Took a ride in a Mercedes sedan with an I6 diesel a couple of days ago and it was smooth and quiet. The tach put the redline at 5,000 RPM and I don't know the hp or the torque.

My question is why would this I6 design have a redline of 5,000 RPM and the newer CTD have a redline of only 3,200 RPM?

Please keep the answers simple so I might be able to understand them

 

The Mercedes engines are much lighter in every way. And are smaller displacement. Smaller and lighter parts can run faster than big heavy parts. The heavier the moving part the more the forces build up from acceleration and rotation of those parts, like pistons, rods, etc. So big engines must turn slower than smal engines to stay together. But, having said that, it is amazing how high Mercedes diesels rev. They seem like they run better at higher speed where Cummins engines like to turn slow and do their job with high torque.

Wetspirit

 

The redline for the Cummins is also artificially low... I don't think you risk breaking the bottom end until 4,500-5,500 RPM, do you?


Granted there are harmonics and valve train problems starting at 4,000 RPM.

 

Peak piston velocity and G force has a lot to do with redline, and an engine with a longer stroke will hit higher piston velocity at the same RPM, but it will also produce more torque.

 

I think Cummins says somewhere around 4300... don't remember for sure though.

 

Begle1,

I don't have any idea what we could get away with. But wow, 5,500 would be scary. I wonder what the torque would look like at that speed? I can't imagine it would have much torque at that speed. The Mercedes 5 cylinders will rev right up to 5,000 and just keep pulling, but my Passat starts to fall flat at about 3,500 or so. It has a torque curve like the Cummins, nice and strong down low. For passing on the highway I don't let it downshift.

Wetspirit

 

Those small diesels have a much shorter stroke. You can tell by the hp to torque ratio, the numbers are much closer together. The greater the distance from TDC to BDC, the higher the speed the piston must travel at to get there in one crank rotation.

 

yes it would be



Click here to see Video

 

the torque would be a little lower than what the engine would be making for HP... 5252rpm on all engines have matching torque and hp values. below 5252 torque is always higher, above 5252 hp is always higher

HP = [torque x rpm] ÷ 5252

 

Thanks for the answers guys. I totally understand the mass factor and stroke length.