Under-revving the engine

[Zayyan]

What does under-revving do to the engine, i.e. what are the results of "straining" the engine and what exactly is strained?

The manual says keep it over 1,500rpm when the engine's under heavy load (e.g. going uphill) but what would be a safe lower limit for 3rd, 4th & 5th gear (it's happy accelerating from 1,000rpm in 1st/2nd so not worried about these) for everyday driving, i.e. flat surfaces with only the driver and no luggage?

I'm wondering this as I get quite a few extra mpg with the speedo on around 32/33mph if I'm in 5th as opposed to 4th

[320Touring]

tbh mate, if its on the flat my m20 is in fifth at 30mph -gives me about 40mpg on the gauge...

'Lugging' is only really a problem if the engine is under strain- in situ's such as climbing a hill or accellerating rapidly

generally I shift at 2.5k when its cold and shift down if the revs go below about 1200

[astondg]

My 323i has trouble going below 1800rpm in 3rd, 4th or 5th. Sometimes I can get it to 1500rpm but even if there is light or no load on the engine it usually starts to jerk and shudder. I think that is might be because of the standalone ECU not being tuned properly though.

You only go to 2500 rpm when cold? I hate to rev it hard when it is cold but if I don't go to at least 3000rpm I don't really get moving. Probably because of the 4.1 diff and not helped by the hill I have to go up right in front of my house, might be better with a 325 or something as well.

Aston

[320Touring]

yeah I shift up at 2500rpm when cold... means you're safely below the 3000rpm threshold for revving it when its cold

as for the car stuttering... my 320 will pull cleanly from 1500rpm in 5th on level ground

defo sounds as if you have a fuelling/ECU problem...have a look at Andy_majic's guide on cleaning all the air/ fuel side eof an m20... may iron out some of the stuttering

You certainly dont seem to be experiencing the same gains in drivablilty and performance as other standalone ECU users on this forum

may be worth getting someone to do an 'on the road' fine tuning!

[mrLEE30]

I believe straining the engine causes simptons similar to pre-ignition or pinking which creates hot spots and can lead to piston failure. but you would have to do it all the time i guess

[320Touring]

thats true-hotspots and borewash are the problems, but as you say, you gotta lug it a LOT to develop those problems

[Ade]

too low shuld only be a problem for good carburation. But if the engine is set up for a relatively high powerband, the gas speed at much lower revs is going to be too slow to get good atomisation of the fuel. a small engine with lots of valve overlap will be wasting its time and effort as fuel may start to be scavenged straight through to the exhaust port or "drop out" of the mixture.

Picking up the revs slightly wont make much difference to fuel consumption but may help fuel efficiency and carburation when under load. Its when your trying to accelerate that the mixture goes increasingly rich and the fuel economy drops.

just my thoughts - feel free to put me right.

[320Touring]

sounds about right squire!

borewash is caused by there being too much petrol in the ores and 'washing' of the oil...

therefore if more throttle is applied and the engine revs are too low, there is too much fuel and not enough air....

[astondg]

Yeah too low rpm is bad that's why they have that red bit on the tachometer, it is were you are supposed to keep the needle. Personally, just to be safe, I don't let it drop below 9000rpm.

Aston

[320Touring]

Yeah too low rpm is bad that's why they have that red bit on the tachometer, it is were you are supposed to keep the needle. Personally, just to be safe, I don't let it drop below 9000rpm.
Aston

[Demlotcrew]

You only go to 2500 rpm when cold? I hate to rev it hard when it is cold but if I don't go to at least 3000rpm I don't really get moving. Probably because of the 4.1 diff and not helped by the hill I have to go up right in front of my house, might be better with a 325 or something as well.

Aston

Where does your 323 red line? i bet its not at 6.5k like on the 325's in this country.

The effects of bore wash will not be from putting the car under load or not revving it high enough.

The problems you will get is premature wear of the bottom end shells.

Andrew

[Karan]

yep load at low rpm shags shells especially if u do it all the time

[Ritchie]

It will cause the crank journals to go oval as well so if you shag the shells your crank wont b too hot either.