Quote:
Originally Posted by pete c
Ever look at an old Benz diesel tach? There is no redline.
I read somewhere that the reason Mercedes didn't bother with painting one on there was that their engine simply could not rev high enough to do itself any damage. You could put it in neutral, drop a brick on the accelerator and walk away. Diesel plain and simply can't burn fast enough to get it past a certain rev point. I think it was somewhere between 4-5K revs.
I have seen other diesels that did have redlines. I don't know if these engines actually were capable of spinning that fast or if the manufacturer just decided that all tachs should have a redline.
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I can tell you right now, that's the hugest load of bull. Diesel DOES and WILL ALWAYS burn fast enough to spin engines VERY MUCH faster than 5k RPM.
In fact, the biggest cause of blown engines in the VW IDI family is modified governors and over-revving. You can safely take a 1.6D to 6-7K, but much more, or a quick snap to that speed, and you're gonna grenade it.
The reason that engines have a rev limit has nothing to do with fuel type, and everything to do with component and build type. An oversquare engine will never be able to rev as high safely as an undersquare engine will. R/S ratio, B/S ratio, and several other factors I won't bother explaining here, including airflow and cam specs, as well as dynamic compression ratios (determined by the former) all play parts in redline on an engine.
Diesels are literally kept within certain ranges by the engine manufacturers installing governors in the pumps, or in the case of common rail and most DI style engines, electronic limiters which literally just cut the amount of fuel being injected at certain RPMs.
On my VW 1.6 IDI, I only get about 50% of the available fueling (that the pump can provide) at 2500 RPM, and it's cut by 80% at 4,000 RPM, which is basically the rev limit of the pump (It'll go slightly higher, but not enough to matter). Simply removing the throttle plunger and shimming the fat spring (main fuel cut) increases fueling available (so you have manual control over it, via the pedal), but also removes the rev limiter to some extent (depending on how much you shim it).