Question for people who know cars

[Dari]

1. At what horsepower is it prudent to go from FWD to RWD/AWD?

2. At what horsepower is it prudent to go from RWD to AWD?

3. At what horsepower is it prudent to go from manual to non-manual?

4. At what horsepower is the engine better served in a semi/plane/train/boat?

 

[Throckmorton]

1) RWD is better with any horsepower except on icy roads etc
2) 1000hp
3) Never
4) 3000hp

 

[Dari]

1) RWD is better with any horsepower except on icy roads etc
2) 1000hp
3) Never
4) 3000hp

For question #1, asked another way, what is the highest HP for a FWD where the rest of the car is stable and not being dragged along by its front wheels?

 

[jlee]

For question #1, asked another way, what is the highest HP for a FWD where the rest of the car is stable and not being dragged along by its front wheels?

That's how FWD works...by dragging the rest of the car along.

A lot more plays into your equation than horsepower. For what purpose are you using this vehicle?

 

[SyndromeOCZ]

The last question would probably be better to use the TORQUE figure instead of horsepower. I myself only pay attention to torque, horsepower doesn't make nearly as much sense to me.

 

[Yuriman]

I've read that marginally more power is lost between the engine in the wheels in a front engine RWD vehicle when compared with FWD, but the driving experience is very different and RWD is still preferred by most even with less horsepower.

In a FWD vehicle you'll have a more difficult time getting power from the wheels to the road because of the the change in center of gravity when accelerating (less grip). Because of this, FWD vehicles have a much lower effective horsepower cap, but it depends entirely on the vehicle as to what this is.

A manual is always going to be a more efficient transmission and you'll have a better feel for the drivetrain than with an automatic.

 

[Yuriman]

The last question would probably be better to use the TORQUE figure instead of horsepower. I myself only pay attention to torque, horsepower doesn't make nearly as much sense to me.

A 1.5L engine that produces 150HP at 8000rpm with a gear ratio of 2.0 will drive exactly the same as a 3.0L engine that produces 150HP at 4000rpm with a gear ratio of 1.0. What comes out of the other side of the transmission will be the same.

HP = Torque x RPM (divided by a constant), so halve your torque and double your RPM and you have the same power being delivered to your tranny.

The trick is finding the right transmission to match the characteristics of your engine.

 

[billbobaggins87]

to the OP. those questions are better answered for what is the true use and purpose of the car...
in other words is a commuting only car?
Are you going to be driving up north through snow?
Do you pull trailers launch boats?

the answer to those questions will get you to your cars drivetrain.

 

[LTC8K6]

Does the front wheel drive hypothetical car have a limited slip diff?

 

[Dari]

The vehicles I am talking about here are those that you drive on the street or at a track. Not really talking about utility vehicles...

As for the other, more specific, questions, I'm not a cary guy so explaining the differences and the outcomes would be grateful.

 

[nerp]

Manual is not "always going to be better" than automatic. The latest automatic transmissions can have 8 or more gears and can shift in milliseconds. Most supercars mated with automatic gearboxes post better times in automatic mode than when manually shifted, or the automatic version does better than the manual in identical scenarios. And the mileage on the newer automatics is better than what you get with a manual, too.

 

[Fenixgoon]

Does the front wheel drive hypothetical car have a limited slip diff?

MS3's and Focus RS's have some trick diffs, but at the end of the day you are still liable to get torque steer somewhere in the range of 250-300hp.

of course most people don't drive in a manner where it actually matters.

 

[unokitty]

1. At what horsepower is it prudent to go from FWD to RWD/AWD?

2. At what horsepower is it prudent to go from RWD to AWD?

3. At what horsepower is it prudent to go from manual to non-manual?

4. At what horsepower is the engine better served in a semi/plane/train/boat?

Its not really about horse power.

From my perspective, there is an elegance to RWD. That is, you can have the front wheels optimized to provide steering and the rear wheels optimized to drive the vehicle. My favorite cars Mazda Miata and Fiat 124 Spider were both RWD.

On the other hand, FWD is easier to package and can offer some space and weight advantages. My current car is FWD. But it is a transportation appliance rather than something that I might enjoy getting out on a track with...

At the same time, when I was in the Army, I went through their off road jeep training. And that was great fun. Though, I don't think driving a Grand Cherokee to the mall would offer the same level of fun.

While in the service, I had a deuce and a half license for two different bases. And there is nothing like a six wheel drive vehicle for certain conditions... I worked on a hill. And my dog squad went to and from that hill in a deuce and a half...

Might I suggest that your choice of drive for your vehicle is less a matter of prudence and more a matter of choosing what is appropriate for your environment and taste?

Uno

 

[Zenmervolt]

A 1.5L engine that produces 150HP at 8000rpm with a gear ratio of 2.0 will drive exactly the same as a 3.0L engine that produces 150HP at 4000rpm with a gear ratio of 1.0. What comes out of the other side of the transmission will be the same.

