At what injector duty cycle does upgrading become necessary?

[turbosam6]

My inj. duty cycle was up to 104% on one of my runs, mostly around mid 90's the rest of the time. I plan to go faster, hopefully. Do I need bigger injectors? Thanks for your help!

 

[bobc455]

Rule of thumb is about 80-85% maximum, and you should design based on 70-75%.

-Bob Cunningham
bobc@gnttype.org

 

[Brooks]

It's not just the injector duty cycle that indicates whether you need to upgrade but also what fuel pressure you ran that duty cycle at. I run 009's at considerably lower fuel pressure than what they were rated at and if you were to look at my duty cycle you might think I was out of injector with the stock turbo

What base pressure are you running?

There's a pretty handy little chart on gnttype.org put together by Joe Lubrant that shows 50's good for high 10's at 100% duty cycle and at the rated fuel pressure.

I'm sure some of the guys actually running 50's will chime in with real world experience to help you out.

Charles Brooks

 

[bruce]

Originally posted by turbosam6
My inj. duty cycle was up to 104% on one of my runs, mostly around mid 90's the rest of the time. I plan to go faster, hopefully. Do I need bigger injectors? Thanks for your help!

There seems to be a real fetish around here for hammering small injectors to go fast, and there is no need to. Just get a decent sized injector and tune.

I spent a day fooling around with a car with 009s, and there was just nothing to tune, he was out of injector. I have a WB so I could actually see what was going on, rather then guessing.

With a stock combo, other then exhaust we're right up at 85% DC with 55PPH injectors now.

There is a slight error in DS it seems like, in comparing my DC reading to DS, DS seems to report 4-5% low, so your right at, being static with your injectors.

I've worked with stockers, redtops, 009s, and 55s, the 55s are a good place to start. Thou you need a good chip.

Speaking of which that's why so many folks like or are forced to use small injectors. There are lots of chips that just set the injectors static, and then you just dial in enough FP to try and get it to run right. Note the word, TRY. Why do you think so many folks blow head gaskets around here?. Yep, the fuel curve is wrong, and they try to run hard, with a chip that has lean spots in it, and then wonder why the headgasket blew. Oh and they had good O2 numbers, but no one pointed out, that O2 voltages are bogus, as hell to begin with. Please look at the thread going on in the Scan tool area to see my post about the O2 voltages and then look at the direct scans of the various runs.
Sorry to rant.

 

[turbosam6]

I run 45 psi with the line off, per Bobs recommendation. I have a single walbro 340, should I bump the fp when racing to lower the duty cycle? Thanks for the help.