can change of AFM give more power?

[magpies]

some friends are building a rally car with the Ford Cosworth V6 from a Granada (BOA). They have been told that replacing the AFM with one from the Mondeo ST24 will give more power as that venturi has a larger diameter. However they are using the standard Granada ECU, so my question is: will the ecu with info from the engine and exhaust sensors increase the fueling to give the correct air fuel ratio (assuming the injectors can give the extra fuel. Figures of upto 20bhp extra have been bandied about.

[stevieturbo]

magpies said:

some friends are building a rally car with the Ford Cosworth V6 from a Granada (BOA). They have been told that replacing the AFM with one from the Mondeo ST24 will give more power as that venturi has a larger diameter. However they are using the standard Granada ECU, so my question is: will the ecu with info from the engine and exhaust sensors increase the fueling to give the correct air fuel ratio (assuming the injectors can give the extra fuel. Figures of upto 20bhp extra have been bandied about.

If the new unit does actually remove a large restriction then there is the potential for gains.

And no, there will not be any automatic compensation for air/fuel, and the chances of it running even remotely correctly with a totally different AFM, is rather unlikely.

[Pumaracing]

An AFM outputs a voltage signal proportional to airflow, between 0 and 5 volts usually, from which the ecu can look up the mass air flow rate and inject the correct amount of fuel. If you change just the AFM then a given voltage will now relate to a different mass airflow but the ecu won't know that the AFM is not stock so it'll continue to inject the amount of fuel that would apply to that voltage from a stock AFM.

However, if you're running fully closed loop with a cat and lambda sensor then the ecu will quickly learn to adjust the fuel trims to bring things back to stoichiometric at cruise and medium throttle and provided it also applies those same fuel trims at full throttle you should be good to go.

If you're not running closed loop then the easiest way to compensate for the different AFM is probably to change the fuel pressure with an adjustable regulator.

Whether a different AFM will give any more power is another matter. The hot wire types are not usually restrictive.

[Pumaracing]

stevieturbo said:

And no, there will not be any automatic compensation for air/fuel, and the chances of it running even remotely correctly with a totally different AFM, is rather unlikely.

If the engine is running in closed loop mode, which one suspects it must be if the air/fuel components are all stock, then I beg to differ. The ecu will quickly adjust the short term fuel trim which is only used in closed loop mode for stoichiometric operation and then after a while it will also adjust the long term trim which will bump up the full throttle fueling too in open loop mode.

So basically don't give it full beans until it's had time to learn the fuel trims from part throttle operation and then it should also be correct at WOT and high rpm.

[fatjon]

Fuel trim, both long and short term is usually restricted to a defined percentage before the ECU sees a problem and pops up an engine problem light and maybe goes into limp mode. Not sure where that percentage is on that model but if the difference in output voltage/mass of air is greater than the ability of the ECU to trim it will not work at all well. Some I have seen are as low as 5% some or much higher. It will quickly become apparent when you try it.

[stevieturbo]

Pumaracing said:

If the engine is running in closed loop mode, which one suspects it must be if the air/fuel components are all stock, then I beg to differ. The ecu will quickly adjust the short term fuel trim which is only used in closed loop mode for stoichiometric operation and then after a while it will also adjust the long term trim which will bump up the full throttle fueling too in open loop mode.

So basically don't give it full beans until it's had time to learn the fuel trims from part throttle operation and then it should also be correct at WOT and high rpm.

Narrowband CL is for emissions only, and when at larger throttle openings/loads it will be running OL with no "learnt" corrections being applied. I would doubt that these long or short term corrections are applied/maintained even when in open loop ?

So in this case, randomly swapping the AFM is a stupid thing to do really, unless you have some means of tuning the ecu afterwards

[Sardonicus]

stevieturbo said:

Narrowband CL is for emissions only, and when at larger throttle openings/loads it will be running OL with no "learnt" corrections being applied. I would doubt that these long or short term corrections are applied/maintained even when in open loop ?

So in this case, randomly swapping the AFM is a stupid thing to do really, unless you have some means of tuning the ecu afterwards

<This the replacement is not going to scale/output correctly compared to the original anyway its either going to need a re-map or alternatively fooling the voltages the ECU sees with some kind of piggy back circuit to correct the offset

[LarJammer]

This is an early 90's engine, its not going to have time based fuel trims. it might not even have a lambda sensor....

[Pumaracing]

LarJammer said:

This is an early 90's engine, its not going to have time based fuel trims. it might not even have a lambda sensor....

Ah - bugger. Just Googled it and it has the Ford EEC IV management system and no cat until the BOB engine came out. So changing anything isn't really an option without some retuning. As I said above it should be possible to get things close with a bit of fuel pressure tweaking but management systems this old are not really in my wheelhouse.

[DVandrews]

Closed loop correction towards stoi is normally clamped at a fairly low correction percentage, in my experience between 5 and 8 percent. This could well be outside what is required.

Dave

[anonymous-user]

The AFM isn't a restriction anyway, so forget about changing it.........

[stevieturbo]

Max_Torque said:

The AFM isn't a restriction anyway, so forget about changing it.........

but the rally boys say they changed it and can now hit 90 in 2nd.....

[magpies]

stevieturbo said:

Max_Torque said:

The AFM isn't a restriction anyway, so forget about changing it.........

but the rally boys say they changed it and can now hit 90 in 2nd.....

thats the whole point of asking ....as I didn't think it would be THAT easy

and of course the latest cars hit that in 1st (and a half.........)

[Locknut]

magpies said:

... They have been told that replacing the AFM with one from the Mondeo ST24 will give more power as that venturi has a larger diameter...

Without going into the properties of the management system it seems to me that the above statement is wrong on the basis of simple physics.

A venturi will accelerate the air passing through it. If the venturi is made larger the air will accelerate less and there will be a lower air flow speed read by the meter. This in turn will cause the ECU to think there is less air entering the engine and reduce the fuel injected. If I'm right it is pointing to the need for a re-map.

[Nick1point9]

magpies said:

Figures of upto 20bhp extra have been bandied about.

Along with the 20bhp from the exposed cone air filter