No, exactly the opposite. The rod bolts are not under stress on the downward stroke. They are under stress on the upward stroke, at or near the very top, when the piston starts to decelerate and all the weight of the assembly is still moving upwards. This is particularly true on the end of the cycle, when there is no compression in the cylinder to push down against the piston.

So, the deceleration of the upward movement at the end of the exhaust stroke is the portion of the cycle that puts the most strain on the rod bolts.

It's only after the rod bolts stretch that problems manifest themselves in the rest of the cycle...

Get it?

 

I understand. If I had plotted the position, velocity, and acceleration I would've seen that long ago.

So....this leads me to my next question.

If decelerating the piston on its exhaust stroke causes the largest load... how does this load compare to accelerating the piston on the intake stroke?

F=MA right? It is the acceleration in the Y direction that produces a tensile load upon the connecting rod fastener... and your piston, rod, wrist pin, and ring pack all weigh the same. the force applied to the fastener is the same on the Exhaust stroke decel. as the Intake stroke accel.

I guess the big difference between the loads will depend upon the forces applied in the y direction at the piston (cylinder pressure) which will depend upon valve timing mostly.

Seem reasonable?

 

The highest load on the fasteners (rod bolts) comes as the piston has to be (essentially) stopped from its highest speed the top of the exhaust stroke, while bringing it to a stop is not assisted by the opposing force of compressed charge in the cylinder (like it is during the compression stroke).

While the accelleration phase of the intake stroke may be similar, the big difference is that when changing direction from high speed going down to stopping and going up, the rod bolts are not under tension during the reversal of the motion, when the change of direction and inertia is the greatest.

 

I get it...

If you plot the rate of change of acceleration you'll see twice the rate of change in acceleration over 90 degrees of crank angle on the exhaust stroke as you would on the intake stroke.

The rate of acceleration is the same, the rate of change of acceleration is different.

thank you for an intelligent conversation on Evom.

 

ah... and what you were saying before about it being on the compressive side... you meant...

That the highest rate of change of acceleration on the intake stroke occurs between 90 degrees ATDC and BDC...where the connecting rod is in compression.

 

The load is still the same...

F = MA.

the third derivitive of position (yank) does not change the load upon the fastener. Therefore the load upon the fastener on the exhaust stroke will not be any larger than the load upon the fastener on the intake stroke.

it's similar to a maximum speed limiter. It doesn't matter how fast you get to the speed (accel), but your actual velocity does matter.

 

With that being said I have some stock rod bolts to send you to torture and a stock rod and piston as well.

 

CO VR4 is saying this...

When the piston is moving down the rod applies a load directly to the crank, the rod bolts see basically nothing. This could also be said about the upward stroke because the crank is loading the rod.

The rod bolts get loaded during the acceleration change at TDC and BDC.


Aaron - E85, seriously.

 

Impressive.

 

Cant work on my own stuff if I am busy working on everyone elses 70hrs a week doesnt leave me much free time. Plus it still runs great, its summer, and 574 is plenty fun. I might start mixing in some racegas and push it over 600whp until my fuel system is done (one of the current hang ups).

 

WRONG. On the intake stroke the crankshaft is pulling the connecting rod down...that force is transmitted to the cap on the con rod, which puts the fastener in tension as it accelerates the rod, wrist pin, ring pack, and piston down the hole. I'm talking about where the piston is being accelerated downward from TDC on the intake stroke...not when it is being decelerated.

You're talking about the power or combustion stroke.

SUCK, PRESS, BANG, BLOW. you're talking about the bang...I'm talking about the suck.

Also what you said, "The rod bolts get loaded during the acceleration change at TDC and BDC." Is fundamentally wrong. A change in acceleration from accel to decel does not produce a load. The quantity of acceleration produces the Force upon the fastener. F=MA.

 

R/T Ernie, I think you understand. I agree with you about the fastener being in tension as the piston starts to go down on the intake stroke. The difference is that the most abrupt change when it de-accellerates to -0- at the bottom of the stroke is when the fasteners are not in tension, because the crank journal is taking that downward load. Comparing the upward travel on exhaust stroke, the rod bolts are in full tension then, without any "help" from compression above the piston, so that is when the most load occurs...

I think you essentially said that in your last few posts...

 

Like I was saying before though... it doesn't matter that it has the most abrupt change in acceleration. Yank has nothing to do with Force... We're looking at tensile strength of fasteners which requires a tensile load. The load is the mass of the rotating assembly times its acceleration. The rate of change of acceleration (yank) has nothing to do with this.

Onto your point about cylinder pressure on the intake stroke changing the net force upon the connecting rod fastener...

The intake stroke has the boost pressure applied to the top of the piston. In most turbo applications you'd be lucky to get less than a 1:1 ratio for your pre-turbine pressure to boost pressure. This means that "ideally" you would have the same force opposing the tensile load on the connecting rod on the intake stroke as the exhaust stroke....assuming symmetrical cam lobes and similar IVO/EVO timing.

Now... I said ideally because typically the pre-turbine pressure is higher than the boost pressure. And your exhaust is being pushed out of the cylinder past two tiny valves.... this will result in higher cylinder pressure than the regular exhaust pressure seen in the manifold. (think flow and pressure differentials) Now... lets think about the intake stroke. Your piston is being accelerated downward by the crankshaft while drawing in the intake charge. The pressure in the cylinder will be less than what is seen in the intake manifold because you're pulling down with one 86mm piston through two 36mm intake valves. Again... flowing from High pressure to low pressure. pressure differential causes flow therefor to get flow you have to have a delta P.

So by thinking through the compressive force exerted upon the piston during the last 50 degrees or so of the exhaust stroke and the first 50 degrees (or so) of the intake stroke... I think it's clear that the in cylinder pressure should be higher for the exhaust stroke compared to the intake stroke.

 

do you happen to have a mod list i myself am looking to make good numbers on pump so i am intrested in the combo thanks

 

Stock long block
Stock rod bolts
Stock head bolts
Stock headgasket
Magnus Cast Intake manifold
GSC S2 cams
FP Beehive valvesprings
ETS 4" Intercooler and complete ICP
ETS 3.5" exhaust
ETS 3586 HTA single scroll (.82) w/38mm gate
Stock ECU with SD conversion
Omnipower 4 bar MAP sensor
FIC2150s
Single modded walbro 255
28.8psi boost
92 octane
Tuned by me