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  #1  
Old 06-14-2006, 01:31 AM
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RevTalk and Radiators...

I keep getting car questions on and off but can never reemember them when I want to ask. I'll keep it at these and see if I can think of any later.

1) I've heard that less revs means better quality engine. For example if a Corolla is doing (cruising at) 100 km/h, it would do so at probably 2000 revs but a Lamborghini would do so it at 1000 revs or less. Based on this, I'm very confused as often members here and the Top Gear crew prefer "more revs" as something of a driving experience and I dont' get it at all. Can anyone explain this? Why would anyone want "more revs" if it means the engine is more stressful?

2) I've seen many-a-time water come out of a new bus radiator whenever it stops. Is it a manufacturing fault or is the radiator designed to let stagnant water out, or does it just stop working when the bus brakes? It's fairly common on the public transport busses here.


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Old 06-14-2006, 01:38 AM
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Quote:
Originally Posted by spi-ti-tout
2) I've seen many-a-time water come out of a new bus radiator whenever it stops. Is it a manufacturing fault or is the radiator designed to let stagnant water out, or does it just stop working when the bus brakes? It's fairly common on the public transport busses here.


Cheers.
- spi
are you sure this isn't condensation from the aircon?
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Old 06-14-2006, 02:12 AM
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Quote:
Originally Posted by spi-ti-tout
I've heard that less revs means better quality engine. For example if a Corolla is doing (cruising at) 100 km/h, it would do so at probably 2000 revs but a Lamborghini would do so it at 1000 revs or less. Based on this, I'm very confused as often members here and the Top Gear crew prefer "more revs" as something of a driving experience and I dont' get it at all. Can anyone explain this? Why would anyone want "more revs" if it means the engine is more stressful?
What you have there is gearing, not the engine.

A Corolla that has a maximum speed of, say, 120mph will be geared so that it will presumably be running close to max revs in top gear at 120mph; 8000rpm for the sake of argument.
At 60mph (just under 100km/h) in the same gear the engine would be at 4000rpm.

A Lamborghini is geared to do over 200mph. If the Lambo were pulling 8000rpm at 210mph in 6th gear, in the same gear at 60mph the engine would only be running at 2285rpm.

Gearing is largely linked to the power and torque outputs of the engine.

If you need, say, 100bhp to overcome the friction and drag forces encountered at 100kmh, then the engine in a 150bhp Corolla-esque car will be reving pretty hard. A Lambo will produce 100bhp at much lower revs, which is what enables it to have taller gearing than the Corolla.
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Old 06-14-2006, 03:17 AM
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Quote:
Originally Posted by spi-ti-tout
1) I've heard that less revs means better quality engine.
Thus proving the point Diesel engines are better
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Old 06-14-2006, 06:10 AM
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Quote:
Originally Posted by spi-ti-tout
Based on this, I'm very confused as often members here and the Top Gear crew prefer "more revs" as something of a driving experience and I dont' get it at all. Can anyone explain this? Why would anyone want "more revs" if it means the engine is more stressful?
So when you want to go faster, you don't need to downshift, if you're out for a fast drive you want to keep revs up in the powerband
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Old 06-14-2006, 04:14 PM
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Thus proving the point Diesel engines are better
The same applys to large V8s in American cars, does that mean they are just as good?
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Old 06-14-2006, 07:17 PM
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Quote:
Originally Posted by spi-ti-tout
1) I've heard that less revs means better quality engine.
I'd say a good quality engine is one that lasts and has a good power delivery (both in quantity and quality).

I like my engines to be revy. I get bored with engines that have a linear output (you never find a sweet spot). I also dislike those that run out of breath short of the redline.

By my standards an engine that lasts the same than another and revs higher is a better engine.

About the water in the buses... you are in the wrong forum!
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Old 06-15-2006, 03:32 AM
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Quote:
Originally Posted by clutch-monkey
are you sure this isn't condensation from the aircon?
I have no idea tbh, but the water doesn't just drop, it coems out in a flow from the front and (sometimes) from the back.

Quote:
Originally Posted by Coventry
All that he said.
So, the rev output has absolutely no linking with the quality of engine.

