This is a question for all you people who've installed an aftermarket ECU on your 20B. First let me assume your tach signal is that of a 6 cylinder, thus requiring either an adjustment of the tach, or an aftermarket tach set to 6cyl mode. Second, let me assume all primary injectors are fired simultaneously (is that usually called batch fired?). Lastly, how does a 20B run with three injector squirts per revolution compared to the two squirts per revolution of a 13B? Does it require a different duty cycle due to the third squirt per rev?

I'm basically trying to find out which is better for the engine when all three primaries are fired together, mainly because I have the option of going either way with my flexible ignition system. It's currently setup to output a 4cyl/2 rotor tach signal for my stock tachometer, and since a 13B seems to run well enough with two squirts into each one of its rotors, I figured the 20B would too. Then fellow forum user Mouse (on the Mazspeed forum) suggested that I ought to go for three squirts per rev rather than two, because it might cause some funky mixtures into the middle and rear rotors, due to their phasing compared to when the injectors fire.

Ok, let's assume the front rotor (0º) will be fine since it is phased just like a 13B's front rotor (0º). The middle rotor (120º) is a little further ahead in its phase than a 13B's rear rotor (180º), and the 20B's rear rotor is a lot further all togther (240º). If all three primary injectors were firing every 0 and 180 degrees, I can see where is might cause some concern, especially because they are 'port' style injectors and very close to the rotors. I'm thinking that if I were to fire all three injectors three times per rev, at least one squirt would happen at the correct time for each rotor... I guess. The other two would just sort of be there... I guess. I'm also wondering how this would affect the duty cycle having to fire more often per rev, and so on.

Am I thinking right about this? All this EFI stuff is still quite new to me.

I'd image the secondaries wouldn't suffer from this, being so far up in the runners as they are. Of course they'd only flow well at higher RPM anyway, so I won't use them for lower RPMs. No need to worry about them for now.

So what do you guys think? I've got a 2 per rev vs 3 per rev thing going on. Since I have a choice, which should I go with?

Thankyou for reading this.

By the way, if anyone is unsure how I can have a 4 cylinder/2 rotor tach signal on a 20B, just trust me for now. I'll explain how it works later.

 

On mine the primaries are fired sequentially, the secondaries are in batch mode. I thought the others were similar and thus backwards of what your theory is... ?

 

Does 'sequentially' mean the first rotor's injector fires first, the second injector fires next and so on? I don't have that option.

I have no idea if other ECUs are similar to what you described since I'm still so new to this whole EFI thing. The best I can do on a 20B is batch fired because the ECU I'm going to use only has two injector control banks. I'll use one for primaries and the other for secondaries.

 

Yeah, "sequential" fuel injection means each fuel injector fires at the right time for each rotor.

We're running our Haltech E6K at /3, so it's the same as you have described.
It seems to run fine that way.

For FC3S, all you need to do is crank the adjustment screw on the stock tach to get it to monitor the signal.
We're taking the signal off our MSD DIS-4 for the tach.


-Ted

 

Are you saying that your injectors squirt three times per revolution? If so, that's fantastic! It's what I'll do, assuming I can find myself a six pointed reluctor. I wonder if Racing Beat is willing to sell one out of their 20B dizzy kit?

So you adjusted your tach to read a 6 cylinder square wav accurately? Cool! I think there are instructions on Granny's website.

 

That is correct.
It should trigger fuel injection 3 times per one RPM.


Yes, the stock FC3S instrument cluster (i.e. tach) has an adjustment screw.
It took us 3 tries to get it right to match the Haltech RPM display, but you basically crank it all the way over before it hits the stop - wish we knew that sooner. :P


-Ted

 

That is so cool! I'll probably have to adjust the duty cycle, but that's ok.

I won't need to adjust my tach because the three leading coils will still get a 4 cylinder tach signal. Then the lone ignitor/pickup with a 6 tip reluctor will output the 6 cylinder signal. Here is where it shows how to do it with a GM HEI ignitor. http://www.megasquirt.info/manual/mwire.htm#ignition