MSefi.com read only forum (Jan.15,2018)

Ok I have another thread started asking which gm tbi to use... but i figure i'll start this thread since people might look at it.

So my engine is a 78 mopar 360 LA bored .40 over...so really a 365 ci.

Installed flat top hyp. pistons, comp cam, heads were port and polished by the owner before me (how much and how good i have no idea?), running msd 6al box, blocker hugger headers. Compression is around 10:1

Now I am not sure on the amount of HP i am making but lets just say around 300?


I want to run a tbi setup. If my math is correct I need the bore size to be 43mm with 80lb injecotrs. But to factor in the 80% cycle i would need 100lb.

I know the stock 350 came with a 43mm bore and 55# injectors.

now for the math:

355ci x 5,000 rpm / 2
= 912,500 x .80 ( the 80% is the VE airflow since you never get 100% air)
=730,000 ci / 1728 cf

= 423 cfm

stated in another post someone said they had a 4 barrel tbi that was 9.6 and it would flow 900cfm.

9.6in / 900cfm = x / 420
420 x 9.6 / 900 = x
4032 / 900 = x
x = 4.48in

4.48 / 2 = 2.24in (dividing by 2 since that is how many bores i have)

2.24 = 3.14r^2
sqr root .71 = r^2
.84 x 2

=1.7in = 43.18mm

thats for the airflow, now for the injector rating:

i figured its 25,200 F^3 per hour

.075 lbs/ft^3 is the weight of air

.075 lb/ft^3 x 25,200 F^3/hr
.075 x 25,200
1,890 lb/hr

so we will round to 1,900 to make things easier.

since we are running a 12:1 air:fuel ratio

1,900 / 2
= 158 / 2

= 79.1 lbs/hr injector

Does this seem correct???

Yeah, your math looks good (I didn't go through it in detail, though, just skimmed it). I hate to say, but you're doing it the hard way.

Easy way: Guess ultimate HP, say its 200. Guess a reasonably high BSFC (Brake Specific Fuel Consumption), say 0.5 lb/HP (0.55 or 0.6 might be better, but 0.5 is easy to do in my head). Multiply them 200 * 0.5 = 100 lb total fuel required to make 200 HP. Divide by number of injectors and you've got approximate injector size...

Eric

alrite good Yes i know there is an easy way but this way i can see it...

Now i know that GM makes a 90lb injector for the 454. Would that work in my application and what psi should i run it at? If you up the PSI you get more lb out of the injector, rite??