The MFI Diaries: Correlation at LAST!

[304065]

Some of you may have been following my ongoing efforts to achieve perfect MFI correlation. A couple seasons ago I started by trying to tune out imperfect adjustment by moving the throttle stops. This turned out to be a bad idea, not merely because of the difficulty of loosening the lock nut on the port-side TB (Zyls 2 and 3) but also because the entire starboard TB was flowing at a different rate. Turns out the port TB is a replacement from a sportomatic replete with pipe plugs in the vacuum ports, but that's not key to the story.

Anyway, I determined to fix the correlation, so I removed both TBs(time consuming and you have to use a 1/4" drive 12mm to get certain of the nuts that hold the TB to the head) and used a small vacuum cleaner with a couple wraps of duct tape around the nozzle to seal to the engine side of the TB, and the STE BK synchrometer on the other side. Not perfect, and I would have preferred to fab Grady's suggestion of a manometer on the engine side for more consistent readings, but I was able to get them flowing close, about 3 Kg/hr on each stack with the plate at the stop. Good start.

Unfortunately when I reassembled the TBs I didn't have the right gaskets originally, so the correlation was all over the place from air leaks and things not being in the right position. But it was good enough to get the car on the trailer. So on Friday I went back and did it right, tore everything down to the heads, soaked the TBs in B-12 (Thanks Warren!) alternating ends, pulled the air corrector screws and inspected them and blew through the ports with more B-12 and soaked the rods to remove the old grease.

To remove the TBs you have to remove the injector lines. When you put the injector lines back on it's critical to make sure they are properly torqued with a 17mm flare wrench. That 220 psi fuel will find a way to spray all over the engine compartment which could be the beginning of a tragedy. Be incredibly careful working with Gasoline, it is the most toxic and powerful chemical most people encounter (Unless they work in the Nuclear industry).

One benefit of soaking the linkages is that all the accumulated crud in the threads is removed, which allows you to turn the linkage for micro-adjustment of the length. You CANNOT properly adjust the linkage by only turning one end, you have to dial the locknuts toward the center of the rod and then fit the rod, without grease initially (Thanks Grady!) so that it just pops into place.

It all starts with the pump linkage rod, 114mm with no exception, locktite the nuts and paint the rod ends as a witness mark. With the pump lever at the stop, that sets the distance to the crosslink. Next, with the individual throttles at the stops, the four side rods should be adjusted. Do NOT let the throttles come off their stops or you go back to Square A. Finally, set the side linkages, following CMA's admonition to not allow greater than a 5mm difference side to side, but you can loosen the lock nut at the end of the linkage bar to make up for a little of the difference.

With that "basic adjustment" set it should be pretty close. This is as far as you can go without protractors.

Fortunately, due to my lavishing praise and attention on one of the Good Doktor's neglected offspring (the 1966 911) I earned a little Karma with the Porsche G_ds and came across an original set of MFI protractors.

The protractors attach to the linkages with a magnet, and they have metal tabs that capture the sides of the linkage, and a moon-shaped cutout that captures the ball stud. Once you get them on, they fit tightly, and you can move the linkage through its range of motion to check it. The pointers attach to the studs that hold the stacks, and in my case, I used a self-tapping screw into the unthreaded boss where the '69 enrichment solenoid bolted up to hold the pump pointer on. As Grady says, you can use regular old coat hangar wire to make pointers if you don't have them or if they are bent.

[304065]

This first photo shows the left pointer on the throttle lever in position. The pointer is attached to the stud on the stack and tightened down, then you can bend the pointer so it indicates zero. The tolerance is half a degree but you can get it exact (this may not show in all the photos.)



Next, the pump protractor goes on. I used a screw which cut threads into the pump itself, there were no threads in the boss for the old '69 coldstart solenoid. I suppose one could JB weld a little stud into the hole which would make installation/removal easier. Grady, didn't you mention that you just left that one on the engine?



The right protractor. I don't know if the pointer wasn't oriented correctly, but you have to gauge zero degrees with the pointer shaft, not the end. No big deal.



Now the fun part. Pick a value for the pump protractor in Check Measure Adjust and find the equivalent value of the throttle bodies. In my case, I chose 30 degrees for the pump, which corresponds to 21 degrees for the throttles.



Aha! Note how the throttle is only at 20 degrees.



Ditto.



So, to correct this, I adjusted the side linkages to be slightly longer. There is enough play in the crossbar and the links to make one degree possible. With everything set, 21 degrees at the throttles was achieved.

A final check with the synchrometer revealed a consistent reading of 5-7 Kg/Hr at idle and a rock-solid 11 Kg/Hr at around 3000 rpm. Obviously I'll be checking and re-checking that at the track to make certain.

