Now Here's A Thought

[M3Evo 0]

Was talking to the chap who kindly came all the way from Brands Hatch to help map my car on Friday about various engine managementy things.

 

Anyway, he came up with what I thought was an interesting point:

 

Would the engine management know or care if you replaced the AFM with a throttle pot?

 

i.e. If you were to attach a throttle pot to the throttle and run the +ve, 0v and signal wires to that and ditch the AFM altogether, would the engine run properly and would you gain anything in terms of throttle response etc.

 

Sounded like an interesting (and cheap to try) little project to me so though I'd share it

[DrSeuss 0]

no, an afm measures the air, the throttle pot measures an angle.

 

Essentially the afm track is like a throttle pot track, but calibrated to the ancient jetronic by the laser cut track.

 

So, in answer to your question. No. But you can map a car just on the throttle pot but only if N/A

[M3Evo 0]

Aah but the angle of the AFM flap is proportional to the amount of air going in, as is the angle of the throttle

[DrSeuss 0]

Yes, you can map an n/a engine on the throttle pot. A fixed volume of air at a given engine speed for a given throttle angle. Most mappers map this way for throttle bodies.

 

For a boosted engine this doesn't work at all, because air flow isn't just proportional to engine speed but turbo boost.

[Toddman 0]

no, an afm measures the air, the throttle pot measures an angle.

 

Essentially the afm track is like a throttle pot track, but calibrated to the ancient jetronic by the laser cut track.

 

So, in answer to your question. No. But you can map a car just on the throttle pot but only if N/A

Surely if you take the view that the AFM is just a throttle pot that is moved by the airflow against the flap as opposed to a direct mechanical linkage then M3evo has a very valid point?

The AFM is just a means of moving the pot that sits onside the top of it's housing, throw away the flap arrangement and move the pot mechanically and what is the drawback?

 

Cheers

Luke

[Ahl 4]

Surely if you take the view that the AFM is just a throttle pot that is moved by the airflow against the flap as opposed to a direct mechanical linkage then M3evo has a very valid point?

The AFM is just a means of moving the pot that sits onside the top of it's housing, throw away the flap arrangement and move the pot mechanically and what is the drawback?

 

Cheers

Luke

Surely if that was the case they wouldn't have bothered with the AFM in the first place.

 

Just to throw in some ideas, I don't really know:

The AFM is always measuring the amount of air going into the engine, and adjusting the fuelling to suit, with the exception of the idle setting and maybe full throttle.

 

With throttle pot only fuelling, surely this can't account for the higher or lower revs when the engine is underload or going down a hill etc? So the fuelling won't be as accurate.

[M3Evo 0]

Was thinking more along the lines of standard GTis rather than turbo'd cars.

 

Think Ahl's got a good point about the engine speed requirements which I guess would mean that you could only open the throttle in proportion to the rate at which the engine was accelerating.

 

Still be quite interested in the results. Reckon a working test setup could be done for less than a tenner

[Rippthrough 98]

Was thinking more along the lines of standard GTis rather than turbo'd cars.

 

Think Ahl's got a good point about the engine speed requirements which I guess would mean that you could only open the throttle in proportion to the rate at which the engine was accelerating.

 

Still be quite interested in the results. Reckon a working test setup could be done for less than a tenner

 

 

That and it'd be different on the warming up cycle, and the car would be slower if you went past the optimum point for the conditions, and if you backed off and went through the sweet spot it would give a sudden bit of acceleration that you don't want.

Had to do it with a knackered AFM years back, to get it to the end of a stage, and it's not fun.

[Toddman 0]

But the throttle butterfly controld engine revs not th eother way around.

If you move the AFM pot via alinkage rather than the flap what is the difference?

And just because PSA didn't do it doesn't mean it won't work - they didn't fit the 205 with mi power did they

 

Cheers

Luke

[Rippthrough 98]

But the throttle butterfly controld engine revs not th eother way around.

If you move the AFM pot via alinkage rather than the flap what is the difference?

And just because PSA didn't do it doesn't mean it won't work - they didn't fit the 205 with mi power did they

 

Cheers

Luke

 

Because a throttle butterfly is definately not linear.

[Toddman 0]

But the position of the throttle butterfly determines the airflow so at each point of butterfly opening you map the ign and fuelling in relation to the AFM pot.

I may be missing something really obvious but at the moment I can't see what is the problem with not using a flap to move the AFM pot.

 

Cheers

Luke

[Rippthrough 98]

But the position of the throttle butterfly determines the airflow so at each point of butterfly opening you map the ign and fuelling in relation to the AFM pot.

I may be missing something really obvious but at the moment I can't see what is the problem with not using a flap to move the AFM pot.

 

Cheers

Luke

 

if your going to do that why not just map alpha-n anyway?

[Toddman 0]

That is a good point but we have to consider the opening post of this thread.

All I am saying is that with respect to the original question - Yes in theory you can do away with the AFM flap arrangement and map the engine via the AFM pot and get rid of the restrictive AFM flap arrangement.

 

But I agree it is not an ideal situation but should work.

