I have a lame question.

Let's say you have two identical cars with the only difference in the engines. One of them has a regular gasoline engine with higher HP number at high RPMs and the other is turbo-diesel with good torque at low revs and less impressive HP figures.

1. Which one of these cars would have a better acceleration from a dig?
2. Which one would accelerate faster from a roll?
3. In other words, if we don't consider towing a trailer, which one should have better dynamics (whether it is real or perceived from behind the wheel)?

Thanks!

 

The high torque car will probably get off the line better (assuming traction isn't a problem in either car).

Once the gasoline engine can stay high in the powerband though it will out accelerate the lower HP car (with weight equal as you said). Horsepower is derived from torque:

hp = (torque*rpm)/5252

Meaning a higher hp rating at a certain rpm means the car has more torque at that point than the other one. Torque is a measure of the force the engine is exerting, while horsepower is a measure of the work you can do over time with that force.

 

I'll make some assumptions.

1) It's the same car
2) The turbo diesel will way about 100-150lbs more because of the heavy cast iron block and turbo hardware.
3) The turbo utilizes variable vane technology (BMW, Benz use it)
4) The turbo diesel makes peak power at ~4300rpms (that's high for a diesel) and redlines at 4500rpms.
5) The NA motor makes peak power at 6400rpms and redlines at 6700rpms
6) The NA motor is rated at 280hp@6400rpms and 270tq@4500rpms
7) The turbo diesel makes 220hp@4300rpms and 350tq@2000rpms
8) Both cars use the same gearing
9) Both cars are manuals (autos change things quite a bit because of the different torque converter stall speeds that would be used).

So, what we're seeing is that the NA motor makes a good bit more peak power and less torque and at a much higher rpms. The turbo diesel is the exact opposite. With the NA motor, you need to keep the rpms above 4000rpms for maximum performance and the diesel's max performance occurs shortly after idle. The NA motor's powerband is from 4500rpms to 6700rpms and the diesel's is from 2000 to 4300rpms (turbos loose steam real quick). When you calculate out the average power over the powerband, the NA motor has the advantage; however, the diesel can do the work quicker, but it doesn't have ability to breath near as hard as the rpms go up. From 0-40mph, the race would be close and the diesel may actually have the advantage and it would be far easier to launch. As the MPHs increase, the NA motor starts to stretch it legs. With every 10mph increment, the gap widens, little by little because the NA motor can accelerate longer in it's powerband. By the end of the 1/4 mile, the NA car will have about a .5 second and 4mph lead.

In super simple terms, this is what the run would sound like in the NA car and the diesel car

NA car:
1st waaaaaaa
2nd waaaaaaaaaaaaaa
3rd waaaaaaaaaaaaaaaaaaaaaaaaaa 1/4 mile end

Diesel car:
1st waa
2nd waaaaa
3rd waaaaaaaaaa
4th waaaaaaaaaaaaaaa 1/4 mile end

 

Actually a pretty good question if he's talking about the BMW 3 series turbo dieseil that's being rumored to being imported here.

 

Horse power sells cars but torque wins races!

 

Not true at all.

Higher HP car is going to win the race that is the bottom line.

BTW Wsup Tru-G!!!

 

Carroll Shelby is on the G forums?!


JK. +10 for the quote.

 

HP is how fast you hit the wall, torque is how much wall you take with you.

 

Mr. Shelby was terribly mistaken. Greater average HP wins races.

 

Sup V? I didn't realize that my little comment was going to start such a commotion! LOL

I gotta stop making such broad sweeping statements on this board. You'd think I'd know better by now! LOL

See you at Mile Square!

 

You are correct...... Sometimes..... and not in the way you're saying it.


Peak HP is what most people compare, but it doesn't tell you the whole story. What matters is the area under the curve (hp or tq.... doesn't matter, one is just a calculation of the other).


Torque (at rpm) is what you FEEL when you accelerate. In any gear, the peak acceleration occurs at peak torque. The more torque you have, the faster you accelerate. If you have more torque for a LONGER period of time (more rpms) that's better. HP is basically a calculation of that.

If you're comparing two cars with the same PEAK hp, I'd rather have the one with a much higher peak torque, rather than one with a flat torque curve.

Perfect example of how gobs of torque is GREAT!!

2005 Mercedes E320 CDI (Diesel)

Only 201hp
369 lb/ft of TORQUE!!!

27mpg City, 37 mg HWY!!!

0-60 6.8
1/4 mile: 15.1


Oh yeah..... and it weighs almost 4000 lbs.

So... in comparison to an Acura RSX. It has the same HP, weighs almost 1000 lbs more, is basically as fast, and gets better fuel economy.

Put that Diesel in the RSX, and you'd be waaay faster, and get over 40 mpg.

 

Yeah... sure.... you had no idea.....

Usually the HP vs. TQ threads end up with 30 pages or so.

 

if you have 20hp and 2000 torque. you'll get to 40mph in like .2 seconds, but that'll also be your top speed (40mph). if you have 1 million horse power and 5lbs of toque, you're top speed will be zillion mph, but you'll reach that in 3 months...

 

that's the explanation i've been looking for! sure its far-fetched, but can anyone confirm if this is true?

 

that is exactly right, but at the same time if u rev the engine to where it makes that peak 1 million hp and dump the clutch the car will get there in very fast time.