FAST injectors...pulse width?
 

I have a 440na with a 2000cfm throttle body with 83lb. injectors and 8.5 comp ratio. I realize this is overkill for a NA motor but the blower will be going on in a few months but I want to drive the car now!! An EFI tuner has informed me that with the current setup I will destroy the motor because of excessive fuel wash. Ok I agree...overkill. But with the adjustability with the FAST XFI can't I tune the pulse width of the injectors to make the car run?

FI info
 

Does your FI system come with a star up tune? It should. I would like to find out some of this info also since I am running a bigblock with a procharger, that more then likely I will be converting to FI.

Yes there are about 25 different settings for different engine types that are pre-programmed into the system. I found my closest one and tweeked it a little. Haven't gotten the car running yet because of the above concerns.

there is a minimum amount of time (pulsewidth) that it takes an injector to open/close. If an injector is too large it can cause idle issues as you lose the resolution when you try to get the pulsewidth numbers too small to get the proper a/f. In other words, you can only get the pulsewidth so low.

With that said, I've run bigger injectors than that without issue at idle or anywhere else. Although they were forced induction setups, idle is the problem a large injector usually has the most problem with, and a naturally aspirated or forced induction setup are basically the same there and mine had zero issues. Make sure you boost reference the regulator, it will help at idle by dropping the fuel pressure a bit with the vacuum line installed. My last setup was 75# injectors on a small block with a stock camshaft, idling at 650 rpm and 18" of vacuum. Ran perfect.

Thanks for the help guys. FAST just told me that the injectors are just way too big and no matter how low I set the pulse width, too much fuel wash will occur. Not a good thing. Ah well, I'll just wait a few more months until the blower arrives. :thumbsup:

What's the projected output? 83's will handle over 800hp.
Are you running batch or sequential?

If it's a sequential system, they should be able to idle without issues. The map may be skewed towards the low end vs something that's using the injector but, should work.

Batch/Bank to Bank is different story, they'd be abit much to work at idle, low speed operation.

I'm shooting for 800. But the tech guys all said that the 83lbers are too big for an 8.5comp naturally aspirated 440...now matter how low the pulse width. But then again, the s-charger is pushing a certain amount of air even at 600rpm. That's not a lot of air but yet it's a ton of fuel? The FAST guys were real serious when they told me no.

Oh yeah,I'm running sequential. Still learning about this stuff. Bank to bank is when one side fires then the other and is not as controllable as sequential?

at idle I don't see how the EFI or engine even knows there is a blower. There would be some extra pressure in front of the throttle body, but the engine wouldn't see that; in other words, it would act just like N/A. I'm amazed my 95's idled perfectly with bank to bank...................... :wow:

It's kind of like sticking a 950cfm double pumper on a 350sbc. No matter what size jets you change to, it's still overkill for the motor right? I thought the same thing but hey...it's 2012 and I have full control over the motor via laptop and I can program the injectors to do what I want. At least that's what I thought.

Just thinking out loud here but would it be possible to regulate the fuel pressure down and leave the pulse width a little longer until the blower is on there?

Any idea on what the pressure should be? Kind of important on startup being a new motor and all. Considering all of the negative feedback I've been getting from the tech guys.

If the 83lb injectors will be right with the blower on it, then it will still have to idle and low speed cruise with them. Yes, they are way to big for it as an NA setup, but they still need to be dialed in and can be for sure.

What I would do is have the fuel pressure set lower (40-ish) and use a boost only referenced fuel pressure regulator to gain pressure as boost climbs.

We run 80-100lb injectors on quite a few cars these days on FI LS engines and they drive and idle quite well. All in the proper injector data, proper fuel pressure and proper tune.

Mike Norris

Let's not forget cam specs, typically a blower cam has a wider lobe sep resulting in better idle quality anyway.

Yes you can reduce the pressure when off boost then increase it as boost increases with the suitable regulator.

83's on an 800hp 440 shouldn't be a problem. Sounds like a tuning issue as well. I've run 120's on a 383 Chevy making 1200hp with twin turbos, no issues when tuned right.

Exactly, the thing isn't going to have boost at idle once you install the blower.

If they will idle/cruise correctly with the supercharger than they will do so without the supercharger. It won't be a problem.

Ok, so the injectors will be fine at idle and maybe at a light cruise. I'm assuming the engine will bog down because of too much fuel no matter how much the pulse width is adjusted. That's fine with me because I really want to hear this baby fire. Was only concerned about the excessive fuel wash during break-in.

With the correct tuning it'll be fine at all rpm's.