Someone clarify something for me re: HP vs. Torque

[Guest bastaad525]

Okay... I understand how hp and torque are related, for the most part. But there are some things I'm still a little confused about after a whole lotta reading on this.

 

Torque moves a car. Period. HP is a theoretical number, and the actual number is derived from the torque. Only torque is measured on a dyno. (correct me if I'm wrong on any of this).

 

 

Here is the one theoretical question I have that leaves me still a bit confused.

 

If you take two engines... identical in almost every way, displacement, heads, auxiliaries, just about everything... get them to make the EXACT same amount of torque, same torque peak, same curve.

 

Then take one of those engines, and move the torque curve... dont alter it any way other than just shifting it up... say 1000rpm or whatever. Obviously, this would result in higher HP, though torque is the same, right?

 

So... which one would be faster? Assuming identical cars, gearing, tires... everything other than the rpm range of the power curve. I'm guessing the higher HP one, but why? I mean... if it's 400ftlbs or whatever... it's 400ftlbs... why does it make it faster when it occurs in higher RPM? Assuming identical gearing... you would want to shift sooner in the rev range on the lower hp car, I know that. But if the gears are spaced so that when you shift you're still in the power band, and your'e shifting earlier in the lower hp car keeping it in the power band (putting the same amount of torque to the wheels) why does the higher hp car go faster?

 

I dont really understand the time aspect of the work = force x time that makes the higher hp car faster.

[Tim240z]

http://auto.howstuffworks.com/horsepower.htm

 

 

http://auto.howstuffworks.com/question622.htm

 

http://auto.howstuffworks.com/question381.htm

[MrWOT]

HP is Torque over time.

 

So Torque is actual power, and HP is rate of use of that torque. If you have 2 motors, same displacement, one has 500hp 500ft/lbs at 5000rpm and the other has 500hp 500ft/lbs at 6000rpm they will both have equal acceleration at those points all other things being equal.

 

but..

 

If the gearing is the same, the 6000rpm motor will win, since it will top out later then the 5000rpm motor.

 

In another case you have this:

 

Say one motor has 300hp 200ft/lbs at 8000rpm, another motor has 200hp 300ft/lbs at 8000rpm, which will accelerate faster?

Answer is the 300ft/lbs motor, because it has more force available. Which is why when people has low tq motors but spin them very high, they still get tanked by lower hp V8s with big torque. The little motor has much better efficiency of it's torque (hp) but more torque = more acceleration.

 

So HP is basically just the efficiency of TQ at a given point, high HP means you can maintain TQ. But you can have high HP and low TQ and not accelerate for beans compared to lower HP and high TQ 8)

[blueovalz]

This has been discussed a while back, and several times I believe. But to answer your question as to which would be quicker in acceleration, the engine with the peak torque at the higher rpm would (for the most part).

 

Place these two engines side by side in identical cars and let them drag race. The car with the higher horsepower (same torque, but at a higher rpm) will wait on the 1-2 shift and stay in 1st gear for an additional amount of time (verses the lower HP car) to get through the extra 1000 rpm the torque curve is set at. Meanwhile the lowHP car is already in 2nd gear, and as we all know, 2nd gear is higher than 1st gear, which means his "at the wheels" torque is lower than the guy that is still in 1st gear. Now, as we continue on down the track, the LowHP car will do the 2-3 shift, but the hiHP car still has an additional 1000 rpm to go, PLUS an additional 1000 rpm before he does the 2-3 shift thing. So now he stays in 2nd gear even longer while the LowHP car has been in 3rd gear, which is again a higher gear (and slower to accelerate than if still in 2nd gear).

 

In essence, you take advantage of staying in the lower gears longer because you stay in each gear longer before a shift than the low rpm car. Now suppose you place a lower final drive gear ratio in the HighHP car so that it now shifts at the same time as the LowHP car. Well, in essence, nothing has changed because now the lower gear ratio will again provide more "at the wheels" torque than the lowHP car has, which again helps it accelerate faster than the lowHP car.

 

Hope you understand my blabbering.