HP = Torque x RPM (divided by a constant), so halve your torque and double your RPM and you have the same power being delivered to your tranny.

The trick is finding the right transmission to match the characteristics of your engine.

And yet, whenever engineers design a vehicle specifically for hauling very heavy loads they use a high-torque, low-RPM engine.

While you are correct in theory, in actual practice there are numerous reasons why low-RPM torque is preferable for sustained hauling. Most mundane is fuel economy; RPM is the 800-pound gorilla in terms of vehicle fuel economy and an engine with more torque at low RPM will yield better mileage for long-distance hauling. Additionally, there is the issue of engine wear. Internal friction increases significantly with RPM and an engine spinning at 8,000 RPM will simply not last as long as an engine spinning at 4,000 RPM.

ZV

 

[amdhunter]

At only 209lbs of torque, my Jetta was enough to suffer from a lot of torque steer, so I'd guess around 200 you'd start to want a RWD car.

 

[SyndromeOCZ]

A 1.5L engine that produces 150HP at 8000rpm with a gear ratio of 2.0 will drive exactly the same as a 3.0L engine that produces 150HP at 4000rpm with a gear ratio of 1.0. What comes out of the other side of the transmission will be the same.

Not really, you have to rev the smaller motor up to the 8k RPM's to get to the same power as the larger engine. In my opinion torque is a more valuable measurement, and as you stated HP is just simply a calculation off of torque. When I'm looking at a vehicle to see if it has enough power to get the job done then I look at the torque curve, not the HP.

 

[T_K]

1. At what horsepower is it prudent to go from FWD to RWD/AWD?

2. At what horsepower is it prudent to go from RWD to AWD?

3. At what horsepower is it prudent to go from manual to non-manual?

4. At what horsepower is the engine better served in a semi/plane/train/boat?

1. From a performance stand point; always.

2. ~500 crank HP, if remaining on street tires. It's not such a performance thing as it is an ease of driveability thing.

3. User preference.

4. As much HP as possible, at as low of an RPM as possible, i.e. high torque at a constant load rpm.

 

[Yuriman]

Not really, you have to rev the smaller motor up to the 8k RPM's to get to the same power as the larger engine. In my opinion torque is a more valuable measurement, and as you stated HP is just simply a calculation off of torque. When I'm looking at a vehicle to see if it has enough power to get the job done then I look at the torque curve, not the HP.

What's wrong with revving a motor that's designed to spin faster? A motorcycle engine that makes 150hp and a big truck engine that makes 150hp both make 150hp, they just need to be geared differently to put the same power out at the same tire speed. The motorcycle engine likely will not last as long at full output but that has nothing to do with power output.

That motorcycle engine will spin for years at 8k while the larger engine will probably fall apart the first time you rev it that high.
HP is not "just" a calculation off of torque, they're forces related by RPM.

 

[KentState]

What's wrong with revving a motor that's designed to spin faster? A motorcycle engine that makes 150hp and a big truck engine that makes 150hp both make 150hp, they just need to be geared differently to put the same power out at the same tire speed. The motorcycle engine likely will not last as long but that has nothing to do with power output.

That motorcycle engine will spin for years at 8k while the larger engine will probably fall apart the first time you rev it that high.
HP is not "just" a calculation off of torque, they're forces related by RPM.

How is said motor suppose to spin up to 8k RPMS from a stand still and get the vehicle moving? Good luck trying to apply the horsepower and not destroy a clutch when trying to pull out a stump. Torque is what gets you off the line.

 

[Yuriman]

How is said motor suppose to spin up to 8k RPMS from a stand still and get the vehicle moving? Good luck trying to apply the horsepower and not destroy a clutch when trying to pull out a stump. Torque is what gets you off the line.

With the same sized clutch and an equivalently longer gear, the result is the same. The issue with smaller motors is that they tend to have smaller or weaker clutches.

 

[jlee]

What's wrong with revving a motor that's designed to spin faster? A motorcycle engine that makes 150hp and a big truck engine that makes 150hp both make 150hp, they just need to be geared differently to put the same power out at the same tire speed. The motorcycle engine likely will not last as long at full output but that has nothing to do with power output.

That motorcycle engine will spin for years at 8k while the larger engine will probably fall apart the first time you rev it that high.
HP is not "just" a calculation off of torque, they're forces related by RPM.

How many motorcycle engines do you see with 500k+ on them?

With the same sized clutch and an equivalently longer gear, the result is the same. The issue with smaller motors is that they tend to have smaller or weaker clutches.

You'll need a ridiculous amount of clutch slip to get an equivalent load moving in the situation you describe. Slip is what wears clutches quickly.

 

[Yuriman]

How many motorcycle engines do you see with 500k+ on them?



