connected by a flex joint between each stack to allow for heat expansion without worrying about throttle plates binding as the engine gets hot. The same suppliers mentioned above can provide these too.
4. Modify the MFI linkage to work with the new configuation. I believe the crossbar can still be mounted in the standard place. The MFI pump actuator and the throttle cable attachments can remain the same, I think. Arms to actuate the throttle shafts would have to be fabricated. The crossbar and actuator arms are steel, so I *think* cutting off, repositioning, and welding the standard arms should work for this. I think the standard MFI linkage and rods ends can be modified to work with the reconfigured setup.
5. Make some adapter plates to allow the flanges at the base of the MFI stacks to bolt to the 911 intake ports.
6. Find some intake trumpets that fit and look the part. I notice that Eurometrix makes some replica high butterfly trumpets. That's one option.

The above would not be easy or cheap. But maybe cheaper than the $6000-15,000 that I see original or replica high butterfly and slide setups going for ...

Thoughts?

Scott

[r lane]

I am 5-6 years late here. Curious as to what you accomplished. I have built a low butterfly system for a shortstroke 3.2. used 2.2 stacks and devised a tapered boring bar to machine the stacks. Built injector blocks for the 3.2 heads and found a longer belt for the pump. I am now building a 3.6 and would like to do a high butterfly setup. Will have to machine from billet material, as the 2.2 stacks will not be large enough I think to handle the additional displacement. I would appreciate any feedback. Thanks, Bob Lane