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jenevers

Rev up or cool down?

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What’s behind the theory of revving up your engine just before switching off. Is it not better to idle for a couple of minutes to let the engine cool down a bit?

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It's just something people do for no good reason. I have heard it's so that "all the pistons are at the top ready for the next start"😆 Very bad for turbos. The Bristol engines fitted to WW2 bombers were run down gently by ground crew after landing to cool them because if they were shut down right away they were damaged by heat soak.

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26 minutes ago, jenevers said:

What’s behind the theory of revving up your engine just before switching off. Is it not better to idle for a couple of minutes to let the engine cool down a bit?

I am with Sir N

 

NEVER ever rev a turbo engine up before shutting down and tray to drive coats the last few yards at low revs, in fact some manuals tell you how long to let it idle for.

 

I don't see it makes any difference at all for a non turbo engine apart from wasting a little fuel.

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7 minutes ago, Sir Nibble said:

 I have heard it's so that "all the pistons are at the top ready for the next start"😆

could be true if it was a Bolinder! 🤣

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1 minute ago, alan_fincher said:

could be true if it was a Bolinder! 🤣

 

Or a two cylinder Kingfisher

 

Richard

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I think some just like to finish on a flourish, there is no mechanical advantage. 

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I always idle for a few minutes before shutdown. But there must have been some theory for the revving up in days gone by🤔

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4 minutes ago, jenevers said:

I always idle for a few minutes before shutdown. But there must have been some theory for the revving up in days gone by🤔

 

I suspect it was to do with 'cleaning the plugs' on high performance petrol engines. A final burst to get the electrode hot and burn off any oil from idling making the next start easier

 

Doesn't make any sense on a diesel

 

Richard

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Revving an engine just prior to switching off is a throwback to the "old" days of motoring.  The theory being a quick blip of the throttle ensured the float chamber was as full of petrol as possible, thereby ensuring better restarting.

It doesn't apply to modern engines and especially not turbocharged engines.  The last thing needed is to set the turbo spinning just prior to starving its bearings of oil.

Hope this helps.....

Edited by NB Esk
Autocorrect
  • Greenie 3

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17 minutes ago, BWM said:

I think some just like to finish on a flourish.. 

I blame Chuck Berry, who "gunned the motor twice" for that very reason in 'No Money Down'. Not sure he's the best role model, mind.

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2 minutes ago, Athy said:

I blame Chuck Berry, who "gunned the motor twice" for that very reason in 'No Money Down'. Not sure he's the best role model, mind.

I don't think I would take motoring advice from him, his work ethic was well worth following

 

Richard

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It is not good practice to shut an engine down immediately after a period of 'loaded running', the last thing an engine needs is a good revving prior to the immediate shutting off of the cooling system.

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When switching off there is a short term climb in cylinder head temperature as the coolant circulation stops. Proof of this is sometimes after you've switched off and walking away the cooling fan switches on to check it.

  • Greenie 1

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Turbo diesels in hot climates are often fitted with a turbo timer that shuts the engine  down after you have wandered off and locked up. (To protect the turbo bearings. ) At 40 degree ambient the engine can run for some time.

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42 minutes ago, bizzard said:

When switching off there is a short term climb in cylinder head temperature as the coolant circulation stops. Proof of this is sometimes after you've switched off and walking away the cooling fan switches on to check it.

Or boil over

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45 minutes ago, bizzard said:

When switching off there is a short term climb in cylinder head temperature as the coolant circulation stops. Proof of this is sometimes after you've switched off and walking away the cooling fan switches on to check it.

and some have small electric cooling pumps to keep the coolant circulating.

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After World War Twice my dad left The RAF and drove buses for a while.

Western National instructed their drivers to .....

- Leave their engines on tick over when stopping for breaks or at terminus turn rounds as it used less fuel than starting them up again and hammered the batteries less.

- Rev the engine before shut down to blow out all the clag and soot from around the exhaust valves.  To avoid sticking valves and poor compression on start up.

 

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As already said by others, no point in revving any engine or letting the engine "cool down" prior to shutdown for normally aspirated engnes.

 

When commissioning turbocharged diesel engines,  it was usual to run them at 100% load for 11 even hours, followed by the 12th hour at 110% load tp prove they met the specifcation.

 

At the end of this test the turbochargers would be glowing red hot, and typically at 1200-1400°C. It was necessary to let the engine idle for 3-5 minutes until the temperature had dropped and stabilised. Failure to do this damaged the turbos as the hot turbo carbonised the oil left in it, which soon wrecked the turbo bearings.

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On 19/03/2020 at 18:55, cuthound said:

As already said by others, no point in revving any engine or letting the engine "cool down" prior to shutdown for normally aspirated engnes

Who said no point cooling down?

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My Dad always blipped the throttle prior to turning off the igntion.....not sure why, he thought it 'primed' the cylinders prior to starting (1930's thinking). I always idle the engine I have as it has a turbo.

The boat diesel just get the fuel shut lever pulled.

 

Interesting about the Bristol engines - radial?, the other thing to remember before staring is to drain the bottom cylinder to avoid oil compression problems, my mate forgot this and caused some expensive problems on a Pembroke plane he had.

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The Bristol Pegasus aero engine was the first aero engine to develop 1hp per lb in weight, which was governed and met by the petrol companies increase in petrol octane rating.  For a while in the 1930's a monoplane with a Pegusus engine held the alitude record of 56,000 ft or so so, around twice the height of Everest.

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25 minutes ago, LEO said:

My Dad always blipped the throttle prior to turning off the igntion.....not sure why, he thought it 'primed' the cylinders prior to starting (1930's thinking). I always idle the engine I have as it has a turbo.

The boat diesel just get the fuel shut lever pulled.

 

A lot of people used to do this without really knowing why.  The last time I came across it was 20 yrs ago when I recovered a Rolls Royce that had been driven through the back of a garage.  The old fella must have got mixed up with his routine, which should have been something like enter garage, knock it in neutral, rev up, switch off and brake.  Anyway, whatever went wrong the front end of the roller was stuck through the end of the garage.  Funny thing was there'd been some shelves up above the window with tins of paint on, a couple of which had burst open and given the Spirit of Ecstasy a new look. 

Got some really odd looks as I drove round the Huddersfield ring road with that on the truck.

 

 

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