what would you say an engines cruising rpm is...

[E21_Ross]

just curious what people think an engine should be able sit at all day long, as maybe a % of it's max rpm (e.g. 70% of it's max rpm - so if it revs to 6500, should it be able to sit at 4500rpm all day) or whatever it can do in 5th gear...it'll sit at in 6th gear etc.

random question i know

cheers

[busta]

Edited because I misunderstood the OP.

Aren't most engines run at max rated power for days on end during development? I'd expect an engine to be able to sit happily at it's maximum power output for quite a while. All depends what the engine is and how long you want it to last I guess. My Peugeot 'cruises' at 5000rpm for 30 minutes straight at least twice a week, and has done for 4 years now with no problems. That's 66% of max rpm.

Edited by busta on Sunday 14th November 14:29

[randomwalk]

I would think it depends on piston speed, therefore depends on stroke and also where max torque occurs.

[E21_Ross]

no idea on piston speed but max torque on my engine is at 4000rpm, and the redline is 6400rpm. engine is, however, 28 years old but has no issues at all. BMW M10 engine is renowned to be tough as old boots. still pulls up to 6200rpm absolutely fine (never taken it above that) and i've had it sit at 4k rpm all day but doing more long trips lately and just curious whether it should be able to sit higher. owners manual states max constant rpm 6000, but that was almost 30 years ago

also interested as i would like to know what my dads car could do. revs to 8400rpm so would think 6k all day would cause it no real issues.

cheers again for any further input chaps

Edited by E21_Ross on Sunday 14th November 14:33

[busta]

I'm not really sure how you'd accurately compensate for age/wear. I intend to keep cruising at 5000rpm until something goes bang. I don't subscribe to the school of thought that says because something is old it shouldn't be used as it was intended to. It'll keep doing it until it can't anymore, then I'll rebuild it so it can again.

[TonyRPH]

I once had a MK II Escort 1600 with 4 speed box and 3:89 diff.

IIRC (I'm going back to 1985) it was about 5000 RPM to get 90 MPH. 6000RPM yielded about 110MPH (flat out!)

I once drove 750 miles non stop (well apart from a lost exhaust, pee break and fuel) at about 95 MPH, which must have been around 5500 RPM.

No ill effects to the engine, but I was deaf by the time I reach my destination lol...

However, the engine was blueprinted and lightened / balanced so that may have helped.

[clarkey328is]

I had a Seat Ibiza 1.4 for a first car and that would happily sit at 5000rpm which was about 9 leptons for an 8 hour round trip once a month or so. I don't think I ever serviced it in its 50,000 miles of me owning it either!

[Welshbeef]

clarkey328is said:

I had a Seat Ibiza 1.4 for a first car and that would happily sit at 5000rpm which was about 9 leptons for an 8 hour round trip once a month or so. I don't think I ever serviced it in its 50,000 miles of me owning it either!

bad luck to whoever bought it no oil change in 50k miles and driven at such a high engine speed that certainly has decreased it's total engine life. O

[BeeRoad]

I imagine most modern cars could sustain constant high revs these days. What I always prefer in a car is a direct top gear, ie max revs in top = max speed, not overdriven. My daily driver Mazda 6 MPS has this which means a 70mph cruise in 6th is 3000rpm.

[itch]

My Celica's manual states something along the lines of: Should last forever at 4,000rpm or less, with limited use up to 8,000rpm.

[Cock Womble 7]

Owners of six-speed Caterhams should look away now.

[busta]

BeeRoad said:

I imagine most modern cars could sustain constant high revs these days. What I always prefer in a car is a direct top gear, ie max revs in top = max speed, not overdriven. My daily driver Mazda 6 MPS has this which means a 70mph cruise in 6th is 3000rpm.

Being able to hit max revs in top is as much about power as it is gear ratios. You can have an overdrive top gear and still hit max rpm in it if you have enough power. Conversely, some cars with direct top gears won't be able to pull it to the redline.

[BeeRoad]

busta said:

BeeRoad said:

I imagine most modern cars could sustain constant high revs these days. What I always prefer in a car is a direct top gear, ie max revs in top = max speed, not overdriven. My daily driver Mazda 6 MPS has this which means a 70mph cruise in 6th is 3000rpm.

Being able to hit max revs in top is as much about power as it is gear ratios. You can have an overdrive top gear and still hit max rpm in it if you have enough power. Conversely, some cars with direct top gears won't be able to pull it to the redline.

But as I said, only a car with a direct top gear will hit max speed at max revs in top. . . . . .

[v8will]

If you take the question away from cars and look at engines which will be maintaining a static speed or close to it for hours on end then the cruising RPM is that which delivers the best fuel efficiency.

