Blown and injected Q:

[Stevie G]

Looking at a Centrfugal supercharger and contemplating combining it with EFI.  How do you determine what size injectors to use?

[Pope Downunder]

Looking at a Centrfugal supercharger and contemplating combining it with EFI.  How do you determine what size injectors to use?

I haven't got a lot to offer, sorry.  You haven't given us much info, but in my case more detail probably wouldn't help!

However, from what I understand of EFI, in closed loop operation, the system will be tuned by the O2 sensor and MAP pressure etc.  If the injectors are too small they will cycle longer, until they cannot cope.  I think the MAP sensor is different also to compensate for the positive inlet pressures.

I guess a good place to start would be to estimate the HP increase you can realistically expect.  Let's say that is 20%, then I'd look at upping the rating (lbs/hr) of the injectors accordingly.  Eg. A 20lb/hr injector would be upgraded to 24lbs/hr.

I'd be inclined to ask this question somewhere like here;
http://www.chevytalk.org/threads/postlist.php?Cat=0&Board=UBB12
where there is a lot of experience with EFI.

[Phil1934]

Just went through this on a SBF and ended up with 36#, which may be a little light but you also have to find a MAF that works with a certain size injector so the whole thing can be handled by the computer.  A catalog like Summit lists injectors and recommended HP ranges, but unless you are going totally aftermarket computer you may end up with something you can't tune.

[choco]

There is a formula that you need to look at to determine injector flow rate. If your engine is going to make 400HP, you need 35 lb/hr injectors. You also need a 3 bar MAP. When I contacted BDS about blown injected EFI conversions (when I was writing my book "Electronic Engine Management"), they were adamant that the blower MUST be run wet, that is, fuel must come in at the top of the blower. This means a TBI or CFI setup. A "dry" blower would be a multi-point setup with the injectors in the manifold pointing straight at the intake valves. B&M blowers are OK to run dry, and I know some cool blown Multi Point setups on Jimmy blowers, too, but I think they all had the rotors worked on to run dry (teflon inserts).

[Stevie G]

Looking at a Centrifugal supercharger and contemplating combining it with EFI.

Well, thanks.  It'll be dry.
This is an old VS57 going on a 322 Nailhead.
It'll only be running 4-5 pounds of boost.