Settings for Dual Dizzy Fuel Only V8

[carmaniac37]

Hey everyone,

I've got an Audi V8 engine running MS1 V3.0 fuel only. The trigger is the coil- on one of the two coils. Because the megasquirt is only seeing 4 pulses per engine cycle rather than all 8, I have the parameters set in a way to still run with batch injection. There are a couple of ways I have made it work but I would like to know what is best for accuracy of the MS unit. In all cases, the Megasquirt sees the engine as a four cylinder so as to have accurate RPM and uses four squirts.

Currently, I have the actual engine displacement 3600 cc set with 4 injectors and simply doubled the actual injector flow value.

I have also tried still using 3600cc but with 8 injectors set to their actual injector flow.

Another option would be to set the engine size to half the actual (1800cc) and have 4 injectors with their actual flow values basically having the ECU run half the engine as a four cylinder and the wiring just duplicates the same fueling to the other half.

Does anyone know if one of these methods works better than another? Is there accuracy lost by lying about the injector flow value maybe with duty cycle curves? I would imagine, once everything is tuned on the VE table according to measured O2 sensor values, it will run the same in all cases, but I would like to know if one is a better base line than another.

Thanks

[billr]

No one else is biting on this, so I gotta ask... what is it that you are trying to accomplish?

If you don't have a real crank wheel, only "basic trigger" from ignition events, then I think there is nothing you can do to improve the resolution of the timing.

[carmaniac37]

Hey, thanks for chiming in.

Yeah, I'm not looking for resolution. That's a fixed characteristic.

What I'm interested to know is whether the parameters effect the fuel calculation in a linear manner or not. For example, one 500cc injector responds differently than two 250cc injectors even though they have the same maximum flow capability.

I may very well be overthinking it as well.

[billr]

I'm still not getting it. Two 250s wired in parallel is the same as a 500, except that you have double the chance to have DT off; especially if the 250s don't have nominally identical DT. Or are you talking about staging the 250s so low loads are handled by only one? Yes, that probably helps by keeping the injector PW out of the low numbers where DT and voltage comp are more critical.

But honestly, I'm confused about your original post. I get it (I think!) that your "basic trigger" is from only one dizzy, so you can only have injection events every 180 deg of crank rotation, but it gets fuzzy for me beyond that. I don't think there is any way to improve the accuracy of the math, except to keep the number of DT intervals ("number of squirts") to a minimum and to flow the injectors to get the most accurate DT, flow rate, and voltage comp; and use individual trims if available.

[DaveEFI]

No reason I can think of not to take a feed from both cols via diodes. Same as a you do with say EDIS to derive a signal for a rev counter.

[carmaniac37]

No reason I can think of not to take a feed from both cols via diodes. Same as a you do with say EDIS to derive a signal for a rev counter.

This would eliminate all of my questions for sure.

My question, most simply, is this... Which of the following options will work best? 8 cylinder engine with only four ignition events going to the megasquirt because of dual dizzy. Injectors are paired between banks.

a) 3600cc engine displacement, 4 cylinders, 4 injectors, 500cc injector size.

b) 3600cc engine displacement, 4 cylinders, 8 injectors, 250cc injector size.

c) 1800cc engine displacement, 4 cylinders, 4 injectors, 250cc injector size.

Thank you everyone.

[DaveEFI]

A V8 running fuel only from a 'normal' 8 cylinder dizzy was one the earliest MS applications, given where it was invented. Since it would be easy to derive the same tach signal from two four cylinder dizzies, I've not really thought about your options.

But it's worth looking at the manual for tips on how to group injectors on a V8 following the firing order. Rather than just left and right bank. It made a subtle improvement to my RV8 changing from factory grouping, mainly a better idle.

[racingmini_mtl]

Since you're using this for a fuel-only setup, that makes absolutely no difference which combination you use. TunerStudio will compute a required fuel value from the combination you enter and then the ECU will use only this req fuel value for fueling.

Jean

[carmaniac37]

Thanks for all the responses.

I got a couple diods and picked up RPM from both Dizzys. It was easy enough and renders my question unnecessary.