I’m hoping someone out there with some tuning experience can help me with this one. I’m not sure what the optimal air fuel ratio is for my engine during wide open throttle. Prior to a possible future visit to the dyno to get it fine tuned I just installed an Innovate wide band O2 sensor kit to better tune my 600 Holley. Prior to the new sensor I had been jetting the carb up to get some slight color in the spark plugs, (OE Autolite 45). This is apparently not an exact science. By the time I had achieved a slight tan color on the plugs the fuel economy had gone down from 20 mpg to around 12 mpg at cruise!! With these conditions it also ran much better prior to reaching operating temperature, which told me it was too rich.
My engine is a non-roller 302. It has a mild cam (222/234 @.050), tubular headers, 2-1/4” pipes back to the 2-1/2” turbo mufflers and 2-1/2” out the back. The carb is model 4160, 600 cfm Holley with vacuum secondaries.
When I purchased the car it came with a Holley model 4150, 650 cfm which is a dual line, double pumper, with a mechanical secondary. This was just too much carb and I never could get it tuned to be comfortable on the street. I’m now beginning to suspect that the current 600 cfm is still a bit on the heavy side.
With the O2 sensor I learned that my AFR was around 9.5:1 at cruise! I now have the 600 cfm jetted back down, (#68), to maintain an AFR of around 13.5 to 14.5 at cruise. At WOT during low rpm the AFR still drops down as low as 10:1. I just changed to a 3.5 hg power valve and it didn’t make any difference in the AFR at WOT. I have also installed the lightest spring in the vacuum secondary, from a kit, with no noticeable change. I attached a clip to see if the secondaries were opening and found that if I “stay in it long enough” they will just begin to open. I should also mention that, under the current conditions, there is obviously still more power at WOT prior to reaching operating temp---too rich.
I have done some experimenting and am convinced that the engine runs off of the idle circuit in the carb at least up until cruising at 70 mph, (3,000 rpm). Does this indicate that the carb is still too big for a mild 302? Could it be that the idle circuit, at the point when the transition slots are utilized, is feeding too much fuel? If so then this is happening even before the booster venturies are beginning take over.
Could ignition timing affect this O2 sensor reading? My static timing is 10 degrees. The vacuum advance, (manifold vacuum), is 14 degrees. The mechanical advance maxes out at 18 degrees @ 3,500 rpm. FYI this is a total of 42 degrees at no-load cruising speed of 75 mph.
Any help, comments, or recommendations would be greatly appreciated.
My engine is a non-roller 302. It has a mild cam (222/234 @.050), tubular headers, 2-1/4” pipes back to the 2-1/2” turbo mufflers and 2-1/2” out the back. The carb is model 4160, 600 cfm Holley with vacuum secondaries.
When I purchased the car it came with a Holley model 4150, 650 cfm which is a dual line, double pumper, with a mechanical secondary. This was just too much carb and I never could get it tuned to be comfortable on the street. I’m now beginning to suspect that the current 600 cfm is still a bit on the heavy side.
With the O2 sensor I learned that my AFR was around 9.5:1 at cruise! I now have the 600 cfm jetted back down, (#68), to maintain an AFR of around 13.5 to 14.5 at cruise. At WOT during low rpm the AFR still drops down as low as 10:1. I just changed to a 3.5 hg power valve and it didn’t make any difference in the AFR at WOT. I have also installed the lightest spring in the vacuum secondary, from a kit, with no noticeable change. I attached a clip to see if the secondaries were opening and found that if I “stay in it long enough” they will just begin to open. I should also mention that, under the current conditions, there is obviously still more power at WOT prior to reaching operating temp---too rich.
I have done some experimenting and am convinced that the engine runs off of the idle circuit in the carb at least up until cruising at 70 mph, (3,000 rpm). Does this indicate that the carb is still too big for a mild 302? Could it be that the idle circuit, at the point when the transition slots are utilized, is feeding too much fuel? If so then this is happening even before the booster venturies are beginning take over.
Could ignition timing affect this O2 sensor reading? My static timing is 10 degrees. The vacuum advance, (manifold vacuum), is 14 degrees. The mechanical advance maxes out at 18 degrees @ 3,500 rpm. FYI this is a total of 42 degrees at no-load cruising speed of 75 mph.
Any help, comments, or recommendations would be greatly appreciated.
