MFI reversion part 2 - tuning

rs911t · 03-08-2005, 06:01 AM

I drove my reverted-back-to-MFI-from-carbs "T" this weekend for the 1st time in 4 months and was really surprised by how well it ran! [ At least after getting the plug wires properly sorted ...

] It sounds better than it ever did with carbs and pulls smoothly to redline. Idle was easily adjusted to ~1000. The only obvious issue is bogging at take-off in 1st and seeming to run rich. Easily an 8 out of 10 right out of the box.

I don't have a CO meter so plan to tune with an AFR meter. I installed bungs on each heat exchanger just before the muffler. CMA says to get the partial load mixture correct before moving to the idle. My preliminary results were 13.1 on partial load and 14.4 on idle. That works out to 3.7% and 0.4% CO levels, respectively, or a bit rich and way lean.

My primary concern is AFR at WOT, due to the engine's high compression ratio. It was way too high, between 15 to 16 AFR at 7000. Assuming a propertly calibrated pump (recently rebuilt to spec), should I expect adjustment at partial load and idle to bring the WOT levels down? Also, does a stock MFI pump move enough fuel for 2.8L 10.5:1 engine?

Jim Richards · 03-08-2005, 06:12 AM

Greg, I have a Gunsten (sp?) gastester if you'd like to borrow it.

jluetjen · jluetjen's Garage

rs911t; The "Part Throttle" adjustment changes the volume of fuel in each "squirt" by changing the amount of time that intake port to each of the MFI Pump's cylinders is open. As such I believe that it will affect your mixture across the board which is why Porsche specifies that you adjust that first. Once you have that set, you can then fine tune the idle from that baseline.

john walker's workshop · 03-08-2005, 07:56 AM

you definitely have to go a lot richer on the idle to get over the low rpm hesitation problem. your mid range of 3.7% still needs to go up to at least 5%, maybe even 6%. be sure the plugs are hot enough to handle a somewhat rich system without fouling. denso irridium plugs work well.

rs911t · 03-08-2005, 08:40 AM

Quote:

Originally posted by john walker's workshop
you definitely have to go a lot richer on the idle to get over the low rpm hesitation problem. your mid range of 3.7% still needs to go up to at least 5%, maybe even 6%. be sure the plugs are hot enough to handle a somewhat rich system without fouling. denso irridium plugs work well.

I assume the 5-6% midrange is needed to get the high-end a reasonable AFR, rather than the factory spec 2.5-3%. Does the idle setting require a similar upward adjustment? Or, is 2-3% ok?

john walker's workshop · 03-08-2005, 09:04 AM

i've never been able to get the idle CO down that far and still have it start readily, idle smoothly and have decent transition. the factory spec was back when we had decent gas. same goes for the original plug recommendation.

BK911 · BK911's Garage

You can double check the idle mixture by disconnecting the throttle linkage to the pump, then slowly opening the throttle valves. If the idle increases a little at first, then dies, then it is set rich. I lean it out a click and try again, until the idle does not increase and the car just dies. Then I richen it up a click or two, depending on the street test. I usually end up around 6% at idle.

Have fun!
BK

304065 · 03-08-2005, 11:26 AM

Hi Greg.

I have an MFI 71E with an LM-1. My Gunson professional is for sale for $100.

Forget, for now, the idle. You want to make sure you have correct enrichment under load. Keep richening up the main rack until you get rich of stochiometric, into the 12's. This will, of course, affect your idle mixture adjustment. Truth is, only the factory was ever able to get them to run within spec. These days, it's more like 5-6% at idle and 7-8% at 3000 RPM.

I use a capacitive discharge ignition with the plug gap opened up so wide you could throw a cat through it. If a cooperative feline isn't handy, something like a matchstick width is a good place to start. That should keep the plugs from fouling.

One last thing, did you CMA?

rs911t · 03-08-2005, 11:37 AM

Quote:

Originally posted by john_cramer
One last thing, did you CMA?

Yep! I haven't made any mixture adjustments yet. More or less bolted the system on and set the idle.

You're approach is pretty much what I decided to do. Make sure it's safe at 7000 let the rest fall where it will.

Btw, Crane, MSD 6AL, hot plugs and wide gaps.