Max flow for MFI injectors?

[stownsen914]

What is the maximum flow rate and/or hp rating (assuming 6 of them on a 911 motor) for MFI injectors? I think I've seen references to conversion of stock injectors to RSR or turbo specs? Is that needed to support 350-400 hp? Thanks.

Scott

[356RS]

Scott, I just recently finished modifying a 013 pumps space cam and flyweights along with readjusting the governor springs for a 2.8 RSR spec motor. Using stock injectors I was able to achieve a flow rate of 68ml - 70ml/1000 strokes/per injector @ 3750 pump rpm with WOT. So adding up all 6 would be 408ml-420ml per 1000 strokes. This would produce around 1,640ml per min. if my math is correct.

[911st]

Then at twice the test rpm would that be apx twice the fuel delivery quantity?

[HawgRyder]

Doesn't 1000 strokes per min equal 6000 RPM?.
After all....1000 (each injector) X 6 Cylinders = 6000/min.
Just a thought.
Bob

[356RS]

The MFI pump is driven off the cam so it turns half the engine speed. All testing data from Bosch is given at the MFI pump speed of half the engine speed. So the 3750 rpm on the pump test equals 7500 engine rpm.
As for the 1000 strokes, all the test data for proper flows is recorded per 1000 strokes at pump rpm's from 400 to 4000 rpm's. If you are testing the flow for idle @ 400 rpm and zero degree throttle the test period will take 150 sec. or 2 min. 30 sec. to achieve the 1000 strokes of fuel. Testing at 1750 rpm would be 34.3 sec./1000 strokes; testing at 1000 pump rpm is 1 min. to get your 1000 strokes of fuel. Hope this helps.

[tadd]

Just curious... does anyone know the duration of the injection event? I would assume relevant units would be camshaft or crank degrees I assume?

Rough order of magnitude would due.... I'm curious if the event is short enough that changing the advance/retard would be useful for trying to tune out reversion with narrow lobe cams (ie 906 grind).

t

[kenikh]

Rumor has it the MB MFI injectors have more headroom than the Porsche ones.

As far as the injection event time, it would be degrees and RPM that decide, right?

[911st]

Interesting question.

I wonder if the duration is bassically fixed but the pressure increases?

If duration dose not stay bassicaly the same I would suspect the finnishing point of the duration probably stays the same and the pulse justs starts sooner.

Just a guess.

Remember the injection event comes from a piston activated by a cam that pushes out a given quanity of fuel. Thus, the event would start slow, increase to a peak, and slow near the end of the cams ramp.

Thus the end point of the injection piston should alwas be at the same point.

[kenikh]

MFI injectors open at a fixed pressure - since liquids do not compress, the expectation is that the duration is fixed, in degrees.

[911st]

My thinking is the injection pump piston and stroke are a fixed in size, the supply pressure is constant and that the variable is how much fuel is allowed to enter the injector cylinder.

If this is right my vote would be that the duration increases but begins earlier and that pressure during the event also increases to a degree.

Also, at higher rpm each injection event has less time so the pressure will also increase with any rpm increase and assuming the TQ level is the same, the duration will actually shorten.

Again, interesting question.


I would be wrong if the stroke length of the injector pistons changes. I have never seen the barrels and injector pistons. I guess it would make more sense for the volume above the injector piston to change.

Bassically I have no clue but the question tickeled my brain.

[356RS]

Keith, Barrels & pistons............

[911st]

So how dose it work?

Dose the piston move up and down on the push rod to change the volume of the injection stroke?

[tadd]

Staring at my pump innards, it seems that the piston stroke is fixed and the main rack rotates the barrel around the piston increasing or decreasing the volume.

Since everything is all fixed, pump cam/cam/crank I should think one could advance or retard just like the valve cam shafts.

Would it be a matter of just measuring the degree/lift of the roller crank in the pump? Since it is a roller design I would think the cam is really aggressive (fast lift).

t

[356RS]

I'll do the best I can. Your correct on the fixed stroke and fuel supply. How does it work! Well, each piston assembly is connected to roller tappet and rides on a cam lobe. The cylinder is fitted in the upper section of the pump and is immersed in fuel and there are small holes on the sides of the cylinders for fuel to enter and flow through the cylinder. The inlet fuel hole is larger than the exist hole in order to keep the pistons top lands full of fuel. As the plunger is forced up by the cam lobe, fuel contained on the top piston lands is forced out through a check valve into the injector lines and the injectors. The amount of fuel is controlled by the main rack which is connected to the plunger via a toothed clamp which is engaged on to the main rack. Movement of the rack forward (towards the front of the car) rotates the plungers and opens up the metering lands which allows more fuel to enter delivery plunger. Pictures below may help in what I was trying to explain. This was a very fast explanation and only 50% of what "how it works"

[tadd]

356rs:
Nice write up. I had to go dig out my box of disassembled pump and look - it's been a while . Rack rotates outer barrel which intern is fixed to the piston which is spiral cut. Thanks for the correction.
Eyeballing the roller crank it looks long duration with low lift. So I would think, given we are working with reasonably incompressable liquids, that the injection event will be on the order of the intake valve event. Thus short of regrinding the roller crank all we can do is move the injection event so that it starts after the exhaust closes (or in that ballpark).

How does one actually set the relative positions of the motor and pump? Is there timing marks on the MFI pulley?

[Flieger]

So does the spiral cut change the volume inside the barrel chamber or does it just "throttle" the fuel that is allowed to enter through the little port hole?

[356RS]

tadd: There are timing marks on the engine crankshaft pulley as well as the MFI pump and MFI pulley.
Flieger: Good question. As I see it, the spiral cut allows more fuel to collect in the piston land when the plunger is rotated toward more throttle. I will try to post a better picture on the plungers today. The 2.0 - 2.2 plungers are different than the 2.4 - 2.7RS plungers. The later plungers have a much larger spiral cut in them.

[HawgRyder]

I do seem to remember that the Porshe engineers tried varying the timing on the pump on the 917.
If with drastic changes in timing, the result was about the same in HP.
Is that correct?
Bob

[stownsen914]

Originally Posted by 356RS

Scott, I just recently finished modifying a 013 pumps space cam and flyweights along with readjusting the governor springs for a 2.8 RSR spec motor. Using stock injectors I was able to achieve a flow rate of 68ml - 70ml/1000 strokes/per injector @ 3750 pump rpm with WOT. So adding up all 6 would be 408ml-420ml per 1000 strokes. This would produce around 1,640ml per min. if my math is correct.

Thanks for that info. Mark. Is the 70 ml limited by the pump or the injector in this case? Understood that the pump needs to be modified to supply a 400 hp engine. Going on that assumption, do stock injectors need to be modified too?

Scott

[Flieger]

Is the barrel just a simple cylindrical bore or is there some interesting geometry to work with the spiral piston groove to vary the amount of chamber volume. Unless the spiral groove causes the piston to move up or down relative to the follower, I do not see how the volume of fuel delivered can change with a fixed stroke.