KevinK2

Your reply is only true if both engines have about the same hp rating. See yaw post, pg 1.

SPOautos

BUT, its going to have to make enough tq at that low rpm to achieve that same hp as the other car. In addition if we're talking about tq at the wheels its already gone thru the gears.

As for Top Fuel, they actually turn around 8000rpms and make and estimated 5000+hp. In addition, from the way they jack the rpms up off the line I'd say they are wanting to get higher rpms, eventhough they probably make the same Tq at 1500 as they do at 5000. Hmmmm

STEPHEN

DamonB

They do not. The tires can't hold it. Hmmmm.

SPOautos

How would we know that? They rev the engines up like mad off the line. As a matter of fact they launch at almost redline.

Here is some info on a top fuel dragster http://www.rhondatf747.com/crew&spec...Specifications

It says its estimated peak tq is 4500 and hp 5000 with a redline of 8000rpms. This means that peak tq would have to start dropping before 6000rpms and go all the way down to 3200lbs or lower at redline. It would make sense to me to launch from the redline since you'd be at a lower tq rating alowing your tires to stick in addition to launching within your HP band. The other way to do it would be to drag it out of the hole at idle with a lower tq and way out of the hp band like they're leaving from a stop light but I dont see many dragsters doing that and winning the hole shot.

STEPHEN

jimlab

Because even beyond the torque peak of the engine, torque at the axles can still be higher in the current gear (through gear multiplication) than it will be in the next higher gear at torque peak, and therefore you'll accelerate more quickly by staying in the current gear until that's no longer the case, or you reach redline, whichever comes first.

Torque at the wheels is directly related to torque at the engine, and the rate of acceleration of a car (g-force) follows the torque curve, not the horsepower curve. And yes, given equal weights, the one with higher torque at the wheels is going to accelerate faster, assuming adequate traction.

I think I confused a few people by not specifying that by "rate of acceleration" when referring to two cars in different gears, I was referring to the rate of acceleration of the engine, not the car. If the rate of acceleration of the cars were identical, then obviously neither car would pull away from the other. That's not what I was saying.

If two engines accelerate (gain rpm) at the same rate of speed (let's say 1,000 rpm every 10 seconds) while one is in 2nd gear and one is in 3rd gear, even though they may have the same maximum torque at the axles (highly unlikely, but let's just make believe), one is obviously more powerful than the other (to make the same torque at the axles with less gear multiplication), and because of faster rotation of the wheels due to the higher gear, will quickly pull away from the other, even though in theory, they had the same maximum torque at the axles. Get it? If so, stop reading now.

Car A is in 3rd gear (1.5:1), has a 4.0:1 differential and P275/40-17 tires, and is at 5,000 rpm, where its engine produces 500 lb-ft. Maximum torque at the axles, with drivetrain losses for a manul transmission will be about 2,550 lb-ft.

Car B is in 2nd gear (2.0:1), also has a 4.0:1 differential and P275/40s, and is at 5,000 rpm, where it's engine produces 375 lb-ft. Maximum torque at the axles, with drivetrain losses for a manual transmission will be about... 2,550 lb-ft.

Assuming that the more powerful engine accelerates at an identical rate of speed in 3rd gear as the less powerful engine does in 2nd, if engine rpm climbs by 1,000 rpm every 10 seconds until an 8,000 rpm redline, which car is going to pull away from the other?

Car A, because in 3rd gear from 5,000 to 8,000 rpm, it will be turning the axles from 833 up to 1,333 revolutions per minute. Car B, in 2nd gear, will only be turning the axles from 625 up to 1,000 rpm. Still following me?

At any point in time during the acceleration run, even though the engines are producing identical torque at the axles and the engine's rate of acceleration is the same, the wheels of Car A are turning much faster. Since both cars have a tire with a nominal circumference of about 80.62", you can figure out for yourself that the car turning that wheel more often is covering more ground.

But... it's just a long-winded way of saying that Car A, which obviously would make more torque at the axles in 2nd gear than car B does in the same gear (3,400 lb-ft. vs. 2,550 lb-ft.) is obviously going to kick its ***. Badly.

My apologies, I should have been clearer about what I was trying to describe.

jimlab

Which is why they use multiple clutch packs which progressively lock up over the course of the quarter mile to limit power to the wheels on the starting line and gradually apply more over the course of the race. The clutch pack is consumed in the process, of course, and rebuilt after each run.

At the starting line, they estimate a maximum of about 17% lock-up to avoid tire shake or going up in smoke. The clutches almost never fully lock-up, even at the far end of the track. If traction is bad on the track that day, they "take some clutch out" and they may not even reach 85-90% lock-up at the far end.

SPOautos

Seems to me the reason is cause car A is sporting 475HP which car B only has 357HP. Also, it would be hard for those 2 cars to race since they are starting at different speeds. You should adjust thier rpms to reflect them starting at the same speed (wheel rpm).

The entire point in Paul Yaws write up about time is totally left of out of these examples, well it wasnt left out it was estimated. That is THE most important part, the length of time it took one car to go from 5000-8000, that is where HP comes into play.

STEPHEN

jimlab

I have a handy spreadsheet that I made to illustrate many things, and torque at the axles is one of them.

This chart was generated from an actual dyno sheet for a stock FD (RWTQ).



