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Air Fuel and Low Dynamic Compression

1067 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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What carb are you running? With your jetting arrangement it sounds like a 750 dp. With your lower compression ratio sometimes you can get away with more total timing. To get more complete combustion have the spark happen a few degrees earlier it may have more time to get it done. Whats your A/F at cruise? Your primary jetting seems a bit high.

Yikes a 850 dp comes 80 square new. So you are running 4 sizes smaller in the front and approx. 6 sizes smaller in the rear. Its hard to get your A/F's that close when you are that out from the stock jetting. A 950 HP will pick your car up I bet. What is your compression ratio? If its like 8.5-1 lets say, I wouldn't be scared to bring your total up to 38*. I think if I remember right you have a 406 correct?

Let me know
Ok your compression ratio isn't that bad. When you said you had low dynamic compression, I thought we might have been dealing with something around 8.5 to 1. Holley 4781's are the most ignorant carbs to run lean or even run good on the street. They run so fat all the time. So since you are at 4500-5000 ft. you are on the right track for jetting. It just seems like alot of jet to take out to me. I already assumed you had the rear power valve out , thats why I said you had the jetting 6 sizes smaller than stock and the primaries 4 sizes smaller than stock. The 950 HP's are a more responsive carb with the smaller venturi's of a 750 and they seem to run alot better than the 4781's.

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