Thread: 300 TDi Torque/Power Curves

Power, boost and AFR plots. Perfect. That's exactly what I needed.

Can we get a run down in a language we can all understand about the different plots on the dyno sheet and what they mean. Pat

Originally Posted by PAT303

Can we get a run down in a language we can all understand about the different plots on the dyno sheet and what they mean. Pat

Boost I'm sure you've all got. A/F or AFR is air to fuel ratio. Basically how rich/lean the engine is running. You can see it richens up as the turbo is spooling, then leans out at max boost before richening up again as RPM builds and the cylinders get a little less air through the valves.

These plots are power at the wheels, which is why the power flattens out. The drag from the drivetrain reduces recorded power at higher pm.

So how do we know for instance the A/F ratio is correct?,it's a line on the sheet so what do we compare it to?. Pat

Originally Posted by PAT303

So how do we know for instance the A/F ratio is correct?,it's a line on the sheet so what do we compare it to?. Pat

There is no single correct A/F ratio on a diesel. It varies with load in the general range of 18-80.
My 4BD1T idles at 80:1 and at full load I'm around 19:1. Cruise is ~37:1.

The full load A/F shouldn't be richer than about 18:1 to avoid smoke and high EGT. Most stock vehicles are around 20-22:1 to stay clean, some of the new cummins industrials are ~28:1. They run that lean to meet higher emissions classes.
As you run leaner, power drops. So full load is a compromise between emissions, efficiency (smoke really slaughters efficiency) and EGT.

The real challenge is to get enough boost for drivability and to stay lean enough at the highest normal altitude (to avoid smoke and high EGT) without the turbine side of the turbo turning into an exhaust brake at higher rpm.

The stock turbo looks like the turbine side does become an exhaust brake at higher rpm. A smaller turbine would give more low end boost and better performance at altitude, but would make the exhaust brake problem at higher rpm worse.

Here's a copy of the DTS/MTQ MHI TD04-11G power/torque curves.

Not very detailed, and they are scans of a fax, but it gives an idea of the difference between the T250 Garrett and TD04 MHI (and I'm guessing a little pump tuning)
The story I was told years ago was that one of the bosses at MTQ had a Disco 300Tdi that he towed a big boat with and so the reason they developed a kit.