Injector Sizing
Turbo Mini Forums

Right, trying to work out what size injectors for the MS system.

Say 4 injectors, 2 per port, 200 hp and a BSFC of 0.45 cc/hp.

I have looked at various cams and it appears that the maximum injection period would be about 290 deg. This is the arc of engine rotation during which there are inlet valves open more than 0.050" in a port.

Allowing for a maximum IDC of 40%, based on a maximum opening period of 290 deg, I get a injector size of 600cc!!!

Problem is now to work out if they can be turned down low enough for idle flow from 2 injectors.

Any clues anyone???

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Axel,

I assume that you meant 0.45 lb/hr/HP which is quite low for a boosted engine. And 40% IDC sounds about right. So that might be even more than 600cc. Probably more like over 700cc.

You basically need to double the size you would have for a "normal" engine since we only have 40% max IDC instead of the usual 80%. And people do manage to get 400HP 4 cylinder engine to idle with those big injectors.

You could also use 2 different size injectors. A small one for idle and low load and a big one for the top end. Then you have to see how you're going to position them and tune the injection timing and what's the best method for staging them.

Jean

http://www.jbperf.com/

I doubt you'd be able to get these to idle at any reasonable RPM with gasoline. They'd be great for secondary injectors and that's what I've seen them used for (on turbocharged rotaries). I haven't heard anyone use them as primary injectors with gas even on big V8s but I'm no expert.

From what I've read from other's experience, anything above 800cc is probably too big and even that would be pushing it especially for an engine the size of an a-series engine. I personnally wouldn't plan on using anything larger than about 600cc and if the target max hp is larger than 150-160 then I would go with smaller primaries (400cc) and much larger secondaries (1600cc). That would provide good idle and good top end.

By the way, I assume that everyone with a turbocharged a-series engine will go for 2 injectors per port. Unless someone is going for a near stock turbo Metro engine, it's not going to be possible to size the injectors for power and idle with just 2 injectors.

Jean

http://www.jbperf.com/

I'm doing dual 700cc injectors - and playing with fuel pressures to my advantage. 1600cc units are almost definately not going to idle.

Edited by TurboDave16V on 26th Mar, 2006.

Playing with fuel pressure is probably a good idea.

What kind of pressure variation are you planning on and what are you using to get it?

http://www.jbperf.com/

Peter,

If you have the 190cc injectors go ahead and use them but 2 of them are only good for less than 30hp at 40% IDC and 0.5 BSFC. That's enough for idle but maybe not for cruising speed.

From the article you mention above and this:
http://www.autoperformanceengineering.com/pics/gsl393.jpg
it seems that the low cost 190lph Walbro is more than enough for 200hp and over 30 psi boost. And I assume it's best not to go overboard on the pump capacity in order to not heat the fuel needlessly.

Jean

http://www.jbperf.com/

Pete / Jean...

I'm going to be using a Walbro 255 - as this supports 6-bar rail pressures. I'll be using 4-off 700cc - as i mentioned, but running the pump through a resistor for when on the first injectors, then seeing full voltage as i switch to the larger injectors. I've got another little trick aswell; but you have to keep some cards close to your chest I'm not sure as yet how well it'd work either, so you'll have to bear with me until I've got it up and running!

Peter, Dave,

Since you're both planning on using dual voltage to control your fuel pump, would a PWM controller be a good idea? This would allow a much finer control than a simple resistor. I was thinking about making such a controller that would be connected to MS through CAN. It could also control a couple of electric water pumps if needed.

Let me know what you think.

Jean

http://www.jbperf.com/

PWM is used effectively for all my model speed controllers, and those motors pull quite a few amps. You'll probably need flyback protection as you're effectively feeding an inductor. Some of these new power MosFET's can take a lot of load.

Chief advantage of PWM is that you maintain the torque characteristics of the pump motor.

As I understand it, by placing a resistor in line with the pump, you'd be restricting the current, the voltage would remain the same.

Andy

Edited by Andymini on 27th Mar, 2006.

BMW use PWM on their fuel pumps alredy.

So the problem must be surmountable!

Alex

AlexF

Those don't look like genuine Walbro's to me... If it's not got Walbro witten on the side, along with the walbro codes, be VERY suspicious.

http://www.jbperf.com/