Question regarding exhaust

[Outlaw383]

Forgive me for this odd question but I wanted to get some of the really smart individuals input on this. At our local asphalt circle track there's a guy running a late model
car. Its your typical 500-540hp 355 the car seems to run decent (usually a top 3 car) but every race around lap 30 it starts blowing fire from the exhaust all the way down the straight. I was joking with him in the pits after the race one night and told him he was going to have to ease up on his Nos tune because of the flame lol. He proceeded to tell me they have tried everything they can think of to figure out why the car is doing this. Timing is good , plugs look good , he said engine was ran on dyno and air fuel ratios were in the 13 to1 range thru the pull etc. The car has headers 4 into 1 collectors into a single straight pipe. Any ideas what's causing it?

[steve cowan]

I am certainly NOT smart but would like to contribute-
If it was lean the pipes would be glowing ??
Must be rich and by 30 laps of nearly WOT engine must be heat soaked.
Possible air filters blocked causing rich mixture after so many laps,when you guys dyno your engines do you pull for a long duration or is it like you normally see.
Interesting to hear other people opinions

[BOOT]

What he said, not that smart either but asking A lot of dumb questions has helped me be less dumb

Did they read each cylinder on the dyno or was that the ave AFR? Could be a misfire that comes on with heat soak. Maybe a leak in the exhaust that loosens up n lets air in. Be interesting to see with someone with exp says it is.

[mag2555]

2 things come to mind.

1) since on the dyno you can’t get the motor top to bottom as hot as at 30 laps into a race , then maybe the heated carb has expanded enough and also the fuel in it to cause the motor to run rich?
A easy / simple 1/16” drop in fuel fuel level could be the cure

2) cylinders 5 and 7 ( if stock firing order/ cam is used) may be misfiring due to inductive coupling of the plug wires running neatly parallel to each other.
This can many times be cured by twisting wire 5 around wire 7 ( or visa versa) 3 times.

Condition 1 and or this misfire issue ( especially combined ) would allow a good amount unburied fuel out the exh system only to collect up and light off on the way out where the exh system is hot enough and has met up with enough air to do so .

I first ran into this in the early 70s when on my 671 super charged street motor I had a float in one of the two 4 bbl carbs stick open at a stop light.

I managed to keep the motor running, but this dumped enough fuel into the exh system that once I got back on the throttle hard to get the hell off the road and into a parking lot it all lit off with the result of blowing both of Mufflers on the wide open!

[barnym17]

Is here nice g an alternator?If not maybe voltage is dropping and misfire is allowing excess fuel into 5th exhaust.

[Outlaw383]

Thanks everyone for the ideas. I will let him know what was said and maybe something in these replies will help get this situation cured.
Thanks so much !

[lefty o]

well , dyno's dont run 30 laps or more, and they dont turn corners, so imo id start with some o2 sensors after each collector. without any data, its pure guess work.