Finally drove MFI
 

I finally drive an MFI 911. Wow! I wanted to experience the legendary "immediate" throttle response. Yup. Immediate is right. As Tyson Schmidt had told me, it feels like you are pushing the car with your right foot. Anyone thinking their non-MFI car has immediate throttle response should drive an MFI car for comparison. Just WOW!

Yes, all air cooled Porsche owners should have that MFI experience at least once. Put yourself back in time to the early 911's period and the other autos on the road. I had a lot of fun seeing my Chevy and Ford friends reaction when driving my MFI 911. The look on their faces when they were already past 4000 RPMs and than pushed the gas pedal to the floor.
I loved the 911-T but the 911-S was a race car at 4500-7200 RPMs

You're now part of the MFI inner circle. It does make you question the wisdom of removing the MFI from all of those old E's and S's and replacing it with Webers because they are "easier".

If you want "easy", buy something else. If you want immediate throttle response, buy an MFI'd 911.

Fun stuff, huh? There is nothing else like it. Maybe a Tesla, but there you don't get the sounds and smells... "MFI rules, CIS drools"... ;)

I've only driven 2 MFI cars, so I have assumed they all are like that

They're not all like that. Because of the distance between the air flow sensor plate and the throttle, and from the throttle to the cylinders, and because of the elasticity of air, CIS throttle/fuel systems in 911s have some of the slowest throttle responses in all of motoring. They feel a bit like a turbine in the sense that power "spools up" for a time after opening the throttle. We are talking whole seconds here.

With MFI, the lag time is shortened to....uh....zero. Full power happens NOW.

I've driven a stock '74 Euro Carrera and a '71 hotrod with a 2.8 MFI - both kinda ran like $hit... doggy out of the hole and came on around 3700 RPM. I'm guessing these cars suffered from deferred maintenance because the "MFI throttle magic" just wasn't there.

And yeah, a stock CIS SC isn't a very thrilling experience when it comes to throttle response. But there's plenty you can do to jazz it up. Uncork the exhaust, install more aggressive cams, lighten up the flywheel, ditch the decel valve, etc.

Like most things German this system will function beautifully, but only if the parts and adjustments are in spec.

The cams in my CIS 911 have the most overlap which CIS will tolerate, which is not nearly what MFI will accept. My car actually runs about as well as a CIS 3.0 can run. It is a blast to drive. But power does come on slowly, like a turbine. Like a slingshot or a rubber band. MFI is like being rear-ended but quicker.

[QUOTE=shoooo32;11442034]I've driven a stock '74 Euro Carrera and a '71 hotrod with a 2.8 MFI - both kinda ran like $hit... doggy out of the hole and came on around 3700 RPM. I'm guessing these cars suffered from deferred maintenance because the "MFI throttle magic" just wasn't there.

Early 911s were not designed for drag racing, they were not engineered for racing from a dead stop. Porsches won races on a road course where the engine RPMs stayed in the 4000-7000 plus range.
When I picked up my 1972 Porsche 911-T at the factory in September 1971 they assigned a technician to go over a factory information checklist. One item that the technician indicated was very important was to keep the engine RPMs over 2800 when moving. Of course letting the engine idle at a stop light is okay.

I think we need to define some terms here...When I think of "throttle response" I think of the engine revving while not in gear. It sounds like you're describing...power. Like having immediate power when you step on the gas while driving.

Like, just having more power from any NA motor will "push the car with your right foot"...I wouldn't call that throttle response. Similarly, having lots more juice above 4k RPM is just...standard in these cars (unless we're only talking about Ts and ignoring the 80s).

Am I reading the wrong book?

No, you are thinking correctly. The MFI does have better throttle response than most engines. It responds to sudden throttle changes quicker, and without any band-aids like accelerator pumps. The reason is that MFI is the only fuel delivery system that does NOT measure airflow, or require the Venturi effect to pull fuel into the intake charge. When you open the throttle on an MFI-equipped car, you are not only opening the throttle plates, but also directly connected to the fuel map. Opening the throttle moves the main rack in the injection metering pump to provide more fuel.

