Maximum rev range

[annageek]

I was just looking into the idea of changing the gauges on the boat over the coming winter to refresh the dash a bit. I got to the rev counter, and thought I'd double check the maximum RPM our engine should run at (4400-4800) in order to get the right gauges and had a thought that a quick google didn't answer. Why do you have a max rev range? Why is it not an absolute fixed value?

I understand many of the various reasons why the limit exists (sort of), so my question is not about that... just why is is 4400-4800 and not just 4800?

My best guess is that it's a means of providing some tolerance to the whole buisness of setting up a boat prop / car gearbox / generator load / whatever it is the engine is powering. Therefore, aiming for a nominal 4600rpm would be the best bet?

No biggie... just my incurably curious mind needs to know!

[Ianfs]

What an excellent question and I have no definitive answer, but would like to throw in my thinking.

A 3.0l and 4.3l for example both have a max rev range of 4400-4800. This is where their max bhp is reached. These engines are made for hundreds of different boats of all sizes including uses for twin installs.

If they peaked at say 4,800 rpms then it would limit the number of applications the engine could be used for simply that the set figure might be difficult to achieve even with correct propping. An example would be that a 4.3 l 225 bhp fitted to a Sea Ray 19 with a 21p high five might give 4700 flat out. Fit a 19p high five and you get 5000 rpm. Now fit a Laser 11 21p and you get 4650.
Now take a larger boat say a 31' Regal with twin 4.3 l 225,s fitted with twin Vengace props 19p's and you might get 4750 flat out.
So you see I think manufacturers leave that range in to cover the different types, hull shapes and weights.
Or I might be talking out of my stern drives.

[ColinR]

The Max rev's will vary for a number of reasons. Assuming flat water and a light load and the right prop you should get 4800. Add a couple of passengers and a full fuel tank you might only get to 4400. Add a few chips out of the prop and a mucky hulll and you will lose rev's. It's a bit like driving a car. In theory it might have a top speed of 100mph but you will only get that on a flat road with a light load and no wind.

[_Ed_]

Its as simple as stating a range by which Mr. J Bloggs member of public who knows absolutely nothing about engines can know that if WOT his engine achieves RPM between X and Y then all is well.

If you imply that precisely X then if it varies it could be assumed to be incorrect.

Ordinarily (and I don't have the dyno plots of these engines) you would tune it to rev as high as possible. Power is a result of torque * RPM. So if you can get more RPM then usually that means more power, (thats very basic the relationship is BHP=Torque(lbft)*RPM/5252) but if your engine has a flat torque curve at 4-5K rpm of 250lb/ft then only revving it to 4k would mean you are only using 190hp. If you change prop and can rev now to 5K you will be going faster and the engine will be producing 238bhp. Torque by the way is DIRECTLY related to the amount of air entering the engine (and being consumed by fuel) at any given moment.

Overall I guess it just gives a way for people to simply work out the above an know that everything is ok.

[betty boop]

did some one say simple?

[Centaur]

Okay, simply put...

It's the rev range in which the engine is happy to work (at full throttle):

- any more and you are thrashing it (causing premature wear)

- any less and you are lugging it, a bit like trying to ride a bike up hill in too high a gear (putting huge strain on various components)