Hi,
As question above. Trying to settle a pub conversation which was that high revs damage engines.
would driving at a constant 90-95mph cause any damage to the car/engine, on a mg 2.0 turbo diesel.
Car is sitting at around 3800-4000 revs at those speeds.
Would engine wear increase due to those speeds and would it affect engine life?
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yes
like any engine you need to change the revs every now and then or you get resonic somethings building up which can cause a rod etc to fail
im sure NC could explain it better
...........................................................calling nc'''''---------
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Driving the above-mentioned MG for many hours at 4,000 rpm would do less damage to the engine than lots of cold starts around town, eg driving to Tescos and back.
OK, if you could over-ride the rev limiter and drive the car at, who knows, 8,000 rpm for hours on end then it might ruin the engine.
Basically, an engine running at its normal temperature at sensible revs suffers very little damage. An engine stopping and starting and never reaching its operating temperature, running on an over-rich fuel mixture, will be damaged relatively quickly.
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It's quite good for an engine to be run at varying speeds, as it tends to even out wear patterns.
BB is right about resonance build up, and if an engine is designed to run at a fixed speed, perhaps in a generator application where the engine speed is directly linked to the supply frequency, then, great care is taken to ensure that any damaging resonances are avoided, tuned out, or damped. For automotive use, typically only the most damaging resonances are dealt with and the assumption that the engine speed will be varying effectively covers the lesser damaging vibrations.
In general terms, running an engine at these kinds of speeds is in itself not particularly damaging - the engine will reach a good thermal balance, where if the development testing has been done well, the oil will not be prone to breakdown, and deposits will not tend to accumulate on the engine's surfaces. The lubricating oil films tend to become thicker, in terms of the seperation distance between moving parts, the faster the engine turns.
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Car is sitting at around 3800-4000 revs
That seems low-geared for a diesel. My 1.6 is only just over 2000 at 60mph, so I would have expected yours to be a shade over 3000 at the speeds you mention - or are you being shy?
3000rpm is pretty comfortable for any kind of engine IMHO, unless it overheats.
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3800-4000 rpms isn't what I'd consider high revs.
My bike has no ooomph below about 4000rpm so runs noticeably faster than that and has over 90k miles on it without and problems so far.
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As dc says, continuous highish speed running will do an engine less harm than most things that happen to it, provided of course it is in correct fettle with fairly clean oil and has been warmed up gently first. However if it never runs at any other speed it will develop what NC identified as 'wear patterns'. Best example I can think of of one of those is a 'step' worn in the cylinder walls at the top of the piston stroke (or the top of the area swept by the top piston ring). Revving such an engine beyond its accustomed speed may cause the top ring to foul the step and break it.
That is why when rebuilding engines one used to be supplied with piston rings one of which had one of its outside edges 'reverse-stepped'. That was the ring you put at the top.
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Best example I can think of of one of those is a 'step' worn in the cylinder walls at the top of the piston stroke (or the top of the area swept by the top piston ring). Revving such an engine beyond its accustomed speed may cause the top ring to foul the step and break it.
Not that I'm disagreeing - but I'd like to know how revving any engine (at constant speed or very high speed) can alter the length of the stroke in such a way? Or are we talking extremely small amounts due to 'bend' or give in crankshafts or gudgeon pins etc?
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Yes, very slight increase in stroke length at high rpm resulting from the distortions you mention and slight elastic stretching of the conrods, even the breakdown of oil film in the conrod bearings which leads of course to damage... a piston 'weighs' a surprising amount when it is going up and down 100 times a second.
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Right thanks. On another (maybe) related point - when running-in (especially in the olden days) it was often mentioned that 'flat-spots' could develop if the running-in period involved alot of constant rpm work. How might that happen?
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Flat spot is usually used to describe a performance problem or a tyre that has been locked at high speed. But (as NC says above) varying the speed at which an engine is driven 'helps to even out wear patterns', for example by not forming the 'steps' I mentioned.
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Newguy,
The high revs you describe would definitely damage your engine once it has been transposed into the boot area of your "MG" after you've hit a bridge support because you lost control during an unavoidable "accident".
The maximum road speed permitted in the UK is 70 mph. Everyone knows this.
Edited by Jon P on 19/05/2009 at 21:53
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Without going into the technicalities ('cause i don't know !), one of the reasons that the Le Mans 24hr race was such a car breaker (pre-chicanes) was the amount of time cars spent flat out down the Mulsanne straight. Same comment is still sometimes made in Formula 1 commentaries.
The Nascar and Indy boys seem to have got it sorted though !
maybe the answer is .. dump the MG and buy a Chevvy ? :-)
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>>Car is sitting at around 3800-4000 revs at those speeds.
Max revs is about 4500rpm on the L series diesel and top speed is 115-120ish I think in the top state of tune - are you sure its revving that hight at 90-95?
I would have expected somewhere around 3K revs would be more like it - unless its had a gearbox transplant at some point with different ratios!
Depending on useage and length of time at high revs I would think that it will do the engine more good than short, stop/start driving. Take a marine engine that will chug along for many hours at a time at 70-80% of max output and sometimes a lot more with no ill effects - I remember when we had a boat that Perkins-Sabre recommended running at high revs to keep the turbo's healthy!!
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