 

Say one motor has 300hp 200ft/lbs at 8000rpm, another motor has 200hp 300ft/lbs at 8000rpm

This could not be as 200ft/lbs @ 8000 = 304 HP and 300ft/lbs @ 8000 rpm = 457 hp. Torque X RPM/5252 = HP.

Greatest torque is great, but in close matches, the car that has the higher torque curve, even if the total amount of torque is a bit lower, will have an advantage. You'll never see a full racing engine developing it's peak torque at a low range. Look at F1 with it's 18,000 rpm shift points.

[Guest bastaad525]

http://auto.howstuffworks.com/horsepower.htm

 

 

http://auto.howstuffworks.com/question622.htm

 

http://auto.howstuffworks.com/question381.htm

 

 

I've read those pages... more than once. So maybe I'm slow... it still didn't explain to me why simply moving torque up in the rev range makes a car faster.

 

BlueovalZ - okay... what if both motors have the same redline, and the lower hp driver decides to hold on and not shift until 6000rpm as well.. then they are shifting a the same time. And yeah, I realize the higher hp car would pull ahead as both cars enter the higher rpm band, but wouldn't the lower hp car have pulled out ahead first, since it's torque is down low?

 

Well okay.. maybe that would only apply to 1st gear... because I guess if the lower hp driver stuck with it until 6000rpm, he'd be slower right off the bat when entering 2nd, since he'd already be past a good deal of his power curve.

 

 

I still don't understand why 200ft lbs or whatever is more powerful at a higher rpm. If your motors putting out 200ft lbs of torque, what difference does it make how fast it's rotating? it's still 200 ft lbs of torque.

[JMortensen]

I still don't understand why 200ft lbs or whatever is more powerful at a higher rpm. If your motors putting out 200ft lbs of torque, what difference does it make how fast it's rotating? it's still 200 ft lbs of torque.

 

Torque X RPM/5252 = HP

 

The bigger the number you divide by 5252, the higher the hp rating will be.

 

So the higher the RPM that you multiply by torque, the larger the result. So more rpms for a given torque is always going to mean more hp.

 

Or if you think about the torque side, lets take your original example and move the torque band 1000 rpm higher. So lets say an engine makes 100 ft/lb torque at 3000 rpm. 100*3000/5252 = 57.12 hp. Now we take that same torque at 5000 rpm. 100*5000/5252 = 95.20 hp. So if you make the same torque number higher in the rpm range that makes more hp. The formula only measures at one spot in the rpm range, so you really need to compare the whole range to get any meaningful numbers.

 

If you want to know which car is going to be faster, I think that boils down to which car is setup to USE the torque available. You can maximize the transmission ratios and the gaps between the gears, rear end ratios, tire sizes and traction. Either one can win the race. It depends on who is set up to use their power the best. Put both of these motors in the exact same car and the one that is best suited to the transmission the rear end and the tires and suspension will win.

 

BlueovalZ - okay... what if both motors have the same redline, and the lower hp driver decides to hold on and not shift until 6000rpm as well.. then they are shifting a the same time. And yeah, I realize the higher hp car would pull ahead as both cars enter the higher rpm band, but wouldn't the lower hp car have pulled out ahead first, since it's torque is down low?

 

If you've got a diesel engine that peaks at 2000 rpm you will have no benefit to revving it to 6000 rpm (if it can). So if you have a lower peak torque RPM, then revving higher doesn't do you much good, because the torque starts to decline after the peak. Continuing to rev doesn't accelerate the car as fast as shifting to the next gear, because the engine's power declines.

 

If you put a cam in your car and raise the peak torque 1000 rpm you'll have less off the line, but you'll be able to rev it higher and force that top number to be higher in value relative to the 5252, meaning more hp.

[Pop N Wood]

Torque moves a car. Period. HP is a theoretical number' date=' and the actual number is derived from the torque. Only torque is measured on a dyno. (correct me if I'm wrong on any of this).

[/quote']

 

Most of this is wrong. Unfortunately there are so many incorrect and misleading articles on the web that this problem becomes very difficult.

 

HP and torque have very precise mathematical definitions. Unfortunately this is very poor way for most people to understand it. But it gives the best understanding of the problem when the math is understood. The biggest problem with this argument is the conflicting definitions people give to torque and power.

