That AFM airflow restriction issue

[Chassis 33]

You know that old chestnut where some folks argue that the AFM doesn't restrict airflow until very large BHP figures are reached, and others who say there is a noticeable restriction in flow as low as 150hp.

How can you prove it either way, what will, say, the dyno graph of a restricted and unrestricted engine look like? Has anyone ever done a back to back test of a hotwire setup against a fuel only ECU mapped against manifold pressure and TPS (ignition remaining unaltered from the dizzy)? Will the power trace or the torque trace show the restriction best? I'm assuming torque becasue you don't have the added complication of engine speed.

Any thoughts, other that I should stop worrying and crawl back under my stone?

Regards
Iain

[The AJP Griff]

We(trackcar and i)put my car on the rollers and run it up without any afm,with the std afm,and with the larger afm.You could see the smaller afm was restrictive from 150 bhp,and that the large one was restrictive from 200bhp,this is where the lines on the graph parted,proving the restriction.I have talked to a number of tuners now,all of whom dont really think that changing to a large afm is really worth the expense,and some of them resent the fact that there are people out the who sell them in a very misleading way as a performance 'upgrade'.If you want to make BHP/Torque,then ditching the afm is one of the first steps,ie get an alternative ecu and enjoy all the other benefits it will also give you.

Edited by The AJP Griff on Sunday 12th August 11:20

[Chassis 33]

Cheers, so it was found by overlaying one run over another and not from any single run? How did you ensure things like the CO trim voltages between the different AFM's was controlled so they didn't affect things, or was it "this is what a standard AFM can be optimised to", "this is what a larger AFM can be optimised to"? What ECU/mapping did you use to run with no AFM?

The crux of the matter is what am I best spending £xx on to improve engine performance, polishing and porting, ECU running no AFM, etc, etc, etc.

Regards
Iain

[trackcar]

Iain, how we did it (and w've done it on quite a few cars now cos one of the accusations to us was that we'd only done it on a sample of one car which was unrepresentative , well it's the same on all cars so it's very representative ..) was we first had a car which was mapped on an aftermarket ECU with no AFM fitted .. this was then our baseline figure as the true aiflow potential of the whole system ..

then we added the larger bosch AFM and remapped teh aftermarket ECU to get the ture potential of this new system

then we fited the std AFM and remapped again to get the true potential of that system ..

the results therefore are for an optimised car map for each of the three systems .

the output with NO afm is always the best
the output wit teh bosch is the next best
the output wit the std AFM is the worst

you coud probably guess that anyway, but the results are backed up by the dyno plots.

If you run a std AFM even on a std 4 litre car you are robbin your car of hp everywhere the engine makes more than 150hp

on tuned and/or larger capacity engines then even the bosch AFM is a limitation above 200hp worth of airflow

The other noticeable thing was that the supposed gain i midrange that's supposed to come from using an AFM never showed in our tests .. using an AFM only ever reduced hp once the non-limiting air flow potential of the AFM had been achieved.

here's the graphs:

[The AJP Griff]

Chassis 33 said:

The crux of the matter is what am I best spending £xx on to improve engine performance, polishing and porting, ECU running no AFM, etc, etc, etc.

Regards
Iain

The first thing i would buy is a different ecu.This will give you gains straight away,will have less things to go wrong,and will make future mods cheaper when you need to re-map (you could even do this yourself).
If i had a race car,i would just run wasted spark with coil packs,and map off the throttle angle,cheap,reliable and makes very good power

[Chassis 33]

Exactly what I was looking for Joo. I've seen it alluded to by yourself and the other engine tuners/builders out there but never seen the results presented.

Regards
Iain
Edit: Can I just pick up on this point

trackcar said:

The other noticeable thing was that the supposed gain i midrange that's supposed to come from using an AFM never showed in our tests .. using an AFM only ever reduced hp once the non-limiting air flow potential of the AFM had been achieved.

My understanding of the perceived norm is that and AFM helps mid range and cruising as it more accurately meters the air in slow flow conditions, low revs, part throttle etc., however on a full throttle power run you're not running at these very low air flows hence not seeing the midrange gain expected. At the end of the day this Lucas system was built to sit in 3.5 and 3.9 Landrovers which struggled to get 150 or 200hp as the emphasis was on torque, so while the standard AFM is perfectly adequate for this roll but is a big compromise when used in a sports car.

Edited by Chassis 33 on Sunday 12th August 12:40