I've seen cylinder bias mentioned in the forum a lot. I recently did a once over on my 95" '03 FLHRCI, replacing the SE204 cams with TW37G, and getting the heads ported by AMS. Had a SERT installed, and a dyno tune done by the dealer. Even though I asked for the cruising range tune along with the full-throttle runs, they didn't do it, as the only cells that were changed in the base map were the 100% throttle, 100 kPa columns, and some minor stuff in the spark advance at low throttle setting. Now i'm having a go at it to improve the mileage. I look at the map the dealer used as a base. 105HG018-D0, which is recommended for FLH with 95", forged high-compression pistons, SE257 cam, SE high-performance heads, etc. There's a map (105HD019-D0) for 95" Softail, cast flat-tops, stock (or performance) heads, SE203 cam. With my cast flat-tops, TW37G, AMS heads, I think I'm closer to this setup, so I took the full-throttle part of the dealer's map (AFR, cylinder VE's, spark advances), and pasted it into the Softail map. Mileage jumped 7 mpg, I think it's because of timing. Now I'm doing plug readings trying to tweak the AFR, but the rear cylinders VE numbers seem really low. Question is why there's so much difference in VE between front and rear cylinders (front VE is much higher, more fuel)? I know we're trying to fool the ECM, but I've always heard the rear cylinder runs leaner and hotter. Don't we want to richen it up? Also, the two maps I'm using have big differences in spark advance at cruise. SE257 map is 34 degrees at 2500 rpm, 50 kPa, softail map is 43 degrees. I don't get any pinging, and SERT shows no knock retard. Any scientific theory on this stuff?