Hi all,

I'm just wondering whether the injectors I've got at the minute are right for my EFI conversion.

I'm converting my 6 cylinder Mercedes to use BMW E46 M3 thottle bodies, the standard injectors have a flow rate of 24lbs/hr (252cc/min?).

My target power is around 200-220bhp as it is now, with a plan of taking it up to 280-300bhp in the future with a turbo conversion later.

I'm also planning to run 5 bar fuel pressure, using the M3's standard fuel pressure regulator.

Any advice would be greatly appreciated.

Cheers Frank.

This will help linky

Cheers for that link, according to that I need 23lbs/hr injectors.

That calculator uses 3 bar as the operating pressure. Injectors are designed to run at a specific fuel pressure for correct mechanical operation and spray pattern. Operating outside their design pressure range can give odd results.

I'm using the BMW M3 throttle bodies, fuel pipe, 5 bar regulator, and filter.

The M3 is 343bhp as standard, so I'm assuming that the M3's injectors will be just right for when I turbo it, but too big for it while it's naturally aspirated.

Has anyone got any idea what size injectors I'm going to need for about 220bhp at 5bar?

Cheers Frank

This looks like a good calculator, as you can alter the fuel pressure too, injector calculator

I've just had a mess about with the calculator in the link above.
According to that, if I swap the regulator and drop the fuel pressure to 3bar, it says I need 248cc/min injectors.
So, would the standard 252cc/min injectors be OK to use?

Also, does anyone know if it's possible to alter the standard M3 regulator to run 3bar, instead of 5bar?

Cheers Frank.

Are you sure the stock reg is 5bar? Seems high, I thought thr stick reg was 3.5bar.

It's no problem to have more flow that required, the problem starts if you don't have enough, not too much!

The ideal situation is to have your injectors at 80% full engine load and rpm.

Hi,

Yes the E46 M3 runs at 5bar, whereas nearly every other model BMW runs 3-3.5bar.

Could I work with 5bar through these injectors at around 220bhp?
Or, should I try to get hold of a 3-3.5bar regulator?

Cheers Frank.

We normally run high fuel pressure on our conversions, 4.5bar.

The problem with running high static fuel pressure is when you turbo your boost will increase the fuel pressure (as regulator should be referenced to the manifold)
So, 5bar fuel pressure plus 1bar boost = 6bar. The pump may struggle to maintain flow at that pressure (some pumps lock out at 6bar)

The chose of injector and fuel pressure depends on the projected horse power, in NA form I'm sure you could run the stock injectors at 5bar without a problem.

When you turbocharge it maybe better to run 3.5bar reg and go for larger injectors.

So, running 5bar N/A will be fine, but I'd probably be better dropping it to 3.5bar, that way it'd be ready for turboing in the future.

Yep, but you may have to use larger injectors if you use 3.5bar fuel pressure.

But, depending on your merc engine power output you may get away with the stock injectors, worth a try.

Cheers, you've been a big help, all this efi stuff still baffles me most of the time.

No problem. Give me a shout if you need any help / info.

Www.baileyperformance.co.uk
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I've just had another go on that injector calculator, and if keep the pressure at 5bar, but reduce the duty cycle to 60%, it comes out that the injectors I've got are tge right size.
Is a 60% duty cycle acceptable?