Custom High-Butterfly MFI???

[YTNUKLR]

This is an odd question. I was looking at the RSR high-butterfly system on the Eurometrix site. It seems like the RSR system is basically a shorter stack w/ a much taller throttle body. If the RSR system is capable of delivering almost 110hp/L (I know, cams, compression, twin plugs..), other engines don't even seem to get close. A 2.8 with the same everything else besides the MFI seems like it would "only" muster about 102hp/L. SO...

I just met someone that thinks they can fabricate something for me that is basically a throttle body "spacer," like a Weber manifold, but for MFI. It would take a stock 2.2 or 2.4 system and raise the whole set up higher on the engine. Since the actual throttle butterfly would be higher, you'd make more power, correct? With some flexible lines and creative linkage mods, I would even go so far as to tap the "manifolds" for MFI injectors, like the RSR/ST/908/910 etc. (rather than run them through the heads) I'd also probably hacksaw the stacks off a little shorter, so you could fit it more comfortably in the engine bay.

Am I completely retarded, or does this make sense?

Thanks

[Shuie]

I asked a similar question a few weeks ago.

MFI injector placement

Check out the pic of the TWM 3003 ITBs on top of the weber manifolds using the weber linkage. Looks like the injector bungs in the 3003s could easily be re-tapped for MFI. all you would need is a custom set of fuel lines and you would have modern high-butterflies! Seems like it would work great for 3.0+ heads that are not machined for MFI injectors. It would definitely be cheaper than a set of 3.0 RS high butterflies

[Chuck Moreland]

Call John @ John's Fuel & Carb

510 786 2505

He is working on something like this, tell him I sent you

[YTNUKLR]

OK! Cool

[tctnd]

Be aware that while custom MFI setups have a high "cool" factor, tuning them can be an exercise in futility. EFI systems typically use a digital "map" with all the engines possible operating conditions from idle to redline, no throttle to full, cold to hot, and so forth. Any point on the map can be easily changed with a keystroke to dial it in to suit whetever engine you've built. MFI, on the other hand, encodes most of this info on the surface of a "space cam". This is a three dimensional cam that is rotated and moved lateraly in response to speed, load. temp and so on. The cam must be specifically contoured for the engine configuration in use, a job typically requiring the resources of an auto factory. The further you stray from an engine build for which a a space cam is available, the more you will need to kluge your setup and the poorer your chances of sucess. Hope I don't sound like a spoilsport here, but if you are embarking on a quest like Don Quixote's you should know what you're up against.
regards,
Phil

[Mike Bonkalski]

I agree with tctnd, his assessment is right on the money. Although the cool factor would be through the roof, spending big $$$$ on a custom MFI setup doesn't make much sense.

You could look relatively period correct with a set of Weber IDA throttle bodies (guys with 3.0's and 3.2's can get 46's) with integrated fuel rails and everthing would be completely tuneable using a laptop.

[Scott Clarke]

So, what is the advantage of high butterflies anyway? It must reduce throttle response. Does it have to do with port shape and size?
-Scott

[82SC]

Trust me I don't have my panties in a bunch but whenever there is an MFI thread...there always seem to be people who have to tell us that EFI is better and MFI is buring your money...

I think anyone who is looking into MFI is someone who realzes the pros and cons but wants to pursue this arcane outdated inefficient FI for a reason...

yes EFI is better...well so is water cooling and so in mid engine...

I love MFI I think it is nostalgic...like our cars...it sets your engine apart from other...it was used on all those georgeous race cars we dream about...and nothing looks cooler in a 911 bay then a 12 lead dizzy and MFI setup...

(although the previous posts are very polite about it...)

MJ

[silverc4s]

Saw an engine the other day with the MFI stacks set up ON TOP of a set of 930 intake risers. Quasi-high butterfly on the cheap!!

EDIT: I guess you know that length of the inlet from the injector nozzle to the valve is the main advantage - not really the butterfly location. Butterfly just have to be above the injector, so higher is higher, right?
The longer path is more conducive to high RPM performance tuning/torque. Lower location betters low RPM performance.

[jpnovak]

Do the same with injector blocks and add either MFI or EFI to any engnie. Of course, you still have to worry about stack/throttle body diameter.

[silverc4s]

Jamie & i are on the same wavelength once again!!! ;-)

[Zeke]

Cool factor aside, it's for racing. Can you justify that? I'd like a little more low end, not less, and I plan on acheiving that with a motor that I have on the floor ready for a rebuild. Basically a higher compression 2.4 E. with possibly Solex type cams. I'll use the metal stacks if I can find another set.

