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Air Fuel and Low Dynamic Compression

1068 Views 9 Replies 3 Participants Last post by  Sandy
I posted my LM-1 air fuel/rpm results on another forum and got a very interesting feedback.

Based on top end mph my optimum jetting is 76/82. Either way the mph will go down slightly. But at this jetting my air fuel ratio is only around 11.75 and the line is quite "ragged".

The response suggested that my dynamic compression was too low and that I was not getting very complete combustion. They also said that in order to use the 12.5 air fuel as a target, there needs to be more complete combustion.

Their observation was pretty good because I did not post the engine particulars until after their response.

Bottom line is that there is a lot to learn and a lot to know about using air fuel and building engines with less than ideal dynamic compression ratio.

In my case I did it to run pump gas, which is fine, so long as you understand the limitations and some of the tuning considerations. Also explains why a light car with a big engine does not go as quick as you would expect.
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I've read your thread over there. IMO, your cam/compression should not be causing your rich AFR. I have tuned simuliar combos without an issue. I tuned a 489 with a 264 @.050" cam and 10:1 CR. It had a 1050 Dominator and I had no issues tuning it. It runs 10.50's in a 3300# Camaro without optimal gear/converter, and too big of heads.

I looked at your RPM/AFR traces and they are both choppy. Could be noise interference?? Have you tried recalibrating your O2 sensor? Have you checked the plugs yet?
How about the crankcase ventilation tubes that connect into the headers? My AF bung is downwind of these tubes. But if anything they should contribute to a lean condition right?

So the air fuel ratio is still a mystery????

If they are sucking a mist of oil, it could be a problem. They really should have been down stream of the sensor. I would not attempt to tune with the wideband until that is fixed. You don't want any more air or anything contaminating the sensor.

PS. Check your post over on Innovate.
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