Alternate MFI injectors
 

A couple of questions (untried, untested, no liability etc) on MFI injectors

The MFI systems were derived from diesel technology (which required very high pressures to achieve atomisation in a direct injection combustion chamber).
The result was the 250psi pintle nozzle injectors of the well known MFI configurations used on Porsche, Mercedes & Aston.

Bosch later moved to low pressure (K-Jet, CIS etc) systems (about 3.5 bar 35° conical spray - some featuring the same M12 threaded connection as MFI)

Given that 3.5bar is more than enough to give good atomisation, could they be used with MFI ?

Does the MFI pump rely on the high residual line pressure after closing the pintle at 250psi ?

An interesting aside, Porsche did use some other injectors with MFI along the way....check out the Type 771. (Anyone got a 771 parts list lying around ??)

http://forums.pelicanparts.com/uploa...1214958263.jpg
(C) 2007 Pelican Parts
Be warned - mounting the injectors this far upstream is just inviting a fire.....and is only really required for high rpm, maximising evaporation.

Any thoughts on using low pressure, CIS (or alternate diesel) injectors with MFI ?

The Fuel Injector Pump is what supplys the pressure. Using a 'normal' low pressure injector with 250lbs forced thru it would most likely blow the inards out. You might be able to find some diesel injectors that you could use, but a quick search shows that most seem to be no cheaper than new Porsche MFI injectors

Robert...thanks for your ideas....but I'd challenge your thinking on this (but I'm happy to be proven wrong..)

The pump is a positive displacement pump with a non return check valve at the piston outlet.

The 250psi is the opening pressure of the standard MFI injector - CIS injectors would open at a much lower pressure (about 3.5 bar or 50 psi) - and hence not see 250psi (statically at least - dynamically may be a different question.)

250 psi operating on a small diameter (about 6mm for the injector ID) does not amount to an enormous amount of force...in terms of blowing the end of the injector out (about 14 lbf, or 6.5kg)

I believe the MFI pump delivers its fuel over a duration of up to about 90° of the pump camshaft - less at part load due to the bypass of the helix in the pump cylinder. So there may be dynamic considerations like excessive lash in the piston due to lack of retained pressure in the pump (but then the check valve at the pump outlet should take care of this)

In any case - typical (electronic) petrol injectors can deliver the required fuel volumes at duty cycles of about 80% at WOT. so again, I don't think dynamic pressure would be an issue.

Any more ideas ?

Here's the CIS injector for reference, uses same M12x1.5 connection for the fuel as MFI, with push fit/o'ring channel under the hex. Bosch 0 437 502 004. The standard CIS injector is shown below.

The turbo's are thread fit (0 437 502 009) rather than o'ring (to stop them popping out under boost, with dangerous consequences)

I would expect the high pressure of the injection pump would overpower the injector. To clarify, the injector spray pattern would be too narrow due to the "pintle" being forced to far away from it's seat. You can see this when the spring inside a normal MFI injector has weakened.

Call Henry Schmidt with your ideas. If anyone knows, he would. There are others, I know, but you need to experience Henry. :D

Aaron - I'm contending that the injection pump is simply delivering a given VOLUME of fuel (positive displacement) - it's the resistance of the spring/pintle nozzle in the MFI injector that allows the line pressure to build up to 250psi (at which point the pintle opens, maintaining roughly 250psi throughout the duration of the injection).

If the MFI injector were replaced with a "low" pressure CIS style injector, the line pressure would rise to about 3.5bar (52psi), its pintle would open and maintain the pressure throughout the duration of the injection stroke (simply passing the same VOLUME as the std MFI injector.)

A CIS injector can handle the fuel volume (albeit delivered in a shorter duration (say 45° at cam) than as continuous flow). Its designed to atomise at 3.5bar...but seat height may be an issue (I referred to this in the previous post - dynamically..)
A pressure v flow rate diagram would answer this..

Any thoughts Mr Schmitt and others (esp anyone with access to equipment to bench test ??)

Wanted - dead MFI and CIS injectors for autopsy and comparison.....I'll post results to this thread

Thanks,
John

I missed something here. CIS = Constant Injection System. They never close when the car is running. Same as the old Corvette injection. The high pintle pressure allows the injector to pulse as an EFI one does.

I don't know if I can find it, but I posted pics of an MFI injector that I disassembled for the fun of it. Must have been 2 years ago now.

Yep, here it is: http://forums.pelicanparts.com/porsche-911-technical-forum/203134-ever-wonder-whats-inside-mfi-fuel-injector.html?highlight=injector

http://forums.pelicanparts.com/uploa...1106630813.jpg

Thanks Zeke

I had already seen you posted pic, and it's the similarity in construction and operation to the MFI injector that's captured my interest. (thanks for adding the image to this thread)

Both the MFI injector and the CIS injector are simple spring and pintle devices - both open once they see an opening pressure - and close when (or if) the pressure drops below that point (albeit they each have different opening pressures).

Both injectors oscillate promoting atomisation. They operate as a spring, mass (pintle, retainer + 1/2 spring) , damper (fuel) system, and exhibit that characteristic "squeal" of CIS injectors and the "chirp" of MFI.

Here's the construction of the CIS injector...looks familiar right ?

http://forums.pelicanparts.com/uploa...1218172791.jpg
(C) 2000 Robert Bosch GmbH

I'm looking for a couple of dead injectors to take apart (CIS and MFI) to do a very detailed comparison - component weights, dims, spring rates, pintle diameters, orifice dims, natural frequencies, flows etc etc.

Regards
John

Thinking out loud, the CIS injector is designed to spray continuously.

I suspect that with what we ask of the MFI injector having them operate at a higher pressure is an advantage that would not want to be lost.

Because of the higher pressure the duty cycle is greatly reduced and we can have our shot of fuel at just the right time in the intake cycle as the intake valve opens and the air is rushing by.

High pressure MFI dose not have to start spraying fuel a head of the intake valve's opening to deliver the needed amount of fuel.

Just a guess.

PS. Some EFI systems need to run up to 70% more to deliver the needed amount of fuel and max HP.

subscribed.

subscribed.

Hello!

In dyno-comparision, phasing the injection have yielded neglible increase in power. It's good for emissions on low load though.

Apples and Bananas

Dyno tests do not test throttle response.

;)

Let me know if you can help...

Thanks
John

Anyone have a Bosh MFI book?

Might be interesting to look up the diagnostic section and see if it has a symptom for a low pressure injector.

Aaron suggeted the following in an earlier post

"spray pattern would be too narrow due to the "pintle" being forced to far away from it's seat. You can see this when the spring inside a normal MFI injector has weakened. "

I've noted all issues I can think of in this thread - I guess its time for a bench test...just looking for some old injectors as test candidates.

John