MFI injection pump modifications for 3.6/3.8 motor?
 

Every once in a while I see a cool mechanically fuel injected 3.6/3.8 rsr motor and wonder what work is needed to get the pump to flow more fuel for more displacement. I've heard that the MFI pumps can satisfy the fuel requirements for such motors but not sure what modifications are necessary to the pump, if any.

Does anyone here have knowledge as to what injection pump work is needed to modify a MFI injection pump (e.g. 69 911S pump) to provide enough fuel throughout the RPM range for a larger displacement race engine?

To get the fuel flows needed for a 3.6 / 3.8 RSR type engine the MFI pumps space cam has to be re-mapped. Many other mods are needed in the pump to help achieve this flow. Also a single electric fuel pump will have a very hard time suppling fuel @ 7000 rpm up for the 3.6 / 3.8 so a second pump is needed, (plumed in parallel) to mantain fuel supply.

Thanks Mark. I have a core 3.6 long block and was potentially thinking of using MFI as part of a long term project. I'll contact you offline about it in the future if I go that route.

Mfi
 

I have to give Mark an attaboy for a project some years back, we had the task of building a 3.5+ Hi Butterfly pump gas MFI engine that went into a narrow body street car for a Kelly-Moss customer. Mark took all the info and built the pump and we ran it on the engine dyno made the adjustments and needed more fuel, sent him the dyno sheets and pump, we ran it again and it was perfect. Its one thing to get a pump good enough for a track car but quite another to make good power and driveability for the street.
Thanks, Mike Bruns

I heard once that the Porsche 911 MFI injectors don't flow enough for some high hp motors. One MFI pump rebuilder suggested that injectors off a Mercedes 230SL have more flow?

Scott

Mfi
 

We ran an IMSA 911 into the mid 90's as a 3.6-3.8 with stock injectors at close to 400 hp
Mike Bruns

Scott, The Mercedes 230SL , and most of the other MFI Mecedes, injectors had the same size Pintle ( 1.2mm ) as the Porsche MFI injector. The 6.3 Mercedes had a larger 1.8mm Pintle injector. I think there was a size or thread issue that made it a little hard to fit the 6.3 injector in a Porsche head but on top of the stacks was the way to go. Also the 6.3 injectors are almost impossible to find now.

Thank you, Mark. Maybe it was the 6.3 ones then. Re: fitment, I recall the guy said something about machining the Mercedes injectors for the Porsche threads, so I guess that's an option.

Scott

Injector Location
 

I was never a fan of locating the injector in the top of the stack.Scott I ran a Slide Valve MFI motor on the car you have for IMSA GTU and it worked well but I always had the thought of fire possibility with fuel still spraying when you closed the throttle.I never did a dyno test to compare the two locations though.My feeling is below the throttle plate would be more efficient.Ciao Fred

Mfi
 

Fred , I have had a big slidevalve Engine’s on the Dyno and tested them in the port and up top, we noticed some gains especially when playing with pump timing but not enough to get excited. The reversion looked like a fuel tornado above the stacks at hi RPM, I didn’t like the thought of that.
Mike

Fuel Reversion up top
 

The standoff you see at high RPM is a result of the exhaust style and size.Every motor size uses a different length and size to the collector according to the amount of CC,s.Placing the antireversionary cone downstream of the exhaust port is determined by a mathematical formula using a percentage of total cc,s to get the location correct.Once found better power and no flat spots in the torque curve.Ciao Fred

Interesting discussion.

That brings up one other question on what would be a good throttle body/stack set up for the 3.6/3.8 motor. I see that PMO sells a MFI kit with 50 TB but would be interested in what else might work well for these large MFI motors. Some seem to use a high TB system that looks similar to those used on the vintage 2.8/3.0 RSR motors but not sure if those are custom made or sold by a particular vendor?

Rothsport sells some very nice Throttle Body Jewelry

A 3.8 MFI motor is on my dream list.

Those look great!

The only issue for me might be that my 3.6 is a 3-bolt to the heads instead of a 2-bolt as in the pictures and used on 3.2 engines and earlier. I think it's possible to get adapters but that would raise the height of the set up.