Limiting factor for RPMs?

[Bob Holmes]

The absolute answer

The absolute limiting factor for RPM is:



CASH



They spin F1 engines to a rule limited (!!!!) 18,000rpm. 2.4 liter NA engines making upwards of 850hp.

[squirrel]

So we're talking about the moment when push goes to shove??, which I guess would be a degree of crankshaft rotation??..

Not quite..... the moment of inertia is the object's resistance to angular acceleration, about an axis. In other words, how much "rotating mass" it has.

[dieselgeek]

The article by David Reher of RM racing engines, which TC linked, was a good read. It says NOTHING about rpm or crankshaft accelleration benefits by going with wide bores - he attributes the entire reason for short stroke and huge bore is the ability to use larger valves.

[TC]

The article by David Reher of RM racing engines, which TC linked, was a good read. It says NOTHING about rpm or crankshaft accelleration benefits by going with wide bores - he attributes the entire reason for short stroke and huge bore is the ability to use larger valves.

It was a good read wasn't it..... Though not directly saying it, he did say this.......

A short crankshaft stroke reduces parasitic losses. Ring drag is the major source of internal friction. With a shorter stroke, the pistons don't travel as far with every revolution. The crankshaft assembly also rotates in a smaller arc so the windage is reduced.]

http://www.rehermorrison.com/blog/?p=220

[OldMachinist]

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.

[ksj2]

I was a good read wasn't it..... Though not directly saying it, he did say this.......


A short crankshaft stroke reduces parasitic losses. Ring drag is the major source of internal friction. With a shorter stroke, the pistons don't travel as far with every revolution. The crankshaft assembly also rotates in a smaller arc so the windage is reduced.]
Key word Directly!.Think of your reply,wait one minute and decide if that what you want all of us to see.Ive have ate more popcorn and drank more beer because of your inabilty to actually read and comprehend what others have pointed out.Its great that you ask questions but be prepared to listen to those that have more smarts and experience.Observe and learn.I do.

[OldMachinist]

I hesitate to mention that there are a fair number of "high-rpm" OEM engines for sale with a payment book and a warranty, such as the BMW M3 V8 and the Boss 302 . . . .

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.

[TC]

This is an interesting read to...... it's a interview with Darin Morgan of Reher-Morrison

"An oversquare motor is absolutely superior at high rpm compared to a long-stroke motor of the same displacement. Formula One and IRL motors are hyper-oversquare, meaning the bore is twice the size of the stroke. If I could do that in a Pro Stock motor, I'd do it overnight, and the power would go up accordingly. Once piston speed hits a certain range, frictional power losses sky-rocket. By decreasing stroke and increasing bore, you're not only dragging the rings up and down the bore that much less, but you also have less windage, the crank counter-weights get smaller, rod angularity decreases, deck heights get shorter, and the induction system package looks a lot better. With a bigger bore and a shorter stroke in a high-end engine, you don't need a tall-deck block. That lets you move the valve-train closer to the deck, shorten the pushrods, reduce the resonance frequency of the pushrods, and wind the motor higher with less valvetrain flex. In a high-end engine, you always use the biggest bore and shortest stroke you can get. However, there are some exceptions to the rule and you have to look at the entire engine package as a complete system."

Read more: http://www.chevyhiperformance.com/te...#ixzz1zuLdMuI8

[ksj2]

Oh look. Shiny.

[OldMachinist]

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?

[TC]

Last one, and this guy is talking about what I'm talking about....

This also means a 4.25 stroke crankshaft, with the same piston speed as the 3.75 stroke crank, rotates at a slower speed. The slower crankshaft rotation results in a slower rate of engine acceleration, which results in a slower rate of vehicle acceleration.

http://www.rollingthunderz.com/rfs_formulas_piston.shtml

[TC]

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?

I think this article addresses how I feel......

The simple explanation is that raising rpm effectively increases an engine's displacement. This might seem nonsensical because the volume displaced by the pistons doesn't change, but consider the effects of filling and emptying the cylinders faster in real time. An internal combustion engine is an air pump, and if we turn that pump faster, we can theoretically burn more fuel in a given amount of time and consequently produce more power. For example, an eight-cylinder engine running at 6,000 rpm fires its cylinders 24,000 times in one minute (assuming perfect combustion). Increase the engine's speed to 8,000 rpm and it will fire 32,000 times per minute, a 33 percent increase. The volume of air and fuel that moves through the engine is now equivalent to an engine with a much larger displacement. There are also 8,000 additional power pulses per minute transmitted to the crankshaft that can be harnessed to turn the wheels and accelerate the car.

http://www.rehermorrison.com/blog/?p=201

You should check out the blog very good info to be had......

[TheSilverBuick]

Last one, and this guy is talking about what I'm talking about....



http://www.rollingthunderz.com/rfs_f...s_piston.shtml

How about that, you found an article with the same meaningless logic that you have. If you want to cripple the longer stroke engine with less fuel to slow down the piston speed to match the short stroke, then it matters, but given the same energy input (on the same displacement), the longer stroke is going to move faster.

[OldMachinist]

[QUOTE=TC;658143]I think this article addresses how I feel......

You really need to read your quotes. As Randal pointed out you want to handicap the long stroke by keeping piston speed the same. Totally irrelevant.
The Rehr-Morrison blog is also irrevelant as it talks about maximum RPM which you earlier stated and I agree is valvetrain related not stroke related.
Again I will state what you refuse to address. Use the maximum c.i. that the package will support so your short stroke benchrest engine will make less power than a longer stroke maximized c.i. engine will. You stated you want short stroke for faster RPM in your benchrest Monte Carlo. I am trying to point out the obvious. Go big or go home!

[TC]

[QUOTE=OldMachinist;658151]

I think this article addresses how I feel......

You really need to read your quotes. As Randal pointed out you want to handicap the long stroke by keeping piston speed the same. Totally irrelevant.
The Rehr-Morrison blog is also irrevelant as it talks about maximum RPM which you earlier stated and I agree is valvetrain related not stroke related.
Again I will state what you refuse to address. Use the maximum c.i. that the package will support so your short stroke benchrest engine will make less power than a longer stroke maximized c.i. engine will. You stated you want short stroke for faster RPM in your benchrest Monte Carlo. I am trying to point out the obvious. Go big or go home!

Did you even read the quote?? You asked why I like RPM, that quote gave you the answer.......