Does any one know if Bosch ever built an AFM that was bigger than the 928's L-jet AFM, but the circurty being similer enough to be a plug and tune application?

IIRC, the 928 has a weird base voltage at 8 volts, and I haven't seen a bigger AFM on other cars, HOWEVER, I haven't looked on that many cars, so I'm curious if Bosch ever built a bigger AFM that would be a nearly plug and tune application.

Maybe some sort of a Benz, or BMW engine? Porsche couldn't have been the only company to try to use the L-jet fuel system with a V8. Maybe a Jag, I think some of the V12s might have used L-jet, but I can't rember now.

Thanks.

 

IIRC - the AFM on my 944S was physically larger than the unit on my 81 928. If I can find it (car is in pieces) I'll see if the passageway is larger. Maybe the 951 has a larger unit.

Anybody care to check the PET for AFM part numbers of these cars? I'm at work so it'll have to wait until I get home.

 

It won't work, those are Motronic AFMs, not compatible with L-Jetronic. Your best bet is to do the MAF hot wire conversion and then install a larger injector or increase the fuel pressure and pull some signal out. Trying to tune with an AFM won't yield the results you want. The hot wire is so much faster on transient response, so you get a much better throttle response and lower restriction. There is no comparison. All the highly modded 951s are on MAF now. The L-Jet 928s day has come. It is a win win situation. AFM is 1970s technology.

 

I realize that John, clearly the MAF is a superor solution to the problem. However how much did you end up spending on your MAF conversion? If I rember correctly, just to get the peices to set up the system was a pretty nice peice of coin.

If there is a suitable AFM, that is a plug and tune application, the price might be sigificantly cheaper for a similer, if not identical result. Anyways, it can't hurt to look.

 

John,

I would wholeheartedly agree the AFM is primitive tech whose time has long come and gone (just like carbs, roots blowers, and points).

I would disagree that ALL the hot 951s run MAF. Some of the better ones run MAP instead. In fact, there's a fair number of engineers I know who will tell you that MAF is coming up for extinction just like the AFM. MAF is just not incredibly accurate, and it's an expensive piece - MAP sensors are more accurate and an order of magnitude cheaper, and most importantly, tells you the condition of the manifold pressure in real time.

Currently the aftermarket is (somewhat) still hooked on MAF, either because they don't know any better, don't understand EFI, or simply don't have the technical resources to pursue MAP (though honestly, I have the tech sitting on my workbench, it's hardly expensive). But there are guys in the 951 tuner community who know that MAF is also an outmoded tech, and are offering a MAP solution set.

Greg

 

there is a way to plumb 2 afms in parallel, and that would be a nice set up.

maf conversion is the best way, but no one makes a conversion kit. However I have built the physical parts to adapt to the aFM and air box for a US conversion. those parts I still have. however, you need to find a 5-8 volts range MAF, vs the motronic 1-5volt system.

mk

 

I recently spoke to Todd at Protomotive about doing a parallell MAF for the S4/GT/GTS. He said it is possible, with some sort of resistor circuit. I am still trying to find out what parts I need to make this work.

Tod also told me that if the 928 S4 MAF goes over 5 volts, then the EZK goes to "limp mode" Someone needs to figure this out without going to a MOTEC etc.. I would gladly purchase a parallell dual MAF setup for my S4. It would fit perfectly with my supercharger setup.

The S4 MAF is 85mm ID.

Cheers,

 

Now im intrigued for sure!

Im playing around with building a 101.6mm,(4 inch) but its a bitch the way the put the MAF together. For all practical purposes its a sealed unit.


I bet Jon Speake or Rich Andrade could lend some instight to how to do it Blau?
http://www.jdsporsche.com/23fiecu.html
http://www.electronikrepair.com/

 

Tony,

I already asked Rich Andrade about building this circuitry, and he advised me to go to Motec etc. I do not want a 4inch, but may be interested. I would ideally like 2 85mm stock with the special circuit. I will use dual TB, and could mount the MAF immediately befor the TB's...

PS. I am the other Rich Andrade (De Andrade)

Cheers,

 

How about alpha/n setups? I have a friend with an E30 M3 ('88) running the alpha/n system and carbon-fibre airbox. It sounds wicked and he got very good gains with it. He's pusing 235hp i believe (up from 196 or so).