[Grady Clay]

I agree with your 110HP/L figure for a stock RSR (2.8-3.0). Some of the trick engines got 10-15% more. For a street engine 102HP/L is a bit optimistic – that is 306 HP from a 3.0 liter, S cam, MFI. Yes it can be done but it takes a lot of work and everything must be perfect.

Remember, RSR high butterflies were larger diameter than stock S intakes. Another issue is the “reverberation plate” above the intake. If done properly today it increases drivability with race cams and perhaps adds a little HP with street cams.

With “high butterflies” I think Porsche was trying to place them at a resonant node in order to get peak power. In ’74 they went to slide valves with the 3.0RSR. At WOT it doesn’t matter where the slide throttle plates are.

Phil, you are not a spoilsport. That is exactly right. There is a limit for suitable applications for stock MFI. The 2.8S is easy, 3.2 might be a slight challenge. Larger I can’t say. The MFI pump is capable of putting out enough fuel for a 4.0+. The issue becomes drivability at other than WOT and high revs.

The MFI space cam is a 2-dimention mechanical computer. It has two inputs – throttle position and RPM. Yes there are some auxiliary inputs for barometric pressure, starting, and cold running. There isn’t any feedback from how the engine is actually running.

MJ, you are correct. The pros and cons of MFI need to be considered. This is pre-war technology (for the younger generation that means pre-WWII.) When used in an appropriate situation it has outstanding performance. It has among the best throttle response – as good or better than the 997 I drove a bit ago. It also has drawbacks. As much of an enthusiast as I am about MFI, the most serious deficiency is the lack of feedback. There is at least one Pelican trying to develop a MFI high pressure fuel pump with EFI technology. Talk about the best of both worlds.

SO, where is MFI appropriate? Certainly on all the 911s where it was originally installed. I am dismayed by the attrition of MFI systems. Some of the best high performance early and mid-year 911s have MFI. Yes, it can be successfully adapted to larger displacement engines. Is it perfect – no, but it can work great with some TLC.

Best,
Grady

[mpdevelopment]

The key to the high butterfly system is the 50mm throttle bore and the length of the system from the intake valve to the top of the intake bell. As tested on my flow bench the street car thottle diameter and the thottle shaft greatly reduce the intake port flow. If you want power you must bore out the throttle bodies, but you are limited by the thickness of the castings. If you raise up the throttles you will have to work with the throttle linkage in order to get the proper syncronization between the pump and the butterflies.

[Shuie]

My favorite thing about MFI, other than the sound of course, is the fact that it takes a WWII skill and tool set to adjust.

mp, what size intake ports should the 50mm stacks taper down to? The TWM ITBS are available in a 50mm bore. The old mag stacks can only be bored out to 46mm or maybe, IIRC.

What do you guys think about retapping the injector bungs on a set of these to work with MFI injectors? The linkage should be do-able. The fuel lines could be a challenge, but this almost looks too easy.

[randywebb]

"...greatly reduce the intake port flow."

- This must be at wide open throttle, right?

Everybody can figure out how often they are at or near W.O.T.

[mpdevelopment]

Yes wide open but thats the idea right? The 50mm throttles taper to about 43mm at the port. This is a large port diameter and on the flow bench you can get about the same flow with a smaller port, Porsche seems to have wanted a greater port volume to feed a race motor, thus the very large port and the very large manifold runner. Making the linkage work may be more of a challenge than you think. I can make the injector lines with the proper 2mm ID plastic hose if it is still avalable. What size engine are you building? a 3.6 will need an even larger throttle.

[randywebb]

"thats the idea right?" Well, only in our dreams. Even on a race course ... I'd like to see somebody instrument this a record it on a datalogger on whatever course they use. Sonds like a Jack Olsen project...

Anyway, it isn't as common as we think... I'll agree that the ability to have more port flow is a good thing tho.

[r lane]

I love MFI, but then again, I am an old guy. I have a 2.7 street car with that system. Couldn't ask for a more responsive engine. Am tracking a late 70s car with a 3.2 mfi set up. Taper bored a set of stacks, opened up the throttle bodies, made recievers for the injectors, raised the pump (longer belt), and the car is a blast. Have not run on the dyno, but has enormous power and torque. I am now in the process of building a 3.6 with MFI. It will be a dedicated track car, where as the 70s car can be driven on the street. I have 965 with EFI. No slam dunk there. Both systems require an enormous amount of dedication and time to really dial them in. Like some of the responders here, am still trying to figure out which intake system to go with. Bob Lane

[Matt Monson]

12 years is a long time to be thinking about it...