optimum speed.
Discussion
I had a rather enlightening conversation about engine and engine speeds with The chap installing the 27litre merlin engine into his sd1, during which he reckons the optimum speed for a piston to travel through a bore is around 1300 feet per minute....
in his merlin this translates IIRC to 800rpm at which point he has geared the car to be cruising at around 70mph
in my car this translates to 2385rpm however at this my car is only travelling at around 50mph. i am considering swapping the diff to give the car longer legs. but i was wondering if anyone knew where this figure of 1300 feet per minute came from, and whether it was purely theoretical, and whether anyone had a different opinion as to what the optimum speed for a piston travelling through a bore is....?
in his merlin this translates IIRC to 800rpm at which point he has geared the car to be cruising at around 70mph
in my car this translates to 2385rpm however at this my car is only travelling at around 50mph. i am considering swapping the diff to give the car longer legs. but i was wondering if anyone knew where this figure of 1300 feet per minute came from, and whether it was purely theoretical, and whether anyone had a different opinion as to what the optimum speed for a piston travelling through a bore is....?
1300 ft / min = 6.6 metres / sec which is very slow by modern standards most modern engines see about 30 mtrs/sec at 7KRpm.
Bear in mind the merlin is over 60 years old now, things have moved on a bit
However there is something to be said for a big engine revving slowly as opposed to a small engine screaming away
Matt
Bear in mind the merlin is over 60 years old now, things have moved on a bit
However there is something to be said for a big engine revving slowly as opposed to a small engine screaming away
Matt
Firstly, optimum speed for what? Whilst that may be true for the big old Merlin lump, I doubt the same is true of modern 4 valve per cylinder engines which are designed from scratch to turn a high(er) rpm. Many engines/cars feel like you have to rev the balls off them to make them go anywhere. That's just how they're designed and should be driven as such.
In my Alfa engine, 1300 ft would translate to 2901 rpm. Which is barley getting going when it turns to just over 7K. Incidentally, in a stock Alfa 2.5 V6 engine, peak torque is at 5K and peak HP at 6.3K rpm.
In my Alfa engine, 1300 ft would translate to 2901 rpm. Which is barley getting going when it turns to just over 7K. Incidentally, in a stock Alfa 2.5 V6 engine, peak torque is at 5K and peak HP at 6.3K rpm.
Edited by Snake the Sniper on Sunday 6th July 22:54
most modern engines are in the region of 10 to 13 m/sec
but it's not much of a consideration. Your engine will be cammed to give your power output in a certain part of the rev range, so you're best off gearing that part of the rev range to what you want to do ( acceleration, speed or economy )
but it's not much of a consideration. Your engine will be cammed to give your power output in a certain part of the rev range, so you're best off gearing that part of the rev range to what you want to do ( acceleration, speed or economy )
BB-Q said:
Rocket Pepper said:
Optimum piston speed differs for every type of engine. Limiting factors, engine design, life expectancy and materials used. Not much will survive for long at 6000fpm - think top fuel dragster!
Yeah, motorcycle engine don't last 5 minutes, do they?Just cos its hitting 16k dosent mean its average piston speed is excessive as it depends on stroke.
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