 

To take the statements one at a time.

 

Torque moves a car. Period.

 

This should read

 

Torque at the rear wheels moves a car. NO Period. Much more to it.

 

The trick here is gearing. A lower gear will multiply the engine torque yielding greater rear wheel torque. Pick up a good book on vehicle performance and you may find an equation to compute rear wheel torque from the engine HP. What the equation shows is the maximum rear wheel torque for any given vehicle speed is produced when the gear ratio is adjusted so that the engine is spinning at the HP peak. And the higher the rear wheel torque, the higher the acceleration.

 

HP is a theoretical number

 

No. HP is not a theoretical number. It is just as “real†as torque. HP is a measure of power which is the amount of energy dissipated in a period of time. It is energy that is covered by the first law of thermodynamics (energy cannot be created or destroyed, only altered in form). If an engine is producing 300 HP, that HP has to go somewhere. If it doesn’t go into accelerating the car, it is consumed as heat in the driveline losses or in smoking the tires.

 

Only torque is measured on a dyno.

 

Torque is frequently measured on most common dynos. But I have seen dynos that measure power directly. This was on a submarine power plant that used a generator instead of a transmission to gear down the steam turbine. For testing, the electricity from generator was dissipated into a bank of resistors. Measuring the current and voltage at the resistors yielded the power produced by the turbine. Electrical generating stations compute power in the same way.

 

You are correct in that torque and HP are mathematically related. But the equation works both ways. You can just as easily compute torque from power and RPM as the other way around.

 

By it’s very definition, it is power that determines how much work can be done over a period of time. The more power you produce, the more “work†that can be done (meaning the faster you can accelerate). This statement could have a period after it.

 

Go back to the equation computing vehicle acceleration from engine HP. Assuming driveline losses can be controlled, the fastest way to accelerate a car would be to use a continuously variable transmission that keeps the engine spinning at the HP peak. With a 5 speed tranny, you need to match the gearing to the engine HP curve so that shifting keeps the HP from dropping too low.

 

So what you want to do is build an engine that produces the maximum amount of HP over the range of RPM's that it will be used. If your gearing is too wide that a shift takes you out of the power band, then you will be slower at the speed.

[Guest bastaad525]

Jmortensen - yes, torque is declining as the revs increase past the peak torque point. BUT, shifting into the next higher (lower numerical) gear, also causes the multiplication of the torque to go down, resulting in less power to the wheels, and less forceful acceleration (what car accelerates faster in 5th than in 4th, etc.?).

 

 

So here's another question, at what point has the torque dropped off enough in first gear, that shifting into 2nd would NOT cause the rate of acceleration to lessen? Even though my motor starts falling off in power after 5000rpm, I still feel that if I pull all the way to 6500 in 1st, I'm still accelerating faster, from 5000-6500rpm, than I would be accelerating in 2nd gear if I had shifted earlier and brought the rpm back into the power band. How high would I have to rev before acceleration in 1st would actually become less than the acceleration in 2nd? Of course that point moves as well with a regular trans... as if I had revved to 8k in first and shifted I'd already be out of the power band in 2nd anyways...

 

 

Another question, using the continuously variable transmission example. Again going back to my two identical yet different engines. If they were each mated to continuously variable transmissions, that never 'shifted' but rather, kept the engine at the RPM where they were making the most torque (but one would still be making more hp as it's making it's torque higher in the power band), would the higher hp one still be faster or would they acclerate the same?

[Pop N Wood]

The one with more HP, not torque, would be faster.

 

Forget torque curves. Only look at the HP curve.

 

at what point has the torque dropped off enough in first gear, that shifting into 2nd would NOT cause the rate of acceleration to lessen?

 

When the shift is performed such that the HP rating both before and after the shift are identical. As you round the top of the HP curve you start to go down in HP. When you shift, you will drop back to the other side of the HP peak. If the HP is the same at both points, the acceleration will be the same. Or so says the math.

