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Jeff Higgins Jeff Higgins is offline
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Join Date: Mar 2004
Location: Higgs Field
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Quote:
Originally Posted by chris belyea View Post
Hi all,

So Iím new to the MFI world. Iíve been trying to read everything I can on these systems and have a few questions regarding Correlation and Measuring Exhaust Emissions in the Check Measure Adjust Manual.

Regarding correlation, Iíve saw the Porsche came out with a revised correlation process that does not require the protractors. Iíve also read Lee Riceís article regarding correlation as well. From what I can tell, the procedure is as follows:

After checking all the previous items in the CMA manual, one needs to synchronize the throttle plates by using a Synchrometer (or something similar). Remove linkages, set all the air bleed screws the same, read all six stacks with Synchrometer, adjust the air flow by adjusting the throttle plate screw until they are all pulling the same air. Set the push rod between the MFI and the throttle crossbar to 114mm +/- 2mm, then adjust and reconnect the remaining push rods between each of the throttle butterflies and then to the throttle crossbar without an increase in RPM that was set in the previous set using the Synchrometer. Does that sound about right?
No.

Doing it this way creates the real danger of having one or more of the throttle plates making contact with the throttle body bore when at rest in the idle position. They absolutely need to be resting on their stop screws, not the throttle body bores. The only reliable way to set the resting position of the throttle plates is with the throttle bodies removed so you can verify their position. It is imperative that they not rest on the throttle body bores. If they do rest on those soft magnesium bores, the vibration transmitted to them when the engine is running at idle will damage that bore.

It is true, however, that no one (or at least no one I know) actually uses the protractors. It seems a superfluous step. With the 114mm rod disconnected, and both the left and right rods to the throttle bodies from the cross shaft disconnected, and the fore and aft rods on both left and right disconnected, simply make sure all throttle plates are resting on their screw stops. In other words, simply disconnect all rods so the throttle plates move independentently from one another.

Then simply reconnect all rods, starting the two short ones on each throttle body. Make sure that when you connect any one rod that you have not introduced any preload in either direction - adjust each rod accordingly, until it snaps freely in place without moving anything.

Once the short ones on each throttle body are connected, skip to the 114mm rod and connect it, setting the cross shaft position relative to the pump. The pump needs to be on its idle stop. Now you have the throttle bodies with no preload, and pump to shaft with no preload. Once both of those are established, you can connect the cross shaft to the throttle bodies on each side. Again, with no preload. Don't be surprised if these two rods vary considerably in length - CMA allows up to 5mm variation.

The entire system should now be sitting with no preload anywhere, with the pump on its idle stop and each throttle body on its idle stop. From here, follow the Synchronizing Throttle Valves procedure outlined in CMA, using the air bleed screws.

Quote:
Originally Posted by chris belyea View Post
Regarding the measurement of Exhaust Emissions, it appears that a protractor is needed for this step. One needs to use the protractor attached to the bellcrank on the MFI pump so that the throttle can be set to an opening angle of 7 degrees. Is there another way to do this?

Iíve also read that you donít really need an air-fuel gauge to perform the Part Load and Idle Speed measurement. All that one needs is an O2 sensor and a multimeter. Has anyone done this before? I would be setting up an MFI for a 2.2 1970 911E.

Thanks

Chris
I don't know anyone who does it this way anymore either. I think the CMA procedure was a product of the times, when all they had available were CO2 meters. I use exhaust gas analyzer that provides A/F ratios via an O2 sensor in the tailpipe. Mine is the old Innovate Motorsports LM1 model, but there are more modern versions available. I set the A/F ratio under load at mid to high RPM to what I feel is "safe" (between 12.5:1 and 13:1 at the most) and let the rest of the operating range take care of itself.

The idle mixture adjustment doesn't really have much of an affect. The main rack adjustment pretty much overwhelms it. I only use it after the main rack adjustment is where I want it, and only to smooth out the idle. I don't get too hung up on the idle mix - it's not all that important, and just as soon as the motor is up at any "driving" RPM range, it really doesn't have an affect anyway.
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Jeff
'72 911T 3.0 MFI
'93 Ducati 900 Super Sport
"God invented whiskey so the Irish wouldn't rule the world"
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