GM TBI setting Verification
Posted: Tue Oct 27, 2009 1:56 pm
Hello all,
I've spent hours reading, searching and watching a few videos. I just want to make sure I'm on the right track here.
The hardware:
Megasquirt v2.2 board with original 68HC processor. No flyback mods, just basic MS assembly.
GM TBI injection unit with 65lb/hr injectors.
Engine it's feeding will be a Ford 351W 5.8L engine, all stock.
Now for setting up the TBI Injectors, last time I used the MS way back was on my honda with a turbo and hi-z injectors, and the GM TBI is of course a low-z, so I just want to make sure I got this right so I don't blow anything up.
In Megatune for the fuel constants I have it set for:
8 injections per engine cycle, alternating.
Injector opening time is at the recomended 1.0ms, Battery voltage correction is 0.10ms/V
PWM Current limit % is 75
PWM Time threshold is 1.0 ms.
Based on my reading, it appears that the stock v2.2 board should be set to 75% current limiting, but a flyback mod or V3 motherboard are set to 35% current limiting. Is this correct?
With the above settings, the Req fuel calculation comes out to 7.1ms Based on playing with the Stimulator this would give me a 5.4mSec injector pulse width at an 800 RPM idle, and a 93% duty cycle at 5000 RPM should my plow truck ever reach that (doubtfully!). This seems to fall pretty well within the limits.
If I switch to 4 injections per engine cycle, I would get a 9.6ms pulse width at an 800rpm idle, and 83.3% duty cycle at 5000 RPM.
So, My questions are:
1. Is the 75% current limiting the correct setting for my MS hardware?
2. Is there a benefit to adding resistors to disable the need for PWM
3. Should I be using 8 injections per engine cycle or 4? 4 seems to be a better fit for idle pulse and top end duty cycle if I'm understanding this all correctly
Thanks all!
I've spent hours reading, searching and watching a few videos. I just want to make sure I'm on the right track here.
The hardware:
Megasquirt v2.2 board with original 68HC processor. No flyback mods, just basic MS assembly.
GM TBI injection unit with 65lb/hr injectors.
Engine it's feeding will be a Ford 351W 5.8L engine, all stock.
Now for setting up the TBI Injectors, last time I used the MS way back was on my honda with a turbo and hi-z injectors, and the GM TBI is of course a low-z, so I just want to make sure I got this right so I don't blow anything up.
In Megatune for the fuel constants I have it set for:
8 injections per engine cycle, alternating.
Injector opening time is at the recomended 1.0ms, Battery voltage correction is 0.10ms/V
PWM Current limit % is 75
PWM Time threshold is 1.0 ms.
Based on my reading, it appears that the stock v2.2 board should be set to 75% current limiting, but a flyback mod or V3 motherboard are set to 35% current limiting. Is this correct?
With the above settings, the Req fuel calculation comes out to 7.1ms Based on playing with the Stimulator this would give me a 5.4mSec injector pulse width at an 800 RPM idle, and a 93% duty cycle at 5000 RPM should my plow truck ever reach that (doubtfully!). This seems to fall pretty well within the limits.
If I switch to 4 injections per engine cycle, I would get a 9.6ms pulse width at an 800rpm idle, and 83.3% duty cycle at 5000 RPM.
So, My questions are:
1. Is the 75% current limiting the correct setting for my MS hardware?
2. Is there a benefit to adding resistors to disable the need for PWM
3. Should I be using 8 injections per engine cycle or 4? 4 seems to be a better fit for idle pulse and top end duty cycle if I'm understanding this all correctly
Thanks all!
