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Why does my rev limiter cut in at 6500 when the red line starts at 6000 and max power is at 6200?
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dave
The rev limiter is designed as the 'back stop' to protect the engine from excessive wear and mechanical damage, by preventing it reaching excessive rotational speeds.
No surprisingly the recommended rev limit is somewhat above maximum power.
Why is the red line below max. power? Two possibilities:
Cost - the same tacho is used for a number of engines, some of which have lower speeds for max power;
Durability - it's not recommended to use over 6000 revs for extended periods
Regards
John S
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As John says, the limiter is intended as a 'last stop' protection for the engine and most drivers would probably not want it to operate every time the point of maximum power is reached.
The 6500 ultimate engine speed limit may well be to make absolutely sure that the engine doesn't run at a speed where the crankshaft goes into a critical vibration. For example, the crankshaft may be OK at 6500 and 6600 but not at 6570.
The red line is probably advising you to change up at 6000 because you would be better off in the next gear anyway, i.e. you would be into a high torque area of the power curve.
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Dizzy
I'm going to stick my neck out here.
Surely this 'change up early for better acceleration' is a fallacy. The best acceleration must obtained by changing up at peak power, or even a bit above, depending how quickly the power curve drops off above peak power.
This always gets you further up the power curve of the next gear, and it's excess power that gives acceleration, not torque. Torque can't be separated from power.
HP = 2 x pi x N x T/ 33,000
Where N = engine speed in revs/min and T = torque in lbs-ft.
(No, although I'm metricated, I've not converted on this one)
So, even on a turbo engine with a managed flat torque curve, it will produce more power at that torque the faster it's rotating, because the power output is higher. I'm pretty sure for any car that at a given speed, which can be reached in say two or even three gears, the acceleration available will always be better in the lowest of the gears that the car can use at that speed. Why? Because there is more power available.
Regards
John S
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Id guess that best acceleration is gained by changing AT max power, which in my case would be 6200 despite red line being at 6000?
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I'd think that John is probably correct in as much as the best accelleration wil be obtained by revving beyond peak power, because that will put your higher up the power curve for the next gear.
My addition to the discussion is - the power curve usually drops off very rapidly from peak, so the best accelleration is probably achieved by revving beyond peak power, but not by very much.
(Not that in the real world it makes any significant diference to your journey time!)
Andy
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Andy
That's right. I should have said that it's only worth going above peak power if the power curve doesn't drop off too rapidly. In practice you're right - most do fall off quite quickly, so there's rarely any point.
Regards
John S
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John, I recognise the formula but can't get into the mood for in-depth calculations and analyses - I retired to get away from all that - however what you say does make sense.
I might be a bit biased because I can't stand buzzy engines and would normally prefer to be in a higher gear, even with less acceleration. In fact I have rarely experienced peak power driving in the past thirty years. I didn't discover the break-neck acceleration of my BMW at the top end until at least a year after I bought it!
My Triumph 2500 has little more than half the peak power of the BMW (112 bhp against 192 bhp) but has lots more low down grunt, which I much prefer. The Triumph pulls quite strongly from tick-over in top gear yet still has quite lively acceleration up to at least the legal limit. The strong low-down torque of the BMW 320d/530d and the Saab 9-3 turbo-petrol is leading me to consider one of those as the eventual replacement for my present BMW.
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Turbos do not have a managed flat torque curve, they tend to peak at the low 3000's as the turbo spools up pressure.
True that power and torque are not separable but its common to have peaks of the two that are a long way apart across the rev range.
Torque is the vital factor for acceleration, therefore its important to drop into the peak area of torque when you change gear to maximise acceleration (not into peak horsepower).
Again although related, peak power has much less to do with acceleration and more to do with top speed.
Thre is a saying, torque wins races not power.
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To get max acceleration you should change gear when the power output from the engine has fallen to the same power output as the engine would be making at the same speed in the next gear up. think of it as maximising the area under the curve.
Ben
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I agree with Ben.
I think, by the way, some people are confusing torque at the flywheel with torque at the back wheels, which do not care whether the shove is coming from x revs at y engine torque and z gear ratio, or 2x revs at y/2 engine torque and 2z gear ratio!
(It occurs to me that it would be VERY interesting to change the engine prescription for F1 from a capacity limit with other regulations, to a power plant weight limit, any layout, with a specified fuel tank capacity. Do you go for a turbo screamer pitting every ten laps, or something that will run the race distance? I imagine the optimum would be somewhere in between, but where?)
Tomo
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For best acceleration the engine should "sing" between the speed of maximun torque and maximum pawer - say from 3000 to 6000 with the average engine.
Change up 500 to 1000rpm below the red sector on the rev-counter for car sympathy. At very high revolutions, inertia and wear and tear on the valve gear is extreme.
There is no point in revving past the power peak, which is usually 500 revs below maximum engine speed.
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The primary function of the rev-limiter is to prevent use of revs which are potentially damaging, or fatal, for the engine. The power drops off so quickly after max power that it serves no purpose to be revving in that range.
The red line is there for a purpose. Unless the engine has been rebuilt by a tuning specialist, there is NO POINT exceeding the red-line, ever.
Most turbo'd engines have their max torque electronically limited to give a flat torque-curve, from say 2000 to 5000 rpm, to reduce the need for a very high torque-capacity gearbox.
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