Limiting factor for RPMs?

[squirrel]

That's what I figured you were interested in knowing.

[TheSilverBuick]

All else being equal as in the force pushing down on the piston, the frictional force resisting motion, the moment of inertia of the crank and whatever else is attached to it. The only difference being the length of the stroke. One will accelerate faster than the other, or they'll accelerate at the same speed. Oversimplified, yes, but it can be proved or disproved with math. I don't feel like doing it, but it can. Obviously there are other factors in the real world, but that's what I mean by all else being equal.

I want to see that math ran.

[The Outsider]

Bob, I know you're a Ford man, but the fine engineers at General Motors agree with you. In discussions with the ALMS team mechanics today, they said that the little 336ci motor in the (hairy ass, freaking awesome) race Corvette makes peak HP at a measly 5,800RPM and peak torque at 4,500 RPM (or thereabouts...specific details weren't flying around like skittles as you can imagine).

The bore being, "a little over 4-inches" and the stroke being, "a little over 3-inches"

That makes some sense if you're endurance racing with a restrictor plate and ancient 2-Valve Kettering technology. I suspect the answer would be a tad different down in the Porsche pits (and they've won a lot more endurance races than "Government Motors.")

The Vette mill supports the oversquare theory for competition engines. BTW, the only reasons I can think of that you'd want to run a small bore/long stroke are: (a) increased detonation resistance and reduced emissions on cheap pump gasoline; (b) the rules require it; (c) tooling/bore-spacing requires it; (d) You're trying to optimize the lever-arm length for maximum low r.p.m. torque and efficiency; (e) you're building a vintage engine.

[The Outsider]

Pretty simple to measure these parasitic losses. In a 4" bore SBC install a 3.750 stroke crank and turn with a torque wrench and record the value. Then install a 3" stroke crank with longer rods so piston has the same deck height, turn and record the value. I venture to guess the difference would be negligable. Realize that in the larger bore the rings will have more resistance as the circumference is larger.
As DG said the article says nothing about the crank accelerating (gaining RPM) faster and attributes the larger bore advantages to larger valves and unshrouding the valves which several pages ago you said was not an advantage.

While that would measure frictional losses, I'm uncertain whether or not it would provide sufficient data on mass or windage. Maybe it would, but I'm skeptical without trying it.

[moparmaniac07]

I want to see that math ran.

That's what I was trying to avoid. Trying to figure out the moment of inertia on a crankshaft is way more than I want to do, though I suppose since they're equal, any number should work. I'll get back to you in a few days.

[The Outsider]

Interesting as the 5.0 has a stroke of 3.649 and bore of 3.629. The 5.8 stroke is 4.165 and bore is 3.68 with flame sprayed cyls. as there isn't enough room to install sleeves. Both are undersquare and the 5.8 drastically so. That must be why they need multiple valves and forced induction to make power with the small bore. It would follow that according to TC these engines have too much stroke to RPM quickly.

Grand-Am DP engines are good examples of "all motor" modulars making good power. Moreover, the design parameters included more than just ultimate power (i.e. FWD compatibility; detonation resistance, tooling compatability).

As has been repeatedly proven over the past 100 years, 4-valve engines tend to have drastically more valve curtain area and low-lift flow, relative to bore size.

Here's an ARAO Engineering chart comparing a 4-valve pushrod head on a "big bore" SBC to conventional 2V heads at low lifts.

[The Outsider]

I certainly agree with Mr. Morgan for classes limited in cubic inches. You should always use the largest bore available in the block you are using. Why have a 396 when you can have a 427? The whole reason I questioned you TC and got this started is your use of that short stroke NASCAR crank in your benchrace turbo engine. Why limit yourself to smaller c.i. than the block will support? More c.i. will make more h.p. and accelerate the car faster which is the goal. If you want 8000 rpm you will spend the same on hard parts regardless of final c.i. In the example above why have a 427 when you can have a 482 with the standard bore or 489 at .030 or 496 at .060? I will say I prefer to bore a block as little as possible to clean up so it can be used for a longer time.
You will admit that more cubic inches will accelerate a car faster all other things being equal won't you?

Zora Arkus-Duntov once famously said (tongue-in-cheek) that the reason they replaced the Mark IV 396 with the 427 was for weight reduction. Certainly, unless limited by fuel economy concerns, available cash, durability issues, piston speed, or rules, you want to build the biggest mill you can with the platform you've got.

[BKBridges]

Dang this thread got re-routed badly... I dont think Ive ever actually had to calculate the moment of inertia of a crankshaft unless it was something that I designed myself from the ground up. Youve got a lot of other more important things to bench race when you are souping things up (like torque converters...etc.) But if you really feel the need, try the Bosch Automotive Handbook isbn 0-89 283-518-6 or its latest revision. Its got some decent thumb nail charts for free forces and moments of inertia of the 1st and 2nd order as defined by the ignition intervals in various configurations (60 deg, 90 deg etc.)
Carry on
BKB

[The Outsider]

Hell might as well add to the bashing.......

This has always been my thinking.......

8000rpm will produce 8000 more power cycles in the same minute than 6000rpm will do.....

A 5.88 gear will deliver the power to the rear wheels twice as fast as a 2.73 gear.......

And a 31" tall tire has a bigger contact patch and delivers more power to the ground during one rotation of the tire than a 28" tall tire..........

But then I'm sure you'll disagree with me on this to........

Can't go with you on this one. In the Otto-cycle, the number of power strokes is r.p.m. /2.
The 5.88 gear will likely get the mill into (and often past) the powerband quicker than a 2.73. But often "taller" gears load the turbo(s) better, allow for better trap speed (assuming that the engine traps at its power peak with the taller gear), and reduce frictional losses. See http://www.hotrod.com/techarticles/t...er_comparison/

The size of the contact patch of a tire is a function of weight and pressure, not how "tall" or "wide" the tire is. http://www.hotrod.com/techarticles/t...er_comparison/ Certainly "tall" and "wide" can affect how low you can run the pressures (which does affect the contact patch size), how cool or hot the tire runs, and the shape, high-speed stability and replenishment rate of the contact patch.

[dieselgeek]

Dang this thread got re-routed badly... I dont think Ive ever actually had to calculate the moment of inertia of a crankshaft unless it was something that I designed myself from the ground up. Youve got a lot of other more important things to bench race when you are souping things up (like torque converters...etc.)
Carry on
BKB

Always grab the Low Hanging Fruit first ;-)

The biggest problem with guys who build their combos using internet knowledge - they're worried about some stupid small thing (that they believe is huge) while the dumbest mistakes go ignored. I see that a lot at Bonneville too. Guys focusing on the "best aero" and they have the totally wrong engine package. Real experience really changes your approach, IMO. Your mileage may vary.