 

You want to shift in such a way that you keep the highest HP possible, but you also need to account for the time it takes to shift. If the HP curve is too narrow and peaky, you will need a whole bunch of narrowly spaced gears to stay in the power band. So the spacing between the gear ratios will dictate you shift points as much as the shape of the HP curve. Ideally you will select gear ratios in conjunction with the engine power curve.

 

One other thing. Shifting into a higher gear will cause the torque multiplier to go down. But not power (provided the drive train losses don't change much). Power can neither be created nor destroyed. Torque times RPM equals power. This is true for the rear wheels was well as the engine. So if the engine is pumping out 300 HP, you can divide that by engine RPM to get the torque rating. Multiply the engine RPM by the gear ratio (including diff), you can get the revs of the rear wheels. Take the engine HP, subtract for driveline losses, then divide by the rear wheel revs and you now have the torque value at the rear wheels.

 

... if it's 400ftlbs or whatever... it's 400ftlbs... why does it make it faster when it occurs in higher RPM?

 

Because a higher engine RPM means you can select a lower gear ratio for any particular vehicle speed to create even MORE torque at the rear wheels.

 

Or more simply, because it is making more HP. HP is good. More is better.

[Guest bastaad525]

 

... if it's 400ftlbs or whatever... it's 400ftlbs... why does it make it faster when it occurs in higher RPM?

 

Because a higher engine RPM means you can select a lower gear ratio for any particular vehicle speed to create even MORE torque at the rear wheels.

 

 

VERY good answer I wish I was so good at boiling the things I'm trying to say down into one little sentence like that. It took that one line to really make sense of a lot of the info you guys have given me so far .

 

I'm a bonehead for saying HP can't be directly measured... I forgot for a second that hp can be applied to a lot of different things, not just to cars. So it can be directly measured just not on a car.

 

I don't agree with what you said about torque going down after shifting but not power. That doesn't seem to make sense. How could you not have lost power as well? You're gonna acclerate less rapidly after you shift no matter what, so power at the wheels must be less as well. But I have a feeling I'm not 100% understanding exactly what you said there.

 

I've heard a lot of different viewpoints on exactly WHEN you should shift. I've heard that you want to shift to keep the torque peak in the middle, or to keep the hp peak in the middle as you say, or to shift AT the hp peak, right as hp starts dropping, or to shift so that the rpm spread you cover in the next gear will fall right in between the hp and torque peaks. This drives me nuts! I just try to shift when I feel that the car has stopped pulling so hard.

 

So IS it the HP number that truly determines the rate of accleration??

 

Did anyone read that article... I'll be darned if I could remember... a magazine I think, where they compared a BMW (Z3 3.0L L6 I think) and Honda S2000. Both had 240hp, but the BMW had way more torque. I believe they weighed very simliar to each other as well. That article lead me to believe that torque was the number that really determined how fast your car acclerates... citing how the BMW was much easier to get moving fast and how the Honda felt relatively powerless. I think the BMW was the faster of the two though given I dont know all the factors involved I don't think that means much.

 

Reverse my example. What if you had this kind of situation? If you had two cars making the same amount of peak HP, but vastly different amounts of torque, but the cars were basically identical otherwise, oh but they are geared to take advantage of their relative power curves (i.e.: the S2000 with it's 6 speed, relatively short gears and 9000rpm engine). THEN given identical conditions which one should be faster or would they be about the same given the same HP numbers? Shouldn't the torquier motor be faster?

[JMortensen]

Jmortensen - yes, torque is declining as the revs increase past the peak torque point. BUT, shifting into the next higher (lower numerical) gear, also causes the multiplication of the torque to go down, resulting in less power to the wheels, and less forceful acceleration (what car accelerates faster in 5th than in 4th, etc.?).

 

I think you're hung up on the torque numbers (maybe I was too) and not looking at the other factors like the ability of the engine to accelerate. Obviously Pop and I agree that gearing is crucial, even if we don't agree that the car with the most hp wins the race. Having ridden in a 1978 Toyota Pickup with a 20R with 44 inch tires, dual 44mm Mikunis and a huge cam, I'm pretty confident in the strength of my argument. HP doesn't always win. I'm not kidding by the way, that truck really existed back in 1990, and a stock 20R would have been very much better at getting that thing moving. "Show" truck. What a turd. Had one hell of a polished valve cover though...

