Getting ready to get started with a 383 LT4 builduup here very soon. Just have a couple questions on the fuel system.

Question 1: What size injectors to use??? I was planning on 38 lb/hr, but I just realized I know NOTHING about duty cycle and what duty cycle I should be running at. This will be an NA motor, maybe spraying a 100-150 shot down the road. PLEASE...IF YOU GIVE ME A CALCULATOR FOR INJECTOR SIZE, EXPLAIN WHAT BSFC IS AND WHAT DUTY CYCLE I SHOULD BE RUNNING!!!

Question 2: What type of injectors are good?? I have seen a lot of people using SVO injectors, but I have also heard that Lucas makes some of the top injectors. Opinions on some brands...MSD, Accel, Lucas, etc.

Thanks

 

http://www.iroczone.com/calcs/injectorsize.htm

 

My advice after my extensive reseach and pondering(seriously) is to go with 36lb NA and possibly 42lbers if your set on spraying. Its easier and safer to "tone down" an injector than to stretch one to its limit. The calculator I used said I needed 36.4 lb injectors with my set up and Im gunning for 450 rwhp NA...

Go to www.corral.net and get your injectors, I got mine cheap.

 

30lb svo's are more than adequate and you will save about $100 over 36 or 38 lb injectors

 

Injectors 101:

BSFC = brake specific fuel consumption. This is the amount of fuel, in pounds per hour (#/HR) required to make one HP. A well-tuned N/A setup will probably have a BSFC of about 0.45#/HR/HP. A blower motor, or a nitrous motor may have a BSFC of about 0.52#/HR/HP. Remember, nitrous only affects the injectors if you are running a dry system.

This means that for an N/A engine, making 500flywheelHP, you will need:

500HP x 0.45#/HR/HP = 225#/HR fuel

Since you have 8 injectors, each one needs to supply:

225#/HR / 8 inj = 28.125#/HR per injector.

An injector achieves its rated flow only if it is continuoulsy open, and only if it is supplied with at least the rail pressure that is was flow tested at. But, an injector can not be constantly open. The computer is turning it on and off. It takes time for the injector to open, and time for the injector to close. The times are "offsets".

Testing has shown that once the computer is calling for the injector to be open more than 80 - 85% of the time available (this is the duty cycle), the injector can become unstable. It can start to "flutter" and the flow will drop significantly. It can also overheat and experience shortened life expectancy. To keep away from this condition, it is a good idea to design your injectors for less than 100% duty cycle. The popular online injector sizing formulas use 80% as the recommended duty cycle. They also typically use a BSFC of 0.50#/HR/HP, so they are VERY conservative.

To size an injector correctly, you need to know the BSCF for your engine. This is normally calculated during an engine dyno test. That is where I got the numbers quoted above. If you don't have the actual BSFC, you have to make "allowances". You also need to remember that dyno HP is typically rated in SAE "corrected" HP. This means that in actual operation, with colder, denser intake air, your engine is fully capable of making more HP that the dyno has indicated. That means it will need more fuel.

Since you don't know the actual BSFC in most cases, and you need a little fat in the system to cover the nice days when you get high air density, it is always better to go larger than to go smaller.

For an engine making that 500flywheel HP, and requires 28.1#/HR of fuel, it would be foolish to use a 28# injector. You divide the fuel requirement by the desired duty cycle to get the injector size.

28.1#/HR / 0.85 = 33.1#/HR injector

It would not be a good idea to use a 30# injector on a 500HP engine just to save $100. A 36# injectopr makes a lot more sense. A lot of people will argue that they have dyno tests that indicate 400+RWHP and 100% duty cycle, and their engines run fine. My response to that is to consider the fact that the typical chassis dyno pull subjects the injectors to max fuel demand for about one second. That isn't enough for the shortcomings of an undersized injector to show up in lost power. And, isn't it possible you would have made more peak power with an injector operating at 80% duty cycle, rather than 100%. How much power will you use on the track, when the injector is required to provide max fuel fuel for a longer period of time? If you are running at high speed for an extended period of time, is it going to run lean? How much have you reduced the life of the injector by repeatedly over stressing it?

There is no downside, other than cost, to using a 36# injector where a 30# injector would be marginal. Using an SVO instead of an OEM RP will net you a small increase above the flow rating, because of the differnces in fuel rating pressure. Running an injector at a higher pressure than it is flow rated at will also by you some added capacity. But to me, it seems foolish to drop $5-10K on a high HP storker, than skimp on injectors to save $100.

I run Bosch 64#'s at 58psi (=74#/HR).....

 

I can get you 39# SVO's for $199. Email mike@axincorp.com

 

38 lb/hr was my original plan, but now I am thinking maybe just stepping up and getting the 42 lb/hr just to be on the safe side. According to desktop dyno, the motor is supposed to make around 577 fwhp which would be 490 rwhp assuming a 15% drivetrain loss, but I guess I could take that for what its worth...he he

Thanks for the help, that really lets me know what is going on. I think I will probably stick with 42# injectors. And I have one vote for the SVO's, which I know are popular, but has anybody heard anything about the Lucas injectors???

Thanks