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.

Rob Petrie E395 · 07-30-2015, 08:59 PM

How far down the primary should the 02 sensor be? If I were to try and put some in.

MOTORSPORTS Innovations · 07-31-2015, 09:06 AM

Quote:

Originally Posted by Rob Petrie E395

How far down the primary should the 02 sensor be? If I were to try and put some in.

10-12 inches form the cylinder head. A little closer is ok if necessary to deal with clearance issues.

Greg Kelley - 609-265-2110

pbp1 · 07-31-2015, 11:44 AM

No matter where you have the sensor mounted, the two step is skewing your readings. Misfires are always read as lean on the O2 sensor. I dont know exctly why but they do. What has helped me the most is looking at what the A/F ratio does immediately after you let go of the button. If it dives really rich, that tells you it was rich when it was on the 2 step. If it stays lean after you let go of the button, it was lean on the 2 step.

Ed Wright · 07-31-2015, 01:51 PM

A misfire in a street car does the same thing, often skews the fuel trims until fixed.
Reason is oxygen out the exhaust ports when misfiring. The wide band only knows oxygen content, and converts the oxygen level to air/fuel ratio. High oxygen = lean, to the wideband.

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: 574 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: 319 Liked 311 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: 2,659 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

07-30-2015, 08:59 PM	#7
Rob Petrie E395 VIP Member Join Date: Dec 2007 Location: Woodlawn IL Posts: 1,092 Likes: 162 Liked 216 Times in 77 Posts	Re: A/F Ratio How far down the primary should the 02 sensor be? If I were to try and put some in.

07-31-2015, 11:44 AM	#9
pbp1 Junior Member Join Date: Mar 2013 Posts: 80 Likes: 12 Liked 27 Times in 6 Posts	Re: A/F Ratio No matter where you have the sensor mounted, the two step is skewing your readings. Misfires are always read as lean on the O2 sensor. I dont know exctly why but they do. What has helped me the most is looking at what the A/F ratio does immediately after you let go of the button. If it dives really rich, that tells you it was rich when it was on the 2 step. If it stays lean after you let go of the button, it was lean on the 2 step.

07-31-2015, 01:51 PM	#10
Ed Wright Veteran Member Join Date: Mar 2005 Location: Sand Springs, OK Posts: 8,132 Likes: 896 Liked 390 Times in 170 Posts	Re: A/F Ratio A misfire in a street car does the same thing, often skews the fuel trims until fixed. Reason is oxygen out the exhaust ports when misfiring. The wide band only knows oxygen content, and converts the oxygen level to air/fuel ratio. High oxygen = lean, to the wideband. __________________ Ed Wright 4156 SS/JA