[URV8SUX]

Is the fuel cell vented?

If not I'd bet that is why it drops!

If it is vented maybe the vent isn't big enough.

For what it's worth. I ran e85 and ran twin walbros with a boost a pump on them has the stock vent on a stock tank. Made 830 @DB never miss fired or showed it was out of fuel. Went to the track first pass I messed 3rd gear. So te next pass I get in 4th and the fucker is miss firing and showing its lean in the wide band so I lifted. So I pulled the log showed it ran the same boat we ran on the dyno so I turned it down 3 psi and did the same thin once I got in 4th. So then i turned it down again and changed plugs and the same result.

When I started I had about 5/8's of a tank and after the passes it was about 1/4 so I figured I'd add some fuel. Cracked the cap loose and I could hear it pulling air into the tank from just cracking the gas cap and it all made seance. the tank was being shrunk or collapsing or why ever he proper word is.

Left tHe gas cap off and boom no more missing and running the AFR it did on the dyno.