It's great when everything works. Even the warmup thermostat works great, when the engine starts, the mixture is extremely rich until the engine heats up, then it leans out just like it's supposed to.

Next up: using the LM-1 to measure fuel mixture at full throttle, full load RPM. According to Dan Jacobs, an excellent mechanic and all-round nice guy who is doing some suspension work before the race, the MFI pumps tend to lean out at the top end on the dyno. So with the LM-1 I should be able to get real-world full load data and adjust accordingly for best power mixture.

The protractor exercise opened my eyes to the reason why you can't just set the links with the throttles and the pump at the stops and the operating rod at 114mm-- a higher degree of precision can be achieved using the protractors.

Standing on the shoulders of giants.
More fun with MFI and LM-1

[David E. Clark]

I missed this thread and I'm sure some others did as well. Since John provides such great information (and pictures) about using the MFI protractors I thought I'd bump it back to the top so others could see it.

I included a link to this thread in the MFI message board index.

[mobius911]

John,

You have really gone the extra mile to get this perfect- most impressive!

Is the example above of 1 deg. off what you really experienced? I'm trying to gauge how much from perfect correlation I will be without the protractors, but having adjusted the throttle linkage and side push rods to the proper lengths. If they are all correct (114 and 149 respectively), is the system likely to be correlated?

Thanks,

Jeff
73 Carrera RS #1282

[304065]

David, thanks for putting it on the index page. The photos could be a little better but next time I take off the rods I'll update it.

Jeff, the 1 degree off was an accurate, first-time measurement following completion of the CMA routine and the following steps:

1) Set the pump link to 114 +/- 0.2 mm, you might as well put a witness mark on the both ends with some yellow paint to keep it from ever moving once the angles of the ball cups are set.

2) Removal of side links and throttle links and soaking in B-12 to remove all caked grease from the ball cups;

3) Removal of the rod ends and jam nuts and cleaning of the threads;

4) Reassembly with light oil instead of grease; and

5) With the throttles at the stops, reassembly of the throttle links with absolutely zero preload. This is ONLY possible if you have the ability to freely and smoothly rotate the linkage rods (which are left-hand and right-hand threaded) in minute amounts to center the ball cups over the studs.

So once you have the throttle rods set and the pump link set, the only thing that remains is the side links. CMA calls out an initial measurement of 145mm but says that you can vary the side link length to achieve the right angles, to a limit of 5mm difference between the side links.

So I made the side links the variable in the setup.

To review:

Pump Link Set
Throttle Links set
Adjust side links for zero preload.

That should get the basic correlation set. Of course, we know that Porsche subsequently said that the use of protractors was not required, but that was probably as the systems began to wear out and the tolerances could not be maintained anymore.

Another technique that I'm going to try is to remove all the throttle linkages, start the engine and take the exhaust gas measurement. With the engine at operating temperature, it should be possible to set the correct idle mixture (they like 5-6% CO or an AFR of 12.64-12.25, pretty darn fat if you ask me) and then use the air corrector screws to balance the stacks. With that done, then put the rods on. Of course, that will never happen if the plates or TBs are worn or if the throttle stops have been incorrectly adjusted, back to the top for that.

[72doug2,2S]

John, Thanks for posting your findings and the pics.

If you ever do an MFI clinic please let me know, I'm 6 hr.s away.

[Jim Richards]

Great thread, John. I don't recall seeing a spec for setting the throttle stops. I haven't touched mine; however, I wonder if they are set up properly. Thanks in advance for any advice you can lend.

[Zeke]

Originally posted by Jim Richards
Great thread, John. I don't recall seeing a spec for setting the throttle stops. I haven't touched mine; however, I wonder if they are set up properly. Thanks in advance for any advice you can lend.

There is a preset spec so that you start off with them even and off the stop just a wee bit, which is necessary. This is all in the CMA. but, to achieve even flow, you must readjust on the car with the engine running if the air correction screw won't even the flow. It should, however, or something is likely to be worn past its tollerance.

Thanks for bringing this back up. It gives me some impetus to get my protractor project moving along. I've been lagging due to various reasons. My personal feeling is that the protractors are handy, but not to the point of having to shell out the hundreds it takes to acquire some. Hence, my plan was to reproduce an adequate set for simply checking to see that things are in order and you don't have the wrong rods, or some such malady. The specs call for a half a degree tollerance in the correlelation. To me, it's one of those deals where it's either real close, or out of the ballpark. There isn't a lot you can do to change things. You'd have to be where John was to understand.

Or know way more than both of us.