 

I think maybe I underestimated the usefulness of the hp curve though, and Pop's post is definitely helping me to understand. It seems to me that the area under the peak of the hp curve is where the engine accelerates the fastest. So it seems to me that you want to place your shifts and gear your transmission so that the shifting hp and the after shift hp are as close to the same as they can be. That way you are always driving over the very peak of the hp curve with max acceleration.

 

So thinking back to the show truck maybe if that thing had a 60 speed automatic transmission which could allow the truck to be run from 5000-6000 rpm and moved from a stop at that rpm it would have been OK to drive. As it was with the limits of the 5 speed and the differential gearing torque would have won that race hands down.

 

I'm thinking the same would be true of your S2000 example. If it was possible to keep that sucker in the 6-8000 rpm range for the whole race with the Z3 maybe it would win. But it loses a lot off the line to the Z3, and has to shift more which also costs time.

 

I guess what it boils down to is that you have to prioritize the engine to work with the rest of the car. If you are like John C and can afford a Quaife sequetial dog ring box with whatever ratios you want, then you can have a car with a peakier power curve. The rest of us are better with a wider power band.

 

In your case Bastaad a bigger cam is going to get you a torque shift, which will get you higher hp at a higer rpm, the question then becomes can you keep the engine in the rpms you need to maximize acceleration. As Pop said, the peakier your hp curve the closer your ratios need to be. Time to break out this old link... http://www.geocities.com/z_design_studio/transmission.html

[Pop N Wood]

If you had two cars making the same amount of peak HP, but vastly different amounts of torque, but the cars were basically identical otherwise, oh but they are geared to take advantage of their relative power curves (i.e.: the S2000 with it's 6 speed, relatively short gears and 9000rpm engine). THEN given identical conditions which one should be faster or would they be about the same given the same HP numbers? Shouldn't the torquier motor be faster?

 

This type of discussion is common on web boards so I have had a good bit of time to think about it.

 

Some of the problem is wording. Most people use the word "torque" like you do above. It is interpreted as having a wide power band. I think of torque as HP divided by RPM. Thus having a wide torque curve is the same as saying you have a lot of HP over a wide range of engine speeds.

 

When you think about it, peak HP is doesn't mean anything unless the engine is always spinning at that speed. Shifting takes the engine revs away from the HP peak. So when you shift you have less HP to convert into rear wheel torque. A car with a wide power band doesn't have to shift as much and can thus be faster, simply because it spends more time at higher HP.

 

I think it is Drax240 who always says it is the power under the curve that matters.

 

I personally can't tell you what the best shift points are. This takes more driving experience than I have. I would imagine it varies with different types of racing. Obviously you don't want to power shift in the middle of a turn or it will upset the balance of the car.

 

But I can tell you from the math involved when the maximum theoretical acceleration occurs, and that is when the HP is maximized.

[Guest bastaad525]

heheh I've had that trans calculator page marked forever. I am thinking of a possible cam swap, but I do like having so much torque down low, and I remember NOT liking the way my N/A motor lost a lot of that down low grunt when I went with a hotter cam. With so much torque though simply moving the curve up in the rev range would obviously give me a LOT more HP, and from what I"m reading here would make a dramatic difference in the cars acceleration.

 

I was hung up on torque when I started this thread... now that I'm understanding things a bit more it is seeming a bit less important. I decided to go reread the articles at HowStuffWorks, and after reading the info here, the stuff on those pages makes more sense to me now, as far as how HP actually means the true amount of work (acceleration) the motor is doing. I'm also understanding a bit more just how important looking at the WHOLE of the HP curve is. In the Z3 vs. S2000 example, for instance... I tended to focus on the matching peak hp numbers. But now it makes sense that the Z3 should be faster, at least off the line. Since it makes more torque, you can bet the HP curve is much more flat, and the S2000's hp curve is probably very peaky. Though with what you guys are saying I figure they probably accelerate very similiarly at their respective peaks, after 1st gear where their gearing (should) keep them both close to those peaks, and both doing the same amount of work, regardless of torque. IF they are geared right, right?

