B20 Vtec power.

[string]

I don't think that thats the correct way of looking at this though because it isn't just a push pull force but also a rotational force as the rod moves with the crankshaft.

All of which can be represented by simple orthogonal forces.

- and there's never a "straight down" force applied except when the piston is TDC right? The rods are almost always at an angle.

The force from combustion is most certainly straight down onto the piston. From here it splits into two components; one directly down the rod at whichever angle it happens to be at. The other can be computed by vector subtraction, and is the force of the piston against the sleeve.

The top of the rod supports the piston, "added weight", the rod (and ultimately the bolts) are supporting this load whether the piston is going up or down

There are two elements of rod load:
1) Inertia property of the piston/rod. Changing the direction of the mass requires acceleration, which requires force. When the piston is accelerating downwards (upper half of the stroke) the rod is in tension. When the piston is accelerating upwards (lower half of the stroke) the rod is in compression.
2) Combustion forces. Always compressive on the rod and has a the majority of it's power in the upper half of the stroke.

The rod bolt must be able to hold the rod cap on under the extreme tension stress of the rod. When the rod is under net compression, the rod bolts don't have anything to fight - the rod-cap bearing is completely unloaded.

Conclusion: Combustion forces are in the wrong direction to have any effect on the rod bolts.

[string]

the force never changes direction does it?

As long as we're both talking about internal combustion 4 stroke motors, I am quite certain that combustion forces are always downwards.

or the caps and bolts are weightless and don't have inertia?

Tensile bolt stretch from cap and bolt mass changing direction is a function of piston acceleration - which is independant of engine power output. Since superposition is in full swing, I can say that when talking about combustion forces, it's irrelevent.

now i have heard everything!

A stunning jibe, but no substitution for intelligence.

[tinkerbell]

re-read what you quoted, then think of way to explain why "increased power" must only relate to downward forces created by combustion.

you are just being silly.

[string]

If you have an argument, let it out - don't beat around the bush making me guess what you're thinking.

The context is a stock rev-limit B20 block. Increase power without adding more revs? Add torque; a synonym for combustion force.

[tinkerbell]

nah, you win,

or something like that...

[string]

So is there any reason you came into this thread other than to block it up with claims you won't substantiate, or to add some childish manner?

[Chr1s]

I think string means the main reason for rod bolt failure is the tensile forces implied are alot more to consider than a bolt which performs better in shear stress.

edit: ffs, just missed that whole load of crap above.

[tinkerbell]

edit: ffs, just missed that whole load of crap above.

yeah, the moral of the story was that "increased power" doesn't have any physical effects on any part the bottom-end beside the top of the piston, the piston pin, the rod (but not the rod cap) the top half of a rod bearing and the top half of the rod journal, the bottom half of the crank bearing and the main cap...

[string]

My point is that from a geometric perspective, combustion forces cannot cause tensile rod forces.

yeah, the moral of the story was that "increased power" doesn't have any physical effects on any part the bottom-end beside the top of the piston, the piston pin, the rod (but not the rod cap) the top half of a rod bearing and the top half of the rod journal, the bottom half of the crank bearing and the main cap...

Are you going to have a discussion like an adult now? A condescending attitude does the masses few favours in an academic debate.

[tinkerbell]

My point is that from a geometric perspective, combustion forces cannot cause tensile rod forces.

i agree.

but your comments don't really assist do they?

they stop at the first part of a four stroke cycle...

[string]

i agree.

but your comments don't really assist do they?

they stop at the first part of a four stroke cycle...

We're comparing an Engine with a slightly more powerful version of itself - the differences between the two stop after the first part of a four stroke cycle...

To make myself crystal clear: The context of my argument stems from the combustion force as the variable in question. If you want to discuss how increased power affects more than the power stroke, be my guest (though if this were the case, I'm sure you would have raised it in post #12).

[Chr1s]

yeah, the moral of the story was that "increased power" doesn't have any physical effects on any part the bottom-end beside the top of the piston, the piston pin, the rod (but not the rod cap) the top half of a rod bearing and the top half of the rod journal, the bottom half of the crank bearing and the main cap...

I beg to differ, manufacturers, or should I say engineers, are more inclined to investigate the tensile forces of the rod bolts. If you mean incease in HP with a result in twisting as such that could cause shear across the bolt between the cap, well thats another story - and how often does this happen before the rod gives up?