[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.

 

[Dave427]

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.

Dave

 

[Sandy]

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!

 

[Dave427]

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
Dave

 

[Sandy]

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.

 

[Camaro_fever68]

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?

 

[Dave427]

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.

Dave

 

[Sandy]

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????

 

[Camaro_fever68]

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.

 

[Sandy]

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