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duty cycle based loads

Practical Motorsport Wiring - Professional Level

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Im trying to wrap my head around powering my ign1a coils correct me if im wrong here. as per Race Grades Spec Sheet on these coils they recommend not pushing them past 40% duty cycle. my car; 2JZ powered will turn at 9000 rpm max, has a 16V battery with no alternator. using a duty cycle formula found in Greg Banish's book "Designing and tuning high performance fuel injection systems" where duty cycle = (injector pulse width x RPM) / 120000 assuming i can substitute injector pulse width with coil dwell i come up with a value of 5.3ms of dwell 0.4 = (Dwell x 9000)/12000 re arranged to dwell = (0.4 x 120000) / 9000 = 5.33ms.

again referencing the same datasheet with a dwell time of 5.3 second at my 16V the car uses i come up with an amperage of 14A

my understanding is that because these coils charge in pretty much a linear fashion until they become saturated you can use the average current draw of the graph.

average would be 14/2 = 7A

because of the 40% duty you can take the 7A and multiply by 40% to get constant draw of 2.8A per coil

6coils means 2.8 x 6 = 16.8A

is it as simple as that? if so then why are all these guys suggesting to power these coils with 16awg wire? using the motec pdm user manual, m22759/32 wire with an ambient temperature of 100 degrees C according to the chart 24 awg wire would be rated for 4A at that temperature which is more than sufficient to power a single coil. to me this seems very small. a single 14 awg wire would be rated to 18A and should be sufficient to power all 6.

on to my next question. if all that checks out then i should be able to use a single 25A output on my ecu master pmu16-AS to power all 6 coils? most kits i see for these coils with fuses and relays for 2jz have two 35A fuses or two 40 amp fuses coupled with two separate relays.

it should be noted that the ECU (motec M130) and PMU (ecu master pmu16-as) are mounted inside the cabin at the firewall and these runs of wire are very short.

can the same be said for injectors?

where my injectors are from fuel injector development measure out at an average of 8 ohms

ohms law V=IR

I = 16/8

I = 2amps

2 x 80% = 1.6

6 injectors X 1.6 amps = 9.6

does that mean i need roughly 10 amps of current capacity to power all 6 injectors?

G'day Jean.

Your math checks out, I did it a slightly different way, but came to the same conclusion. I havent got an IGN1A datasheet infront of me, but 14A sounds about right for 5.33ms of dwell, maybe even a little high, but that will be an error on the conservative side, not the dangerous side.

Yeap, you're right, 16.8A total effective draw for those 6 coils operating under those conditions.

I'd want to put a bit of a factor of safety on that, and call it 20A.

What's the purpose of the car, how long will it see sustained high revs?

I'd be wanting at least a 14g main power feed, that branches out to a 20 or 22 gauge for each individual coil. Yes, smaller is possible, but the cost of running on the ragged edge doesn't outweigh the risk usually... To finish first, first you have to finish.

Another key consideration is a reliable crimp join. IGN1A connectors aren't going to really like being crimped onto 24awg wire, and I'd personally double back the 22 gauge wire if that was what we being used. 20 gauge should be fine as is.

Your injector calcs are solid. There is a bit more going on, as they (like the coils) are an inductive load, but they usually spend quite a bit of their time in the saturated zone, so flowing max current. Once again, any errors in the calcs are on the conservative side, not the dangerous side, so they're all good :-). I'd probably run a 16gauge main feed to those and 22gauge to each injector.

Sounds like a sweet project, pop up some details in the build threads section!

appreciate the response Zac!

the purpose of the car is very light street/drag.

i never had the intention of powering the coils with 24 awg wire. i was just wondering why most of the "kits" are running such large wire. i guess major safety margin? i had fully intended to use 20 awg wire for the power and grounds. The sensor returns and signal were going to be 22awg (smallest wire i have on hand and my dexterity can tolerate lol)

works out perfect because i have a Souriau 8sta 14-19 connector set here i intended to use for the sub harness for the coils. 6 powers 6 signals 6 grounds and a return and of course the coils grounded to the head.

sub harness for the injectors i have a 8sta 10-35 set here that should be up to the task!

again thanks for reassuring me i haven't completely lost my mind and im on the right track with sizing my wires appropriately for the loads!

you mentioned you did your math a different way would you mind sharing the steps you used? i really should go back and re watch my courses!

Just looking at a single cylinder, 9000rpm = 4500 ignition events per minute, as a four stroke engine.

4500 / 60 = 75 ignition events per second.

1s / 75 = 0.0133*s between ignition events, or 13.3*ms.

13.3* x 40% Duty Cycle (0.4) = 5.33* ms.

That's about the max time you'd want to dwell them for, much longer and they're probably just going to heat up excessively.

Note that if the engine is idling at, say, 1000rpm, then 40% duty cycle at this engine speed would be much longer that 5.33* ms, so the 40% DC thing only really makes sense if you calculate it at your max planned RPM.

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