Other systems have to measure the airflow before reacting to deliver more fuel. That delay is the "less immediate throttle response" that other engines have.

That said, MFI is not better in all ways. That fuel map is not easily changed, other than moving the entire map richer or leaner. And it is absolutely NOT fuel efficient. Best mileage I have ever got from my stock '72T is 17 MPG. That was on a gentle back road tour with no high speed segments.

So like...can anyone compare the throttle response of $15k MFI setups to $15k coil-near-plug EFI ITB setups from Rasant and X-Factory...?

I understand the legend of MFI (particularly in the carbureted era), but I find it hard to believe it still bests what's available today.

ITB with Efi using TPS and tuned correctly should feel the same as MFI. EFI doesn’t wait for a vacuum signal like carbs and cis. MFI is basically ITB with a diesel pump to squirt the fuel straight in.

I agree MFI has a different response that is addictive, even the lowly Touring version.
Get em tuned right using a Gunson Gastestor and strict adherence to the Check, Measure, Adjust bible and they perform amazingly well and hold a tune for a good long while.

http://forums.pelicanparts.com/porsche-911-technical-forum/254520-mfi-adjusting-tech-manual.html

Gas mileage sucks for such a small light car, but you are in the wrong car for that...
I get about 15 MPG out of a 72 2.4

They run way better if you take the rubber stoppers out of the stacks
http://forums.pelicanparts.com/uploa...1630380863.JPG
http://forums.pelicanparts.com/uploa...1630380863.jpg

Very true. Modern systems don't wait to measure the airflow, their computers and sensors are fast enough to "predict" what the airflow is.

On the other hand, MFI is still pretty amazing for 50+ year old tech.

Yes.

Spot on. Great explanation.

These EFI setups still have to "read" parameters and "decide" (all electronically) what to "do" next. "Oh, he just opened the throttle plates (the only direct mechanical connection to anything). How far did he open them? How fast is the engine revving? What is the current A/F?" All questions these modern EFI systems have to "ask", then "answer" before "deciding" how much more fuel to provide. And, when that fuel is provided, it is delivered at quite low pressure, up in the throttle bodies. Which then has to meet incoming airflow to make it into the cylinders.

MFI doesn't "ask". Air and fuel delivery are mutually "blind", so it can't. It simply delivers fuel with no "visibility" of the changing air flow. And at the same time... And at about 240 PSI, directly past the intake valve and into the cylinder. About as close as we came to "direct injection" in the 1960's and 1970's.

No one has ever said it is the "best". A/F ratios will vary by up to two full points in even the best tuned systems. As mentioned above, it gets less than stellar mileage. It's really stinky. It only really excels in two areas - throttle response and power. The throttle response because neither half of the equation - neither fuel nor air - has to wait to see what the other half is doing before it responds. They respond independently. Power, because of the thorough atomization of the fuel charge at that 240 PSI injector pressure - we can run a richer A/F ratio and still burn all of it.

MFI is certainly not "perfect" by any stretch. It's a dated system. It can be tough to live with. Most people, weaned on more modern systems, wouldn't want to live with it. The only thing we really notice, its "claim to fame" is that throttle response. It's the difference between turning on the dining room lights by turning up a dimmer switch vs. simply flipping a light switch.

There's nothing in an MFI engine that a modern EFI+ITB engine can't replicate, right?

(Other than the nostalgia and period-correctness, of course, I don't want to minimise the importance of subjective feelings when talking about classic cars.)

They are quite a beautiful sight to behold as well IMO. Great pics Skip!

It seems that this claim to fame is now fully attainable with modern systems, with fewer compromises (or "character" if you prefer), for the same money.

My goal isn't to downplay the impressiveness of this technology; it's to understand it in context of current tech—to dispel the mythology without discounting the value of what vintage cars bring.

Nothing! In fact, EFI + ITB is much better than MFI.

MFI rocks, plain and simple. If it's at all an option, do it.

For a traditional Sports Purpose 911, I can't recommend MFI enough. For one thing it's completely fascinating, and how well this old tech works to make power, especially on a 3.0 or bigger motor, eclipses the tuning advantage that a more modern EFI/ITB system may have.

