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Old 05-24-2009, 04:03 AM   #1
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Default Injector Duty Cycle

I noticed today when looking at a datalog in MLV that my max duty cycle (DutyCycle1) was 107. I'm taking that to be a percent, correct? Upon further inspection it appears I'm typically around 80 toward the top of the rev-range, then on some pulls it will jump to 95+ (often the datapoint before I left off the gas). Is that normal or should I be concerned?

For example:
A 3rd gear pull, last 7 datapoints
Duty Cycle: 81.1, 80.3, 78.8, 81.9, 80.3, 104.1, 7.2

I'm running a MSPNP, 550 RC injectors, and a stock fuel pump. I dyno'd 262, 240 a few weeks ago. I haven't had any problems leaning out or anything, air/fuels have been consistent and reasonable. If the high duty cycle is a problem would a fuel pump be a wise investment? From what I've read a Walbro 190lph seems to be the most popular on an NA.

I can post a datalog tomorrow if you guys want to see it for yourself. I'm a little embarassed of this one, I was working on interstate (high rpm/low load) stuff that I haven't really messed with much and it's not pretty.
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Old 05-26-2009, 05:19 PM   #2
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Any duty cycle over 90% or so isn't really happening. A data log and MSQ will be helpful here. 550 cc injectors shouldn't need to max out.
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Old 05-26-2009, 05:23 PM   #3
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Yup. The displayed value is derived after-the-fact by dividing the cycle time by the calculated required duration. Based upon the current VE value, the Req_Fuel number, and any relevant multipliers, the MS computes the required duration. If this durations happens to be longer than the actual cycle time, you get duty cycle values > 100%.

If you're seeing this with 550s, I have to ask- WTF kind of power are you making?
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Old 05-26-2009, 05:39 PM   #4
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i maxed injector at 200whp...then put in a new fuel pump and now I'm at 74% duty and well into 300whp on a dynojet (estimation).
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Old 05-27-2009, 01:02 PM   #5
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Thanks guys. I guess the 20 year old fuel pump has been taking a beating lately. I ordered a new fuel pump yesterday so hopefully that takes care of it.
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Old 06-04-2009, 03:05 AM   #6
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I replaced my fuel pump with a 190 walbro and retuned the car with the new pump. It was running really rich on the old tune. The highest number in my VE table dropped by 11 points, however I'm still seeing duty cycles around 100%.

I know it's going to vary by car and tune but what kind of power are 550cc injectors typically good for? I was under the impression I had a good bit of headroom at 260ish whp. Is there anything else I could check for that could be causing the problem?
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Old 06-04-2009, 09:15 AM   #7
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Couple basic things come to mind.

Fuel filter?

FPR?

Have you done a pressure test at the inlet to the rail? If the factory FPR is going bad, you'd see that as lower-than-stock pressure even at idle. A filter problem might manifest itself at idle, or it might cause pressure to decrease as load increases.

A set of 550s ought to be good for 280 at the crank.
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Old 06-04-2009, 09:31 PM   #8
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Quote:
Originally Posted by Joe Perez View Post
Couple basic things come to mind.

Fuel filter?

FPR?

Have you done a pressure test at the inlet to the rail? If the factory FPR is going bad, you'd see that as lower-than-stock pressure even at idle. A filter problem might manifest itself at idle, or it might cause pressure to decrease as load increases.

A set of 550s ought to be good for 280 at the crank.
280 at the crank? So something around 240ish to the wheels, maybe that is my problem afterall. I thought for sure they were good to 300+ whp.

I considered the fuel filter as well. I can't remember changing it since I bought the car (10000 miles ago) so who knows how long it's been on there. I'll give that a try and check the fuel pressure as well. Thanks for the help.
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