Off the wall RV8 air flow meter question

[chassis 33]

Some RV8's have twin throttle butterflies, eg the Rover SD1 twin plenum.

If, for arguements sake, instead of the standard set up where the combustion air enters a single air filter, passes through a single AFM then splits off to the two plenums/throttles, you have a setup where you had two disctinct air filters and AFM's serving the two plenums/butterflies...is there a simple was to alter the wiring to compensate.

My worry is that if you kept just the one AFM, it would give only half the air flow reading. However if you had two and merged the wires together, in a combination of series or parallel, it would have to assume that hte currents or voltages that the ECU sees are linear with respect to airflow, which i doubt is the case.

Has anyone attempted combining two AFM outputs into one ECU? Can it be done if so how?

Regards
Iain

[GreenV8S]

From comments that Mark Adams has made in the past, I get the impression that he has been involved in doing something of the sort.

[trackcar]

No point in this iain, the std 5am air flow meter isn't restrictive until over 290hp airlfow is reached, and then of course it's only a restriction at hp values over 290, which isn't really going to worry your 4 litre car unduly. Why go to all the hassle? (other than to be different, which is often reason enough for me, lol!)

[chassis 33]

trackcar said:

No point in this iain, the std 5am air flow meter isn't restrictive until over 290hp airlfow is reached, and then of course it's only a restriction at hp values over 290, which isn't really going to worry your 4 litre car unduly.

I'm working on an idea to use what would be crudely refered to as a twin plenum set up but unlike say an SD1 set up the inlets will be some distance from eachother. Plus it will 'look' vastly more balanced have two, however there is a way it can work with just one AFM, its just a bit of a less elegant solution.

[quote=trackcar] Why go to all the hassle? (other than to be different, which is often reason enough for me, lol [quote]
Becasue if everyone did the same thing and never tried something different, we'd still be huddled round a fire, club in hand ready to bludgen anything that invaded our cave...then again who was the first person to figure that smacking something with a lump of wood can hurt whatever youre smacking, or indeed who was the first to figure if you strike two bits of flint together you get a spark that can light tinder wood.

[GreenV8S]

Assuming you're prepared to map the car afterwards, I don't see any fundamental reason why all the air that goes in has to be metered. If you only measure half of it it'll still work (assuming appropriate fuel map) as long as the proportion of air that goes through the meter remains fixed. If you're physically separating the two halves of the engine you would need to ensure it did remain fixed since the two sides get the same amount of fuel.

[chassis 33]

Assuming everything was properly sealed and no restrictions due to foriegn objects etc, the air flows should be 'identical', the system would be physicallly symetrical up to the point of the throttles, except for the AFM.

Is it possible to tinker with the hotwire setup for this, or is it a full remap??? I'm drinking this afternoon with some former collegues who are electrical designers, they my be able to come up with a solution to tell the ECU the engine has twice the air than the AFM is reading.

Regards
Iain

[eliot]

Not thinking of doing the siamese plenum by any chance, i.e. cutting and welding two plenums together so theres a throttle body on either side.

Got such a project on a TVR on the drawing board right now; although were going to be using megasquirt therefore doing away with the AFM's altogether.

[chassis 33]

eliot said:

Not thinking of doing the siamese plenum by any chance, i.e. cutting and welding two plenums together so theres a throttle body on either side. Got such a project on a TVR on the drawing board right now; although were going to be using megasquirt therefore doing away with the AFM's altogether.

Did consider that, but have an even better solution (IMHO )!!! The Thor plenum box/manifold thing/trumpets. Basically its two castings, one which is the trumpet section, the other mounts the throttle butterfly and sits on the front of this running at 90deg to the crank axis.

What I'm proposing is to remove this front section, leaving two forward facing openings, mount a pair of butterfly throttles to this and duct the air to these from two inlet ducts mounted on localised stagnation points on the front of the car. The trouble im having is how to I tell a Lucas hotwire setup how much air is passing through!

Regards
Iain

[eliot]

Way ahead of you. Already had one of them on the bench with that exact plan in mind. Usual story - would look cool and different.

The problem is that its not exactly what i would call a 'performance' piece. Look at the size of the runners and the inlet ports. Its optimized for high torque (long narrow runners) at low rpm - we wanted to put it on a 5.0L TVR but changed our minds after having a good look at an actual manifold. Also getting a round throttle body to smoothly transition into the rectangular plenums would be interesting.

So then we moved onto the siameese plenum idea. Bike throttle bodies are also being considered.

edited to add: May work better on your 4.0L, as they use the thor on the 4.6 - but I still dont like the tiny radius at the end of the runners. And Would need a porting at the cylinder head end.



>> Edited by eliot on Friday 30th September 11:33

[chassis 33]

But I only want 155bhp at the wheels (150bhp/1000kg)...assuming 15% transmission losses, approx 180bhp at the flywheel, I could do that with a well sorted SD1 3.5EFi, dare say with a bit of fettling i could get that power on twin SU's or Strombergs!

I want as much torque as low down as possible from the 4litre already fitted to the Chimaera, because if I went to a bigger engine I'd only be able to run at idle or else i'd be over the power limits!

Regards
Iain

[chassis 33]

eliot said:

Also getting a round throttle body to smoothly transition into the rectangular plenums would be interesting.

Dead easy, 3D CAD model...DMLS, as a 2 off prototype, or the same for a casting mould for production runs. Or other crude methods...



Regards

[350matt]

eliot said:

Not thinking of doing the siamese plenum by any chance, i.e. cutting and welding two plenums together so theres a throttle body on either side.

If you're dead set about going down this route then you'll want to consider putting some kind of baffle / plate to seperate the two halves of the plenum as when this was tried at TWR ( for the SD1 racers) it actually lost power over the std intake.

The theory at the time was the two airstreams collided against each other and set up so much turbulance the net airflow into the trumpets was actualy reduced.
This is why Rover went the side by side route

Matt

[HarryW]

Another air flow within the plenum point to note from my experience, for what its worth;
I experimented with fitting a spacer between the two halfs of the plenum (the trumpet base and plenum top) to increase the internal volume with the idea i might get a little torque. However, I found that I lost power and torque and came to the conclusion that either the height between the trumpets and the plenum roof was outside of the 'sweet spot' and or the side inlet was now drawing its air straight across the the top of the trumpets restricting them. One of the many things done that had a negative yield .

re the 5 AM and max power, I've found on mine the max air flow metered output reading, 5 volts, occurs before peak power can be achieved , i.e. as far as the standard ECU map is concerned it has already reached max and won't give more fuel. On mine this occurs at around 5k8 rpm, peak power is around 6k1. So without re scaling the fuel maps and extrapolating beyond that point mine wouldn't make more than 240/250 ish bhp, it currently is 270 ish .
Removing the metal mesh on the i/p of the 5AM lets slightly more air through, however the 5AM metering for the ECU can get a bit erractic and affect idle and response, again IMHE.

Harry

[eliot]

chassis 33 said:

But I only want 155bhp at the wheels (150bhp/1000kg)...assuming 15% transmission losses, approx 180bhp at the flywheel, I could do that with a well sorted SD1 3.5EFi, dare say with a bit of fettling i could get that power on twin SU's or Strombergs! I want as much torque as low down as possible from the 4litre already fitted to the Chimaera, because if I went to a bigger engine I'd only be able to run at idle or else i'd be over the power limits! Regards Iain

Then the thor may work for you.