PWM settings for injectors

[Zmanco]

I had the car out on the track yesterday for the first time since installing MSnS on MS1. After the first few laps I lost power until I backed off the throttle. Up to that point it ran great. Ultimately I found that by changing the PWM Current limit for the injectors from 35% to 25% the problem went away. I think I the drivers were overheating by running at WOT for long periods as I had not experienced any issues prior on the street.

 

I'm running stock turbo injectors on my NA car with them split 3 each across the two drivers. I'm a bit surprised that I had to drop to 25% as I've seen quite a few threads here that suggest 35% is a normal setting for others. Any thoughts?

[naviathan]

Are your drivers properly heat sinked? If you didn't put enough transfer grease in there they would overheat much quicker than most.

[Zmanco]

I used all of the grease in the kit. I wonder if I should have bought some more?

 

Interestingly, I can't tell any difference (except for the shutting down) with running different settings of PWM. From other threads I thought I'd have experienced something.

[z-ya]

What was the max duty cycle you were hitting at the track? If you are close to 100%, the injectors and drivers will get quite hot. Limiting the current more will keep the injectors running cooler. So my guess is that you are close to 100% duty cycle at max power, and the injectors are overheating. Adding more current limiting solved this. The problem is that at lower duty cycles, the injectors may not open up all the way with the extra limiting. So around town, it may run a little lean. Best thing to do is increase the size of you injectors, and re-tune.

[Zmanco]

Pete, please follow my reasoning and let me know if this makes sense:

 

I have a n/a engine with a reground cam and was running a little above 5000 feet elevation. I'm using stock turbo injectors with a redline of 7k rpm. My Required Fuel is 11.8 msec and I was seeing just under 10 sec max pulse width on the display.

 

7k rpm equates to 117 rpm per second or 8.5 msec per revolution. As I have the injectors set up for 2 banks alternating, I think that means that at 7k rpm each bank of injectors is firing for ~10 msec duration every 2x8.5=17 msec. This would mean a duty cycle of roughly 59%. I wouldn't think that would lead to overheating of the injectors.

 

The only thing I can think of is I am mistaken and the injectors are firing EVERY revolution, which would then mean I was beyond 100%.

 

I'm traveling in Europe and am a little sleep deprived right now, so I'd appreciate the help to figure this one out.

 

Thanks,

[z-ya]

Pete, please follow my reasoning and let me know if this makes sense:

 

I have a n/a engine with a reground cam and was running a little above 5000 feet elevation. I'm using stock turbo injectors with a redline of 7k rpm. My Required Fuel is 11.8 msec and I was seeing just under 10 sec max pulse width on the display.

 

7k rpm equates to 117 rpm per second or 8.5 msec per revolution. As I have the injectors set up for 2 banks alternating, I think that means that at 7k rpm each bank of injectors is firing for ~10 msec duration every 2x8.5=17 msec. This would mean a duty cycle of roughly 59%. I wouldn't think that would lead to overheating of the injectors.

 

The only thing I can think of is I am mistaken and the injectors are firing EVERY revolution, which would then mean I was beyond 100%.

 

I'm traveling in Europe and am a little sleep deprived right now, so I'd appreciate the help to figure this one out.

 

Thanks,

 

I not sure about the logic behind your math, but after setting up my stimulator with your settings with 150 in the appropriate VE table bin, at 7000RPM you are at 100%. What are your VE bins set to at 6000, 6500, and 7000RPM at 100kPA?

 

You may not be hitting 100%, but if you run 95-99% for long periods, the injectors will cause the drivers to overheat. The driver circuit has a thermal overload feature, which basically shuts down the injector drivers. By lowering the PWM setting, you are limiting the amount of current that the injector can draw (hold current). The problem is that if you set the PWM too low, the injector won't open because the peak current is being limited too much.

 

If you have an adjustable FPR, you could increase the pressure, then retune using the required fuel. what is your fuel pressure BTW?

[Zmanco]

I'm using the stock FPR and at idle the pressure reads around 30 psi. My largest VE value is only 106.

[z-ya]

You fuel pressure at idle should be around 38psi. This could have something to do with it.

[Zmanco]

You fuel pressure at idle should be around 38psi.

I'm using the stock FPR connected to a vacuum port on the manifold and thought that at idle with high vacuum that I would see 5-10 psi less less than the target of 38 psi at WOT. I thought that as the vacuum in the manifold reduces (in other words, as it approaches the outside pressure at WOT and high revs) that the FPR would increase the pressure to keep the difference between the fuel pressure and the space into which it's sprayed about the same. Is this not correct?

 

Even though it's old, I'd hate to replace the FPR if it's ok.

[Mack]

DISCONNECT the FPR, and it should run a constant 38psi. you dont need an adjustable FPR with a standalone unless you are running a turbo. With the fuel pressure changing all the time, its making your fuel injectors flow differently, thus making tuning a PITA.

 

DATALOG it and throw that file up. that'll give us a clue as to your duty cycles and what not.

[Zmanco]

Thanks Mack, I'll try that. I'm out of the country until the end of the month, but will post the results here. It may be hard to verify thngs are fixed without a track as the overheating didn't occur until several laps into the sessions. Hard to get that much WOT throttle time on the street