Finally drove MFI

[Superman]

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!

[kltarga72]

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

[jluetjen]

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.

[Jeff Higgins]

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"...

[gptom]

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

[Superman]

Originally Posted by gptom

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.

[shoooo32]

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.

[Superman]

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.

[kltarga72]

[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.

[Tremelune]

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?

[Arne2]

Originally Posted by Tremelune

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.

[Tremelune]

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.

[Coastr]

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.

[Skip Newsom]

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.

MFI adjusting tech manual

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

[Arne2]

Originally Posted by Coastr

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.

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.

[Jeff Higgins]

Originally Posted by Tremelune

Am I reading the wrong book?

Yes.

Originally Posted by Arne2

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 app 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.

Spot on. Great explanation.

Originally Posted by Tremelune

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...?

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.

Originally Posted by Tremelune

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.

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.

[WP0ZZZ]

Originally Posted by Coastr

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.

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.)

[Matt Smith]

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

[Tremelune]

Originally Posted by Jeff Higgins

The only thing we really notice, its "claim to fame" is that throttle response.

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.

[winders]

Originally Posted by WP0ZZZ

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.)

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