Why are stronger rod bolts required as horsepower is increased?

[CyFi6]

I'm not engine expert, so this is probably a pretty simple question, but something I have never quite understood. I see a lot of people recommending replacing stock rod studs over xxx horsepower with stronger ARP studs.

Now if I am not mistaken, the rod bolts would be under their highest load at after snapping the throttle closed at redline RPM. This is when the greatest force would be on the lower half of the connecting rod big end (I would think). High piston speeds combined with high vacuum would make it seem like lots of pressure would be on the rod bolts.

Under full power/boost, pretty much all the force of the combustion is being exerted on the top half bearing shell/top of the rod (very little side loads on the bolts I would assume), correct? If so, all the cap/bolts are doing is pulling the piston down on the intake stroke, and with high boost pressures in the cylinder, it would seem that would further reduce load on the bolts and cap.

When using lightweight forged pistons and wrist pins, it seems that loads on the bearing cap and bolts would be reduced even less than stock at high piston speeds because less mass has to be yanked downward.

So my question is, why is that stronger rod bolts are recommended/required on higher horsepower builds, where redline, stroke, and piston speeds are no greater than stock? Seems like power output has less to do with rod bolt stress than people make it seem, though I might be missing something.

 

[GrimJack]

You're *almost* to the logical conclusion. You're thinking in terms of a single cylinder, though, instead of a complete engine.

If you think about several cylinders working in unison, is there perhaps points in time where the piston is being pulled down due to firing in a different cylinder?

 

[SupraCorwin]

Why leave a weak link? Chances are if you upgrade the rods you have to replace the bolts anyway.

Also there is duty to consider, the longer you stay at higher rpms the more wear. Casual driving with a bit more hp on oem bolts might not be an issue. Constantly hearing the rev limiter in autocross might be.

 

[bioskyline]

same reason why head studs are used vs stock bolts, its normally just better safe than sorry. but is it worth having a stock bolt break and wreck 4k+ worth of engine work?

other thing to think of is the material there made from. shear strength, tensile strength etc. we know the stock rods dont hold up to high power, so why would the stock bolts be any different. also think installation pre-loads - the amount the fastener is stretched during installation. cheap bolts never torque right, and will back off slowly. while this is not overly critcal on some parts, like the altanator and PS pumps, if you dont get the bearing caps torqued right, you can end up with the rod knock with the play, or worse a blown rod.

as to side load, there is quite a bit of it, as the crank rotates it moves the rod from side to side as well as up and down as it travels in its circle, so the crank is pulling the bottom half down and to the side on intake and combustion.

 

[Supracentral]

I wrote this up a long time ago:

http://www.supramania.com/forums/showthread.php?16034-how-much-hp-for-boring&p=180719#post180719

The tensile loads I mention in that post apply just as much, if not more, to the rod bolts as they do the the rods themselves.

 

[bioskyline]

i never though about piston speed that way before.

 

[CyFi6]

I wrote this up a long time ago:

http://www.supramania.com/forums/showthread.php?16034-how-much-hp-for-boring&p=180719#post180719

The tensile loads I mention in that post apply just as much, if not more, to the rod bolts as they do the the rods themselves.

Well what you mention in that post basically backs up what I am saying. Toyota designed the rod bolts to withstand the stock RPM, piston speed, stroke etc. without issue, so why is it that stronger rod bolts are required for higher HORSEPOWER levels with no change to redline, piston weight, or piston speed?

 

[Supracentral]

Well what you mention in that post basically backs up what I am saying. Toyota designed the rod bolts to withstand the stock RPM, piston speed, stroke etc. without issue, so why is it that stronger rod bolts are required for higher HORSEPOWER levels with no change to redline, piston weight, or piston speed?

Piston acceleration is the one factor not really covered in that post. Remember that for every piston that's accelerating downward due to power, there's another accelerating upwards towards the top of an exhaust stroke.

Think about how quickly a modified car moves up through the RPM range than a stock one. Pistons have to accelerate quicker to accommodate that. More acceleration = more stress.

Also, it's cheap insurance. Compared to the costs of all the other upgrades, rod bolts are stupid cheap.

 

[te72]

Thanks for the link, and good read all around. Never really thought about it, and reaffirms my thoughts that I want a mid-range monster rather than an engine I have to rev the ever-loving shit out of it. Granted, 72.5mm stroke is nice for piston speeds...

 

[bioskyline]

according to his chart, at redline, the pistons are doing almost 45mph in the block. thats fast!

 

[te72]

Ever seen an open block being spun, without the head on it? Kinda neat, you can't even touch the cylinder walls at those speeds, it's as if it were a solid surface.