why dont use stop the arguement lol b20 stock rod bolts are shit and will fail when you want to rev more
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why dont use stop the arguement lol b20 stock rod bolts are shit and will fail when you want to rev more
It's a good discussion Sam, well heated between tinker and sting :zip:
what about if you don't rev MORE,Quote:
Originally Posted by GSi_PSi
but rev HARDER? i.e. due to more torque? (i.e. being created by higher combustion pressure)
will the rods (and bolts) suffer higher forces then?
True story. The primary factor is how high you want to rev. The force required to reverse the direction of of the piston and rod mass at TDC grows exponentially as you rev higher - and it all goes through the rod bolts.Quote:
Originally Posted by GSi_PSi
Tinkerbell, is there another way to explain the theories, I still can't see how power has a direct relationship to the stresses on the rod bolts as opposed to RPM.Quote:
Originally Posted by tinkerbell
imagine a nitrous B20B.
stock RPM limit.
combustion goes bang, rod goes down.
where are the other three rods?
there is another piston/rod going down too, but it is being pulled down by the crank with no force on top.... (NB - this rod and piston has inertia)
imagine what is happening to the rod bolts on that rod?
are the forces on those rod bolts going to be the same if the forces on the other piston are higher due to the higher combustion pressure?
Strongly advise you change your rod bolts.Quote:
Originally Posted by Terrano08
If the forces pulling the other rod down were somehow greater with the higher power, then you would be accelerating the piston faster (since mass is obviously conserved) - which of course does not happen in a state of constant engine speed.Quote:
Originally Posted by tinkerbell
The forces on the remaining non-powered rods are determined by the angular speed of the engine, not the torque onto the crank-shaft. For a given engine speed: the physical nature of the rod and piston assembly determines how much force is required to overcome it's state; the reverse is not true: increased torque on the crank pushes the piston harder. If you want to balance the equation, increase the "load" on the engine - which involves increasing the angular acceleration of the load - the exact expected result!
The forces on the rod can be examined one at a time and then summed to get the final result. Comparing two otherwise identical engines at the same operating speed means you can cancel out the inertial loads as they are equal in each case.
Summary: You can push an object a given distance in a given time with only one constant force. You can't push twice as hard and have it take just as long. (I.e. you can't push/pull a rod harder yet have it move at the same speed).
of course. that is why the car with more power (force) accelerates faster!Quote:
Originally Posted by string
way to change the subject.Quote:
- which of course does not happen in a state of constant engine speed.
we (i assume everyone except you) are talking about an engine in dynamic use, for example drag racing, not sitting on a constant 2389rpm...
yep, at a constant speed, lol!Quote:
The forces on the remaining non-powered rods are determined by the angular speed of the engine, not the torque onto the crank-shaft.
yep, again at a constant engine speed...Quote:
For a given engine speed: the physical nature of the rod and piston assembly determines how much force is required to overcome it's state; the reverse is not true: increased torque on the crank pushes the piston harder. If you want to balance the equation, increase the "load" on the engine - which involves increasing the angular acceleration of the load - the exact expected result!
yep, exactly, at a constant engine speed.Quote:
The forces on the rod can be examined one at a time and then summed to get the final result. Comparing two otherwise identical engines at the same operating speed means you can cancel out the inertial loads as they are equal in each case.
yep, that is why the nitrous one accelerates faster :thumbsup:Quote:
Summary: You can push an object a given distance in a given time with only one constant force. You can't push twice as hard and have it take just as long. (I.e. you can't push/pull a rod harder yet have it move at the same speed).
(and why the forces on the rod bolts are higher...)
you cant have an engine accelerate to 7200rpm faster than another (identical) once without the forces on all the parts being higher, can you?
The forces resulting from "inertia" are completely and only dependant on the rotating speed of the engine. A 500hp B20 at Xrpm has the same "inertial rod loads" as a 200hp B20 a the same Xrpm.
The power stroke of one piston only affects the single rod, and the engine load. If you simultaneously increase the torque by 10% but also increase the load by 10% - acceleration remains identical, and the "inertial rod loads" curve vs time remains identical.
It doesn't matter how fast the engine is accelerating, the loads on the rod from the mass of the piston is dependant only on the speed at which the crank is rotating.
When the crank attempts to move the pistons/rods upwards in their bores, it asks them "how much force do I need to give you such that you move at the same rate that I do" - the rods answer and only the required force is applied - no more, no less.
If you agree that the only tension [rod bolt related] stress results from the piston/rod's mass, then you are agreeing that the rod bolt's stress is dependant only on the rotation speed of the engine.
If the engine is accelerating faster because it has less load, then it will accelerate without more force on any part.Quote:
Originally Posted by tinkerbell
If the engine is accelerating faster due to more torque then the difference will be the compression rod stress resulting from increased cylinder pressure. Because of the shorter time scale, you'll reach the peak 7200rpm "inertial rod loads" faster but the peak rod tension loads will be identical.
Summary: Revving harder doesn't pull the 'idle' rods down harder - revving higher does that.
i am sorry, it is hard to follow what you mean here?Quote:
Originally Posted by string
I mean that a faster acceleration doesn't mean greater force on the rods from the mass of the piston - it means the forces grow to the same size, faster.Quote:
Originally Posted by tinkerbell
When I say the "inertial rod loads" are dependent on the engine speed, I mean that the engine speed can be anything. It can be changing, or constant.
A higher torque otherwise identical engine transmits a greater torque to the load thus it's angular acceleration will be greater. The "inertial rod loads" are exactly the same as a function of engine speed, just that the engine speed builds up quicker.
All the forces can't be the same otherwise the result would be the same. The forces arising from the mass of the rotating assemblies will be identical in magnitude however, regardless of the engine's angular acceleration. Since the time from 0-MAX rpm will be less, the power absorbed/transmitted by/through the various means, will be larger since you have reduced the time to do the same work.