You'll need a ridiculous amount of clutch slip to get an equivalent load moving in the situation you describe. Slip is what wears clutches quickly.

This is fallacy.

Consider that the engine is running at twice the RPM with half the torque. Couple it with a gear that is half as long, resulting in the same power curve on the other side of the transmission. With the same clutch, it'll last just as long because (due to shorter gearing) the clutch needs only work half as hard to start the load moving.

The only real issue is that yes, you can't run a motorcycle engine for 500,000 miles. Motorcycle engines are also more expensive for the power they put out because they need to be light and compact and survive a reasonable period at high rev's. You're not going to wear out clutches any faster however if it's geared correctly.

Torque is what gets you off the line.

Torque x gearing is what gets you off the line. Half the torque with half the gearing accelerates exactly the same.

Of course you can put a bigger motor on the same transmission and get off the line faster, but the same thing can be achieved by shortening the gears.

 

[jlee]

This is fallacy.

Consider that the engine is running at twice the RPM with half the torque. Couple it with a gear that is half as long, resulting in the same power curve on the other side of the transmission. With the same clutch, it'll last just as long because (due to shorter gearing) the clutch needs only work half as hard to start the load moving.

The only real issue is that yes, you can't run a motorcycle engine for 500,000 miles. Motorcycle engines are also more expensive for the power they put out because they need to be light and compact and survive a reasonable period at high rev's. You're not going to wear out clutches any faster however if it's geared correctly.

Peak torque figures for motorcycle engines (if we're talking inline 4) seem to be above 10k.

Peak torque for truck engines are what, 2k?

I think you'll need more than twice the gearing.

 

[KentState]

This is fallacy.

Consider that the engine is running at twice the RPM with half the torque. Couple it with a gear that is half as long, resulting in the same power curve on the other side of the transmission. With the same clutch, it'll last just as long because (due to shorter gearing) the clutch needs only work half as hard to start the load moving.

The only real issue is that yes, you can't run a motorcycle engine for 500,000 miles. Motorcycle engines are also more expensive for the power they put out because they need to be light and compact and survive a reasonable period at high rev's. You're not going to wear out clutches any faster however if it's geared correctly.




Torque x gearing is what gets you off the line. Half the torque with half the gearing accelerates exactly the same.

Of course you can put a bigger motor on the same transmission and get off the line faster, but the same thing can be achieved by shortening the gears.

You can only shorten gears so much before they become to brittle to be worth a damn.

Secondly, the size of a clutch has a huge impact on heat dissipation and clamping force. Yes you can put a smaller one on a high revving engine and hope it can withstand a 8k launch, but that will only work so many times before it's toast when trying to move anything heavier than a few hundred pounds. Even on a light motorbike, they can only take a few hard launches.

Your argument is only taking a few factors into consideration, but leaving out a lot of other factors. If it was just as simple as building a high revving engine to move stumps do you think that Honda would be king of truck engines by now? Small engine/high revving is more about packaging or the need for lower power.

 

[JCH13]

At only 209lbs of torque, my Jetta was enough to suffer from a lot of torque steer, so I'd guess around 200 you'd start to want a RWD car.

My MS3 has over 275 hp and over 300lb-ft of torque and I don't really notice torque steer often. Torque steer management is a science, particularly balancing the stiffness of the left and right drive shafts. My MS3 has a 3 drive shafts, one is fixed to the engine and the two moving shafts are the same length. This helps to control torque steer. It's even a problem in some RWD setups, but only really on the race track.

What's wrong with revving a motor that's designed to spin faster? A motorcycle engine that makes 150hp and a big truck engine that makes 150hp both make 150hp, they just need to be geared differently to put the same power out at the same tire speed. The motorcycle engine likely will not last as long at full output but that has nothing to do with power output.

That motorcycle engine will spin for years at 8k while the larger engine will probably fall apart the first time you rev it that high.
HP is not "just" a calculation off of torque, they're forces related by RPM.

There are other forces too... like the force on engine bearings, vibration forces that increase with rotational speed squared. Also, running an engine twice as fast, or more, doubles the number of cycles that each bearing sees. This reduces their load carrying capability, increases heat generation, and halves their useful life in terms of hours of operation. Adding gearing stages causes extra energy loss through the gearing stages and additional vibrations and forces the system must endure. Engines that last a long time are slow and have low specific power output for these reasons and more.

There is a real, tangible benefit to having torque, and thus power, available from low RPMs through higher RPMs. The vehicle is more comfortable to drive, fewer gears are needed for normal driving, fuel economy is improved, etc. Clutch slippage is not a viable option for overcoming a low-torque motor, clutch material goes away too quickly and a larger/heavier clutch adds some imbalance and inertia that will really hurt a high-revving engine.

Your ideas are correct in the simplest of theoretical environments, are you a physicist who likes to live and work in a friction-less vacuum?