The car I'm buying is set up with 4 720 CC injectors with a pump and fuel pressure regulator, T04 and FMIC running 10 lbs boost, running 250ish at the rear wheels.

Now, at 58-63% max duty cycle with the stock ECU, doesn't it seem a little odd that it can put out that much power without an upgraded ECU?

It's got a Fuel Cut Defender so that points to "probably not upgraded" but due to the fact that it shouldn't be able to put down that much power without a higher duty cycle on the ECU.

I'm wondering if the previous previous owner (the guy I'm buying from didn't do this) upgraded the ECU somehow by allowing a higher duty cycle on the injectors.

How can I determine what max duty cycle the injectors are running at, and see if they're beyond stock (63%ish).

 

Part answer: Obtain something similar to a Fluke 88 meter and back probe a primary injector at the ECU and go for a drive. This type meter has a *duty cycle* position and you can watch it as you stomp the pedal.

http://www.tequipment.net/FlukeDigital88Multimeter.html

It also has a min/max buttom where you can capture the min/max of the signal. That is a good feature. Watching the boost sensor output visually I found out that I did not see the max with my naked eye....but going back to min/max it showed that the sensor had peaked a bit higher than what I saw in real time.

By the way...does this car not have a safc with those large injectors?? It must, and that would explain a lot about the duty cycle.

Where did you come up with the idea the stock ECU will only hit 58-64 percent????????? Ain't so. I'm not reading that part of your post right, I'm sure.

 

I think I know what car you're talking about it. The stock ecu only driving injectors to 60 percent is completely false. Henrik on teamfc3s proved that with a datalog of the stock computer.

4 x 720 is enough fuel for mid - high 300 rwhp depending on fuel pressure....swap the stock ecu out for a e6k/wolf/lt8 and you're good to go.

 

so you're telling me I'm safe with 4 720's on the stock ECU? How the heck isn't this thing running pig rich at low RPM?

And do you have any more info on the duty cycle being disproven?

I'll have to check the fuel pressure, what's the stock pressure and any specs on that? I don't have the manual right in front of me here at work. And if I increased the pressure over stock (it's got a fuel pressure regulator), it should be able to run more fuel with the same size injectors, is that something people do often?

 

you need a comp to effectively control the fuel

the stock ecu thinks there are 550cc and so when in fact there are 720cc it's dumping more fuel that it needs.

duty cycle refers to the time the injectors are open.

when the injectors are actually open it flows WAY more than stock so that's why it's rich.

i guess u coudl think of it as a valve.

the 550cc would be like a 1/4 pipe and the 720cc 1/2 inch.

when they opperate at the same duty cycle the 1/2 pipe will put more fuel out at the same pressure.

100 percent duty cycle is open constantly. 0 percent is no fuel flow.

saftey margin is what mazda was doing keep'n the duty cycle so low.

 

about fuel pressure- it's fairly common for that but prefered is just large capacity systems.

put more pressure behind the hose it will force more water out faster.

 

i htink he said it wasnt running rich, which is why the concern.

 

People how state "the injectors operate at 60% duty cycle" or similar nonsense simply don't know what they're talking about. That's just not how EFI systems work. ECU's do not calculate injector duty cycles; they calculate injector opening times. If you divide the opening time by the cycle time (seconds per rev) you get a percentage that we call the duty cycle. For there to be a duty cycle "limit", when you reaches that limit you would need to reduce the injector opening time as revs increased to maintain a constant duty cycle. Does that sound logical to you?

BTW, flow through pipe or duct increases approximately to the square root of the pressure increase. So if you increase static fuel pressure by say 20% (for FC's that'd be from 250kPa to 300kPa or 37psi to 44psi), you'd get ~9.5% more fuel from your injectors. So Turbo injectors are 550cc/min @ 250kPa, 600cc/min @ 300kPa, etc. The pump needs to be able to maintain this pressure plus boost pressure at the required flow though, so increasing the static fuel pressure with a new FRP should be done with caution to ensure your pump (which must also upgraded) can keep up.

 

with four 550's

58% = 183hp (hmm max of S4)
63% = 200hp.... (hmm max of S5)


that is just the duty cycle of a stock car.... there is no max.

 

aside from actual physical test data, why would anyone buy injectors and then have the ecu use half of them?

 

most ppl like to see a peak of about 80.

That give them a little head room for a spike or some other malfunction.

if you demand more fuel for some reason and you can't provide it b/c the cycle is at 100 percent somethign goes without.

 

Good God.. so with the 4 720cc injectors, I should be able to safely run 332 hp at the flywheel, with 80% duty cycle on all 4 injectors.

OR...

286 at the flywheel with the primaries only opening at 58% duty cycle, and secondaries opening at 80%.

 

HOLY good point Batman!!!!!!

 

LOL,you've been playing with that fuel calculator on the RP website again, haven't you?

 

http://www.rx7.com/cgi-local/2ndgencalc.cgi

 

Actually those injectors can flow enough for ~380hp @ 80%, assuming the pump can keep up.
There's one major flaw in those numbers. When all four injectors are firing (under load above 3800rpm) they are all being fired at the same time. So they have the same pulsewidth and thus the same duty cycle.

You guys need to get this duty cycle "limit" crap out of your heads. It does not exist!