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Question re Webinar 92 Fuel Systems Designs

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I have a car that is just used as a track car, NA Subaru 3'3 litre and I was told by one of the ECU companies as well as other that when a injector gets to 90 % duty the injectors have reached full duty. Mine hit this at 7,000 rpm, so I have now gone from 330 cc to 740 cc and got a lift in power.

My question,

Is the right duty for a race engine injectors to have them match valve opening at higher revs. In other words if the intake cam is open for 40 % of the time then the injector duty needs to be around 40 to 50 % and no more?

Your thoughts please?

When an injector gets up to about 85-90%, the injector flow becomes non linear and somewhere in this region the flow essentially reaches the same flow as if the injector was at 100% IDC.

No, there's no need to match the injector duty cycle to the intake valve opening. This would give you the ability to match the injector phasing to the intake valve opening, but this really isn't essential and many OE engines choose to inject against a closed intake valve once the engine is at operating temp anyway.

There can be advantages in some engines from moving the fuel injection timing around and to get the biggest advantage from this we obviously need some headroom in respect to IDC - What I mean is that if you're already reaching 90% IDC, there's very little room to move/adjust the injection timing as the injector is essentially open all the time anyway. You also need to balance this with the ability to get good control of the AFR at idle - If you're using an injector large enough to only reach 50% iDC maximum, you're going to be trying to use very small pulse widths at idle and this may compromise your ability to achieve a target AFR.

Lastly in my experience the gains I've seen with injector timing are usually minimal (in the range of 1-2%). If you saw a gain in power from fitting larger injectors I'd be inclined to say it's more likely your old injectors were compromised in some way.

Hi Andre,

In simple terms if sequential injection is better then batch are we saying that the gain is only 1 to 2 % between the two methods. I would have thought that there is more gain from Sequential then that? Not sure if you see were I am coming from but the 1-2% doesn't reconcile. Why would the industry say a good maxim duty cycle is 70% max if you can flog your injectors to 90 % and only lose a couple of %.

Sorry to be a pain, I have dyslexia which means that if this don't make senses it does my brain in.

Hey Desertrunner, no problem - I'll try to explain a little clearer :)

The advantages of sequential injection are more relevant at idle and low engine speed when air velocity is low and typically the injector pulse width will be quite short - this gives a large window for us to move the injector timing around relative to the valve opening. In saying that though, the advantage of sequential injection over batch fire still doesn't always come down to power, but rather is about providing a more uniform charge of fuel and air to each cylinder as the injection timing is the same on all cylinder - With batch fire one cylinder may receive the injection pulse with the intake valve open, another with it closed, and others somewhere in between, and this doesn't make for consistent and uniform charge preparation. With this in mind, sequential injection can improve economy and more importantly to the OE manufacturer, reduce emissions.

I've never seen a maximum IDC of 70% quoted as being ideal although obviously everyone has their own preference. My personal recommendation would be 85-90% max. As I mentioned above, the reason for this is to do with maintaining linear flow from the injectors so the ECU can control the air fuel ratio correctly. Once you get to perhaps 90% IDC, the injectors are essentially wide open and you're not going to see an increase in fuel delivery from there up to 100%.

I hope that helps clear this up?

Thanks Andre,

Mark at Motec ran some numbers here and used 65% duty, 5 posts down.


As I said, everyone is likely to have their own preference and really it depends what you're trying to achieve - Do you want to be able to move injection timing under high load/rpm and be able to get a worthwhile change? If so you'll need to limit your maximum IDC to perhaps 50-60%. The flip side is that as your max IDC drops, you may struggle to control AFR at idle. As with a lot of aspects of EFI tuning, this is a compromise.

Reading Mark's comment though it appears the value of 65% was an off the cuff example as opposed to a strict rule - 'And just to finish off you would obviously choose an injector that was bigger rather than smaller. You could even add in a duty cycle adjustment, lets say 65%'

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