HP vs Torque

[Phantom]

I am all the time seeing weight-to-HP comparisons between vehicles but never see weight to torque comparisons or real discussions about troque curve comparisons. I know the torque curve on the L28 moved significantly between the 280z and ZX models resulting in a car that would accellerate faster with no more HP and a bit extra weight. I also know I can hammer a ricer if i have close to the same HP because of the gobs of extra torque. I also know that if it is taken to the extreme stage I'll end up with the accelleration of a diesel. So where is the balance? What are the factors that limit one vs the other?

[Mikelly]

This has the makings of a VERY good topic discussion.

 

Not sure I can answer your specific question, but one thing I've always tried to do was not focus on Peak torque or Peak HP. Although One L28 motor I built was extremely peaky and didn't perform consistantly. The Goal should always be to figure your target RPM performance range and match the torque and Peak HP to that range. In other words, I would much rather have less torque, but more of the torque I have available through a broader RPM range, and that range to spread into the peak HP zone. This gives a much better, more tractable and usable range for the motor to perform at.

 

Add these two factors into the mix of a 2500-2700 Zcar, then factor in the range of gearing, both in the transmission, rear differential, and the tire selection, and you can see that LOTS of planning must be done, compramises made, and stuck to, or you end up with a project you may not be happy with as an end result (like the T56 in my 383 stroker project!).

 

Now, I'm not sure what I just typed helps any, but I've nexer heard of X Pound = #Torque... I'm hoping to learn something as well!

 

Mike

[Mike C]

Seems to me the point of maximizing torque is really to maximize average horsepower. Making more power through a broader range is the goal. For instance, 350ci V8 with 400lb ft at 2000 rpm is 152 hp, but say the L28 makes 100 lb ft at 2000 rpm and that is just 38 horsepower! Say both motors peak at 250 horsepower, all of the extra time it takes for the L28 to get there, the V8 is just running away and hiding. Since horsepower is a function of torque, torque is what we are really wanting to optimize. This is where the best average torque numbers will be the best performer. My "butt-o-meter" says the 331 originally in my Camaro and currently residing in my Jimmy that now has a 266 210@ .050 cam would easily outrun the 284 228@ .050 cam that was in the motor when it was in the car. Max rpm has been reduced by 2000, from 6500 to 4500, but average torque in that range is significantly higher. Just for a real world example.

Weight is just about inertia. If you could take a new powerstroke diesel and install it in a 2000# car, it would absolutely scream, regardless of the "acceleration of a diesel". Actually, you would be absoutely amazed at how fast my buddies powerstroke dually is with the complete Banks powerpak on it.

[Guest Your Car is Slow]

It would come down to what you want the car to do.

 

For all intents and purposes..the ultimate goal would be a car that continues to pull (meaning hp keeps increasing) all the way to redline so you can take advantage of longer gears or a higher redline to basically "stay in it" longer while the other guy has to shift more

 

Sure its great to have a broad powerband and a ton of area under the curve if you are talking about a street car...or an autoX car where it will see nearly every bit of the RPM range.

 

However if you are only concerned with dragracing or whatnot....I only care how much power the car makes above a certain RPM all the way to redline...because thats the only range the engine will see during the race.

[Guest vegeta]

Everyone seems to think that acceleration is either all about horsepower or all about torque. But its both. Torque is simply a measurement of the actual TWISTING FORCE of the engine; in other words torque is how HARD it turns. Meanwhile, horsepower is a measurement of the actual RATE at which FORCE IS APPLIED to the flywheel (or where ever). In other words, horsepower is how FAST does the thing turn. So, technically, horsepower is a function of torque, but without horsepower, your car would not accellerate.

 

So in order to move a mass (the car) from a standstill, you need a certain amount of torque just to get it moving, but once it gets going, there is not much FORCE to overcome (because the car has momentum), and you are relying on the horsepower- the RATE of force- to make the car accelerate. The amount of torque you need depends on the mass (weight) of the car, as well as the gearing (because numerically higher gearing multiplies torque). A heavy car or truck will need much more torque just to overcome the forces of inertia (correct word?) than a lighter car. AND it will need more horsepower. A broader torque/hp curve is necessary because accellerating from coasting at a given RPM obviously requires some torque/hp at that RPM.

