What is the limiting factor in RPM/redline and powerband?

[spidey07]

I've never really understood why motorcyle rev so high.
I've never really understood why F1 cars rev so high.

I think it is because of more power, wider power band.

But then why are passneger cars so different? The free reving S2000 probably holds the mark for passenger car, but all others are much lower.

What is holding them back? Piston size/weight? Precision on the cams/valves? Too much force on the crank? Is there any real advantage to revving high (obvisoulsy there must be)

Anybody know why motors in cars have such low redlines when compared to racing/bikes?

 

[BatmanNate]

Bearings, crank/piston/rod materials, valve train are what limits the RPM, IIRC.

 

[spidey07]

Originally posted by: BatmanNate
Bearings, crank/piston/rod materials, valve train are what limits the RPM, IIRC.

so basically all the moving parts of the motor?


I guess I could start a train of though until the real gurus show up...

1) Much smaller car/bike = smaller motor
2) less weight to move
3) allows higher RPM without the associated stresses a larger vehicle would entail from the sheer weight of the moving components?

 

[gsellis]

Yep, mass in motion. That is why motorcycles rev so high, lighter pistons and valves in motion. Other things come into play, at high revs, springs cannot return the valve to seat fast enough, so they float and meet the pistons. F1 cars don't have springs, they use compressed air instead of springs. But, the pistons changing direction and the twisting of the cylinder walls do still limit speed.

HP is a factor of RPM and torque. Want more HP, spin it faster. That is why a normally aspirated 3.0l engine at 18000 rpms can put out 800+ HP. If you think about it, a 3 liter engine at 18000 is moving about the same fuel as a 6.0l at 9000.

 

[thomsbrain]

the faster you rev, the great the forces on the parts. to rev high, you need to have a strong engine, and it's easier to do with small engines because smaller parts have less momentum. hence motorcycle engines (that and they don't last nearly as long as car engines). rotory engines can rev high because they don't have so many directional changes, it's more just a constant spinning kind of deal.

 

[91TTZ]

Originally posted by: spidey07
I've never really understood why motorcyle rev so high.
I've never really understood why F1 cars rev so high.

I think it is because of more power, wider power band.

But then why are passneger cars so different? The free reving S2000 probably holds the mark for passenger car, but all others are much lower.

What is holding them back? Piston size/weight? Precision on the cams/valves? Too much force on the crank? Is there any real advantage to revving high (obvisoulsy there must be)

Anybody know why motors in cars have such low redlines when compared to racing/bikes?

It's mostly about reciprocating mass and airflow. It's very easy to get a small engine to rev to 20,000 rpm, but it's much more difficult to get a larger engine to rev that high.

For a F1 car to rev that high, it takes a hell of a lot of engineering and materials with a high strength/weight ratio. For a small RC car engine to rev that high, you can just use steel and brass, and a 1920's engine design.

 

[91TTZ]

Originally posted by: spidey07

Originally posted by: BatmanNate
Bearings, crank/piston/rod materials, valve train are what limits the RPM, IIRC.

so basically all the moving parts of the motor?


I guess I could start a train of though until the real gurus show up...

1) Much smaller car/bike = smaller motor
2) less weight to move
3) allows higher RPM without the associated stresses a larger vehicle would entail from the sheer weight of the moving components?

It's not about the vehicle weight as much as the mass of the engine internals like valves, lifters, connecting rods, etc. There's nothing stopping you from putting a 18,000 rpm motorcycle engine in a SUV, it'll just be a torqueless wonder with bad gas mileage and a short life.

 

[91TTZ]

Originally posted by: thomsbrain
the faster you rev, the great the forces on the parts. to rev high, you need to have a strong engine, and it's easier to do with small engines because smaller parts have less momentum. hence motorcycle engines (that and they don't last nearly as long as car engines). rotory engines can rev high because they don't have so many directional changes, it's more just a constant spinning kind of deal.

Also, rotary engines aren't really revving as high as the RPM indicates. When a rotary engine is revving to 9,000 rpm, the rotor inside is only moving at 3,000 rpm. Only the output shaft moves at 9,000 rpm.

 

[spidey07]

well I guess that's what I was postulating - the weight of the components.

Now if I could only get a turbofied R1 motor into a miata or porsche 914 I'd be in hog heaven.

 

[MrWizzard]

Ya know those F1 engines are rebuilt after every race, some don't even make it threw 1 race. Smaller engines rev higher easier, also quality of engine parts.

I drive a s2000 the original 9000rpm one. From what I have read they really overbuilt the engine on the car so that it still has Honda reliability. As long as I take care of it.

If you could rev a new mustang to 9000 rpm you would put a pistons through your hood.

 

[Shawn]

RX8s can rev to well over 10000rpms. People have accidently downshifted at redline in their RX8s without any engine damage.

 

[91TTZ]

Originally posted by: Shawn
RX8s can rev to well over 10000rpms. People have accidently downshifted at redline in their RX8s without any engine damage.

I did that do my Z before. Instead of shifting from 4th to 5th, I went from 4th to 3rd. The RPM's went right off the scale.

But like I said about the rotary engines, they're not really revving to 10,000 rpm's. The output shaft is turning at 10k, but the rotor is spinning at 3333 rpm.

 

[gsellis]

Originally posted by: Shawn
RX8s can rev to well over 10000rpms. People have accidently downshifted at redline in their RX8s without any engine damage.

