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Air Fuel and Low Dynamic Compression

1069 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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It is a 850 DP with the choke horn milled off.

I am running 35 degrees total.

On the return road the air fuel is between 12 and 13 and pretty erratic.

The car starts and runs excellent.

But it is not a rocket!
I am aware of stock jetting on a 850, that is why I have been reluctant to go much lower in jet size. It is not unusual to have density altitude of 4-5,000 feet here.

Secondary jetting is with no power valve.

The HP950 flows about the same as a 850 DP.

Compression ratio is 10.2

It is a standard bore 454 with LS6 pistons and iron oval heads.
Ray, thanks for checking the post over on the Innovate forum. I don't know why the data points are so choppy.

Interesting to hear that you have a big cam low compression combo that works well and has the air fuel where it is supposed to be.

I can't see taking out much more jet to get up to 12.5 air fuel. It is probably going to take four sizes all around which just does not seem right.

The other possibility is that there is some fault with the O2 sensor, but it seems to be giving a correct free air reading. I have not checked the plugs yet this year.

Dave, the track is around 2,600 feet elevation but density altitude can be quite a bit higher.

The 950 HP may be more responsive, but from the time I release the tranny brake my foot is on the floor and the carb is wide open. The Innovate log file shows a bit rich through first gear probably because of the extra fuel from the double pumper. I tried removing the link for the secondary accelerator pump thinking I would lean it just a bit for the launch and guess what. When I mashed the pedal expecting it to go to the 2 step it just died on the line. So the link is back on and running normally again. One qualifying round experiment that did not work. I don't know who was more surprised, me or the starter.

I am pretty close on my jetting for power having got to it by monitoring trap mph. As I reduced jet size, my mph went up until the 75/81 set which appears to have slowed 0.5 mph.

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????
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Thanks for the comments, I have checked the Innovate forum.
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