 

The point is you can have all the horsepower in the world, but unless you have enough torque, it will take a lot of gearing or a long time just to get your car rolling, especially as the weight of your car goes up.

BUT you can have all the torque in the world, and once your car is rolling, you still need HORSEPOWER to keep the mass accellerating! That's why diesels are so damn SLOW is they have plenty of torque but they don't have hardly any horsepower, so once the torque gets them moving, there's nothing to accelerate the damn thing! It also explains why the Honda S2000(you know that rear wheel drive honda sports car?) is so fast even though it has hardly any torque (240hp, but only 160 lb-ft torque, 2650 lbs, 14.0 second 1/4 mile) it is light, so it doesn't NEED much torque as long as it has enough hp, and numerically high gearing.

[KiD-ViD]

a buddy of mine gave me the idea of figuring average hp and tq rather than just looking at the peak numbers and eyeballing the tq curve. I was messing around with dyno2k and he suggested that I average all the numbers. I found this to be a decent way to decide on which cam profile was better. I think it all depends on preference as well.

 

I learned to drive in my integra and have been driving it since. I have grown very acustomed to the 8100rpm redline and the near lack of power below 4k. I have also driven some more tourqy cars and I really like the feel. so for my datsun I am planning to go for somthing in the middle. I think peak power around 6500 with a 7k shift point would be real nice.

 

What one car lacks in low end tq it can make up with high end hp and vise versa. I think it differs from case to case. Anyway I think it would be nice if this topic was moved to high tech forum or misc tech forum. I think alot more people will see it that way and we will see alot more diverse opinions. most people here in the chevy board are low end tq biased so lets see what the L6 guys have to say.

[Guest Anonymous]

Phantom and Gentlemen;

 

The Power/weight ratios we use are an exellent "off the cuff" mental yardstick picture that can indicate the overall peformance difference to be expected between vehicles. The idea is not of HP alone but Torque also and the resulting preformance of the two combined.

 

For a give engine size, a cam ground to peak at 6000 can only wind up as fast as the available torque can move the load, and that depends on the weight of the load. Thus we know that this same Power applied to different loads will accellerate those loads at different rates of speed.

 

Then to answer questions resulting from this statement we have to turn immediately to so called "power band" (of a dyno chart), gearing and tire size and go through an explanation of that; And then it's on to the study of tuning intake and exaust and finally even the weather.

It's just a matter of getting enough of this information going around in your head to cause an explosion which results in "the light".

 

If the guy across the street has the same new make and model as you, you can race to the corner every morning and always be within a foot of each other but, if you put smaller tires on the rear or a numerically higher ring and pinion in the rear end, you will beat him flat every morning. This is better accelleration because you have lessened the torque load on the engine by effectively multiplying the torque leverage which in turn allows the cam to accellerate it faster and that's how that works.

 

If however you try him on the highway, the following happens; You pull away at first but at the end he beats you. This happens because you used torque leverage to get more accelleration which has caused your peak rpm to leave you at a lower speed. In short, you traded accelleration for top speed. In racing, this phenomen is used to set up a "Sweet spot" in the power band that will get us from point 'A'to point 'B' the fastest....I hope I haven't presumed upon you too much.

 

Perhaps you know all this already but:

 

Stock Bike: 350 lbs @ 40 hp = P/W ratio of 8.75:1

Stock Car: 3000 lbs @ 250 hp = P/W ratio of 12.00:1

 

The figures mean that in an accelleration test, Bike beats car hands down. You can tell at a glance. Top end is another matter but these day it is likely the bike will again prevail if geared correctly plus the fact that his wind profile is grossly less than the car.

 

This is pure Power/Weight ratio thinking.

[Guest Your Car is Slow]

Actually while the frontal area of a bike is less...the Cd of a bike with a rider is generally more than most cars.

 

Also keep in mind that while cars have a constant frontal area (and for the most part smooth) the rider on the bike provides an ever changing aerodynamic shape (unless somehow you could get yourself and your leathers to stay perfectly still).

 

Its much harder for a bike to cut through the wind at high velocity than it is a car.

 

Of course this really only applies to speeds above 100mph where this factor begins to play a more important role.

[Guest Carlissimo]

I know this is sacrilege, but I when I rebuilt my 327, I intentionally sacrificed low end torque for a wider powerband and more RPM on the top end. (I have to admit driving a friends 302 powered Z-28 and shifting at 8000 RPM might have swayed my opinion a bit.)

