A/F Ratio

Tgreen3724 · 07-27-2015, 12:17 PM

454/425 Super Stock BBC running a two step button and footbraking. AF ratio runs about 16:1 on two step then dives all the way to 11:1 immediately after release of the two step button. Then two seconds into the run AF curve flattens out and remains in the 12.5 to 12.7 ratio range. Is this normal for the AF ratio to dive from 16 to 11 after the two step is no longer activated? Ratios remain under 12 for two seconds of run then flatten in the upper 12 range for remainder of pass. Is this normal? Thoughts?

Kevin Panzino · 07-27-2015, 06:13 PM

Yes, Mine does exactly what you describe, however I'm not seeing nearly that long for mine to get back above 12... p.m. me if you like.

Kevin

Bob Shaw · 07-27-2015, 06:29 PM

Have you tried checking the A/F ratio without using the 2 step? Mine keeps getting richer the longer I am on the 2 step.

Tim Lisson · 07-27-2015, 06:43 PM

If you have the sensor towards the end of the collector, you will not read an accurate reading on the two-step, and it will always reflect a lean signal.

The fuel is puddling in the intake and after launch it should show rich due to the blast of fuel. Sounds like you could pull a little fuel out of it but then again, each combo likes something different.

Dave Noll · 07-27-2015, 09:10 PM

The AF ratio is calculated by input from the O2 sensor. Wide or narrow it's still an O2 sensor. The O2 is seeing lean while on the limiter because the engine is missing & oxygen that normally gets used up in combustion, isn't. It would be interesting to see the curve on a run without the two-step.

MOTORSPORTS Innovations · 07-30-2015, 08:20 PM

One of the key elements on this is where the O2 bung is located. In the collector, carb car, guaranteed crazy reading (on 2-step) - in a primary tube - it might be ok or will straighten out much more quickly. The reason for this the back-pulses of clear air coming into the collector at lower RPM's.

Big cube bracket cars can get away with it in the collector. Stock & SS - put it in the primary.

Also, EFI cars seem to have more stable O2 readings on the 2-step.

Here is a log of a car showing pretty stable O2 readings bungs are in primary tubes. BUT it is an EFI car.

07-27-2015, 12:17 PM	#1
Tgreen3724 Junior Member Join Date: Oct 2007 Location: Shepherdsville, Ky Posts: 38 Likes: 0 Liked 1 Time in 1 Post	A/F Ratio 454/425 Super Stock BBC running a two step button and footbraking. AF ratio runs about 16:1 on two step then dives all the way to 11:1 immediately after release of the two step button. Then two seconds into the run AF curve flattens out and remains in the 12.5 to 12.7 ratio range. Is this normal for the AF ratio to dive from 16 to 11 after the two step is no longer activated? Ratios remain under 12 for two seconds of run then flatten in the upper 12 range for remainder of pass. Is this normal? Thoughts?

07-27-2015, 06:13 PM	#2
Kevin Panzino Senior Member Join Date: Oct 2002 Location: Pittsburgh Posts: 575 Likes: 298 Liked 747 Times in 136 Posts	Re: A/F Ratio Yes, Mine does exactly what you describe, however I'm not seeing nearly that long for mine to get back above 12... p.m. me if you like. Kevin

07-27-2015, 06:29 PM	#3
Bob Shaw Member Join Date: Jan 2002 Location: Sedalia, Mo Posts: 436 Likes: 328 Liked 316 Times in 61 Posts	Re: A/F Ratio Have you tried checking the A/F ratio without using the 2 step? Mine keeps getting richer the longer I am on the 2 step. __________________ Bob Shaw V/SA 515

07-27-2015, 06:43 PM	#4
Tim Lisson Member Join Date: Aug 2003 Location: Solon Springs, WI Posts: 166 Likes: 0 Liked 0 Times in 0 Posts	Re: A/F Ratio If you have the sensor towards the end of the collector, you will not read an accurate reading on the two-step, and it will always reflect a lean signal. The fuel is puddling in the intake and after launch it should show rich due to the blast of fuel. Sounds like you could pull a little fuel out of it but then again, each combo likes something different.

07-27-2015, 09:10 PM	#5
Dave Noll Senior Member Join Date: Nov 2007 Location: Div. 6, Eastern, WA Posts: 709 Likes: 2,671 Liked 238 Times in 111 Posts	Re: A/F Ratio The AF ratio is calculated by input from the O2 sensor. Wide or narrow it's still an O2 sensor. The O2 is seeing lean while on the limiter because the engine is missing & oxygen that normally gets used up in combustion, isn't. It would be interesting to see the curve on a run without the two-step. __________________ Dave Noll, EF/S ,?/SA 6526

07-30-2015, 08:20 PM	#6
MOTORSPORTS Innovations Member Join Date: Dec 2009 Location: Hainesport, NJ Posts: 123 Likes: 0 Liked 15 Times in 2 Posts	Re: A/F Ratio One of the key elements on this is where the O2 bung is located. In the collector, carb car, guaranteed crazy reading (on 2-step) - in a primary tube - it might be ok or will straighten out much more quickly. The reason for this the back-pulses of clear air coming into the collector at lower RPM's. Big cube bracket cars can get away with it in the collector. Stock & SS - put it in the primary. Also, EFI cars seem to have more stable O2 readings on the 2-step. Here is a log of a car showing pretty stable O2 readings bungs are in primary tubes. BUT it is an EFI car. __________________ MOTORSPORTS Innovations www.motorsportsinnovations.com