Limiting factor for RPMs?

[squirrel]

We're just trying to figure out the moment of inertia of the crankshaft, first....

[TC]

Well, I guess I'm weird. I want the lowest "revving" engine that will get me to my goals.

I don't make enough money to want to rev the wee out of an engine.

Reher-Morrison has a different view.......

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

[38P]

Well, I guess I'm weird. I want the lowest "revving" engine that will get me to my goals.

I don't make enough money to want to rev the wee out of an engine.

That makes sense.

On the other hand, the trend toward higher-reving engines (and "EcoBoost"-style turbo mills) is driven by necessary increases in "power density." In other words, a smaller, higher-reving engine can be lighter for a given torque rating and can burn less fuel at part-throttle during times of "off-peak" loading (which is the overwhelming majority of the driving cycle). On the other hand, larger engines are easier to spec (and often cheaper) for increased low-rpm torque (at the penalty of higher fuel consumption in Otto-cycle designs).

An unintentional benefit is that engines that turn over 6,000 r.p.m. tend to sound "racy" whether they are or not. Now you can buy'em for almost no down and reasonable monthly payments.

In the early days of the car, huge 500+ cubic inch engines were not uncommon in competition. For example, Ford's 1902-1903 "999" and "Arrow" racing cars had 1,155.3 cid inline-4 engines (bore of 7.25 inches and a stroke of 7.0 inches) which didn't rev much over 1,000 r.p.m. and made between 70 to 100 horsepower at the crank. The general trend over the past 120 years has been toward increased efficency, as measured by torque per cube (h.p. merely being a measure of torque output over time) and reductions in fuel consumption.

[38P]

Reher-Morrison has a different view.......

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

Of course they've probably got different goals, too.

[squirrel]

Reher-Morrison has a different view.......

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

"My subject is racing engines, not street motors"

I guess if you're racing, then you have to do what it takes to win that class.

I'm with Bob, I can't afford high RPM

[TC]

How about if you put some numbers on it? Do you know how to calculate the moment of inertia of a crankshaft?

Not off the top of my head.... But I could learn........

[38P]

"My subject is racing engines, not street motors"

I guess if you're racing, then you have to do what it takes to win that class.

I'm with Bob, I can't afford high RPM

For most racers in classes with minimal rules (e.g. bracket racing), a cheap, relatively low-reving engine of modest displacement with a cheap, consistent, tractable power adder tends to yield the most "bang for the buck." Huge increases in size or r.p.m. in common 2-valve OHV engines tend to cost a lot.

In restricted classes, high r.p.m. is often necessary to win.

[Brian Lohnes]

Well, I guess I'm weird. I want the lowest "revving" engine that will get me to my goals.

I don't make enough money to want to rev the wee out of an engine.

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"

[Dynoroom]

The above reminds me of the multi-time LeMans winning Dodge Vipers. 6200 rpm max IIRC, 'course if was an endurance race.

[Bob Holmes]

Interesting, that's almost exactly the range I have on my turbo-4 endurance engine, I hit peak horsepower at exactly the same rpm. They must have read the same SAE paper on LeMans racing engines that I did. (Said firmly with tongue in check, LOL)

Anytime I hear that someone's goal is X boost, or X rpm, I figure they can't find their ass with either hand. Those are means to an end, not the end.

[BKBridges]

Not that Ive got the cash, but this thread started me looking for R5 parts on ebay. Lots of good stuff out there for lowish bucks,
at least when compared to new.

[TC]

We're just trying to figure out the moment of inertia of the crankshaft, first....

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

[Dynoroom]

Not that Ive got the cash, but this thread started me looking for R5 parts on ebay. Lots of good stuff out there for lowish bucks,
at least when compared to new.

Yep, that's one reason we've got one between the frame rails of my race car.

By the way, El Mirage next weekend! Bring the Jensen

[Bob Holmes]

Reher-Morrison has a different view.......

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

Nothing I said is inconsistent with their views.

[Bob Holmes]

For most racers in classes with minimal rules (e.g. bracket racing), a cheap, relatively low-reving engine of modest displacement with a cheap, consistent, tractable power adder tends to yield the most "bang for the buck." Huge increases in size or r.p.m. in common 2-valve OHV engines tend to cost a lot.

In restricted classes, high r.p.m. is often necessary to win.

Bingo.