 

Think about it this way, my engine came out of a stock 3500lb 1967 Camaro with a 2 speed powerglide automatic transmission and no tach. A perfect application for an engine with nothing but torque.

 

I then put that engine into my 2500lb 240 Z with a 5-speed and a tach that shows redline at 7000 RPM. It would burn rubber all day long. Kinda cool, but I was not happy with the fact that it would rev up to about 5500 RPM and then just flat run out of steam. It also didn't seem to make much a whole lot of difference what gear I was in. I used to joke that first gear was for pulling tree stumps out of the ground.

 

I now have an engine that has less torque, but pulls strongly up to the 7000 RPM redline. I like it much better this way. I love blasting around, letting it run thru the gears, especially on twisty mountain roads.

 

Unless you have an automatic transmission or plan to pull a tailer, I don't see the advantage of trying to maximize torque.

[80LS1T]

I must say I am enjoying this thread a lot! Keep up all the good info!

 

 

So having a numircally(sp?) lower Power/Weight ratio is good? I thought it was the other way?

 

So if my car weights 3000@400HP then I have a 7.5:1 ratio? What is a good power to weight ratio for a Z/ZX with a V8?

 

Guy

[KiD-ViD]

technically its weight/power ratio. heh just thought I should add that.

 

a weight to power ratio of 15 will get you about 15sec 1/4 mile flat. a w/p of 10 will be somwhere around 13. a w/p of 5 will get you somewhere in the 11s.

[Guest Anonymous]

Well, actually that was a test to see who was awake.....NOT. What happened is that in a standard dissertation I use the slash (/) for the meaning of the word (and or) or (per) and not "divided by". In a math problen we would expect divison proper but is is used somewhat more loosely without nuimbers. My error however, I must stop doing that and I thank yoiu for the correction gentlemen!

-----------------------

 

Ya, 80LT1, I amsuddenly awake and enjoying it too.....LOL.

 

Guy, 7.5 lbs per HP is correct. You would have to weigh the car on public scales to get your number for that Z/ZX V8

 

DATSUN 240Z CURB WEIGHT'S

 

1970=2355

1971=2500

1972=2550

1973=2575 (weighs 220 lbs more than '70).

 

ALL 260Z=2605 (weighs 250 lbs more than '70).

 

ALL 1982 280Z=2875 (weighs 520 lbs more than '70).

ALL 1982 280ZX=2825*** (Ya, 50 lbs less ???).

 

ALL 300ZX=3000+ (weighs 645+lbs more than '70).

--------------------

The SBC is said to weigh 150 more than the original engine. Maby the BBC weighs another 100 more for 250. Take off 90 lbs. for alum heads.

 

***NOTE: There are two ZX's listed above.

[Phantom]

I think what is really coming out is that for the 1/4 mile you want a torque & HP peaky engine that gets as high as possible through a fairly narrow range that can be covered by gearing to keep it in that range for the duration of the run. Conversely, the daily driver / road race car will be willing to sacrifice the peak some in order to broaden the band and improve response over a wider RPM range. This is one reason why advertisers will refer to the width of the 85% peak torque range - especially on boosted engines. They know the advantage of that range for regular driving. That is kind of the curse of the diesel - it has tremendous torque, and frequently very good HP but it is across a narrow RPM band so you wear yourself out shifting to get it going. Do I hear CVT here? Too bad they currently can't take the torque & HP we want for our cars. It was earlier stated that this should probably be on the high tech forum - maybe. Having driven an NA L28 for 17 years and now stepping into a V-8 again I can tell you which I'm going to prefer - the one with the higher HP & torque that is across the broader band. I want to come off the line easily without having to rev the engine way up. I want the car to respond well in whatever gear I happen to be in without having to move it down 2 or 3 gears to find the sweet spot, maybe down one gear if someone really gets me going. And I want to run the quarter in about 3.2 - is that asking too much?

[Pop N Wood]

Open any high school physics book. Power is a measure of the ability to do work. Thus it is POWER that determines how fast a car can accelerate.

 

You guys want to talk torque, but you are measuring it at the wrong place. It is the torque applied to the rear wheels that determines the force in the F = MA equation. What ever torque the engines does or does not produce is meaningless if the car is geared wrong. You can't talk about the engine and transmission/rear end separately...they are both essential ingredients and must be matched to each other.

