Don't UNDERrev your E34 M5! - The Unofficial BMW M5 Messageboard (m5board.com)

Sebring NL · 18th June 2008, 17:07

Gentlemen drivers,

Having been in the passenger seat of several E34 M5's this year and a lot more in previous years, I noticed that many drivers have the tendency to drive the car under revs.

At the Old Goat meet, I rode shotgun with many people, but I noticed quite a few shifting to third gear at very low speeds and then ramping up speed. In itself, its not a problem to keep the car at a certain speed in a high gear, but the problem starts when accelerating.

Same as riding on a geared bicycle, the force that the 'engine' has to put up when the gearing is too high is an extremely high and unnatural one. The momentum needed to spin the wheels is so high that the stress on the internals and the slowing down of the engine through the gearbox is a lot worse than , say , overrevving the car when cold. Try riding a bike up a hill in highest gear, your feet won't be able to get the paddles down although they are hardly moving.

Because of the low revs, people think it can do no harm. Well, the oil pressure on most street-engines is 1:1 related to the engine revolutions. The S38 pump is directly chain-driven by the crank, which means, low revs = low oil pressure. The car has a considerable amount of oil ( 6 litres) so its quite important that , with cold engine and thick oil, the flow is high enough. It may not be needed for cooling, but for lubrication and getting the microscopical metal and alloy parts into the oil filter ASAP.

Its OK to keep the car between 2500 and 3000 revs at any engine temp. Remember, it goes all the way up to 7250 so you'll leave 4250 revs unused. Take a look at BMW's later rev counters where cautions blocks light up at certain revs, IIRC none of them is lower as 4500 rpm in E39 M5, X5 4.8, CSL etc.

To be perfectly clear here, its ofcourse ok to do very low revs in first and second and accelerate, because the gearing is such that the momentum is enough to move the car out of those revs fast. Also, cruising at 2k revs in 3rd or 4th is no problem, just shift down if you want to accelerate.

Wout

m5 deamon · 18th June 2008, 17:26

very valid points made thanks for the write up

Blutarsky · 18th June 2008, 18:04

great info, thx Wout

davidoli · 18th June 2008, 19:14

Wout .......you are a Legend !

Where did this inspirational piece of writing come from ?

Lantz · 18th June 2008, 19:39

Quote:

Originally Posted by davidoli

Wout .......you are a Legend !

Where did this inspirational piece of writing come from ?

Its true, a real fountain of wisdom and always generous to share ( rather undutch like I may add

)

Lantz

Alan Archer · 18th June 2008, 20:09

Old BMW owners manuals always stated a minimum engine speed
of 1500rpm in each gear.
People need to take time & effort to understand the gearing / torque characteristics of their cars....

I must have won 50% of all confrontations between my M5 and other cars
by using the right gear & revs at the right time.

The result of course is two tail pipes, a tricolour badge and a roundle.

Good post Wout

davidoli · 19th June 2008, 00:02

Quote:

Originally Posted by Alan Archer

I must have won 50% of all confrontations between my M5 and other cars
by using the right gear & revs at the right time.

100% agree on the importance of this Alan !

vadas1 · 19th June 2008, 00:30

Quote:

Originally Posted by Alan Archer

Old BMW owners manuals always stated a minimum engine speed
of 1500rpm in each gear.

Good post Wout

I recall the handbook on my 325i recommending that the engine revs should be between 2000 and 3000 as often as possible to preserve the integrity of the engine and drivetrain. I have to admit that in town in slow traffic I don't in the least bit mind leaving it in second for prolonged periods and shifting up to third only when the traffic smooths out. Mind you, on the way in the lack of cars on the road means I get to do the same trip in fourth and fifth. Makes getting up early worth it.

ralph (vadas1)