[Jim Richards]

Milt, I just made another pass through CMA and didn't find it. Of course, I may be blind in one eye, can't see out of the other.

[Zeke]

Sorry, Jim. Maybe it was in a thread here somewhere. I remember something about the starting off of the air corrections at 2.5 turns. The throttle plate arm is just off closed. I'm gonna take a WAG here and say a half turn. At least the subject is up now, so maybe Warren Grady or someone will chime in with the needed info.

The point was to keep the plate from hanging up in the closed position and have enough air by pass to put the air correction screws within the adjustment zone. This could vary from car to car depending on overall condition of the butterfly shafts, plates bores and shaft bores.

[Jim Richards]

No worries, Milt. I set my air correction screws to three half turns (for 2.7RS) and might give your suggestion of 1/2 turn off closed throttle plate setting a try.

[Zeke]

See, there ya go.....it IS half turns, not two and a half turns. Shows to go ya what I know.

[rs dreamer]

Milt, the bit about adjusting the stops is not in the CMA (it should be), but I believe was in an article by Lee Rice that was in the Pelican Parts MFI index a while back (the link no longer works). Is this what you were referring to Milt? I think I cut and pasted it from that article:

Setting idle mixture & throttle body maintenance

It is assumed that your throttle bodies have been cleaned and intakes (throttle plate, air mixture and vacuum) are “surgically” clean. Your throttle bushes should also have no significant wear. Regarding idle mixture tune-up, first turn the air correction screws on the throttle bodies fully closed, then back out exactly 2 1/2 turns. Then adjust the throttle stops by removing all the linkages and, one at a time, loosening each throttle bolt locking nut. Lightly push the throttle plate towards the fully closed position. Turn the throttle stop bolt to allow the throttle plate to close completely. When you get to the point where you have almost removed the bolt, the throttle plate will stick shut. Now screw the throttle bolt back to the point where the throttle plate no longer sticks shut. (This is rather critical as the throttle plate can be closed too far and stick, as engine temps increase and alter clearances). Now that you have the throttle stop bolt set to just keep the throttle plate from sticking, screw the throttle stop bolt another 3/4 turn in the direction that opens the plate. Check for free play by pushing lightly toward the stop. If all is OK, secure with the locking nut and repeat on remaining 5 throttles. ....there is more in the article

Gary

[Jim Richards]

Just what I was looking for Gary. Thanks!

[Zeke]

Now we're back to 2.5 turns. And, I forgot about the most imporant part, that being the bit about thermal expansion. Thanks, Gary. It's no good to do this stuff from memory.

[Jim Richards]

Milt, the air correction screws are turned differing amount depending on the engine. The 2.7RS was 3, 1/2 turns. My original 2.4E engine was 5, 1/2 turns IIRC.

[Grady Clay]

Jim,

Gary is correct, that is what Lee Rice published. The critical issue is where Lee says; “…have no significant wear.”

The best procedure is to use this as a starting point and then use a flow bench (or John’s vacuum cleaner) to measure the actual air flow. Any throttle body that has any miles on it needs to be done this way. This is also how the stops are set when new and I think how all the rebuilders do it.

Best,
Grady

[Jim Richards]

I'm pretty well set now thanks to the help here.

[304065]

Gents, I read that piece by Lee Rice and found it impossible to implement in practice. I even tried setting the plates using a feeler gauge against the side of the bore (the flat gauge will allow a constant measurement even though many TB's have a groove worn in them by millions of reversion pulses from our (comparatively) high-lift cams and it doesn't work either.

The only reliable way is to close the air corrector screws all the way (gently, now!) and to rig up some sort of a flow bench to get them close, then use the air correctors to dial in the same reading on the synchrometer. It's not complicated: practically everyone owns a shop vac and the synchrometer is an absolute must have for this project.

The correct number of half-turns is specified in CMA page 22: 5 half-turns for 2.0 and 2.2, 3 half-turns for 2.4 and 2.7. Some exceptions are noted on page 22 for different air bypass screw bores.

[Eddyvdw]

I'm using above mentioned method with the vacuum cleaner and synchro-meter. Working on the right throttle body. Fully cleaned. Noticed some wear. Air screws set at 1.5 turns (2.4E).

2 cylinders are fine at stop reading of 3 kg/hr. The other gives me a bit more trouble. Can get to 3 kg/hr only if I help it a bit. If I simulate WOT and let it flip back to idle, the reading is more close to 5kg/hr. If i push it in idle by hand the reading drops to 3 kg/hr.

So what to do? Adjust the other 2 also to 5kg/hr and accept a higher idle? Get a stronger spring for the troublemaker?

Thanks,
Ed