Hib Halverson
Technical Writer for Internet & Print Media
Forum members-
Some on the CAC might remember that, three months ago, I put my 71 Big-Block Coupe up for sale (see http://www.corvetteactioncenter.com/forums/thread121684.html), but...even after a price reduction, didn't get a buyer. Had a ton of people tell me what a cool car it was. But–not cool enough to buy, apparently, because I had only a two serious inquiries both of which didn't buy. What turned out to be a short-term financial crunch forced putting it up for sale but then, my Wife, the Fairest Sandra the Red, and I got through the crunch and the car didn't sell, so ole "Yellow Thunder" has been returned to project car status.
My '71 Coupe is the only car I have left without electronic engine controls. It's fed by a big, dumb four-barrel carburetor–a Holley 850 to be exact. In preparation for future carb retuning which maybe necessary if I change the camshaft and heads, I replaced the narrow band O2 sensor and digital display I'd put on the car back in the early-'90s, with the new "Sport Comp Wideband Air/Fuel Pro" made by AutoMeter (PN 3378). This unit is designed for use with gasoline but is adjustable for alternative fuels such as E85, ethanol, methanol, propane and compressed natural gas (CNG). It has data recording ability and has a output for on-board data acquisition systems. It is available in a host of styles that go with AutoMeter's many lines of gauges.
The gauge installation was easy as the car already had a sensor. I pulled out the old O2S and harness and put the new AutoMeter/Bosch LSU4.2 wideband sensor, which has a resolution of 0.1 air fuel ratio, in its place. I ran the wires the same way they were before: from the O2S in the exhaust, though a grommeted hole in the driveline tunnel, under the console trim and then to the gauge on the steering column. I ran the AutoMeter's power and ground wires to the same hot-with-ign-on location and ground I used before.
With this product, AutoMeter demonstrates why its instrumentation is the choice of racers and discriminating hot rodders. The two methods of displaying data are quite intuitive. There is a straight digital display–on which you have the choice of showing air-fuel ratio or lambda–in the center of the gauge and a circular, digitally-generated analog-style display at the perimeter of the gauge with yellow LEDs for rich, green for near stoichiometric and red for lean. The gauge is the standard 2 1/16-in diameter and is intended for in-dash or A-pillar pod mounting but, also, can be mounted in one of Auto Meter's carbon-fiber gauge "cups." I chose to put the AFR gauge in a cup which I mounted on the steering column.
The installation of this wide-band air-fuel-ratio meter paid off almost immediately. The 850 I use is an R4781-5 double-pumper modified with an intermediate circuit in the secondaries. One problem I'd been having was the very first part of the primary accelerator pump shot was too lean causing the engine to spit back sometimes. For a long time, I "covered that up" with a slight increase in primary float level, but I decided, with this new tuning aid, it was time to fix that so I lowered the primary float to the proper level then went testing.
On the first test drive with the AutoMeter, I noticed right away that my idle transfer air-fuel ratio was a bit rich. The wide-band O2S helped me improve that and gain fuel mileage. I did that by increasing the diameter of the wire restricters I had in the idle feed restrictors which control air-fuel ratio at idle and off-idle. Next, I found my wide open throttle AFR was somewhat rich so I leaned that back by two jet numbers such that I saw between 12.4 and 12.8:1. Lastly, I noted that near the engine's rev limit of 6800 rpm, the AFR got a little rich. My guess is my air filter begins to be a restriction above 6250 rpm. Fixing that isn't going to be easy, considering the limited hood clearance I have.
Next came tuning the accelerator pumps. My 850 has two of them, a 30-cc, operated by a white cam, on the primary side, and a 50-cc, operated by a brown cam, on the secondaries. Idling in neutral, if I whacked the throttle to just short of secondary opening, the engine would sag and sometimes spit back. If I did the same thing a little slower, the engine responded properly. I tried a larger shooter. While that mitigated the problem, the AutoMeter Wideband was telling me the pump shot was still lean, right at the start. I went back to the 31 shooter then replaced the white cam with the blue cam which causes the pump to deliver a lot of fuel right at the start of the shot. That solved the tuning problem with the primary pump and the AutoMeter Wideband verified that showing no more momentary lean sag and an AFR during pump action of about 12.5.
The secondary pump was more of a challenge. :W Before I started messing with it, I had a huge 42 shooter. Roadtesting the car on a long hill by rapidly opening the throttle at 1700 rpm in second gear showed the initial secondary pump shot was just a tiny bit rich, but once the engine responded (ie: once all four barrels were wide open and the need for accelerator pump shot was past), the AFR want way rich, like 10.8-10.9:1 . That richness was temporary, gradually ending with the AFR hovering about 12.6 up to about 6250 rpm. I went to a smaller shooter and the engine developed a sag at secondary opening but was still way rich once the engine reached WOT followed by the same, gradual lean-out to 12.5.
