fuel rail pressure/injector consideration

[dwightp]

I've got a set of 6 Siemens low imp injectors rated at 37#/hr, 80% duty, 3 bar fuel press.

How much lower rail pressure can I go for the injectors to remain controllable throughout the range? 2bar? I'm at the opening limit at idle.

[350HP930]

Most modern injectors can handle up to and beyond 4 bar but I do not know about those siemens injectors in particular.

As far as lower limits there are none. As far as I know the fuel pressure does not noticibly effect pintle function when under the limit.

[beepbeep]

Hello!

I wouldn't go lower than 2.5 bar on them. going lower than that might result in bad spary-pattern. Injectors is using fuel pressure to finely atomize droplets before they enter the combustion chamber. Lower pressure might result in fuel drizzling into it instead. Control it by adjusting down opening times instead.

Your injectors are good for around 320hp @ 3 bar... peak & hold injectors are quite good when it comes to distinct opening times, so i doubt you'll have problems even if you runt them in 200hp-range.

I hope you are not thinking about just slapping them on Motoronic without re-mapping and using real high-to-low ohm driver circuitry...?

[dwightp]

I was thinking of just going down in pressure until I get suitable idle control. Then looking at coldstart difficulties due to spray pattern as you mentioned.

I'm in the 320 HP range. Motor was set up on engine dyno with 50lb/hr injectors. We had idle around 1500 RPM and dyno load so I really didn't know what I was up against untill I got installed. I did some calcs and thought the 37# set would work. Much closer, but no cigar. Problem is only at idle. Injectors can't operate as low as I'm commanding. Below 1150 rpm, lambda goes wild, and engine will stall. < 1.5 uSec command.

I've got 2 options as I see it, lower fuel rail pressure or spend another $5-600 for a set of lower flow injectors.

It's a Motec.

[beepbeep]

Hmm...if I understand you correctly you have fully programmable Motec and a set of low-Z injectors that are opening unreliably at short opening times??

Hmm...that's quite weird, considering the fact you are running quite small and fast injectors. If you had 86lbs injectors running with shunt resistors on turbocharged four-pot, then I wouldn't be surprized but 36'ers should open w/o problems at idle.

Are you using resistors on them?

Peak and hold injectors should be run with real peak & hold circuitry in order to retain their fast opening times. They should open with 4A and then go down to 1A hold current. High-Z injectors are opened and held with constant 1A current.

I don't know which type of Motec you run but most aftermarket EFI systems have circuitry to run peak & hold injectors correctly. some of them (Haltech i believe) can only run p&h injectors correctly if connected in parallel two-and-two.

I suggest you to check if they are opening correctly and that they are reaching peak currents. You can check it by monitoring voltage drop over injector(s) with oscilloscope. Check fuel pressure too, it might be too high.

As I said, it's quite uncommon to have idle problems with injectors that small, especially when using p&h injectors which should open even faster than ordinary ones. I mean, my 19yr old car had high-Z 349cc injectors that idle perfectly...

Cheers!
Something is wrong there.

[dwightp]

"Hmm...if I understand you correctly you have fully programmable Motec and a set of low-Z injectors that are opening unreliably at short opening times??"

Yes

"Are you using resistors on them?"

Nope

"I suggest you to check if they are opening correctly and that they are reaching peak currents. You can check it by monitoring voltage drop over injector(s) with oscilloscope. Check fuel pressure too, it might be too high."

Checked ok on these and the 50# Bosch injectors. Verified settings with Motec tech support and the engine builder. Fuel pressure set and verified with mechanical gauge and sender to ECU. Charateristics got better with the 37#.

Looking at the data logs, the opening times look correct off idle and with load. The efficiency of the motor is higher than I anticipated, but that's a bonus. I have gotten confirmation that getting down below the 1.0-1.3 uSec opening times, these injectors will have problems. That's the region I'm in. At least that's what the data is saying. It makes sense from that standpoint.

FWIW, max duty cyle I ever saw with the 50# injectors on the dyno (7000rpm) was 44% (sequential). At the HP numbers I was crunching I'm amazed. -OR- something fundamental is wrong as you say. I'm still looking.


Thanx for the input!

[beepbeep]

...there is also issue of spray pattern. If intake/head walls are sprayed by injector, precipitation will occur and you'll have problems with idle and emissions.

Doing some simple math, you should have max duty cycle around 60% with 37# injectors at full tilt which is fairly OK, considering that max usable duty ratio is around 80%.

It's interesting to hear your expirience as i have a set of 86# Siemens low-Z injectors that i plan on using on turbocharged four-banger with hot cams.

I wonder how my idle will be... not too good i suppose.

May I ask about engine details? 320hp? Sounds interesting...turbocharged? N/A?

[dwightp]

Goran:

Yes. Agree that these issues need to be considered. I am also using CHT as an enrichment input. It's great as a cold start input to the fuel map. I am also playing with inj timing. I can also use it as an ign input as needed (different thread). Emissions aren't a real big deal at my location.

Take my experience with a grain of salt. Many I have talked to are similar in your initial response (i.e. this shouldn't be a problem). Even my engine builder is surprised based on their earlier experiences. However, I can't help but go back to the data the ECU is telling me.

I can't express how much fun I'm having with this set-up. It has turned out (ECU and Engine) MUCH better than I had hoped.

Best of luck on your project!

Post back when you have your experiences with those injectors in your project. I'll keep this up to date on my findings and final solution.

[dwightp]

RC Engineering has also expressed "something is wrong". They even think that 80# injectors should be capable of idle in a 3.6. Now, I'm off to look at other ECU set-up parameters...

[ischmitz]

this might be off topic but doesn't the stock 3.6 engine have a fuel pressure regulator that is controlled by manifold vacuum. Maybe that could help you to obtain variable fuel pressure and thus have more margin to control idle but still get enough fuel into the engine at higher RPM's/loads

ingo

[dwightp]

Ingo:

I believe it does. Keeps the pressure delta across the injector constant (which you probably knew). For my particular application, it's not a stock 993/964 engine. However, I am running the rail pressure relative to manifold for 1 of 2 setup's. The other is an individual TB which is run relative to ambient.

Not necessarily off topic. Cause, if I wasn't running relative to manifold, it could very well be a contributor in certain extremes of operation. Depends on what scenario was present to set the rail pressure as the external gauge would read relative to atmosphere.

Now for something really off topic, a 74 Targa? I had a white one. Never had the case/head stud problems many speak of. It was a blast to drive...light weight, gear ratios, etc. Loved it. Wish the tornado would have left it alone.