I'm planning on rebuilding to a 396ci and I was wondering what kind of injectors you guys would suggest......28#, 30#, 36#, 42#, or higher

I will be re-doing my computer chip with all of this. I've heard that bigger is better, but is there such a thing as too big??? Thanks

 

Get a hold of 89vette, He has a 396. Here is a link of his dyno run and I see he is using 24lbs in his sig..
http://forums.corvetteforum.com/zerothread?id=361733

 

I would run #30s

 

ok.....why??? I'm not sure the limitations/advantages to each # of injector, a little background would be helpful.

 

Depends a lot on how much power you are looking at. If you are looking at 400 to 500hp you can probably go with 30lb injectors without going to high into the duty cycle. Here's a common equation used for determining injector size.


Inj. Size = (engine HP x BSFC) / ( cylinders x .8 )

Using a BSFC of .4 - ,45 should be fairly accurate in your case. .8 is the duty cycle. Most people suggest going up to around %80. Hope this helps.

 

I am running 30# in my 398 and they are right at the line of being maxed out.

 

ok so lets try this

(550Hp * .45) / (8 * .8) = 38.67#

which means 42# injectors, 550Hp might be a little high, but it's room for growth.

Is this right or is there something I'm missing???

 

That's how it works. Oh, I did forget to mention that this is a rating at 43.5 psi. So you can run an injector higher. Take your example that worked out at 38.67 lb/hr. Go out and buy a set of 36lb. injectors and you can run them at ( 38.7 x 43.5 ) / 36 = 46.7 psi. So with your estimated BSFC at .45 you can run about 550hp at the motor on 36lb injectors at an %80 duty cycle. How \ever, I would tend to think that you BSFC is closer to .40 and you can always go a little above an %80 duty clcle, so lets see what 30lb injectors will do.

( 30 lb/hr x 8 x .83 duty cycle ) / ( .42 BSFC ) = 474 hp at 43.5 psi
550 x 43.5 / 474 = 50.4 psi

So even if you only went with 30 lb/hr injectors you could get upwards of 550hp at %83 duty cycle if you are running at 50psi. Plus you still have a little room to go up.

One good thing with going with a smaller injector is that you tend to get better atomization because of higher psi. Then problem is that running 50 psi is getting up there most fuel pressure gauges don't go much higher. Also a 30lb injector tends to cost less than a 36. Just depends on if you get a deal A 36 lb injector can realistically support about 575hp at %80 duty cycle at 43.5 psi, so unless you plan on running up toward 660hp or so I probably wouldn't go higher. Good luck.

 

ok, that makes sense with upping the fuel pressure, However, I'm running my nitrous system off of my fuel rail and by upping the fuel pressure I would be running it richer than it is now, designed rich to help prevent going lean. So I want to keep the stock fuel pressure if I can.

 

The calculation is incorrect. Flow rate increases with the square root of pressure increase so to get 38.7 you would need to use this formula

NewPressure = OldPressure * (DesiredFlow/CurrentFlow)^2

so to get 38.7 you would need a theoretical 50.3 psi.

There is a limit to how much pressure you can safely run. Injectors will not open, not close or open more slowly when the pressure exceeds a certain point. The limit depends upon the injector design. I would limit pressure to 50 psi with 45 being a better choice with the pintle design.

 

I don't know much about nitrous, but I can tell you that going with something as big as even a 30lb injector will probably require computer programming. Otherwise your car will run very very rich, especially down low. If you do get computer work done the program can compensate for the amount of fuel at any specific pressure that you want.
Hopefully this is helpful.

 

Thanks Aqua,
Sorry for the confusion mn, fuel rate is not a linear equation as I wrote earlier. Use aqua's formula if you want to play with the fuel pressure.

 

I'm going with at 396 so the computer reprogram is a must. I'm having trouble with my new heads and headers with it running too rich I couldn't imagine what a bigger CI would do to that poor computer.

So it looks like 42's it is then

 

I personally think 42# injectors are too big unless you plan to put out 480-490+ rear wheel. The largest ones I would go with would be 36# for a NA 396.

Chris

 

can an injector be TOO BIG???

 

Yes.

 

Plan to upgrade to the FMS 30# injectors on my 396 just to see what happens. My 396 runs real strong and like a top with the stock 24# injectors and the fuel pressure at 50-52PSI. There are MTI 396'srunning 400+ at the wheels with the stock injectors and no problems. The Editor of GMHTP magazine thinks the stock 24# injectors are the only way to go. 420+ at the wheels and 45,000 miles on his 396. Driving the car during the week and racing at the track on weekends is the best math calculation you can do.JMHO


[Modified by Red Rocket, 9:57 AM 8/23/2002]

 

Good info!! I was thinking about this as I am planning on building my 400 next year and was worried about using my stock injectors. :cool: