Bottom End Life: Torque -v- RPM

[Tom and Vipers]

Some bored out of my mind ramblings:

TORQUE CONSIDERATIONS:

Obviously torque is a load which deforms parts. The more torque the more deformation and hence bearing misalignment.

Further all the torque of the engine must pass thru the 9/10 rod journal on the crank.

RPM CONSIDERATIONS:

A spinning free crankshaft will distort internally to some shape. Aside form this shape, it poses no load on the main bearings.

However, the reciprocating masses are another story.

Centrifugal force, mrw**2, is proportional to the square of the rotational velocity. So a 10% increase in rpm amounts to approximately a 20% increase in such forces.

The effect of reciprocating mass on main bearing load is a factor, however, I do not know to what extent. I do know that over-balancing tends to react more of the reciprocating load in the horizontal direction of the main bearings.

Since this is a stronger direction (shear loading of the MB bolts) overbalancing has its following.

Then, of course, there is the effect of journal diameter and rpm. Higher rpm likes smaller journals. There is surely a velocity limit for a hydrodynamic bearing.

Motorcycles use needle bearings and avoid that problem. Probably because the journal diameter required for a HDB would be too weak for loads.

NOTE: I have seen 720 deg polar plots of main and rod bearing loads and the trajectories are extremely complex.

$64,000 QUESTION: (assuming a suitable oiling system)

For bottom end durability, what is the trade-off between torque and rpm?

Tom

PS

It is interesting that in the Ford racing parts catalog, their blocks have max HP @ max RPM ratings.

The smaller journals have higher RPM ratings.

 

[VIPR GTS]

You must be BORED. Was any of that english? Am I bored, I read it all?

 

[TOOOFST]

You've been partying with my buddy Paolo.I'd guess a viper engineer's answer would be %$^*^&@#$%^%!

 

[HP]

My recollections touching on the subject have been; for racing, smaller crank main journals are preferred because of less surface area on rotating parts = less friction. Motorcycles that use needle main bearings, use this option to get around lack of consistent oil pressure supply necessary for shell bearings. Shell bearings have closer tolerances under load, and for that reason enable higher RPM's, over needle bearings.
The size of the main journals, is relative, and there is
compromising factors. Example - the cast crank of the 351 windser, has large journals; this gives it the advantage of
added strength, but as a performance platform the Cleveland 351
is preferred because of its smaller journals, thus higher RPM
potential.

 

[Tom F&L GoR]

Add to all this:

Main bearings are full pressure lubrication. Rod bearings get a squirt once in a while. I think that's one reason for needle bearings.

The higher the engine speed, the more likely the bearing pulls oil into in. However, the pump has to a) get it there and b) have enough flow.

Everything I've ever seen shows the reciprocating mass causing more distress on the bearings than the power stroke. Combustion sort of happens at one speed, so it doesn't add much "stress" at higher engine speeds and loads. It's the parts - which is why all those high revvers use short strokes, small parts, and lots of them.

And at the risk of another long discussion, I still would like to know the science behind what keeps our V-10s from another productive 500 or 1000 RPMs.

 

[GTS Bruce]

Oil starvation. Bruce

 

[Tom and Vipers]

Oil starvation.

Would this be classic pump sucking air?

 

[jrkermode]

My understanding, second hand, is that the crank flexes too much. Meaning, rev any higher and the middle journal starts banging into things.

 

[Tom and Vipers]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Tom, F&L GoR:


And at the risk of another long discussion, I still would like to know the science behind what keeps our V-10s from another productive 500 or 1000 RPMs.

<HR></BLOCKQUOTE>

Well, for one, I would bet that the manifold is optimized for the current operating range so it becomes a limitation.

Also, anything near a 4" stroke is simply not good for rpm.

Except for the insane 6500 rpm redline Ford Cobra R 3.55/4.17 bore/stroke which appears to have had its stroke reduced to 3.54 for 2003.

Gee, wonder why?

Tom

PS. You wann' built a ultra high rpm strong as a rock engine?

Studebaker 224:

Stroke 2.81

Rod length 6 5/8 at 3.625 stroke with conventional piston design.
Putting the pin into the oil ring would result in about an 8" rod.

6 head bolts per cylinder.

Forged crank stock.

Main/rod bearing journal diameters 2.5/2.0

Tons of strength everywhere - Stude V8 weighs about the same as Chevy BB.

This animal should be able to contain ultra blower boosts.

Since these are all stock parts and cheaper than dirt, all the engine would really cost you is a set of rods and pistons.

However, when you start talking 8" rod, some of the rod shops start looking at you funny... Just DON'T tell them it is for a Studebaker. Make something up.