Fuel injector size help!!!

[SShep71]

Ive tried several different calculators and have done the long math and still cannot get a difinitive answer on what size injector to use. I want to make 650 hp with a 462 pontiac with a 6-71 roots style blower. Whaf injector size do you suggest. Has anyone run something similiar to this? Im going to run the holley HP EFI.

[Jim Nilsen]

Ive tried several different calculators and have done the long math and still cannot get a difinitive answer on what size injector to use. I want to make 650 hp with a 462 pontiac with a 6-71 roots style blower. Whaf injector size do you suggest. Has anyone run something similiar to this? Im going to run the holley HP EFI.

they say my 30.5# injectors are good for 500hp so you can do the % of the hp and size of the injectors and get good clue. From what I have seen on engines like that you would probaly want something like 60# injectors to keep up with the blower.

[MrForce]

55 to 60 lbs/hr.

Rob

[CarlC]

How much boost are you planning and at what fuel pressure?

[SShep71]

max 10 lbs boost, 45 psi

[Ollie8974]

SShep71
Clear you PM inbox. I just tried to send you a PM and your inbox is full

[SShep71]

Try again, I just emptied it

[68Formula]

Is your regulator vacuum/boost referenced?

[Ollie8974]

68Formula
If you are addressing me,
Yes the regulator I use is vacuum referenced.
sshep71 should be sing a boost referenced regulator.

[68Formula]

Should be and is can be two different things, hence I asked. The other question is when he states 45psi is that with the vacuum reference hooked up, or plugged off? And if it's hooked up, what's his vacuum at idle?

[andrewb70]

If this is a roots style blower and the injectors are on top of the blower, then there is no need to reference the regulator. Referencing the regulator to manifold pressure is done on setups where the injectors actually see boost, and thus need to compensate to keep constant (non boosted) pressure at the injectors.

I just went here:

http://www.rceng.com/technical.aspx

and it looks like the requirement is just over 61lb/hour with 650hp, 8 injectors, .6 bcfc, 80% DC, and 43.5psi

So I'd say 60s should do it.

Will you be running a plate on top of the blower with the enjectors and then some sort of air valve, or will you be using 2 4 barrel throttle bodies? Or something else?

Andrew

[SShep71]

4 barrel 2000cfm Holley HP EFI throttle body, Injector plate on top of the blower.

[andrewb70]

4 barrel 2000cfm Holley HP EFI throttle body, Injector plate on top of the blower.

With that set-up there is no need to boost reference the fuel pressure regulator, because, as I stated above, the injectors don't see any pressure above ambient.

Andrew

[68Formula]

Good catch on the injector position, Andrew. Yes he doesn't need to compensate additional pressure due to boost, since he's injecting above the blower.

However, just a note, he could still do this to improve the working range of the injectors. In his case, he'd see about a 50% increase in flow, meaning he could get away with 40lb injectors if he wanted. He'd have to ensure he has enough fuel pump to supply sufficient volume with the added pressure though.

[andrewb70]

Good catch on the injector position, Andrew. Yes he doesn't need to compensate additional pressure due to boost, since he's injecting above the blower.

However, just a note, he could still do this to improve the working range of the injectors. In his case, he'd see about a 50% increase in flow, meaning he could get away with 40lb injectors if he wanted. He'd have to ensure he has enough fuel pump to supply sufficient volume with the added pressure though.

You are correct, he could definitely do that, but why? Why introduce another variable into the tuning equation when he can simply size the injectors to whatever HP level he needs and keep fuel pressure constant? The added fuel requirement can simply be taken care of in the fuel table.

Andrew

[MrForce]

Good catch on the injector position, Andrew. Yes he doesn't need to compensate additional pressure due to boost, since he's injecting above the blower.

However, just a note, he could still do this to improve the working range of the injectors. In his case, he'd see about a 50% increase in flow, meaning he could get away with 40lb injectors if he wanted. He'd have to ensure he has enough fuel pump to supply sufficient volume with the added pressure though.

A 10 lb increase (approx 25%) in fuel pressure would increase the injector flow rate about 10% or from 40 to 44lb/hr in your example. Boost referencing also introduces an unknown variable into the Holley's self tuning formula.

Rob

[68Formula]

A 10 lb increase (approx 25%) in fuel pressure would increase the injector flow rate about 10% or from 40 to 44lb/hr in your example. Boost referencing also introduces an unknown variable into the Holley's self tuning formula.

Rob

Thanks for catching that. I took the 1.22 squared instead of square root of 1.22. I should have noticed that was way too much.

To Andrews question, the advantage of a smaller injector would be improved idle stability or the ability to reduce the idle speed and still achieve a stable idle. The larger the injector, the less pulse width to provide minimum fuel requirements. If the fuel demand is low enough you could reach the pulse width range where the injector delivery varies greatly between different cylinders. This can happen if you have very large injectors and very low cylinder fill at idle (the injector driver is a factor as well).

On the plus side, he has large displacement, and probably won't be running a huge overlap camshaft with that blower. So given the fact that it would only increase the flow rate by 10%, it's probably not worth it.