 

With all this info it seems rather misleading to measure a cars hp OR torque PEAKS at all. Wouldn't it make more sense to somehow total up all the power available from idle to redline, and rate it that way? If you somehow did that, the Z3 would obviously have more "total hp" than the S2000. Maybe express hp as an average of the hp peak measured at every 500rpm. I think this would give a clearer measure when trying to compare different cars.

 

 

 

About gearing, thinking on what you just said, based on my cars current HP curve, even the 3.54 rear gear seems too numerically high to take advantage of it. Revs only drop about 1000rpm when I shift up a gear. The whole of my power curve is very broad and flat, 200+hp from 3500-6000rpm (peak 233hp). Seems like only covering a 1000rpm spread is wasting a good deal of useable power, a lower rear gear would let me stay in each gear longer, and would use more of that curve by dropping to a lower rpm after each shift. Of course... lower numerical diff, lower torque to the wheels. So which would be better? Would changing the diff increase the amount of rpm drop when shifting to the next gear, or would I have to actual change the ratios in the trans to do that? Would either be more ideal for my setup?

 

How would I figure which gearing would be optimal, on paper, as I can't test and change gears until I do. Well actually it's a moot issue as applies to my car... I don't exactly have the ability to change gearing freely, but I still would like to know how you'd figure this out. Maybe I'd be better off with a 3.36 rear?

[JMortensen]

Changing the diff ratio wouldn't change the rpm drop between shifts, but it would change the speed at which you shift, and the torque multiplication regardless of what gear you're in.

 

So if we were saying that more torque multiplication is a good thing, then you'd want the highest numerical diff that you can get.

 

I don't know if I've been doing the diff gearing "right" but I always looked at the theoretical top speed. My 3.70 diff with my tire size and redline of 7000 gave me a top speed of like 172 mph, which my car would NEVER go to. Likewise your 3.54 probably tops out at 190 or something. I happily changed down to a 4.11 because I still probably won't ever see 155 mph or whatever it is now, but I'll have more torque mulitplication to help the car overcome the wind resistance and get closer to 155.

 

The downside to this is that you spend more time shifting, and especially with a turbo it's better to have a bit wider gearing and let the additional torque from the boost carry you through a longer rpm range.

 

Incidentally the turbo is going to help with your bigger cam too. Once the turbo spools you'll have plenty of pressure to force the air/fuel into the engine. It's the NA's that really feel the effect with a big cam because we have to wait for the intake velocity to come up, and at the same time we're bleeding off compression due to lots of overlap. You won't have lots of overlap, probably just a lot more lift I would imagine. I would think you might feel a little difference before it spools, but as long as the turbo spools at a reasonably low rpm you should have more mid and more top end which should more than make up for the slightly increased lag. That is my impression of camming a turbo anyway...probably would help to get advice from someone that has done it.

[Guest bastaad525]

yeah with an N/A, I agree higher diff gearing is better... I had the 3.9 in my N/A, and wanted a 4.11 but never found one reasonably priced.

 

But I have many times heard what you just said, that you really want a lower gear for a turbo car. The problem is still... how to figure out what diff gear would really be ideal, giving me max torque multiplication w/o any negative effect on boost or whatever? The 3.54 IS really wide, and I"m not even gonna bother trying to find a 3.7 LSD (did the 3.7 come non LSD?).

 

So I wouldn't suffer much in the low end (boost comes on full about 3000rpm) with a larger cam? It's funny because the sticky in the turbo/supercharger section of the forums kinda makes it sound like the money spent on a bigger cam is really money wasted unless I"m over 400hp. But looking at my power curve and how early torque falls of, it seems to be just what I need. Not a lot of people on these forums have put a hotter cam in an L28ET for me to glean info from.

 

I want to switch to a full mandrel bent 3" exhaust first though and see how that affects the curve. I think/hope it is the restriction causing torque to fall off so fast. If that doesn't do it then I'm definately going to start looking at a bigger cam.

[JMortensen]

This is about the point where I should just shut the hell up. But I'll give you this much:

 

I think I'm right about the cam, because once you've got boost it should be good to go because the turbo is just going to force more air in while the intake valve is open longer.