The biggest danger with over-revving a rotary is that the center gears will 'walk'. Never seen it happen though and still not sure how bad it can be.

 

[Shawn]

Forgot to add, there are people who have sent in their ECU on the Miata.net forums to have their Miata's rev-limiter bumped up to 7800rpms from 7300rmps without any ill-effects. It's kinda like overclocking your CPU. Some of the engines can handle being revved higher than others, even though they are the same design. The manufacturer decides what the max rpm is considered safe for most all of the engines and that is where the rev-limiter kicks in.

 

[MrWizzard]

Shawn

RX8s can rev to well over 10000rpms. People have accidently downshifted at redline in their RX8s without any engine damage.

Damage might not be evident untill 3 years later, and your compression is all jacked up. I am suprized RX8s didn't put a rev limiter in their car.

 

[91TTZ]

Originally posted by: Shawn
Forgot to add, there are people who have sent in their ECU on the Miata.net forums to have their Miata's rev-limiter bumped up to 7800rpms from 7300rmps without any ill-effects. It's kinda like overclocking your CPU. Some of the engines can handle being revved higher than others, even though they are the same design. The manufacturer decides what rpm is considered safe for most of the engines and makes it that.

The problem with that is that the car is still cammed the same way it was before, so it's not going to breathe any better. On mine, I have the rev limiter bumped up to 7200 rpm but there's no use revving there since the engine's running out of steam by then. You'd be making more power if you just shifted into a higher gear.

 

[MrWizzard]

The problem with that is that the car is still cammed the same way it was before, so it's not going to breathe any better. On mine, I have the rev limiter bumped up to 7200 rpm but there's no use revving there since the engine's running out of steam by then. You'd be making more power if you just shifted into a higher gear.

You would need a new cam set to take full advatage of it.

Edit: I agree just upshift

 

[gsellis]

Originally posted by: MrWizzard
Shawn

RX8s can rev to well over 10000rpms. People have accidently downshifted at redline in their RX8s without any engine damage.

Damage might not be evident untill 3 years later, and your compression is all jacked up. I am suprized RX8s didn't put a rev limiter in their car.

No amount of rev limiting will prevent over-revving a blown downshift. Engine speed will try to match gear speed for that mph. Remember, rev limiters retard spark and/or fuel. Those aren't involved.

Edit - Oh, compression is not a factor either. The apex seals could potentially be damaged, but a rotary is basically a 2-cycle engine and there is oil present.

 

[MrWizzard]

Originaly posted by gsellis
No amount of rev limiting will prevent over-revving a blown downshift. Engine speed will try to match gear speed for that mph. Remember, rev limiters retard spark and/or fuel. Those aren't involved.

Good point.

Edit: Would be a kind of violent downshift though, like someone hit the brakes, because the rev limiter would not allow the engine to put power out untill the RPMs came back down.

 

[MustangSVT]

are you the guy that spawned the term "SPIDER!"?

 

[Demon-Xanth]

High redline does NOT mean a wide powerband. I don't think F1 cars can even idle below 5000RPM. The S2000's high 8000RPM redline does not yeild a wide powerband, merely a high, peaked powerband.

I have a Dodge w/ a 4.7. The redline is 6000RPM. Above this RPM the oil pump will cavetate. However, my powerband is 2500-5500RPM. Taking it to 6000 is pointless Now, in my dad's truck, he has an RPM of about 3300RPM. His powerband is about 1600-redline. What limits him is the slower burning diesel, and the long connecting rods. My brother in law has a 377 in an El Camino that did 7500, his limitation was the valvetrain. A friend had a 358 out of a Pro Stock truck (now defunct NHRA class) that did 9200 if you wanted it to last forever, 9400 if you wanted it to last the season, and 9600 if you wanted it to last the weekend. At the higher RPMs the reciprocating parts started to have much more stress.

Going back to an F1 engine. They have a very short stroke, so that the pistons don't have to be accellerated as fast. They only have to last about 135 miles. They can be lightened and pushed much further than a car engine that is expected to go 135,000 miles. They cost around $150,000 iirc, rather than the $15,000 engine that goes into the Viper, or the $1500 engine that goes into your normal family car.

Speed costs money, how fast do you want to go?

 

[spidey07]

Originally posted by: MustangSVT
are you the guy that spawned the term "SPIDER!"?

No, that would be syringer.

http://www.urbandictionary.com/define.php?term=spider

 

[Sureshot324]

F1 engines have a really short stroke, meaning the piston doesn't move up and down very far. This means that the piston doesn't have far to go to complete a full engine rotation.

 

[spidey07]

Originally posted by: Sureshot324
F1 engines have a really short stroke, meaning the piston doesn't move up and down very far. This means that the piston doesn't have far to go to complete a full engine rotation.

shorter stroke = less inertia?
Same piston acceleration, but shorter stroke?

 

[Kadarin]

Originally posted by: MrWizzard

The problem with that is that the car is still cammed the same way it was before, so it's not going to breathe any better. On mine, I have the rev limiter bumped up to 7200 rpm but there's no use revving there since the engine's running out of steam by then. You'd be making more power if you just shifted into a higher gear.

You would need a new cam set to take full advatage of it.

Edit: I agree just upshift

Not only a new cam, but the intake and exhaust, along with the cylinder heads, would need to be engineered to handle the amount of airflow at that rpm.