 

Simply put, the more horsepower you have, the more torque you can put to the rear wheels. This is a simple fact.

 

Taken one step further: Whoever has the most horsepower between the shift points wins.

 

Indy car teams know this. That is why they change the transmission ratios depending upon what track they are racing at. In fact, I have heard them talk about setting the ratios based on their speeds coming out of a specific corner at a specific track.

 

One advantage of a torquey motor is you don't have to be as precise with the gearing. A 2 speed power glide means you are going to have a huge change in RPM between shifts. In this case you had better have a broad torque curve to match OR your HP will drop too much after the shift.

 

Similarly, if your engine only makes power from 7000 to 8000 RPM, then you had better have enough gears to always stay in that band.

 

In some ways a street engine/tranny combo is a more demanding problem than a pure race engine. The street set up has to satisfy a wide range of mundane driving conditions without compromising too much flat out performance. In this sense the broad torque curve (read HP curve) of a V8 rules.

 

The one other thing to consider when deciding on a street engine is the availability of transmissions. Very few of us can afford to tailor a transmission to a specific engine/chassis set up. Thus we are limited in our transmission choices to what ever we can find in a JY. Since most of these trannys were designed for a street car, that means they are generally going to favor a wide torque curve.

 

Grumpyvette has talked about this in the past. Z's are so lite that they can't really make use of a lot of low end torque without smoking the tires. Thus a V8Z can afford to trade some low end torque for high end HP. In fact, Grumpy advocates a 383 stroker with a big cam and intake. This way you get a LOT of high end HP with the stroker helping to maintain an adequate low end.

 

Part of the problem with the whole torque vs. HP argument is all of the car magazines are always screaming "torque torque torque..". Mags like Hot Rod and Car Craft are geared toward guys with 3600#, street driven American muscle cars. In this case they are right. Big heavy cars need torque to be streetable. But a Z is a different application. A V8Z needs to read between the lines and move the torque curve up in RPM....to gain more HP!

[Mikelly]

Jim, See my original post??? Exactly what you said... Also, I'm building the 383 stroker to meet Grumpy's theory... God I hope he is right!!!

 

 

Good points guys!

Mike

[Phantom]

Consider this:

T56

1st = 2.66 :1

2nd = 1.78:1

3rd = 1.30:1

4th = 1.00:1

5th = 0.74:1

6th = 0.50:1

 

The odd ratio is 1st. Forget it as a 6-spd. and consider it as a 5-spd. , ignoring 1st gear. Select differential gearing based on that assumption and evaluate the numbers. It ends up with a great, close ratio, transmission that would keep most V-8's in their sweet spot. Then you always have first gear for the burn-out competitions.

[Mikelly]

Yes and no. You take a HUGE COST and WEIGHT hit for the T56. In your explaination you just showed how ludicrous it is to have an over weight 4 speed. The other BIG HIT that typically gets lost in the cool factor of the T56 is the hit to both torque and HP in using the T56. Jim Biondo Swears that the T56 is a huge robber of HP due to the added gears, fluid, and moving parts. Parasidic (SP??) drag creates a hit in the driveline as well. I've also read this claim on a couple of Viper sites, as well as one Corvette site. Mind you, I had a T56 and thought there were issues with the combo that took away from the cool factor...

 

Throw in tires and rear gearing and you can muck things up real good, or have a decent package... Depending on what you are building... If I ever get a C5, I'll get it with the 6 speed, but I'm gonna Keep my Z projects 5 speed only.

Mike

[Guest Anonymous]

Phantom;

 

I can see there's not a dummy in the bunch here and the chat is atill very interesting but you are poking us with a stick when you ask for a 3.2 trap time. Is this a type-o? Ha, Ha!.....(Waitress could you bring us a refilln please?).

 

Europeans birthed the 4+ gearboxes first as a means of navigating their type of roads. Long distance Road racing demands several gears and it too came from euroupe. Proponents of road racing will want all the gear changees they can get.

 

The roads In the states demanded a different approach to the gearbox and so our selection is different. Since the sixties we also have jumped on the euroupean approach so our Detroit sports cars could enter international racing competitively. Lately on the street however it has become a matter of cleaning up the air, so we get the european gears in everything. We all know that the engines have gotten very small and the trans gears have been multiplied to match their lack of punch (torque).

