Whats the advantage of high revs with the same power?

[DGthe3]

I've been looking around trying to answer this question and I keep comming up blank. Lets say there are 2 engines. Each makes 500 hp, each is a V8, each is naturally aspirated, each will weigh nearly the same. One screams up to 9000 rpm, the other makes that 500 hp at 6000 rpm. So the high rever has a peak torque in the area of 350 ftlbs, the stump puller has about 475 ft lbs. What advantages does the high reving engine have over the other one if they both make the same power? I can list off a bunch of disadvantages but I want to know the other side of the story.

I know that a Camaro forum isn't the best place to post such a question. But its the only car forum I belong to, so I'll work with what I have. Besides, there are enough car guys here that should be able to answer it regardless of personal taste

[Sterling]

I much prefer low end torque... but a high rever could take better advantage of a turbo charger and could utilize a low rearend ratio (ie 4.10) better.

[jhitson]

A saying that I have alway heard is "Build for Torque, and talk about Horsepower." I take that to mean that a higher torque number is better. I don't know if there is any truth to that as I don't build engines.

[Mr. Wyndham]

As far as I can tell...with my very biased, and limited knowlege in the area, higher revs = .....


Well, hell. I don't know?! I can't think of ANY advantages...Higher chances of failure and wear. Terrible gas mileage. Strain, god awful noise (imo), and you have to wait for those 8 thousand revs to come... I'm at a loss.

[MTron]

Basically, a higher revving engine is better for some applications and worse for others, it really depends on what you want, but in general, all things being equal, making the same power at a higher rpm is better

There are 2 ways to make good torque at the back wheels.

1. A really torque'y engine or
2. sacrifice rpm's for increased torque, as you gear down an engine, the rpm at the wheels drops, and the applied torque increases...its like a lever, a bigger lever allows you to have lots of leverage, but you have to move to farther to get the same result. The same applies to engines

Also since horsepower = (Torque x rpm)/5252 you will make more hp at higher rev's, allowing you to gear the rpm's down to multiply your torque.

However, certain problems can be run into, first, high revving engines are more complex and costly, as they must be finely tuned and balanced as well as made of strong lightweight components to handle the high rpm's. Also, the vast majority of high rpm engines dont have a very flat torque curve, this is due to the type of camshaft they use that allows them to make power at high rpm's. Techniques such as VTec try to work around this by having different camshafts for different rpm ranges. Because high revving engines have a very narrow "torque band" their power peak is also very narrow (see equation above).

All this equates to two very different engines, if you go with a low revving but very powerful engine you have a very broad torque curve, allowing for good acceleration and driveability at all rpm ranges, a good example is the tv show "top-gear" and their short on the Corvette Z06 going from stand still to 150mph (or somtin) all in 5th gear, a wide torque band allows this.
The high revving engine could probably not do this at all, but if you keep the engine in the torque peak/power peak then you can really thrash it around, and because the torque perk occurs at such a high rpm, when you gear it down, you make even more torque at the wheels

eg lets take the z06 transmission that has a .56:1 6th gear

400 lb/ft at 8000 rpm ->6th gear -> 14 285 rpm and 224 lb/ft at the drive-shaft
400 lb/ft at 6000 rpm ->6th gear -> 10 714 rpm and 224 lb/ft at the drive shaft

as you can see the overdrive 6th gear of the transmission increased the rpm of the drive shaft but decreased the torque, but the higher revving engine has more rpm's in the driveshaft, so for the same speed of the drive shaft, we can change the ratio a bit and the higher revving engine will have more torque in the drive shaft

400lb/ft at 8000 rpm, with a .746:1 transmission ratio, will give us 10 714 rpm (same as the 6k rpm engine above) but it now has 298.4 lb/ft available

all this means, at the same speed, the higher revving engine will put more power to the wheels then the lower revving engine, but all at the cost of having a very narrow power band

[Mythic]

Torque gets you off the line, HP is for speed

Torque is also what is most likely to break your tires loose in hard cornering if you tab the throttle too soon.

