F350 Super Duty vs 3500 Denali vs The Ike Pulling 30k lbs

ShinerBock wrote:
^^^That is not how turbos work. Air density, blade profile, A/R ratio, flow, blade size and other things dictate how much power a turbocharged engine looses at altitude. Turbo shaft speed generally increases with altitude to compensate for the decrease in air density. Bigger turbo wheels that make a lot of power cannot spool as fast as a smaller turbo wheel. So generally a smaller turbo that is at its max potential at sea level will do better at altitude than a larger turbo.

Also, tuners do not often push turbos way past their efficiency zones, at least not a good one. If you go past the turbos efficiency zone then you start making less power and higher EGT's which is not what you want so to make the best gains you have to stay withing the turbo's efficiency. A tuner will generally keep adding fuel and timing until they start to see power numbers start to decrease with excessive EGT's. That is the point where they know the turbo is leaving it's efficiency zone and a bigger turbo is needed to make more power so they back the fuel and timing back down to where it made more power. Diesels are regulated by fuel and not air like gas engines.

Re-read what I wrote, it does not contradict anything you said.

A turbo that is sized to produce full rated power at sea level would start to starve the engine of CFM at higher altitude. Because maintaining the same absolute manifold pressure at altitude, is similar to over-boosting at sea level (higher pressure ratio), which almost ALWAYS pushes you to a lower eff curve on the compressor map.

Again, watch Gale Banks explain the limit of the stock L5P turbo. To go above the stock max power at sea level, essentially you have to "choke" your turbine side by activating the variable geometry nozzles at WOT (to gain turbo rpm, and therefore CFM), which is not within OEM GM parameters.