Back to cars, There is no reason that I can think of why an engine should not be able to maintain it's maximum rated RPM for hours on end providing the engine has been well maintained. However in the interests of mechanical sympathy I prefer to use a bit of common sense. Basically whatever the particular vehicle/engine feels most comfortable at.

[Ozzie Osmond]

In themselves, revs never hurt an engine (within sensible "red line" limits).

Most engine damage occurs when the engine is "laboured" in too high a gear and fairly low rpm (say uphill at 2,000 rpm in 5th, or overtaking without changing down).

On a modern petrol engine I'd see cruising rpm as 3,000 to 4,000. But I wouldn't expect an extended motorway cruise in third gear to cause any harm at all. Lousy fuel economy though.

[davepoth]

All cars sold by the German manufacturers are subjected to non-stop running at vmax for extended periods to make sure they are safe to do so on the Autobahn. I remember this is why Opel never sold the Omega V8, as it overheated after a couple of hours. I guess all the other companies do the same.

[busta]

BeeRoad said:

busta said:

BeeRoad said:

I imagine most modern cars could sustain constant high revs these days. What I always prefer in a car is a direct top gear, ie max revs in top = max speed, not overdriven. My daily driver Mazda 6 MPS has this which means a 70mph cruise in 6th is 3000rpm.

Being able to hit max revs in top is as much about power as it is gear ratios. You can have an overdrive top gear and still hit max rpm in it if you have enough power. Conversely, some cars with direct top gears won't be able to pull it to the redline.

But as I said, only a car with a direct top gear will hit max speed at max revs in top. . . . . .

There's no 'only' about it. If it has enough power, yes. And so will a car with an overdrive, if it has enough power.

Edited by busta on Sunday 14th November 16:03

[E21_Ross]

thing is, where my car has not a great deal in the way of sound proofing, you can hear the engine buzzing away quite a lot at 4k rpm. i think....hmmm surely this can't be doing it much good. take it up to 5500+ rpm and it's much louder, so coming back down to 4k rpm it's like, hmmm not too bad after all

interesting what you say about low rpm in tall gear. i regularly shift very early when the engine is cold, rarely taking it above 2500rpm for a good 5-10mins of driving. this is probably a little too OTT. on a cold engine i'm guessing 50% revs shouldn't cause it too much harm?

i did about 80 miles yesterday with about 25-30 miles sat at 9 leptons (4000rpm is 8.5 leptons ) and it seemed absolutely fine. interesting thing is, it's over 28 years old and between oil changes (i do them every 3000 miles.... though owners book says every 5000) it doesn't even use 50ml or so of oil. never needs topping up which must be a good thing!

thing is, most of my long-ish trips are on quiet motorways (at night) and sat at even 8 leptons feels positively slow. i think between 4000-4500rpm should be fine....

thanks guys

[E21_Ross]

busta said:

BeeRoad said:

busta said:

BeeRoad said:

I imagine most modern cars could sustain constant high revs these days. What I always prefer in a car is a direct top gear, ie max revs in top = max speed, not overdriven. My daily driver Mazda 6 MPS has this which means a 70mph cruise in 6th is 3000rpm.

Being able to hit max revs in top is as much about power as it is gear ratios. You can have an overdrive top gear and still hit max rpm in it if you have enough power. Conversely, some cars with direct top gears won't be able to pull it to the redline.

But as I said, only a car with a direct top gear will hit max speed at max revs in top. . . . . .

If it has enough power, yes. And so will a car with an overdrive, if it has enough power.

an over drive gear is only to do with the diff and gearbox gear ratios. i think an overdrive gear is one where it's ratio is less than 1. but as said, it may be less than one, so technically an over drive gear, yet a car could in theory still reach the max rpm possible for that gear.

on most cars though, it's final gear (which is almost always an over drive gear these days) it will not reach max rpm, it will often only be able to do that in 5th for example.

[Ozzie Osmond]

E21_Ross said:

interesting what you say about low rpm in tall gear. i regularly shift very early when the engine is cold, rarely taking it above 2500rpm for a good 5-10mins of driving. this is probably a little too OTT. on a cold engine i'm guessing 50% revs shouldn't cause it too much harm?

Automatic gearboxes are programmed to "look after" the engine by not letting it labour. In other words, they change down to protect the engine and only change up when the engine is ready.

From a cold start an auto transmission will typically run to a HIGHER rev count before upshifting than it doeas with a warm engine. In other words, the approach you have outlined is exactly the opposite of what manufactureres are building into auto cars. There may be a message there somewhere.

My own approach in a manual car with a cold engine is just to drive normally but avoid using more than about 2/3 max' rpm or wide throttle openings until it's warmed up.