High rev mutli-cam engines typically make their HP with smaller displacement, Though not always lighter than a higher displacement pushrod they are usually more compact. This allows the engine to be mounted closer to the firewall for weight distribution. Though front engine with a 6L V12 the new Ferrai 599 ha a 47:53 weight distribution

Additionally high rev engines typically have much taller axle ratios to get the power to the wheels.

http://craig.backfire.ca/pages/autos/horsepower
take 2 aspirin and a shot of JB

[DGthe3]

Originally Posted by Dragoneye

As far as I can tell...with my very biased, and limited knowlege in the area, higher revs = .....


Well, hell. I don't know?! I can't think of ANY advantages...Higher chances of failure and wear. Terrible gas mileage. Strain, god awful noise (imo), and you have to wait for those 8 thousand revs to come... I'm at a loss.

Thats exactly why I started this thread. I can't think of a good reason to do it, but the Euros love it that way so there must be some reason.

[MTron]

i thought i did a pretty good job of detailing it...LOL albeit rather lengthy....but it is a quite....diverse subject, with lots of room for debate ^^^^^^^^^(points to earlier post)^^^^^^


p.s my preference is a nice big displacement, torque'y engine with a big fat flat torque curve.....LS3 anyone!

[Sterling]

Originally Posted by DGthe3

Thats exactly why I started this thread. I can't think of a good reason to do it, but the Euros love it that way so there must be some reason.

They love it because it's the only way to get the horsepower out of those small displacement engines.
They don't have the torque so they gotta have the high rpm's to make up for it.

[DGthe3]

Originally Posted by MTron

400 lb/ft at 8000 rpm ->6th gear -> 14 285 rpm and 224 lb/ft at the drive-shaft
400 lb/ft at 6000 rpm ->6th gear -> 10 714 rpm and 224 lb/ft at the drive shaft

as you can see the overdrive 6th gear of the transmission increased the rpm of the drive shaft but decreased the torque, but the higher revving engine has more rpm's in the driveshaft, so for the same speed of the drive shaft, we can change the ratio a bit and the higher revving engine will have more torque in the drive shaft

400lb/ft at 8000 rpm, with a .746:1 transmission ratio, will give us 10 714 rpm (same as the 6k rpm engine above) but it now has 298.4 lb/ft available

all this means, at the same speed, the higher revving engine will put more power to the wheels then the lower revving engine, but all at the cost of having a very narrow power band

Out of all that, I think this will illustrate my point the best. Modifying the example you provided, using a low torque/high rev engine making 300 ft llbs.
300 lb/ft at 8000 rpm ->6th gear -> 14 285 rpm and 168 lb/ft at the drive-shaft
400 lb/ft at 6000 rpm ->6th gear -> 10 714 rpm and 224 lb/ft at the drive shaft

Both engines there are making 457 hp in that case but the high torque engine . . . makes more torque. if the final drive ratio is 3:1 for the HT (high torque) and 4:1 for the HR, you get:
HR ->14285 rpm/168 ft lbs -> 3571 rpm 672 ft lbs
HT ->10714 rpm/224 ft lbs -> 3571 rpm 672 ft lbs

They give the same numbers because they have the same power. So with proper gearing, the wheels won't know the differance. So whats the advantage of making a screamer vs making a stump puller, given all of the negatives associated with high revs?

[Mindz]

Originally Posted by Dragoneye

god awful noise (imo)

I take it you're not a F1 fan?

[Mr. Wyndham]

Originally Posted by Mindz

I take it you're not a F1 fan?

No, I am...but I have to wear something in my ears..... They look cool, they Go fast - but that screaming...ugh. I'm more of a roar than a banshee-scream type of guy....

[GTAHVIT]

Hmmm,

Shooting from the hip:

Scenario

Same tranny, final drive gear ratios, single gear (no shifting), HT Redline = 6000, HR Redline = 9000.

If you had a drag race to 6000 RPM, the HT engine would get to that RPM faster/quicker than the HR. Eventually the HR motor would get to the same speed at the same RPM but would be a few car lengths behind the HT equipped car.

Assuming that the HT redline is 6000 and the HR redline is 9000. Eventually, the HR car will pass the HT car with an overall higher top speed as he increases his RPM from 6000 to 9000 RPM.

So, all things being equal, The HT will have better acceleration and the HR will have a higher top speed.

If I'm all gooned up, please re-educate me.

[The_Stache]

Just like sound systems... You build towards a goal.. there is no one true way.

Each has its own advantages/disadvantages.

And just like sound systems, my first question is "What do you want it to DO?"