Does anyone run alpha/n on a 928?

 

Totally disagree on the MAF vs. MAP. Just try to interface that MAP with L-Jet. It will be impossible. MAP is a calculation based setup, i.e. you have to know throttle position, pressure and RPM.

MAF is a direct measurement of mass of air flowing. Sure MAP sensors are cheap, it is nothing more than a pressure transducer, but if these were better we would see them on the high end cars already. Virtually all Bosch OEM system now run MAF sensors.

Both will work, but honestly I think the MAP is used more often because of the low cost and ease of integration with lots of different setups. A lot of the Motec guys run MAP and it works fine, but for a computer that is designed around an air measurement device the MAF is the better choice.

I have completed the MAF conversion on the L-Jetronic with a standard 0-5 volt MAF sensor.

 

"Totally disagree on the MAF vs. MAP. Just try to interface that MAP with L-Jet. It will be impossible."

That's news to the dozens of people running MAP kits on 951s using the stock computer.

"Sure MAP sensors are cheap, it is nothing more than a pressure transducer, but if these were better we would see them on the high end cars already. "

They are on a lot of high end cars.

 

If you’re running a MAP sensor on an L-jet car, you've done ALOUGHT of electronic trickery. Lets see, to convert an L-jet ECU to operate on an MAP set up, you'd need a computer to turn the rpm, and manifold pressure into a flow rate. That’s not all that simple of a computer program, but do able. Just not cheap! It would probably be cheaper to switch to a stand-alone engine management system.

For a MAP to work, it requires, among other things, a mapping of the engine volumetric efficency across the rpm range. This is a "fudge factor." It changed with the camshaft, engine temp, cam timing, and exhaust backpressure. It can also change with engine temp. It is a fudge factor, and by defection, it isn't accurate. It's close, but not perfect. Anytime you change the power of the engine, you need to retune the computer's fudge factor. Finally, a MAP sensor is almost useless on an engine with a wild cam. Such cams cause the engines to have very little manifold vacuum at idle, no matter how open the throttle is. This confuses the computer, and it gets the mixture all wrong.

On a MAF or AFM system, there isn't a fudge factor. The computer uses the input from the MAF, or AFM to directly, and without the use of a fudge factor; calculate the flow rate into the engine. This airflow rate is then used to calculate the fuel flow. It allows much more accurate mixture control than the AMAP systems. The AFMs require correction factors for air temperature, and air pressure, usually these are included. The correction factors are relatively easy for the computer to calculate, as the air almost always follows the ideal gas law. The MAF need a correction factor for humidity, but not for temp or pressure. The humidity is a bit of a problem, as there are no easy to use, cheap computerize able sensor for humidity. The solution is that the humidity is usually relatively constant, so you can use a computer with memory capability, to use the O2 sensor to provide a correction factor. Also, relatively speaking, humidity does not throw the reading very far off. (Just a few percentage points.)

MAP sensors on high-end cars? Not new OEMs. Just about everything I've heard of runs a MAF. Heck, Ford went from MAP to MAF just a few years ago to meet tightening emissions standards.

However, MAP sensors are common in the racer world because they provide accurate enough mixture control, the racers would often have to retune the computer away, because they max out the stock MAF. The MAF requires a bit of plumbing, making it a bit more difficult to install, and finally, the MAP sensors are cheap. Not to mention, the racer types don't need to meet emissions standards, nor are they worried about drivability. Finally, they can compensate for the problems of a wild cam effects on vacuum at idle with the addition of an "idle switch" that tells the computer the engine is at closed throttle, and to supply what ever amount of fuel needed for operations at closed throttle at that rpm.

 

Just throw a big 4 barrel Holley on it, and turn the idle screw until it runs!

 

ViribusUnits:

Check out gururacing.net for a popular MAP kit for the 951... no where near the cost of standalone, and cheaper than a lot of MAFs. You're probably right about more racers using it. Also, it's probably a lot better suited to a turbo car where you are messing more with boost, and less with VE-changing mods like a cam, etc. making it a very bad candidate for an NA car like the 928.