 

Cheers

Luke

[Rippthrough 98]

That is a good point but we have to consider the opening post of this thread.

All I am saying is that with respect to the original question - Yes in theory you can do away with the AFM flap arrangement and map the engine via the AFM pot and get rid of the restrictive AFM flap arrangement.

 

But I agree it is not an ideal situation but should work.

 

Cheers

Luke

 

 

But surely the whole point of doing this is a way of ditching the AFM without changing managment?

Some clarification perhaps M3Evo?

[Toddman 0]

Sorry probably my misunderstanding.

I mean that using a mechanical linkage between the AFM pot instead of the flap will allow you to map the engine correctly thus removing the restriction and not needing a new ecu etc.

After all the signal from the AFM pot is just a value that corresponds to a point on a look up table (MAP)

 

Cheers

Luke

[Rippthrough 98]

Sorry probably my misunderstanding.

I mean that using a mechanical linkage between the AFM pot instead of the flap will allow you to map the engine correctly thus removing the restriction and not needing a new ecu etc.

After all the signal from the AFM pot is just a value that corresponds to a point on a look up table (MAP)

 

Cheers

Luke

 

I think the linkage would be a bugger to work out though, and you still end up with the problem of opening the throttle furthur than you should for optimum, etc.

[dee205 3]

I thought the standard ECU map could not be manipulated thus throwing the idea of remapping out the window.

 

Unless you're using aftermarket managment in which case would a MAP sensor and throttle pot not take care of the air/fuel mixture throughout the rev range. And you wouldn't have an AFM either.

 

 

 

 

Damien

[C_W 3 1 Cars]

The afm flap opening is not directly linked to throttle opening though as it measures load too, I thought it would need a MAP sensor too plus the throttle potentiometer.

[DrSeuss 0]

The mi16 has a throttle switch not a throttle pot so it only uses it for idling and WOT enrichment. The afm as it is measures engine load.

 

By converting to a proper throttle pot you could map the engine alpha-n like you would with throttle bodies. All this talk of linkages to retain original management is foolhardy nonsense. The linkage would have to take into account engine speed. As at 7k rpm with a closed throttle the engine is drawing in more air then at 1k rpm with a closed throttle.

 

What you can do is convert to a hotwire afm which isn't restrictive. You would need to work out the resistance curve of the afm (not difficult) then create a small program to emulate that in the hotwire afm.

 

Now, what have you gained from this little thought excercise. Absolutely sod all. Convert to tb's, map alpha-n and be done with it. Or retain a standard manifold and tune with speed density.

[Toddman 0]

It is only a theoretical discussion and I beleive it can be done although not an easy ask.

I am of the opinion that the std system is fine for road use anyway.

But to go onto another area carbs!

We know that you can run carbs with the motronic ecu but you don't get any ign advance.

So if the only real issue with removing the flap is the load sensing then if carbs are used can't we use a pot to mimick the AFM to fool the ecu into giving us more ign advance when needed?

Just a thought.

 

Cheers

Luke

Edited February 12, 2006 by Toddman

[M3Evo 0]

Blimey, I've started a riot!

 

Was really just something that sounded logical on first thought, but as people have pointed out, it's not really viable.

 

Having said that, I'd still be interested in seeing how difficult it'd be to drive.

[DrSeuss 0]

But if you take that kind of attitude then discussion forums are pointless

The question was posed to be discussed and despite what ha sbeen said I reckon it could be done - granted it would be a pain in the arse to acheive anything worthwhile but with some trickery the result could be acheived.

Maybe I have spent too long in the development engineering area to accept "not viable" as a reason to not do something

 

Anything is possible, some things are improbable. The method of producing a linkage simply isn't sensible. You could build your own afm with a pot linked to a vane that measured air flow. But then your back to square one.

 

Or you could use another system to mimic the afm's output. Also easy enough with a microcontroller.

 

But neither get over the original issue. You use standard injection without alot of electrotrickery.

[dee205 3]

But neither get over the original issue. You use standard injection without alot of electrotrickery.

 

 

In other words, the standard managment is the real restriction.

[PumaRacing 1]

Was talking to the chap who kindly came all the way from Brands Hatch to help map my car on Friday about various engine managementy things.

 

Anyway, he came up with what I thought was an interesting point:

 

Would the engine management know or care if you replaced the AFM with a throttle pot?

 

Of course it would, and no it wouldn't run. The AFM measures total airflow and the throttle pot only measures throttle position. Without knowing rpm and having a lookup map the throttle position on its own is meaningless.

[petert 554]

Would the engine management know or care if you replaced the AFM with a throttle pot?

 

No you can't, but you can replace it with a MAP sensor if you make them appear "electrically" the same. A MAP sensor changes resistance with engine load, as does the AFM. If you're handy with electronics, or know someone who is, it's an easy task.

 

Go to the wreckers and find yourself a cheap GM MAP sensor. Hook it up to a vacuum source and a vacuum guage and record it's resistance under various loads. Do the same with the AFM then compare the results.

Edited February 13, 2006 by petert