Based on what you've said then, I'd like to ask this question. A Porsche Boxster can do, as a maximum, 5500 rpm max (according to James May). Why are the revs so short and does this mean top speed is affected? Or are shorter revs more "brutal" and "aggressive" than higher ones?

Quote:
Originally Posted by magracer
About the water in the buses... you are in the wrong forum!
What?! I don't think I'll find anything in the Aussie forums
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Old 06-15-2006, 04:48 AM
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Quote:
Originally Posted by spi-ti-tout
Based on what you've said then, I'd like to ask this question. A Porsche Boxster can do, as a maximum, 5500 rpm max (according to James May). Why are the revs so short and does this mean top speed is affected? Or are shorter revs more "brutal" and "aggressive" than higher ones
According of the picture attached (dashboard of a new Boxster S), the redline starts at 7,000rpm.
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Old 06-15-2006, 04:53 AM
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Quote:
Originally Posted by spi-ti-tout
So, the rev output has absolutely no linking with the quality of engine.
The rev output of the engine, i.e., the maximum revs that the engine can achieve is linked to quality.

An F1 engine can do 19,000rpm - you need very high quality materials and manufacturing techniques to do this: to stop bits of the engine hitting each other/bending/breaking etc,.

The revs that you achieve at a particular speed & gear in a car is nothing to do with the quality of the car's engine, but is governed by the gear ratios.

Quote:
Originally Posted by spi-ti-tout
Why are the revs so short
At a complete guess I'd say it was due to the flat 6 configuration.

Useful Stuff About Configurations: http://www.e31.net/navmisc_e.html

Quote:
Originally Posted by spi-ti-tout
and does this mean top speed is affected?
Depends on gearing & power.

You need power to overcome friction & drag.

For a certain ammount of power you can get a certain ammount of speed.

You need to have the gearing set up so that the wheels can actually turn that fast at the correct engine revs.

E.g., if you need 500bhp to get your car to 200mph, and it produces 500bhp at 5000rpm, your gearing needs to work out so that when the engine is at 5000rpm the wheels are turning at 200mph.

Obviously if you have top gear set to do 250mph at 5000rpm, you probably won't get to 200mph, because the get the wheels at 200mph, the engine would have to be at 4000rpm, which might only generate 450bhp, which is only enough power for, say, 185mph; that would be your top speed.

With the correct gearing you should be able to get a vehicle to the same top speed whether you power it with an engine that produces 500bhp@1000rpm, or 500bhp@10,000rpm.

Quote:
Originally Posted by spi-ti-tout
are shorter revs more "brutal" and "aggressive" than higher ones?
Depends on what the engine is.

A big 26 tonne marine diesel producing 5000bhp at 1000rpm isn't going to be remotely "brutal" or "aggressive" as the moving parts are very large, therefore more difficult to accelerate.

A 2.0-litre V8 weighing 75kg producing 500bhp to 10,000rpm will be more brutal because the moving parts of the engine are easy to accelerate.

However, a 26 tonne diesel engine with a max. engine speed of 10,000rpm would be just as sluggish, and a 75kg V8 with a max. engine speed of 1000rpm would be just as fast.

(I think...)
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  #11  
Old 06-16-2006, 01:52 AM
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Quote:
Originally Posted by clutch-monkey
are you sure this isn't condensation from the aircon?
Just confirmed, it is. Heat is transferred to the water which is let out so the bus doesn't overheat.
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  #12  
Old 06-16-2006, 02:39 AM
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Quote:
Originally Posted by spi-ti-tout
Just confirmed, it is. Heat is transferred to the water which is let out so the bus doesn't overheat.
lol thought so some of my (admittedly female) freinds freak out because they think they're car has some sort of leak
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Old 06-17-2006, 11:19 AM
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OK, remembered another one. Two actually, forgot the 4th.

3) Why do the revs go down for a second when you change gear? I know it's because torque is suddenly suspended when the firewall disengages with the clutch and connects again with the other gear. I just need to know the thing exactly.
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  #14  
Old 06-17-2006, 04:32 PM
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Do you mean during a gear change or after you've made one?
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Old 06-17-2006, 05:06 PM
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Quote:
Originally Posted by h00t_h00t
Do you mean during a gear change or after you've made one?
I think he means during the actual gear change.
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