That confirmed my early belief that the problem was the secondary pump shot being too long. I decided the engine did not need the large, 50cc pump on the secondary side. Not having another 30-cc pump, I simulated one by switching the blue cam on my primaries with the green cam (the next most aggressive 30-cc cam), moving the blue cam to the secondaries and installing a 35 shooter. Boy, did that wake the motor up and the AutoMeter Wideband verified my lean-then-rich secondary accelerator pump problem was much improved. I finished off with a 37 secondary shooter and the car's response was much more crisp and the Wideband backed up that feeling showing me that AFR during the operation of both pumps stayed around 12.5:1.
I ordered a second set of pump cams along with a 30-cc pump and lever from Holley. Temporarily, I'll continue to use the 30-cc cam with the 50-cc pump, but once I replace it...does anyone on the CAC want a 50-cc Holley accelerator pump kit–cheap?
So...how is it there was a fair amount of fine tuning I had to do with a carb that was supposedly already sorted?
Simple. The narrow band sensor I was using before wasn't as accurate. A narrow-band O2S is just that–narrow band–because it's resolution either side of stoichiometric is poor, that is, you can tell the AFR is rich, but you can tell if it's 11.5 or 12.5. That makes it tough to tune sometimes. I backed-up my narrow-band data by making second gear "runs" while measuring acceleration, but that sometimes results in inconsistent data. Nevertheless, at the time I did my most recent round of carb tuning–about the late 1990s–that was the best I could do without paying a bunch of money for chassis dyno time.
Having this AutoMeter wide-band O2 has vastly improved the quality of data I've been using to make my carb tuning choices. It makes on-road tuning much easier. In fact, it would improve anyone's tuning work regardless of what fuel mixing device their engine has, carburetor or electronic fuel injection. I have yet to enable the AutoMeter Wideband's ability to record AFR data. I need to add a WOT swtich to the carb and I can do that. Then I don't even have to look at the gauge. I can just make my test runs, then play back the data.
Bottom line...AutoMeter's Wideband Air/Fuel Pro is pretty darn cool.
Want more information? :confused
Visit the Auto Meter web site and search for "wideband".
Some on the CAC might remember that, three months ago, I put my 71 Big-Block Coupe up for sale (see http://www.corvetteactioncenter.com/forums/thread121684.html), but...even after a price reduction, didn't get a buyer. Had a ton of people tell me what a cool car it was. But–not cool enough to buy, apparently, because I had only a two serious inquiries both of which didn't buy. What turned out to be a short-term financial crunch forced putting it up for sale but then, my Wife, the Fairest Sandra the Red, and I got through the crunch and the car didn't sell, so ole "Yellow Thunder" has been returned to project car status.
My '71 Coupe is the only car I have left without electronic engine controls. It's fed by a big, dumb four-barrel carburetor–a Holley 850 to be exact. In preparation for future carb retuning which maybe necessary if I change the camshaft and heads, I replaced the narrow band O2 sensor and digital display I'd put on the car back in the early-'90s, with the new "Sport Comp Wideband Air/Fuel Pro" made by AutoMeter (PN 3378). This unit is designed for use with gasoline but is adjustable for alternative fuels such as E85, ethanol, methanol, propane and compressed natural gas (CNG). It has data recording ability and has a output for on-board data acquisition systems. It is available in a host of styles that go with AutoMeter's many lines of gauges.
The gauge installation was easy as the car already had a sensor. I pulled out the old O2S and harness and put the new AutoMeter/Bosch LSU4.2 wideband sensor, which has a resolution of 0.1 air fuel ratio, in its place. I ran the wires the same way they were before: from the O2S in the exhaust, though a grommeted hole in the driveline tunnel, under the console trim and then to the gauge on the steering column. I ran the AutoMeter's power and ground wires to the same hot-with-ign-on location and ground I used before.
With this product, AutoMeter demonstrates why its instrumentation is the choice of racers and discriminating hot rodders. The two methods of displaying data are quite intuitive. There is a straight digital display–on which you have the choice of showing air-fuel ratio or lambda–in the center of the gauge and a circular, digitally-generated analog-style display at the perimeter of the gauge with yellow LEDs for rich, green for near stoichiometric and red for lean. The gauge is the standard 2 1/16-in diameter and is intended for in-dash or A-pillar pod mounting but, also, can be mounted in one of Auto Meter's carbon-fiber gauge "cups." I chose to put the AFR gauge in a cup which I mounted on the steering column.
