Wont rev above 3000rpm in park

[losh1971]

I was told this from a bloke who is doing his mechanical engineering degree. He also has a lot of knowledge on engines and transmissions and rebuilds both.

 

[Brettly-2008]

I was told this from a bloke who is doing his mechanical engineering degree. He also has a lot of knowledge on engines and transmissions and rebuilds both.

Fair enough, I would love to know the theory on this as it seems to fly in the face of logic. Obviously over-revving is dangerous for bearings (with or without load) but I cant see how a free-spinning engine, properly cooled, with adequate oil pressure and within its rev limit, would be at any more risk than the same engine under load. If anything, logic says the opposite would be true.

Not having a go BTW, I just love thinking about this stuff

 

[losh1971]

Yeah I'm not sure on the logic either. I mentioned it to the fella that i did it and he said it wasn't a good idea. Never done it since.

 

[panhead]

Yeah I'm not sure on the logic either. I mentioned it to the fella that i did it and he said it wasn't a good idea. Never done it since.

There are difference combustion chamber pressures on an unloaded engine and it can cause heat to come on too quickly making the rings expand and ultimately causing unnecessary/premature wear or so the story goes.

I had a cop put his foot inside my HX Ute back when I was 18 and pushed the accelerator to the floor and the engine dropped a lifter.

That turned into a fun conversation from that moment on.



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[Big Red VF-SII Go-kart]

In some old GM 3.6L engines from around 1992 onward, a fast 3,000rpm+plus idle for between 20 and 30 seconds was the way to reset the rudimentary engine computer. This was certainly so for my redoubtable first V6, a Toyota Lexcen CSi wagon (VP Commodore-cross). Neither Alloytec nor SIDI engines should be idled at speed because they are nothing like the engines from decades ago, when you could thrash them to gay abandon (I am guilty of that, way-back-when!). I wouldn't want to invite the spectre of involved repairs today caused by foolish playing around at idle or trying to get donuts...

 

[EYY]

Curious where you've been led to believe this.

Rods and rod bolts can stretch further with no load, and bearings can rattle/vibrate and permanently damage/deform them. Lack of load can also cause ring flutter due to reduced combustion pressure.

It's just a bad idea if you don't want to be replacing engines.

 

[EYY]

And then theres also valve float and overrev

 

[Brettly-2008]

Rods and rod bolts can stretch further with no load, and bearings can rattle/vibrate and permanently damage/deform them. Lack of load can also cause ring flutter due to reduced combustion pressure.

It's just a bad idea if you don't want to be replacing engines.

Interesting.

So, an example. Assuming same cooling, oil pressure etc; a stock engine sitting at 5000rpm circling a track/on a dyno/doing a burnout for an hour, is at less risk of damage than the same engine sitting at 5000rpm in neutral?

 

[panhead]

3000 revs for an engine in a stationary vehicle is a lot of noise so why would you want to rev it over that?

Since my daddy was a boy the revving of engines without a load was considered a no, no.

Racer's take great care not to do so and limiters are set on their high strung engines for a reason.

Whatever all the theories are and there are many if you'd like to do a google search, I err on the side of caution simply because it may be cheaper to do so.

Over the years I've seen all manner of parts let go in an engine under revs unload load and not, so why do it when you don't have to.




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[Brettly-2008]

3000 revs for an engine in a stationary vehicle is a lot of noise so why would you want to rev it over that?

Since my daddy was a boy the revving of engines without a load was considered a no, no.

Over the years I've seen all manner of parts let go in an engine under revs unload load and not, so why do it when you don't have to.

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Well, getting to the bottom of why in a logical, scientific way rather than theories and hand-me-down advice is why I kept probing.

Thanks EYY for the reply above.