Been there, got the sticker!

http://forums.pelicanparts.com/uploa...1630415714.jpg

I've been farting around getting the 3.2SS on MFI working as well as possible and it can certainly be very, very good. It does take a bit of fiddling, but in 2021 the knowledge is out there, and the technology exists, with wideband O2 sensors, to color outside the CMA lines and tailor the system to just about any 911 motor. I'm not the first one to map out a space cam, but this is the "T" cam I am using to run the 3.2 in "Project Heavy Metal" which uses a custom machined big bore MFI induction system.

http://forums.pelicanparts.com/uploa...1630416058.jpg

(EDIT) - all that said, I've started purchasing components for an EFI/ITB system for my AutoX car. Everything has a place....

You're on the right track Jonny.

Thanks to You, and Jeff Higgins, and countless others here on the forum!!!

We have covered this ground oh so many times. Those with little or no first hand experience with a properly tuned MFI system will always say "modern EFI/ITB is better". And you know what? They are absolutely right. It is far, far better. More consistent fueling across the range of RPM, throttle position, and load. Better power across the range due to that. Better milage due to that. Cleaner running and less stinky due to that. Much, much better in every regard.

Except for "feel". A "feel" provided by that instantaneous throttle response. There is nothing else on the market that can match it. Modern EFI/ITB does come very, very close. But it is not there yet, and that difference can be felt. This difference is a lot of fun for some of us, some could not care less and won't want to put up with all of the downsides of MFI to get it. I understand that, especially considering just how close we can get these days, with none of the compromises. But, for some of us, those compromises are indeed worth it in the face of the "feel" MFI provides. You simply cannot get it with EFI/ITB setups, even as good as they are.

This strikes me as a religious position from those with a vested interest in MFI being irreplaceable...

That is pretty much it.

I have driven a well tuned 1973 911S and the throttle response was not any better than my race car's MoTec EFI setup with AT Power ITBs.

I get that people want the MFI on cars that came with it or if they just want MFI on their 911 to be period correct or get that period feel from the best injection system available at that time. I would feel the same way if I had a 911 from the MFI period.

Mfi
 

I do think sentimentality plays a big part in our choosing MFI, but the fact is, it works wonderfully. 917 normal and turbo, 935, 936. I have a good friend who has raced for many years with both MFI and carbs. He presently campaigns very successfully a 3.0 RS with carbs and is after me to ditch the MFI, as he feels the PMOS/Webers are less maintenance. I have driven for years as a daily driver a 72 2.7 with MFI and the joy I receive from its performance and sound never fades. I track a 3.0 with MFI and I don't know what carbs could give me that the MFI doesn't but I think nothing, as the throttle response and pull with no flat spots is phenomenal. An image speaks to each of us differently, but if you had never seen one, this could cause one to pause. I prefer MFI for more reasons than performance.http://forums.pelicanparts.com/uploa...1630437853.jpg
http://forums.pelicanparts.com/uploa...1630438090.jpg

Absolutely!! Now you are starting to understand. :D

Nonsense. I've driven an example (or several) of just about every induction system ever offered by Porsche, and most of the modern aftermarket EFI/ITB setups as well. It's subtle, but it's there. I think a lot of guys that spent a fortune on the modern EFI/ITB systems are simply in denial, or simply do not have sufficient seat time with MFI. Of course, the same could be said for the MFI faithful as well. ;)

I've always wondered why so many guys who are ostensibly seeking some kind of a "vintage sports car experience" spend so much money, time, and effort to modernize them. Not just induction systems, but but modern brakes (Boxster caliper kits), modern suspension (coil overs, pick up point relocation, etc.), massive power increases (3.6's in early cars, etc.), modern A/C, and on and on. More power to 'em, just not for me. We all enjoy this hobby in our own way. That's what is important. Of course this "bench racing" is a large part of it as well. Fun stuff...

I'm curious about the slide throttle you have pictured. Are you driving this engine on a race car only, or street, or.....? I'd love to hear an assessment of the slide throttle performance for a non-race application.

