When tuning an engine, which numbers are more important, the tq or the hp? Which numbers truly dictate how powerful the engine is, and how fast/quick the car will be?

Just trying to get a good ALL MOTOR discussion, and trying to better understand some tuning concepts.


All input is more than welcome.

 

Isn't tq needed at low RPMs... ie launch and hp is needed for high rpms?

 

They are both important...a 200whp Engine with 110ft lbs will get smashed by a 200whp Engine with 160 ft lbs. Other things come into play after that such as gearing...an engine with more torque can get away with longer gears.

More on this later.

Suprdave

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Suprdave &raquo;</TD></TR><TR><TD CLASS="quote">They are both important...a 200whp Engine with 110ft lbs will get smashed by a 200whp Engine with 160 ft lbs. Other things come into play after that such as gearing...an engine with more torque can get away with longer gears.

More on this later.

Suprdave</TD></TR></TABLE>

Exactly like I have a ~175whp JDM B20b with around 140 ft-lbs, so I can get away with running an LS tranny where a 175whp b16a would not be able to go over train tracks with that tranny.

 

This could get very interesting... keep the comments coming!

 

You tune the car to make the highest cylinder pressure for as high a RPM as the components can stand. The more cylinder pressure, the higher the torque. The higher the RPM you keep the torque up, the higher the HP. Chances are the breathing system is compromised to make lower RPM reasonable, so the torque curve falls off (diminishing returns) before the max RPM durability threshhold is seriously threatened. So it is then a question of getting the torque curve to fall off slower, assuming you don't want to shift the peak torque RPM to higher RPM.

 

You ought to read this article:

http://vettenet.org/torquehp.html


Horsepower is the total potential to do work over time. At any given instant and point on the RPM range, torque at the wheels (engine torque x overall gear ratio - drivetrain loss) determines how hard cars accelerate.

 

A car with 100 lb-ft of torque with a 1:1 transmission gear (pretend its fourth) and a 4:1 final drive ratio will effectively have 400 lb-ft of force at the wheels. If the total resistance (aerodynamic, rolling resistance, and weight) to change in state of motion (acceleration) equaled 400 pounds, and traction permitted, you could, at that instant, be accelerating at one g of force. Accelerating just as hard as if in a freefall. There would be a shove in your seat equal to your weight.

Edit: sorry for the multiple posts, and there will be more because my computer is a bitch, and it fcks up.

 

Here's an example I like to give. Assume two cars exactly the same except for the engine. Same body, weight, weight distribution, tires, trans, all gearing, except for the engine. It is adapted from the example in the previous linked article.

My computer won't let me do long or multi-paragraph posts, so there's more coming in inelegant segments.

 

Now pretend one is a awesome, short-stroke, ultra-high rev engine that makes 200 lb-ft of torque at 5250 rpm and holds that torque flat to the rev limiter of 10,500 rpm, where it makes 400 hp.

The other is a torque machine, double the displacement. 400 lb-ft of torque at 5250 rpm and 400 hp at the same RPM, with the rev limiter at 5250. A nice, tractable, flat torque curve.

 

The high torque car pulls much harder, basically twice as hard. So off the line, until it is shifted from first gear, it blitzes the low-torque car. The low-torque, high-rev car is geared that same, but revs twice as high, so it shifts out of first gear at TWICE the ground speed the low-rev, high-torque car does. While the high-torque car is in second gear, with less torque multiplication of the taller second gear, the low torque, high-rev car is still pulling hard through first gear.

 

Both cars are just as fast as each other. One pulls twice as hard (high torque), but half as long (low rev). The other pulls half as hard, but twice as long. The way you rate that overall potential to accelerate is horsepower.

That's all unless something needs clarifying/correction.

 

LOL...Einstein, it took you 7 posts to get your point across

I'll bet the orginial Einstein was like that too!

 

So is he the record holder?

 

Horsepower is directly realted to torque. As far as what makes a faster car it depends on the application. On a drag car you can tune for mostly top end because you're going to keep the tach parked in the higher RPMs. On a road racer you're going to want the fattest powerband you can get because you'll be reving throughout the entire rev range depending on the course. I'd say gearing is much more important than outright power, take my Si for example. It's geared pretty damn tight so I don't have to worry about torque once I get it moving the gearing multiplies what little I have.

 

Horsepower tells you how truly powerful an engine is, and the theoretical maximum acceleration of a car.

Torque comes into play because an engine is used throughout varying rpms where you're not at peak power most of the time. If you had some CVT gearbox with a ridiculously huge gear ratio spread, then for acceleration you would be pretty much be only interested in the power. Since it doesn't work that way, people are interested in torque.

 

this is how it was expaied to me.trouqe gets you there,horsepower keeps you there.

 

Don't forget about gearing. A final drives does wonder on a B16.......I should know since I got walk by one awhile back.

 

HP is a function of Torque.

Grassroots motorsports hada great writeup about TQ and gearing a couple moths back. It's the same mag with the RX-7 on the cover being dyno'ed. The cover story was "How to properly get your car dyno'ed.

Good articles in that mag.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by BERT-O &raquo;</TD></TR><TR><TD CLASS="quote">

Don't forget about gearing. A final drives does wonder on a B16.......I should know since I got walk by one awhile back.

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I couldn't imagine my gearing being any shorter than it is now.

 

Thanks for all the input guys.

Ok, so for example, on a B16 with stock gearing, which would one you tune for, maximum tq or max hp? Obviously a fatter power curve is more important, but is it the tq or hp that is more important?

 

I'd tune for midrange torque: High compression pistons, mild cam, light P&P, decent header, 2.5" exhaust, high flow cat, and Hondata. Should be good for about 170+whp.

 

I couldn't explain it much better that what was already said.

But, here's my .02 cents anyway

Torque is the actual perdendicular force the engine makes. Horsepower is the rate at which that force can be applied.

 

Thanks omniman.

 

Informative.