injector duty cycle

philip j · 08:56 AM

I had suspected my injectors were under sized, although my WOT AFR's are fine. Today after some of the local P cars guys did a breaky, I found a willing co pilot. My SDS EFI does not log injector duty cycle, and checking the monitor at WOT is almost impossible. Well my fears are confirmed. I'm running at 109%. My Siemens 57cc injectors should be good for 500 HP by the Marren sizing chart, at 85% duty cycle. With a cam change (993SS) coming up this winter, which should further stress out the injectors, any recommendations on what I should run?
How does the methanol injection change the BSFC? Philip

Speedy Squirrel · Speedy Squirrel's Garage

How high is the rail pressure? Turn it up to 65PSI.
How rich is the A/F at WOT? Anything less than 11.7 is too rich. Lean it out.
The effect of H2O methanol injection depends on how much you dilute it with water.

Tippy · 10-27-2013, 05:06 PM

How do you know your duty cycle then?

Speedy Squirrel · Speedy Squirrel's Garage

I understood it to mean that he was able to watch it on his laptop while somebody else did the driving. He can display it, but not log it.

philip j · 10-27-2013, 05:16 PM

Speedy, I don't go any richer than 11.5, I could lean it out to 11.7 I run the fuel pressure at 3 bar. I run a 50/50 meth/water mixture. Tippy, I had my co pilot read the duty cycle at WOT. Philip

sjf911 · sjf911's Garage

If your fuel pressure regulator is adjustable then the simplest solution is to raise the fuel pressure to 4 bar or greater and then re-tune everything.

Tippy · 10-27-2013, 07:33 PM

I wouldn't trust your readout, mine says the same - 109%.

But as "jpnovak" discovered, if you change this one certain setting in MegaSquirt, it skews the duty cycle analysis. Could be same situation.

TurboKraft · 10-28-2013, 11:27 AM

Are those the fat bodied injectors like a 964 or 993, or the newer style skinny injectors?

xbmwguy · xbmwguy's Garage

i think chris will concur it is CRITICAL to have an exact dead time for the injectors. at wot you could simply be leaving the injector wide open. on my new build i will have a fuel pressure guage on the laptop to log whats going on up top

JM2327 · JM2327's Garage

I agree the 109% seems high unless you are well north of 500 whp. I run 55lb injectors at 80% duty cycle and 466 whp with a rising rate fuel pressure regulator. Keeping the injectors small as possible really helps with idling and low end tuning

philip j · 03:39 PM

These are not the newer style skinny injectors. I have not had the car on the dyno yet, but would have to say it's well north of 500hp. Philip

TurboKraft · 10-28-2013, 03:19 PM

Quote:

Originally Posted by xbmwguy

i think chris will concur it is CRITICAL to have an exact dead time for the injectors. at wot you could simply be leaving the injector wide open. on my new build i will have a fuel pressure guage on the laptop to log whats going on up top

Yes, with this data we make 2,000cc injectors behave perfectly at idle speeds as low 850rpm.

Philip -- what is your fuel pressure? Which regulator?

philip j · 10-28-2013, 03:37 PM

Chris, I just checked, they are the fat style injectors like the 964. I am running a stock Carrera manifold and factory lines and regulator. Philip

TurboKraft · 10-28-2013, 04:17 PM

Okay, we just removed a set of those fat 57# injectors from an engine we tuned (switched for more flow volume on ethanol. We got 557whp/516wtq @ 1.2bar on 91-octane, room to spare in the injectors, and a perfect 900rpm idle.

To keep going on gasoline, you need to swap the regulator for a 964 part, then replace it's lower return line for one with an M16x1.5mm threaded fitting. This will bump fuel pressure to 3.5bar.

If you have plans to run ethanol in the future, you'll need bigger injectors (plus larger fuel lines, possibly more pumps). We have used and sell both Siemens (880cc) and ID (1000, 1300, and 2000cc) fuel injectors, in stock. For your build you would need the ID injectors.

Tippy · 10-28-2013, 05:45 PM

Philip, I've read many, many, many 4-cylinders run 1000-2200cc injectors and have no problems idling.

My next set will be 1000cc once E85 becomes the norm around here and I turn up the boost.

philip j · 10-29-2013, 04:30 AM

If anyone has a lead on a 964 FPR it would be appreciated. Also I think if I'm going to raise the pressure , new fuel lines are in order. thanks Philip

sjf911 · sjf911's Garage

Quote:

Originally Posted by philip j

If anyone has a lead on a 964 FPR it would be appreciated. Also I think if I'm going to raise the pressure , new fuel lines are in order. thanks Philip

LOL, mine was just delivered yesterday. Hopefully I'll get it installed tonight then the retuning begins.

TurboKraft · 10-29-2013, 08:19 AM

Quote:

Originally Posted by philip j

If anyone has a lead on a 964 FPR it would be appreciated. Also I think if I'm going to raise the pressure , new fuel lines are in order. thanks Philip

Pelican Parts - Product Information: 928-110-198-04-M14

If you want OEM style fuel lines, talk to BoxsterGT.

philip j · 10-29-2013, 09:55 AM

thanks Chris.

sjf911 · sjf911's Garage

I have to say that replacing the FPR with the engine in car and not removing the intake is a PITA. I did get 51 psi with the 964 FPR, though. Also, resetting the fuel numbers was made easy by megasquirt's recalculation of "required fuel" multiplier and the excellent injector data provided by Injector Dynamics on dead time, small pulse width non-linearity, and voltage correction.