Why do diesels have lower rev limiter?
Discussion
ZeeTacoe said:
Really? Looking at a dyno-chart for one the 335d seems to make peak torque at 1500rpm then hits peak power at 4200rpm after that it quickly tails off. 4200-1500 gives you a mere 2700rpm of usefulness.
Peak power at 4400rpm so it's 2900 revs of usefulness. I'd say that compares with most petrols. People forget that many petrols are useless under 4-5k rpm.
Vladimir said:
Just thinking about this on my walk around the countryside (as you do).
Now a four pot single turbo diesel (like our old Astra 1.9CDTi 150) can be hustled along fairly quickly BUT the useful power band is 2500rpm to about 4000rpm - 1500 of useful revs.
But then look at a decent bi-turbo six pot like our current 335d - useful rev range is 1500 (when turbo one kicks in) to just under the red line - 5000rpm. That's 3500 useful revs - more than twice the four pot.
However most judge all diesels on mainstream four pots and that's that - fair enough the view of them is fairly dim.
Now let's compare to a mad bad purists high revving car - maybe a CTR, maybe a more exotic 911 Turbo - useful rev range for a CTR? I'd say 6000-9000. And the 911 Turbo - no point revving it about 5500rpm and the fun starts at 3000ish - so 2500rpm of fun.
then you remember that a car has things called 'gears', the actual engine 'numbers' don't directly relateNow a four pot single turbo diesel (like our old Astra 1.9CDTi 150) can be hustled along fairly quickly BUT the useful power band is 2500rpm to about 4000rpm - 1500 of useful revs.
But then look at a decent bi-turbo six pot like our current 335d - useful rev range is 1500 (when turbo one kicks in) to just under the red line - 5000rpm. That's 3500 useful revs - more than twice the four pot.
However most judge all diesels on mainstream four pots and that's that - fair enough the view of them is fairly dim.
Now let's compare to a mad bad purists high revving car - maybe a CTR, maybe a more exotic 911 Turbo - useful rev range for a CTR? I'd say 6000-9000. And the 911 Turbo - no point revving it about 5500rpm and the fun starts at 3000ish - so 2500rpm of fun.
an engine that revs to 10,000 will be geared differently to one that revs to 5000, obviously
Thanks for going off on a tangent then claiming to be clever.
I'm addressing a major gripe of diesels - very small useable rev range; which is evident in most mainstream four pot diesels, not decent six (and above) pot soot chuckers.
Anyway it was a mere thought that was all. Keyboard warriors like Marf don't really add very much...
I'm addressing a major gripe of diesels - very small useable rev range; which is evident in most mainstream four pot diesels, not decent six (and above) pot soot chuckers.
Anyway it was a mere thought that was all. Keyboard warriors like Marf don't really add very much...
Edited by Vladimir on Tuesday 17th May 09:42
ajsphead said:
Or there's a bike engine. A litre V twin will pull hard from about 2500 to 10000, a litre 4 cyl from about 3500 to 11500 which makes 8000 revs of full on power. Oh dear Mr policeman, sorry I broke the national speed limit, but I'm still in 1st gear.
But sadly bu99er all torque. Bike engined cars are notoriously hard work to drive and look after - track only really. Plus you can't stick kids in the back 
quite liked this video from that youtube poster linked earlier in the thread...
http://www.youtube.com/watch?v=mafOvbSfLck
Any ideas of the RPM that 6.4 diesel V8 is reaching?
http://www.youtube.com/watch?v=mafOvbSfLck
Any ideas of the RPM that 6.4 diesel V8 is reaching?
there's no being clever about it, the actual revs are irrelevant
how quickly it picks up those revs, and the torque and therefore power produced are important
I once had a v4 400 Honda bike that revved to 13000, what power there was started at around 8000, so a 5000 spread of revs
at the same time I drove a truck with less than 1500 rev useful range
what does it all mean?
how quickly it picks up those revs, and the torque and therefore power produced are important
I once had a v4 400 Honda bike that revved to 13000, what power there was started at around 8000, so a 5000 spread of revs
at the same time I drove a truck with less than 1500 rev useful range
what does it all mean?
But torque is pretty irrelevant if you don't weigh much. A litre bike engine produces similar or higher levels of peak torque to a litre car engine, about 75-80 lb/ft compared to 65-70 for your average 1 litre car engine. So no, b****r all torque is wrong. It's just a proportionate measure of horsepower (180+) anyway. You can also stick 1 child on the back.
I think diesels work very well on the road for everyday driving. Easy to make swift progress and cheap on fuel. You'd never want one for your weekend fun though, which I think everyone would agree with.
Low reving torque for transport, high reving power for fun. The 2 engines will obviously overlap in the middle somewhere and then you have to make a tough choice.
Low reving torque for transport, high reving power for fun. The 2 engines will obviously overlap in the middle somewhere and then you have to make a tough choice.
Vladimir said:
Anyway it was a mere thought that was all. Keyboard warriors like Marf don't really add very much...
Not being a keyboard warrior, I'd happily say exactly the same thing to you in real life, despite not being powerfully built I am just firmly disagreeing with your assertion that many petrol engines are useless under 4-5k.
Hell, even VTEC engines(which I would assume you feel fit into your "useless below 4-5k rpm" group) produce 80% of their torque from around 2000rpm.
Edited by Marf on Tuesday 17th May 10:25
Vladimir said:
Now let's compare to a mad bad purists high revving car - maybe a CTR, maybe a more exotic 911 Turbo - useful rev range for a CTR? I'd say 6000-9000. And the 911 Turbo - no point revving it about 5500rpm and the fun starts at 3000ish - so 2500rpm of fun.
Tosh.My Porsche 930 pulled hard at 3000rpm all the way around to just under 7500, which didn't take long at all - nearly 3500 of range.
Then there is the BMW 4.4 V8s, from 2000 to 6500 no trouble - 4500 rpm of power
My 19.tdi diesel pulls at 2000 on to 3250, 1250 rpm of range.
I think the reason petrols can rev is because the ignition can be advanced, advancing the fuelling on a diesel has far less effect because it's compression ignition which has to occur near max compression. Pull the advance out of an old car with a distributor and you'll see - it drives just like a non turbo diesel.
jamie g said:
As someone posted earlier, it is the diesels relatively slow burn that ultimately restricts the rev range.
I thought the actual burn/detonation was faster than a petrol burn.It's where the 'knocking' sound of a diesel comes from, and why they are built heavier to withstand it.
Globs said:
jamie g said:
As someone posted earlier, it is the diesels relatively slow burn that ultimately restricts the rev range.
I thought the actual burn/detonation was faster than a petrol burn.It's where the 'knocking' sound of a diesel comes from, and why they are built heavier to withstand it.
I used to drive a ford escort van sometimes for work,one day it started going like the clappers, I was really impressed for about 5 secs,then I had to try and stop ! brakes were not doing too much dipped the clutch revs went sky high came to a stop key to the off position hand brake hard on foot hard on the brake into 4th smartly off the clutch stalled with a lurch.Got out a bit shaken smoke billowing everywhere,police car from the opposite direction pulled up all blues an twos "its all right mate fire brigades on its way" !
The following day on strip down found the seal on the pump had failed the sump had filled with diesel when the level rose above the piston rings fuelled itself .When the exhaust system was pulled off almost a gallon of fluid was poured out.!!
The following day on strip down found the seal on the pump had failed the sump had filled with diesel when the level rose above the piston rings fuelled itself .When the exhaust system was pulled off almost a gallon of fluid was poured out.!!
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