Injector size questions

[bwh13]

According to the RC Engineering tables, our engines (S50/S52) should use 24# injectors to make 240hp.

This is determined by several formulae, using known criteria for HP, fuel consumption, etc.

So why do we have 17.5 and 21.5# injuectors, respectively? And what happens if we were to switch to 24# injectors (providing we had the correct chip to control the air/fuel mixture), and had the same parts (HFM, etc) would we gain significant HP?

OK I think I figured this out. The BFSC (fuel consumption efficiency) for the engines is better than for most others, so they burn fuel at a lower rate to make the same power.

So 21.5# injectors will support 240hp...but still not as HP efficient as some other engines that make 240hp from only 2 liters displacement, NA too...

[MR 325]

I don't know about gains but a lot of S50/S52 guys do run 24lb injectors.

I'm running 21.5's on my cammed S50 and it's been professionally tuned, had enough fuel to work with so i personally don't see the need to upgrade.

[bwh13]

Thanks.

I'l see what kind of numbers I get when I finally have it dyno'd, then decide if the switch to 24#'s is the way to more NA HP.

[z31maniac]

If 21.5s will give enough fuel to acheive the optimum fuel/air ratio, switching to bigger injectors isn't going to help.

[matt]

If 21.5s will give enough fuel to acheive the optimum fuel/air ratio, switching to bigger injectors isn't going to help.

Bingo.

The other missing factor is that BMWs run 3.5 bar fuel pressure and injectors are rated at 3 bar.

[bwh13]

That's true, unless the injectors get maxed out with the duty cycle norm of 80%, then a larger injector, run at a lower duty cycle will still work OK.

One of my friends runs 1000cc injectors in all the Hondas he tunes, all the way from 150 to 800hp. The injectors can be run down to low duty cycles and still provide good fuel atomization.

I guess we don't have that kind of option right now since there isn't a tuneable stock based ECU on the market yet...can only wait until one comes along.

[redmist]

40# with 2.9L 15psi turbo