MFI used systems, are people actually doing anything with them?

[911st]

I see what seems to be crazy prices on MFI systems and am having a hard time understanding it.

Don't get me wrong, I love MFI and have done a full conversion before on a 2.8 twin plug.

However, at these prices it has to be $4000 to $6000 or more for a performance MFI induction change / build.

Are most of the systems sold just sitting on the shelf of dreams that will probably never be?

I am having a hard time understanding $2000 for a box of old parts that were usually taken off because it was better solution to to replace them with Webber's.

I think in the years I have been hanging around Pelican I have only seen one completed DYI project that caught my eye. Yes, there are $20k MFI tuner dream motors out there but for most it is cheaper to go with PMO's or a full 3.2 or 3.6 EFI motor swap.

So, how do we justify used MFI prices?

[BK911]

The only thing I can think of is originality.

I would rather have MFI on my car than PMO, webers, EFI, etc... if my car was originally equipped with them.

[ossiblue]

Originally Posted by BK911

The only thing I can think of is originality.

I would rather have MFI on my car than PMO, webers, EFI, etc... if my car was originally equipped with them.

+1. It goes along with the rising price of original long hoods in general.

[304065]

Used pumps are a few hundred bucks, particularly ones in working condition. Core throttle bodies and stacks, lines and injectors the same. So let's call it $1000 before you get a good worn out system. Gus probably charges $1800 to rebuild a pump and has a several month backlog. Rebuilding the stacks is $1480 at Eurometrix. So you're at $4,280 before investment in any of the tools (LM-2 or other wideband 02 measurement, adjustment tools, synchrometer) needed to make it work right.

THEN you have to deal with the process of getting them to work properly on the motor. For a non-standard set of cams, the tuning process is very time-consuming.

A set of new PMO carbs will run you $3800 and it is generally acknowledged that carburetors give up 10 horsepower to an MFI system.

I think it's the fact that so few of the systems are actually remaining- they are magnesium so they are trying to go back to the mine just sitting there-- and demand among enthusiasts is still high relative to supply. This is a situation of our own making-- ten years ago there were a small handful of people who knew anything about these systems, now, through Pelican, there are hundreds, including guys like Mark Jung who are picking up where the factory left off!

[RSTarga]

Ah but when they are right nothing is so sweet. Incredible throttle response CSI EFI or PMO's don't come close.

[RWebb]

I keep hearing people say that MFI has better throttle response than carbs (PMOs).

BUT I never hear anyone explain why or how they tested to be sure their impression is really true...

John, or anyone else???

[PropellerHead]

All else equal - Physics dictate that throttle response is directly related to the volume of air between the throttle place and the intake valve. Larger volume indicates a slower response. This is why individual throttles are far superior to single throttles with a large plenum when it comes to response.
I can't see a measureable difference between MFI and carbs in this regard. If MFI does a better job of controlling air/fuel ratio in transitions is likely the result of the state of tune.

In short, I don't believe MFI to be superior to carbs in throttle response. Plenty of areas where MFI is great but I don't think throttle response is one of those areas where it exceeds well tuned carbs.

[304065]

Theoretical answer: MFI Fuel pressure is 220 PSI- when the linkage commands WOT the fuel delivery is simultaneous with throttle plate opening. Compare this with the vacuum signal required by even the best PMO carburetor-- you must first open the plate, generate the airflow then pull the fuel out of the bowl and into the airstream, all of which translates into latency vs. the same linkage movement both opening the throttle plate and blasting the fuel into the port.

Also remember this about the 40IDA and derivatives-- and this is a vast oversimplification- the fuel curve is generated by the idle and main jets and the emulsion tubes and the air corrector jet. Say you are running on the main jet and rpm increases, you get into the region where the air corrector jet compensates to keep the mixture from getting too rich. All this in and out of jets affecting the mixture, well compare that with the linear fuel flow coming from the MFI system rack. The rack moves in and out depending on the space cam, which in turn is positioned by RPM and throttle position-- but the movement itself is linear.

Now have I actually done a study doing an integral on the rate of change in rpm given the same motor and operating conditions (baro, temperature, load) and two period induction systems, let's say MFI and Zeniths, well no. . . I own one MFI car and one carbureted one, but that's hardly scientific, right?

