AFR Readings (LM-2) - Idle speed vs. Partial load

[TEC69E]

Just got my LM-2 up and running using a tail-pipe clamp for the o2 sensor. So I'm able to get readings at various RPM's, however, I've seen some factory documentation for my motor (2.7 liter with MFI) that specify's AFR settings at Idle and then under Partial load.

So my questions is - Will AFR readings really be different when the motor is reving at 2-3k RPM with no load . . . vs. reving at 2-3k RPM under load?

BTW . . . no problems with my LM-2 . . . out of the box it is working exactly as I would expect.

Thanks,

[Flieger]

Well, under load the throttle butterflies are open more than free-revving, so I would think there would be more air in the mix. On the other hand, the MFI pump would also be open more because the throttle is open more, so it would depend on the difference between the air vs. throttle angle curve and the fuel vs. throttle angle curve.

[TEC69E]

Hi Max . . . thanks for pointing that out. Of course, more energy would be required to move the weight of the car. It also turns out that my tail-pipe clamp may not provide the most accurate AFR reading. So it looks like I've got another project to add to my list.

Say 'hi' to your dad for me.

[edmayo]

Tom, the no-load readings at a given RPM will be much richer than the same RPM under a load. Porsche made the load readings to a set standard; MFI pump opened to a 7degree angle, (a tool was made to maintain that angle) car in 2nd gear, and use the brake to maintain 2500 RPM while reading the C/O % I use the AFR gauge by driving the car at various throttle and accelleration rates to try and find what setting it's happiest at, too lean will have intake pops and flat spots, you try NOT to be richer than necessary to achieve this. Most times the mixture is set too rich. THEN try full throttle in 2nd and 3rd gears to see what full rich will be, probably in the 11 1/2 to 12 1/2 range, but more like 13 to 13 1/2 at partial load.

[TEC69E]

Hi Ed,
Thank you for your input. I've been pouring over articles on the web about AFR meters, readings, and technique. It appears with our Porsches the prevailing experience says the tail-pipe clamp technique will provide accurate AFR measurements. The issue that I misunderstood with the tail-pipe clamp applied to motors with big cams with lot's of duration/overlap. This caused the AFR meter to read lean at idle near the tail-pipe because the cam profile was causing clean air to get sucked back up the tail-pipe. Makes sense right? So with that understanding, I'm going to proceed with my initial approach (tail-pipe clamp) and just try to secure it in-place so it doesn't fall out while I'm trying to collect data on the road. And at the same time, I'm going to hookup the RPM sensor for this LM-2 so I can monitor RPM and AFR concurrently. Pretty cool stuff! I also discovered that for a 2.7 RS Carrera (not my car, but my motor spec) the factory documented AFR setting at idle and partial load is around 13.5 (%CO 2.0-2.5) which is on the leaner side of your estimate of 13-13.5. And finally once I get some good AFR data, I'll figure out how to adjust my MFI pump to get the AFR readings closer to that sweet spot. But best of all, I'm beginning to feel like this MFI thing is not such a mystery after all.
Thanks again for your support.

[bliebler]

Tom, the no-load readings at a given RPM will be much richer than the same RPM under a load. Porsche made the load readings to a set standard; MFI pump opened to a 7degree angle, (a tool was made to maintain that angle) car in 2nd gear, and use the brake to maintain 2500 RPM while reading the C/O % I use the AFR gauge by driving the car at various throttle and accelleration rates to try and find what setting it's happiest at, too lean will have intake pops and flat spots, you try NOT to be richer than necessary to achieve this. Most times the mixture is set too rich. THEN try full throttle in 2nd and 3rd gears to see what full rich will be, probably in the 11 1/2 to 12 1/2 range, but more like 13 to 13 1/2 at partial load.

Ed, Does this mean you set the AFR at 4500 to 6500 to around 11.5 to 12.5? When I set my 73S there the plugs get sooty with driving around city streets. I use NGK #7's so they don't foul, just sooty along with my tail pipe. The car does run better there, but I worry about fuel washing down my cylinders. When I set mine at 13.1 at WOT the plugs stay pretty clean, even with # 8 plugs, however it's not as smooth around the 3500 to 4500 range.

[304065]

Remember also that the stochiometric ratio for E10 (which is what we mostly get these days) will be richer than pure gasoline.

Or as Ed Mayo put it beautifully in a recent Panorama, "they stopped putting gas in our gas."

[TEC69E]

Remember also that the stochiometric ratio for E10 (which is what we mostly get these days) will be richer than pure gasoline.

Or as Ed Mayo put it beautifully in a recent Panorama, "they stopped putting gas in our gas."

That is a good point. Do we know how much to adjust the stochiometric ratio for E10? Or I suppose we just accept that if the motor runs 'better' with richer AFR's . . . we can chalk that up to E10. I'll keep that information handy as I get into tuning the MFI. Thanks.

[edmayo]

Tom, what exhaust system are you running? hopefully not a Bursch.

[TEC69E]

Hi Ed, I'm running SSI's with an original 2in 1out muffler.