Recently purchased a used MTI built 396 and was told that it was built using the stock injectors. The car ran fine on this set-up (so I'm told) but am curious if this would hold the car back any. Currently it will be used in an N/A capacity, but at what point will I absolutely need bigger injectors (i.e.- even if I used the smallest amount of nitrous, etc). Thanks.

 

There are a couple online injector sizing sites, but they tend to be super conservative. As a guideline, I'd say once you go over 400flywheelHP, its probably a good idea to move up to 30# injectors - have the PCM programmed, of course. There are people here running over 450 flywheel on stock 24#'s, but to me that is pushing the limit for no good reason.

If you use one of the "sizing" sites, use a brake specific fuel consumption (BSFC) of 0.45#/HR/HP for an N/A application. Then use an 85% duty cycle. That will pretty much generate the numbers I mention above.

((flywheel HP x BSFC) / 8) / duty cycle = injector size

As far as using nitrous in the future, what you need to do to the injectors depends on what type of nitrous system you use. For a wet system, the extra fuel will be added directly through the fuel solenoid and a nozzle of some sort, using a "jet" (orifice) to control the flow rate. So the fuel doesn't go through your injectors. If you opt for a dry system, it is not unusual to size the injectors and tune for the N/A condition, then boost the fuel pressure to provide extra flow when you spray.

 

That's what I thought. But I was told that MTI dyno tuned this to 430 rwhp (M6) on this set-up. I can't imagine a place like MTI leaving things to chance like that. Everything else has been replaced with all the extras (billet splayed 4 bolt, forged intenals, etc) so why would they keep the injectors if it wasn't up to it? Also, I have tried contact MTI, but have had no luck. So any advice is more than welcomed. I would hate to have my injectors be at their max and then cause damage down the road.

 

Like I said, there are people here who claim to have made over 450HP on the 24's. Might be an extremely efficient tune in terms of BSFC, or maybe the injectors were on the ragged edge and the one second or so that they are at max flow on the dyno wasn't enough to overheat them, or cause them to start to flutter. My thought is, after investing kilo$$$ in a quality stroker, why skimp on the injectors.

I run Bosch 64's at 4bar (58psi), allowing them to flow 75#. The engine idles like a kitten, and the injectors will flow enough fuel to produce a 66% duty cycle at 786HP, at 0.53 BSFC..... .

 

MTI 396. 420 RWHP/425RwTQ. Stock 24# injectors. 11's in street trim. Idles and runs good. 5k miles and no problems. Go out to the track and see what happens.

 

Listen to Injuneer - you might be able to get away with it, but the stockers are totally inadequate for that kind of power... I'd hate to see the duty cycle on those things - they are definitely static at those power levels, and sooner or later you're going to start having problems with your injectors.

 

I have fms 30#s on my 396, they work great

 

Accell 30's w/ no problem either. I'd trust Injuneer on this. Don't skimp.

 

I have two motors built and tuned by MTI. A 383 in the Z and a 396 in the vette.Both are using stock injectors. The vette has a cc 306 cam and made 402 hp NA, and 450 hp on a 75 hp nx shot. I didnt chose to skimp on the injector, thats what Jason @ MTI recomended. I've got almost 20kmiles on the vette engine without a problem . The Z has almost 30k, I wish I could say without problem, but nothing involving the injectors. Just food for thought.

 

Wow, I won't be going to MTI to get any work done - why on earth would he recommend 24# injectors for a 383 or 396?! I use to hit 100% DC with the stock injectors on the stock bottom end pretty quickly... I don't see how I could use the stockers (even at 50+ psi of fuel pressure) on my 383. If your 383 is only making 402 hp NA, you might be running out of injector.

 

Possibly. I don't think my dual mass fly wheel is helping either. I don't think going bigger is a bad idea, I might even see what it does for me at some point. I just wanted to express that the stock injectors do work to the extent that I described. If I go bigger and pick up power,I'll definately post it.