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Max engine RPM when descending on engine braking

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14 Jun 2018 17:08 #193455 by Bosanek
I am interested to know what should be the maximum safe engine RPM (for M13A VVT engine) when descending down a very steep descent on engine braking?

In other words, at which RPM should the driver begin assisting the engine with brakes?
And is there a risk of damaging the engine if the driver does not assist and the downhill gradient is too great for the engine to cope with?


I tested my M13A VVT Jimny down a very steep descent in 1st low gear. The engine RPMs went up to 4000 (speed to about 10 km/h max) and it did not go above that during the descent.
Did anyone manage to get it over 4000 RPM on engine braking alone and is pushing it above 4000 RPM on engine braking even a sensible practice at all?

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14 Jun 2018 18:00 #193457 by Riccy
From memory I would say 4000 rpm is about normal for very steep, after that grip gives and gravity kicks in so down you go!

the VVT motor likes to rev, so i wouldnt worry in the slightest about 4000 rpm :-)

J999 MNY, M16 VVT, R7me gearbox & Rocklobster,
31x10.5r15 Cooper STT, 2x ARB Locker diffs & twin compressor,
6.5" total lift, Zookbob HD steering shock, Ignis recaro's
Custom front bar & arches, rocksliders & guards,
Snorkel, K&N, 13500lb Winchmax, Buzzweld, etc, etc

Plus ULYSSES (06' basher)

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14 Jun 2018 18:48 #193461 by Lambert
At 4000 you still have a couple of thousand rpm to accelerate into it and regain control if it starts to slide.

It's not a Jimny. It's my Jimny

Mooo said Ermintrude (black)
Boing said Zebedee (blue)

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15 Jun 2018 12:10 - 15 Jun 2018 12:10 #193488 by Bosanek
I am a bit confused with the operation of the petrol engine on downhill braking, so that is why I am asking.

If the descent is sufficiently steep (say super steep), will the M13A engine rev over 4000 RPM (and even over 5000 RPM and even over 6000 RPM) if not assisted with foot brakes?

And is it technically sensible to let it rev that high on engine braking? With my diesel, I would never let it rev above 3000 RPM due to fear of "blowing it" as even on 3000 RPM the engine roar gets quite "intense".
Last edit: 15 Jun 2018 12:10 by Bosanek.

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15 Jun 2018 19:01 #193500 by Riccy
Its perhaps more a question of duration of the high revs.

In my experience of low box descents they tend to only be extremely steep for a brief period. I tend to chicken out and press on the brake, or at least hover my foot over it in case it starts going.

If im sliding off a rock ledge then i would be in 1st low and pressing the brakes to minimise the inevitable slide.

Practically you would be unlikley to rely just on the engine for braking when on extreme descents.

The only thing i would wonder about would be engine oil circulation being adequate during a longer descent, but as most would be short it shouldnt be a problem.

J999 MNY, M16 VVT, R7me gearbox & Rocklobster,
31x10.5r15 Cooper STT, 2x ARB Locker diffs & twin compressor,
6.5" total lift, Zookbob HD steering shock, Ignis recaro's
Custom front bar & arches, rocksliders & guards,
Snorkel, K&N, 13500lb Winchmax, Buzzweld, etc, etc

Plus ULYSSES (06' basher)

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16 Jun 2018 04:12 - 16 Jun 2018 04:14 #193509 by Lambert
One bit of advice I was given more for tarmac than dirt was to use the same gear to go down a hill as i would to get up it. On road this'll preserve the effectiveness of the brakes for longer giving you the ability to stop if necessary in a hurry. We have some fairly serious 1in4 and 1in3 roads round us and one in particular is very long and finishes in a set of exceptionally tight tuns through close spaced houses. Coming down that on the brakes would and has caused quite a few inexperienced drivers to get it very wrong. However a combination of 4k rpm and short pluses to the brakes to assist the engine braking and it's easy to stop at the bottom or if something is coming up.

It's not a Jimny. It's my Jimny

Mooo said Ermintrude (black)
Boing said Zebedee (blue)
Last edit: 16 Jun 2018 04:14 by Lambert.

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