[304065]

Originally Posted by PropellerHead

All else equal - Physics dictate that throttle response is directly related to the volume of air between the throttle place and the intake valve. Larger volume indicates a slower response. This is why individual throttles are far superior to single throttles with a large plenum when it comes to response.
I can't see a measureable difference between MFI and carbs in this regard. If MFI does a better job of controlling air/fuel ratio in transitions is likely the result of the state of tune.

In short, I don't believe MFI to be superior to carbs in throttle response. Plenty of areas where MFI is great but I don't think throttle response is one of those areas where it exceeds well tuned carbs.

The distance between the back of the intake valve and the throttle plate is materially shorter on an MFI engine compared to a carbureted engine, even with the short Weber manifolds.

[PropellerHead]

Originally Posted by 304065

Theoretical answer: MFI Fuel pressure is 220 PSI- when the linkage commands WOT the fuel delivery is simultaneous with throttle plate opening. Compare this with the vacuum signal required by even the best PMO carburetor-- you must first open the plate, generate the airflow then pull the fuel out of the bowl and into the airstream, all of which translates into latency vs. the same linkage movement both opening the throttle plate and blasting the fuel into the port.

Also remember this about the 40IDA and derivatives-- and this is a vast oversimplification- the fuel curve is generated by the idle and main jets and the emulsion tubes and the air corrector jet. Say you are running on the main jet and rpm increases, you get into the region where the air corrector jet compensates to keep the mixture from getting too rich. All this in and out of jets affecting the mixture, well compare that with the linear fuel flow coming from the MFI system rack. The rack moves in and out depending on the space cam, which in turn is positioned by RPM and throttle position-- but the movement itself is linear.

Now have I actually done a study doing an integral on the rate of change in rpm given the same motor and operating conditions (baro, temperature, load) and two period induction systems, let's say MFI and Zeniths, well no. . . I own one MFI car and one carbureted one, but that's hardly scientific, right?

You missed one important item on carbs - the accelerator pump. It squirts fuel for transitions without waiting for the pressure signal response as you describe. This is tuned independent of main/idle jets to cover the transition. If it is tuned well or not depends on the car/owner/set-up.

Also, the integral rate of change study (assuming this is done at some point) needs to be done on the same engine. Bore/stroke, rotating inertia, comrpession ratio, etc. all affect this. I have no doubt that an early MFI 'S' engine might experience a different throttle response than a Zenith 'T' engine of the same year. An observation which is probably where this throttle response superiority theory/rumor originated. But I don't believe that is all due to the MFI system.

The marginal difference in volume between MFI and carbs (I agree there is some difference there) isn't likely to contribute a measureable difference in throttle response.

MFI is great and I'm a fan for many other reasons, don't get me wrong.

[RWebb]

Interesting - and your comments make sense -- qualitative sense.

How does it feel to you when you drive your MFI car back to back with your carb'd car?

[BK911]

Originally Posted by 304065

Theoretical answer: MFI Fuel pressure is 220 PSI- when the linkage commands WOT the fuel delivery is simultaneous with throttle plate opening.

OMG, I think it moved.

But seriously, there is also the atomization issue. The MFI squirts fuel on the back of the hot intake valve at hight pressure right before the valve opens. That has to help combustion, compared to the fuel lazily drawn into the combustion chamber.

[BK911]

I think the "10 extra hp" comes from the Porsche hp figures for the euro 2.4T with zeniths vrs the US 2.4T with MFI.

[jstobo]

I always wonder about the oft quoted statement that a carburated Porsche gives up 10 Hp to MFI. I guess its because the euro 2.4 911T with Zeniths was rated at 130 hp and the US MFI was rated at 140HP. But some of those Euro's had 30mm intake ports vs 32mm for the US. But if you look at the 70 vs 71 racing engines 911/20 & 911/22 respectively the 70 had MFI and was rated at 230hp. The 71 had 46 IDA Webers and was rated at 230hp. Both displaced 2247cc. Had the same cams and the same valve and port sizes and the same torque 170 lb/ft. So I think the oft quote could be wrong.

When I rebuilt my 69 E I went with Zeniths for two reasons. 1)Cost 2) Gus said that unless I had a engine designed to a Porsche engine specs, the MFI would be an approximation and carbs would work as well. The only difference I KNOW I can tell is my fuel mileage with the carbs is a lot worse. Some of that is due I am sure to more displacement, higher compression, bigger valves and essentially more HP. But I can live with that as it is no longer my daily driver.

