MFI pump timing theory

[Zeke]

I was sorting thru the MFI yesterday and it seems my MFI belt is out of time. It is retarded 2 teeth if I'm looking at it correctly. But the motor runs pretty good at this point.

So, I sit and think about this last night. If the pump "squirt" is retarded, it is squirting at an open valve and into a flow of air. If it is advanced, it is squirting at a valve just opening and into perhaps the negative wave. (Extreme).

The length of the "squirt" is apparently shorter than the length of time the valve is open. So, this must be why these motors run with the pump out of time with it actually being less critical on the S because of the long "window" to inject fuel.

That's the hypothosis. The question is: Has anyone, perhaps a racer, intentionally changed the pump timing to suit a particular purpose? I'm going to think at this point that the smog considerations had the pump advanced a bit, but that squirting into a rush of air might be a performance thing.

Rest easy folks. I'm going to try to move the belt today. I was just thinking.

[Jim Richards]

Excellent question Milt. My injection pump is off a little, too, so I hope the experts weigh in on this.

[Early_S_Man]

Bosch found that the timing isn't critical at all during the development of the first EFI system -- D-Jetronic for VW Type 3 engine. That is why all subsequent EFI [including the various Motronic iterations for 911 engines] systems fire the injectors one bank at a time ... i.e., all left-bank and all right bank injectors fire simultaneously!!!

[tobluforu]

Well I'm no expert, but like others I have been tinkering w\this system for 12 years now. My pump timing was also off (retard). Not two teeth. I would say one tooth. But I was able to loosen and adjust. Here's the thing, My car ran worse when I did this. This is on a car with E cams. T stacks and T heads. Pump was rebuilt for E cams. So I spent about 3 days on and off adjusting it to get it right. I got it to run fine. But I decided to go back. So I adjusted it again (one tooth off) and whammo the car ran like a champ. So there is my non-racer, non professional mechanical fuel injection advice dealing with MY car.
I would fool with it. Once you get the hang of it, it's pretty easy to put the pump where you want without taking the belt off-to a certain extent.

[dtw]

I look forward to the discussion on this topic!! Very interesting to me.

My belt was mistimed. I spent about three hours pulling the left-side intake, loosening the pump nuts so I could get slack off the belt, pulling the belt...then lining up the marks, carefully reinstalling the belt, and putting it all back together. Most of the delicate procedures were done with a mirror-on-a-stick. Three hours. Three long, painful hours with my legs dangling out of my engine bay...

Last Saturday my mechanic had to do the same thing on my car when we replaced the injection pump. He simply reached into the engine bay, grimaced and contorted his face for about 30 seconds, and withdrew his hand. He was done. 30 seconds.

[Zeke]

Originally posted by dtw


Last Saturday my mechanic had to do the same thing on my car when we replaced the injection pump. He simply reached into the engine bay, grimaced and contorted his face for about 30 seconds, and withdrew his hand. He was done. 30 seconds.

I don't seee how. I think you need to slip the belt off the cam pully first. And his hand didn't go there, did it? No. there's a cover there to keep the belt on. Anyone else?

[dtw]

He just slipped it off the pump pulley and reset the pump timing...he had already moved the crank/cam to the proper position before reaching his hand back there. I was blown away. With the pump tightened down, I was unable to slip the belt off the pump pulley, much less put it back on.

[Tyson Schmidt]

It isn't really critical for running, but it is very impoortant for starting!

At the low rpm during cranking, it makes a difference. Of course, a few teeth off isn't a big deal, but I've seen some that were 108 degrees out, and they would sometimes start, and sometimes not.

[Jim Richards]

Uh, Dave, I must've been trained by your mechanic.

I also adjusted my MFI injection pump to the nearest tooth without taking anything apart. I'm just off by ~1/2 tooth. I was hoping to make it dead nuts on, but it sounds like it might not be too bad where it is now.

[Zeke]

ABORT!! I just came back in from the garage and I found the REAL timing mark. I'm dead nuts on the mark.

Now let me tell you what had me fooled. I was looking for a punch mark on the face of the pulley; think cams. And there was a poser mark that was faint that is 2 teeth off. It was advanced anyway, once I studied the rotation.

I feel knid of stupid right now, but it was a good discussion over a cup of coffee. I think I'll go drive somewhere where they don't know me.

[Jim Richards]

Milt, you are now free to leave the country.

[dtw]

Milt,
Don't feel bad, that's EXACTLY what I did. There was a dimple and big fat white paint mark on my sprocket. That's what I lined up. Of course as you know now, you are supposed to line up the groove behind the sprocket with the groove in the pump body. My mechanic gave me quite a bit of grief about my error, believe me (He hates cleaning up messes caused by amateur wrenching).
-dtw

[Tyson Schmidt]

Yeah, same here. Back in '91 when I first got my car and put the engine back together, I had no idea where the marks were. Couldn't find the line on the collar that the sprocket bolts to. I incorrectly assumed that it would be on the sprocket.
A few years later, I finally got it timed correctly. It was off quite a bit for quite a while. It started much easier after being timed correctly, but I really didn't notice any difference in how it ran.

[Ho Hum 74]

Well, I still think this is an interesting thread. When I took my 2.7 MFI engine apart for rebuild I noticed that my belt was VERY slack and it was definitely off about 2-3 teeth! Car ran fine but was rich. I was thinking that when I put it back together I'll set it up as Wayne's book describes and see how she runs.

Thoughts?

Tristan

[jluetjen]

If you figure that the Bosch CIS systems are Continuously injecting fuel, as are Carbs, I guess I don't see how it makes much difference (within some broad limits) if the MFI is "nuts on" or not. As long as the volume of the fuel is correct. I could see that if it is grossly out of spec (ie: injecting the fuel after the intake valve closes) it could make a difference on the first 1 or 2 cranks of the engine, but after that, as long as the volume of fuel per cycle is injected, I guess it should work.

[Ho Hum 74]

OK, here's my MFI belt setting. Note the yellow mark which is advanced (clockwise) about 2-3 teeth....what effect does that have on performance?

Thanks
Tristan

[Ho Hum 74]

OR try this for a closer look....

[beepbeep]

Most simpler EFI's use batched injectors and trigger them all at once. There is no discernible power difference but emissions are little worse at idle.

[69 911s]

I remember reading somewhere that Porsche tried altering the timing on the early MFI racers, but that they gave up when none of the drivers could ever notice any difference.

[Matt Smith]

Aside from any theoretical considerations, that's a great shot of the belt you took there Ho Hum. How d'ya get the camera in there?