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

ShinerBock wrote:

RoyJ wrote:


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.

Doing that would create a lot of drive pressure and you would likely pop a head gasket before you had turbo failure. Not to mention a restriction like that would cause extremely high EGT's. It does not make sense to do this because you will not be able to make more power for any real period of time.

I know back when I had the stock VG turbo on my 2014 Cummins, I had the VGT vane position on my monitor. Both in stock form and after being tuned, going WOT the vanes would restrict to about 50% or so for a few seconds to get the wheel turning and then go fully open.

For sure, which is exactly why OEM and a good tuner never closes the VG vanes at WTO. Banks simply showed the stock turbo couldn't produce enough airflow to exceed 445hp while keeping a safe EGT/AFR. The only way to cheat that is by closing the vanes 30%, over-driving the turbo from 110k to 130k rpm. And we both agree that creates all kinds of problems.

At 10,000' elevation, the L5P consumes the same CFM to make the 445hp. However, now the turbo has to do it at a much high pressure ratio. Given a fixed compressor design, the only way to achieve this is to spin faster, and get into a lower eff land on the map.

Since we agree GM OEM programming would never do this, I believe the turbo stays at ~110,000 rpm, and therefore the L5P runs out of CFMs past 2300 rpm. That's why we see a hp plateau.