 

The 3" exhaust is going to do the SAME thing as the cam. It's going to improve the engine's breathing (exhaling instead of inhaling this time), and in doing so it will move your torque band up in the rpm range. You probably want to do that 1st, then decide if you need the cam.

 

The gearing I suppose depends on what you want to do. If you're drag racing you might want to maximize the rear gears so that you come through the traps right at your hp peak. If you're driving on the street you might want a nice cruising rpm on the freeway. If you are doing some of each you'll need to compromise.

[crazy280]

Bastaad's question was pretty much answered, but I can add this very simple explanation of hp vs. torque:

The misconception is that torque is what accellerates the car, but this is wrong. Torque is what moves the car. I say this because 500 lb/ft of torque means you can move 500 lbs for a distance of 1 foot. But it doesn't say anything about how quickly you can move it. And accelleration is all about how quickly you can move something. So heres where horsepower comes in. Horsepower is the measure of torque over time, in other words, how quickly torque is applied to (whatever you apply it to). So horsepower is the only important number related to accelleration. For example, suppose two people were each pushing a different merry-go-round at the playground. And suppose they used the same exact amount of force (Torque) each time they pushed. Now suppose one of them pushed twice as often as the other. Which merry-go-round would spin up more quickly? The one that was being pushed more often. Just like 500 lb/ft of torque at 1000 RPM is not as good at accellerating something as 500 lb/ft of torque at 2000 RPM.

[johnc]

So... which one would be faster? Assuming identical cars, gearing, tires... everything other than the rpm range of the power curve.

 

The one with the better handling car that allows the driver to get on the throttle sooner in the corner.

[wheelman]

Bastaad525,

What Jon was saying about the cam is right. You don't want a lot of overlap with a turbo or you just blow your air/fuel charge out through the exhaust port. What you want is a high lift cam with less overlap and then port your intake and exhaust systems to increase the flow rate. One mis-conception people fall into is that a turbo will cure a poorly designed intake, but the mixture still must flow through the system and restrictions still slow it down and cause turbulence even when it's under pressure rather than vacuum.

 

It shouldn't move your power band up, but it should broaden it toward the high end. Your power is dropping on the high end because the flow rate reaches a maximum. Ported intake, head, high lift cam and larger exhaust should increase the maximum flow rate there-by making more power at high rpm and the turbo should offset the lowend charge velocity decrease once it's spooled up.

 

I hope this makes sense and if you see any glaring errors in my logic please let me know, I'm not an expert in this but it seems to make sense to me. I think a cam with low overlap, high lift and possibly a small increase in duration would probably be your best bet.

 

Wheelman

[Guest bastaad525]

Jmortensen and wheelman - thanks for the clarifications on the cam thing. Yeah I'll do what Jon says... get a good exhaust on there and see what happens. If power still falls off too soon for my tastes I"ll look at installing a better cam. It's good to know I wont lose a lot on the low end by going with a hotter cam on this setup. My N/A motor SUCKED on the street after I upgraded the cam... I really regretted that move even though it gave me 15+ hp... the car definately FELT slower. For the diff... well... screw it I dont see the point in messing with it. I don't want to go back to the manic 3.9 gear, and I'm not gonna make my car slower with a 3.36.

 

 

Crazy280 - thank you for the further clarification. I didn't think this thread was really gonna go anywhere but instead I really got a lot of good explanations and think I do finally understand how hp, torque, and acceleration are all related and work on each other. I found another good one liner in one of my car mags that kinda clicked and helped my (sometimes slow) brain figure it out - "You can't have horsepower without torque, but you CAN have torque without horsepower... it's called a dumptruck".

 

 

Now... just one more question nagging me really... I dont know that I expect an answer to this one, but why is it, even now that I'm making a whopping 300ftlbs of torque at 3000rpm when the boost hits, why the heck don't my tires (or TIRE, no LSD) break loose?? I mean yeah I can spin it if I slip the clutch and launch from a stop, but I'm talking from a roll at any speed... I thought that much torque should easily be able to overpower the tire. ESPECIALLY these cheap all season $25 a piece radials!