Kaitlyn2004

People talk about diesel and the lower redline. On my 2014 Q5 TDI the redline starts at about 5000 rpm... except I don't even get close to that. How on earth do people have this as an issue? Am I driving wrong? haha

idale

The problem is that the number is lower, that's it. People want higher numbers on their tachs and the TDI's tops out at 6K instead of 8K or whatever. The engine I'm sure would handle the higher RPM but you'd be getting nothing out of it so it'd be stupid.

MP4.2+6.0

First, in terms of use, I do get to redline (shiftpoint) at times, whether it's my 6200 or so W12, 6400 or so 4.2, or who knows what on the Hybrid (no tach), but IIRC for 2.0T's it's mid 6's. Having Autobahned it up to redlines (post break in) even in intermediate gears, or here the fast on ramp acceleration or trailer pull or what ever, the upper revs get used at times on anything I drive.

Meanwhile, the basic redline will be a function of the reciprocating masses involved and their speeds. On long stroke (compared to bore) motors, the relevant forces will also be higher, and vice versa for short stroke motors. On diesels specifically, the baseline compressions (put the turbo aside) are much higher--the better part of twice--relative to conventional gasoline. Thus the pistons, cylinder walls, bearings and such all tend to be meaningfully beefier. Could be the conn. rods as well. Well, as soon as you add that mass to reciprocating parts (pistons or rods), the redline is coming down. And on diesels it's always meaningful relative to gas.

Fortunately with both the turbo's and more tranny gear spread allowed with newer trannies like 8 speeds (or even 6), the narrower rev range is still plenty good enough to keep the car on the power curve as it shifts through the large number of gear choices. Having pulled apart really old Audi gas motors that were like tractors and had rev limits only in the mid 5000 range, vs. modern era multivalve gas motors with very cut away pistons (and a relatively light drivetrain too), there is no way a diesel with its compressive forces is going to be built similarly, or it just wouldn't last. Not just picking on diesels either; if you tried to rev a typical passenger car gas motor to F1 (or performance motorcycle) revs, likewise parts would quickly go flying way before their redlines. Again, very different designs with varying reciprocating masses and peak rod and piston speeds and forces.