I finally decided to stop pushing my luck on my old 4L60 and swap in a 4L80 while it was still worth something in good running condition. Through all my digging online (5 minutes) I could not find any real hard dyno numbers as to how much power is lost doing the swap so I decided to test it myself.

Some background on the truck... basically it's an LQ9 with a small N/A cam and a KB Racing turbo kit with a Master Power 76 Q-trim turbo. It's by far not optimal as the Q-trim wheel is restrictive, the N/A cam obviously isn't setup for boost, and it's 10:1 compression which here in AZ during the summer heat with our crappy gas means I'm only running approximately 6 psi right now. On top of that I have 33" tall tires which wrap heavily on the roller at higher speeds in 3rd gear which unfortunately is the only gear I can use for this comparison since the 4L60 and 4L80 have different ratios in the other gears. When dynoing in 2nd gear the power and torque peak numbers are pretty equal to each other, in the 3rd gear dyno graph below you will notice that peak horsepower is quite a bit lower than torque and that's due to the tire wrapping itself up on the roller at the higher speed and making neat (bad) shapes in the sidewall dragging the numbers down. However it's still adequate for a comparison between the two transmissions since it will do it equally as bad since 3rd gear is a 1:1 ratio on both.

I made two pulls in 3rd gear on the 4L60 before I pulled it, swapped in the 4L80, then brought it back to the dyno for 2 more pulls. I made sure to duplicate the running conditions as best I could so that there were no other variables involved that could skew numbers. I had a factory 4L60 converter and I also used a factory 4L80 converter, both locked on the dyno so no difference in stall rpm could change anything. Tire pressures, weather, boost, correction factor, you name it and it was basically identical. The only thing I noticed after reviewing the logs was that the 4L80 runs had a half degree more timing due to the IAT cooling off a bit more between the two runs than the 4L60 had a chance to. I know from previous experience that it's only worth ~5 rwhp/tq on MY setup in this condition so we could just add that to the difference if you would like.



Looking at the graph you can see how noisy the 33" tall tires are on the roller by all of the wavy lines even at smoothing level 5. I took the best of the two runs of each transmission to compare with. It shows an average of 5-10 horsepower and torque all over the curve, add in the 5 horsepower/torque that the extra half degree of timing gives and it all adds up to around a 15 rwhp/tq loss. I always thought that the difference would have been larger than this but after looking over the logs to make sure everything was the same I am very impressed with the power loss or lack there of. This also makes me wonder how much of the power loss is attributed to the additional weight of the factory 4L80 converter over the 4L60 and if some of the losses could be mitigated by running a smaller lighter aftermarket 4L80 converter.

Next up, I'm going to swap in a turbo cam with LSA heads and a factory truck intake and if I'm up to it swapping on the Holley hi-ram intake after that for a direct intake swap comparison.