The installation of this wide-band air-fuel-ratio meter paid off almost immediately. The 850 I use is an R4781-5 double-pumper modified with an intermediate circuit in the secondaries. One problem I'd been having was the very first part of the primary accelerator pump shot was too lean causing the engine to spit back sometimes. For a long time, I "covered that up" with a slight increase in primary float level, but I decided, with this new tuning aid, it was time to fix that so I lowered the primary float to the proper level then went testing.
On the first test drive with the AutoMeter, I noticed right away that my idle transfer air-fuel ratio was a bit rich. The wide-band O2S helped me improve that and gain fuel mileage. I did that by increasing the diameter of the wire restricters I had in the idle feed restrictors which control air-fuel ratio at idle and off-idle. Next, I found my wide open throttle AFR was somewhat rich so I leaned that back by two jet numbers such that I saw between 12.4 and 12.8:1. Lastly, I noted that near the engine's rev limit of 6800 rpm, the AFR got a little rich. My guess is my air filter begins to be a restriction above 6250 rpm. Fixing that isn't going to be easy, considering the limited hood clearance I have.
Next came tuning the accelerator pumps. My 850 has two of them, a 30-cc, operated by a white cam, on the primary side, and a 50-cc, operated by a brown cam, on the secondaries. Idling in neutral, if I whacked the throttle to just short of secondary opening, the engine would sag and sometimes spit back. If I did the same thing a little slower, the engine responded properly. I tried a larger shooter. While that mitigated the problem, the AutoMeter Wideband was telling me the pump shot was still lean, right at the start. I went back to the 31 shooter then replaced the white cam with the blue cam which causes the pump to deliver a lot of fuel right at the start of the shot. That solved the tuning problem with the primary pump and the AutoMeter Wideband verified that showing no more momentary lean sag and an AFR during pump action of about 12.5.
The secondary pump was more of a challenge. :W Before I started messing with it, I had a huge 42 shooter. Roadtesting the car on a long hill by rapidly opening the throttle at 1700 rpm in second gear showed the initial secondary pump shot was just a tiny bit rich, but once the engine responded (ie: once all four barrels were wide open and the need for accelerator pump shot was past), the AFR want way rich, like 10.8-10.9:1 . That richness was temporary, gradually ending with the AFR hovering about 12.6 up to about 6250 rpm. I went to a smaller shooter and the engine developed a sag at secondary opening but was still way rich once the engine reached WOT followed by the same, gradual lean-out to 12.5.
That confirmed my early belief that the problem was the secondary pump shot being too long. I decided the engine did not need the large, 50cc pump on the secondary side. Not having another 30-cc pump, I simulated one by switching the blue cam on my primaries with the green cam (the next most aggressive 30-cc cam), moving the blue cam to the secondaries and installing a 35 shooter. Boy, did that wake the motor up and the AutoMeter Wideband verified my lean-then-rich secondary accelerator pump problem was much improved. I finished off with a 37 secondary shooter and the car's response was much more crisp and the Wideband backed up that feeling showing me that AFR during the operation of both pumps stayed around 12.5:1.
I ordered a second set of pump cams along with a 30-cc pump and lever from Holley. Temporarily, I'll continue to use the 30-cc cam with the 50-cc pump, but once I replace it...does anyone on the CAC want a 50-cc Holley accelerator pump kit–cheap?
So...how is it there was a fair amount of fine tuning I had to do with a carb that was supposedly already sorted?
Simple. The narrow band sensor I was using before wasn't as accurate. A narrow-band O2S is just that–narrow band–because it's resolution either side of stoichiometric is poor, that is, you can tell the AFR is rich, but you can tell if it's 11.5 or 12.5. That makes it tough to tune sometimes. I backed-up my narrow-band data by making second gear "runs" while measuring acceleration, but that sometimes results in inconsistent data. Nevertheless, at the time I did my most recent round of carb tuning–about the late 1990s–that was the best I could do without paying a bunch of money for chassis dyno time.
Having this AutoMeter wide-band O2 has vastly improved the quality of data I've been using to make my carb tuning choices. It makes on-road tuning much easier. In fact, it would improve anyone's tuning work regardless of what fuel mixing device their engine has, carburetor or electronic fuel injection. I have yet to enable the AutoMeter Wideband's ability to record AFR data. I need to add a WOT swtich to the carb and I can do that. Then I don't even have to look at the gauge. I can just make my test runs, then play back the data.
Bottom line...AutoMeter's Wideband Air/Fuel Pro is pretty darn cool.
Want more information? :confused
Visit the Auto Meter web site and search for "wideband".