I still have the MFI setup. Just in case I ever sell(or my kids sell when they inherit it).

[Zeke]

I have driven them all. If you are such a good driver, you can deal with any of it including turbo lag from a single massive turbo.

Originality is the strongest argument to retain MFI. Otherwise, I will out drive you with cubic inches and compression every time. I don't care how the fuel gets in there.

[PropellerHead]

Originally Posted by jstobo

I always wonder about the oft quoted statement that a carburated Porsche gives up 10 Hp to MFI. I guess its because the euro 2.4 911T with Zeniths was rated at 130 hp and the US MFI was rated at 140HP. But some of those Euro's had 30mm intake ports vs 32mm for the US. But if you look at the 70 vs 71 racing engines 911/20 & 911/22 respectively the 70 had MFI and was rated at 230hp. The 71 had 46 IDA Webers and was rated at 230hp. Both displaced 2247cc. Had the same cams and the same valve and port sizes and the same torque 170 lb/ft. So I think the oft quote could be wrong.

When I rebuilt my 69 E I went with Zeniths for two reasons. 1)Cost 2) Gus said that unless I had a engine designed to a Porsche engine specs, the MFI would be an approximation and carbs would work as well. The only difference I KNOW I can tell is my fuel mileage with the carbs is a lot worse. Some of that is due I am sure to more displacement, higher compression, bigger valves and essentially more HP. But I can live with that as it is no longer my daily driver.

I still have the MFI setup. Just in case I ever sell(or my kids sell when they inherit it).

Stock carbs vs. MFI will give up a little power because of the venturi (primary and secondary) in the path of airflow. This isn't there with MFI and thus the same port size has less restriction. Probably where the 10 Hp comes from - I believe it is real.

Fuel atomization is really quite good with carbs. Intake valve targeting with fuel injection helps on a 'cold' engine but is offset on a warm engine by the cooling/evaporative gains you get with a carb.

I'm not arguing that carbs are better than MFI. Not at all. I love MFI set-ups but it isn't because of 'throttle response'.

[PropellerHead]

Originally Posted by BK911

OMG, I think it moved.

But seriously, there is also the atomization issue. The MFI squirts fuel on the back of the hot intake valve at hight pressure right before the valve opens. That has to help combustion, compared to the fuel lazily drawn into the combustion chamber.

As Lee Corso would say:
"Not so fast my friend!"

Resonance time is important for fuel atomization. True, the back of a hot valve is targeted for a reason and that is because if you don't the fuel wets the wall and your fuel delivery is crap. There is a reason the Porsche race teams did a high butterfly injection - the cooling effect of evaporation/atomization of the fuel. This shouldn't be discounted and is why a properly tuned carb set-up can produce just as much power as an injected motor given other considerations.
The key to quality combustion is a homogeneous charge mixture (at desired AFR levels of course). A fully mixed charge is the most important and the engineers spend a lot of time getting swirl and tumble, atomization, etc correct to maximize this. Getting a homogeneous charge is simply easier with carbs where the fuel introduction occurs farther upstream at lower manifold pressures.

The US Zenith carbs had very small venturi sizes whiched choked them relative to MFI stacks.

[Eagledriver]

While carbs have the accelerator injector, they do a poor job of getting the proper mixture during transient throttle movements. There is allways too much or too little fuel depending on throttle position or movement. MFI can get much closer to perfect in all those situations. I've raced my Spec 911 with an MFI engine and now with PMO's. There is no question which one has better/more accurate response. If I could've got enough power out of my mag case 2.7 to compete with the bigger motors I'd have kept it.

-Andy

[911st]

I suspect there is less restriction in an MFI v Carb intake tract. No venturi restriction to accelerate and decelerate which could add drag and effect the air velocity. Also, no tubes sticking out in the air flow.

Atomization and proper timing of the injection event probably speaks to how easily the air fuel mix is going to be lit and how fast it will burn.

With better atomization of fuel and intermixing with oxygen the fuel molecules are going to be more evenly spaced in the combustion chamber and in better in contact with the area between the electrodes of the spark plug.

[RSTarga]

I've driven all of them and the throttle response on MFI is the quickest. Just seat of the pants here. The only thing that came close was a very trick 3.0 with CSI and high compression built by Bob Jones for his own car. He's done enough 911 engines to have a bag full of tricks.