Slides aren't even great for race cars. Butterfly setups, especially like the AT Power ITBs with shaftless butterflies, are better for street and race cars. The only advantage the slide has if ITB size is limited as it will flow more air wide open at any given diameter. The shaftless butterfly design At Power has pretty much eliminates that advantage. The barrel ITB is better than both but has issues of its own including cost and packaging.

https://www.highpowermedia.com/Archive/barrel-butterfly-or-slide

Yeah, slide valves are pretty much just an "on/off switch". Especially the original RSR and the like. I borrowed and played with an original set some 15 years ago, before they apparently got gold plated or something (at least their value started to reflect that sentiment). Really unsuitable for a street car, and I tried, believe me I tried.

Interestingly, a walk through the paddock at the last Rennsport revealed a fair number of cars that should have had slide valves, that had them originally, that now had plain old butterfly throttle bodies. Not even high butterflies, which incidentally create their own problems as well.

I've heard good things about Gamroth's new slide valves, like they don't stick and get gummed up like the old ones did. I have to believe, however, that they add more in "cool factor" than performance. And, yes, they are pretty darn cool...

slide valves
 

They are on a track engine. I have never tried these on a street car but a couple of responders just stated that they don't recommend it based on their experience. It would seem that a half moon knife edge hanging half way out of one side of the stack would create lopsided turbulence, but during on and off throttle and partial throttle, I do not feel flat spots and acceleration feels rapid and smooth. Obviously at full throttle, it is a straight clean unobstructed chimney. I have been at this for 2-3 years so don't have much experience to support what I think. Flow bench info would be interesting. I have a set of 2.2 stacks that I taper bored out to accommodate another 3.0 track motor. 47-42mm I recall. These two engines really feel similar. http://forums.pelicanparts.com/uploa...1630453240.jpg

Even great slides are pointless on a modern build. You are better off with great butterfly ITBs. “Cool” to me is getting around the track faster.

If you need slides for authenticity you probably wouldn’t be able to use “modern” slides anyway. I just don’t get the draw for slides on builds not constrained by authenticity rules. They cost more and don’t work as well as butterflies…

Agreed. Jeff is very much an enthusiastic "mad scientist", though, and I believe does a lot of stuff just because it's so darn cool. His slide valve setup, with its modern common fuel rail and servo fired injectors, will certainly never meet anyone's authenticity requirements. And, knowing Jeff, he wouldn't do it if it didn't make good power. That doesn't mean it would be street drivable, though. Is anyone here actually running this setup? It would be nice to hear some feedback. Beautiful stuff:

http://forums.pelicanparts.com/uploa...1630454243.jpg

My 1970 911E had a 3.0 with Webers. It was 250HP and the car weighed 1900 lb. I've never driven MFI, but the throttle response with the Webers was pretty amazing. It occurs to me I'm referring to the car in the past tense. It still exists, I just sold it, is all. It's now somewhere in France.

This thread makes me sad. Makes me miss my 69E, the only car I've ever regretted selling, the MFI being a big part of that and it looks like I'll never build a 4.0L MFI motor using one of the Catorce cases since no one seems to be getting a case.

Carry on.

MFI is a supercharger for fuel injection.

Chris
73 911 E

T, E and S MFI engines are awesome but a 2.7 RS Spec MFI is a beast.

Yeah, imagine being lucky enough to somewhat, kinda sorta, combine all of that. Things just kind of fell into place for me. Guys were giving me take-off MFI systems when neither they, nor their mechanics, could figure out how to make them work. I have several fully functional systems, of various specifications (T, E, and S of different displacements) sitting on shelves in my garage. Free. All of it. It only made sense to try to make it work on whatever hot rod I was going to build. Which, in the end, turned out to be a 3.0 liter, 11:1 compression, custom cammed (to my specifications, via a custom grind from John Dougherty), twin plugged beast. 2,200 pounds with its 100 liter tank full:

http://forums.pelicanparts.com/uploa...1630462148.jpg

What causes the extra latency that makes the throttle response of an EFI/ITB engine to be worse than that of an MFI engine?