Originally Posted by K-dawg

what size injector, how many of these will I need and where can I get them(a link or a store name i s what I'm looking for, I'm tired of people telling what vehicle I can take them from in a junkyard...)

I have a sbc 400 bored 30 over... I intend on swaping out my 305 tbi for the 400

Originally Posted by Fast355

C.)Proper fueling

Fuel Injector requirement- To mathematically calculate the injector size you will need to have to make your HP is pretty simple. Lets say our engine is capable of 340 FWHP. How much fuel will it need? Simple. Take 340 HP divide it by .85(Max Safe Injector Duty Cycle for TBI), then divide by 4 (easier than saying number of injectors and BSFC). On a TBI engine with two injectors it will always be 4 (.50 lb/hr/HP and 2 injectors) That gives you 100 lbs/hr per injector. Oops, GM only ever made a 90 lb/hr injector in the US, and I have 2 68# injectors, what do we do. Well we take the stock GM fuel pressure of 12 PSI and raise it. How much though. Well this simple equation will help a lot. New flow rate = Old flow rate x SQRT (New Pressure/Old pressure) Lets switch this equation around. We know the old flow rate and the old pressure and the necessary flow rate. Just solve for the new pressure.

100= (Injector size(68)) x SQRT (X/12)
100=SQRT(X/12)x 68
100/68=SQRT(X/12)
1.471=SQRT(X/12)
1.471 squared=(X/12)
2.163=(X/12)
2.163 x 12 = X
X=25.956 PSI

The closest production regulator to this is the GM 30 PSI unit used on the later 454 trucks/vans/suburbans. It is GM P# 17113079, $64.74 gmpartsdirect.com. Using the 30 psi regulator will give you 108 lbs/hr using 68 lb/hr injectors.



You might wonder why you need more than 100 lbs/hr. It has to do with a little routine called Acceleration Enrichment (AE). Lets say you are accelerating at 3,500 and 1/2 throttle. Suddenly a Honda comes along begging to be destroyed. You mash the gas, the manifold goes dry because you just sucked the fuel out of it. The engine bogs and backfires. One kill for the Honda. Lets just say not having enough fuel to cover AE is not fun.

This is a chart of approximate flow and what 30 PSI will do to it.

Engine---12 PSI----30 PSI
2.8/3.1-----33---------52
4.3---------45---------71
5.0---------55---------87
5.7---------61---------96
5.7HF------65---------103
5.7HF------68---------108
5.7 96 Van 72--------114

Just to give you an idea 10 lbs/hr is roughly 20 HP per each injector. For each 10 lb/hr for the pair is 40 HP more potential.

Keep in mind the factory pump is overwhelmed in most cases by a mild 305. I would definitely upgrade to at the minimum a factory replacement TPI pump, with a Walbro 190 being good and a Walbro 255 being better.