 

Unless you want to road race you don't want a lot of gears so as to navigate curves and for braking assist, instead you want brute force, and brute force means Torque, and more torque means you can have fewer gears! Now what have I said so far? That for curves you want many gears and for straights you want as few as possible (because of the time lost in shifting).

 

For our roads you just can't beat a stroker engine for performance! Sure we all love the macho stick and the sound of the engine pulling hard through the gears but, for racing there has to be a limit to this somewhere. To compete, time is the enemy and we have to tailor the car to fit the useage. With a stroker engine you have it all, you can gear any way you want so to fit the challenge and lose nothing, but with the short stroke engines (less torque) you have to have the gearing (more shifts) to try and match the stroker (enter the turbo). Now I am not a supporter of turbo charging at all but I must admit it pulls the smaller engines back into the picture!

 

These days it basically appears to be a matter of taste as to which way you go (Torque or RPM) but I'll stick with the father of it all 'Torque". And Phantom, with a torquer (stroker) the normal driving with the normal transmission is absolutely the same except that wherever you put your foot down more hell breaks loose than with a lesser stroke. (By the way boys, I am NOT considering the DOHC twin turbo in all this chat).

--------------------------

[Guest Anonymous]

Phantom;

 

Oh WoW! You got aholt' of the general idea for sure but the over gears are sure power robbers because of the additional gears required to pull it off. Good for gas economy, bad for racing, but it will all come to you. Let's look at you gear speed seperation at 6000 rpm for this trans.

 

GM T56 6-speed used in 1994-2002 models

 

Gear ratios:

2.66:1 first, 1.78:1 second, 1.30:1 third, 1.00:1 fourth, .74:1 fifth, .50:1 sixth, and 3.28:1 reverse.

 

Torque capacity: 450 lb-ft.

 

Approx. Dry Weight: 125 lbs.

--------------------------

Top of 1st to top of 2nd=36 mph

" " 2nd to " " 3rd=38

" " 3rd to " " 4th=45

" " 4th to " " 5th=66 (Remarkable but useless).

" " 5th to " " 6th=123 (Even more remarkable but useless).

 

This is far from a Close Ratio trans. (About as far as you could get)!

The first three numbers look only "fair" and overall, this is definitly no performance trans! The numbers above show how much speed increase betweeen each gear shift can be expected.

 

With this tranny and a 400 HP engine I wiold have to decide what 0-max speed range to settle on and then find the right ring and pinion to match the trans gears so to stay in the sweet spot while going through the shift pattern. Don't get it wrong here, the sweet spot is always there and you don't have to use it, but knowing it is there makes a difference in our thinking, no?

 

And so I would lower the rearend gearing so as to bring 1st gear into a useable speed range and have 4 gears. If on the other hand I had 600 HP, 1st would again become practically useless and I would probably use only 2nd, 3rd and 4th with the O.D.'s as candy for gas mileage (6th would surely be useless still as too low).

 

This trans will work, don't get me wrong, but it is tailored to the vehicle it came with and for the special purpose of satisfying a certain group of buyers. If we jack things around with a car then we have to do that part over again to satisfy our own tastes, and we had better know what we're doing! You cannot use these OD gears for street

or even highway performance but, for gas mileage.....yes.

 

Figure your weight/power ratio against that of the '99 Corvette with the T56 option:

 

Curb weight: 3240 lb.

Horsepower: 345 hp @ 5600 rpm

Torque: 350 lb.-ft. @ 4400 rpm.

Weight/Power Ratio: 9.39:1

 

 

Now the 1977 'Z' 2605

 

Curb weight: 2605 lb.

Horsepower:170 (139?).

Torque: .....

Weight/Power Ratio: 15.32:1 (You lose badly)!.....LOL.

 

Now the '77 'Z' V8 Hybrid we all love:

 

Curb weight: *2755 lb

Horsepower: 350 stock SBC *(with iron heads).

Torque: .....

Weight/Power Ratio: 7.87:1 (You win nicely).....Amazing!

---------------------------

*****

[grumpyvette]

things to read

 

http://www.69mustang.com/hp_torque.htm

 

http://www.ubermensch.org/Cars/Technical/hp-tq/

 

http://vette.ohioracing.com/hp.html

 

http://www.n2performance.com/archives.shtml