I added the red lines and circles to illustrate. Ignoring the transition spike from the turbos, it's easy to see where torque at the axles trails off and falls below the maximum torque at the axles in the next higher gear. The circles indicate optimum shift points for each gear for maximum acceleration.

jimlab

It's all hypothetical anyway. No two cars are so closely matched that they accelerate at an almost identical rate of speed, (with the possible exception of NHRA Pro Stock) and even starting with identical engines, increasing the power output of one changes the torque curve enough that they no longer bear much resemblance to each other.

SPOautos

Jim, did you read the write up by Paul Yaw that someone posted in the 3rd post on the first page? The first half of the write up is exactly the same as your examples. However, the 2nd half of the write up gets into the merits of hp and how it interacts with all of this. It's very interesting reading. I'd like to get your thoughts on that write up, especially the 2nd half.

STEPHEN

KevinK2

From Jim:

"Car A is in 3rd gear (1.5:1), has a 4.0:1 differential and P275/40-17 tires, and is at 5,000 rpm, where its engine produces 500 lb-ft. Maximum torque at the axles, with drivetrain losses for a manul transmission will be about 2,550 lb-ft.

Car B is in 2nd gear (2.0:1), also has a 4.0:1 differential and P275/40s, and is at 5,000 rpm, where it's engine produces 375 lb-ft. Maximum torque at the axles, with drivetrain losses for a manual transmission will be about... 2,550 lb-ft.

Assuming that the more powerful engine accelerates at an identical rate of speed in 3rd gear as the less powerful engine does in 2nd, if engine rpm climbs by 1,000 rpm every 10 seconds until an 8,000 rpm redline, which car is going to pull away from the other?

Car A, because in 3rd gear from 5,000 to 8,000 rpm, it will be turning the axles from 833 up to 1,333 revolutions per minute. Car B, in 2nd gear, will only be turning the axles from 625 up to 1,000 rpm. Still following me?"

The flaw here rests in your conflicting assumption of equal engine acceleration. If both engines had same fw acceleration, then by gearing the wheels of A in 3rd would have 1.33x higher rotational acceleration than B in 2nd. This requires 1.33x more car acceleration, and based on f=ma, 1.33x more drive wheel torque is needed. This means engine A in 3rd must be putting out 667 ft-lbs at 5K for it to match engine B in rotational acceleration.

Based on your car a and b data, equal axle torque, the B engine has 1.33X more fw acceleration vs the A engine.

DamonB

Uh, yeah. I thought the whole point I was arguing was that if you have the same power it doesn't matter what the combination of torque and RPM are.

DamonB

Yes it will. So what? The other motor will have to spin at much higher RPM to make up for it's less torque. That's the whole point, it doesn't matter as the gearing is setup with each motor in mind; it works both ways. Neither has a distinct advantage over the other from the point of view of making power at the wheels.

That's my whole point! At the wheels is what matters and to get there the power has to go through a gearbox!

I'm not trying to bait you in anyway, I must just be doing a poor job of explaining my point of view.

SPOautos

Thats right, I think it was just missunderstood cause you didnt specify in that post that they had the same hp, you just used the words higher and lower tq and rpms, didnt actually say that they were at a tq/rpm ratio that would provide the same hp.

STEPHEN

SPOautos

Ok, so I think we're getting at the same thing just explaining it differently or something lol

I was under the assumption we were comparing high rpm rotary power to a low rpm tq of a V8. Jim seems to think that tq is all that matters and hp doesnt, or at least thats the impression I'm getting. Thats what I started argueing about. LOL

jimlab

Since it's totally hypothetical, and extremely unlikely to occur in a real world situation, what's the point of arguing? Many factors have to be ignored in order for it to be "true", none of which are likely. The example was given only to illustrate a point.

The point was that wheel speed can make a difference over time. If you can accelerate as hard in a higher gear as another car can in a lower gear, wheel speed alone can be the determining factor. Since cars are rarely (if ever) as closely matched as the example, it's all totally irrelevant. You can go back to your regularly scheduled programming.

SPOautos

But that acceleration is a function of HP and its a very key element as Pauls LONG writup explains in the 2nd half.

STEPHEN

DamonB

Jim, you lost me. Wheel speed is what determines the speed of the car assuming the tires are not spinning. What difference does which gear you are in compared to a competitor make?

jimlab

If you can make more torque at ANY engine rpm, you will make more horsepower at the same rpm. Period. People who think that torque is irrelevant need to learn that, which is why I emphasize torque over horsepower.

250 lb-ft. @ 2,000 rpm = 95 horsepower
300 lb-ft. @ 2,000 rpm = 114 horsepower

400 lb-ft. @ 4,000 rpm = 305 horsepower
450 lb-ft. @ 4,000 rpm = 343 horsepower

550 lb-ft. @ 6,000 rpm = 628 horsepower
600 lb-ft. @ 6,000 rpm = 686 horsepower

"Build for torque" as the saying goes. In other words, build an engine with a broad and powerful torque curve and you'll also have an engine which makes a lot of horsepower over the same range.

Having a lot of low end torque doesn't necessarily preclude the ability to make a lot of torque (and therefore horsepower) at higher rpm, which is one of the most common mistakes I see made. Assuming that "big torque" engines are only suitable for pulling heavy loads is another.

There probably isn't a member on this forum with an engine that could match the performance of a 500 CID NHRA Pro Stock engine, and they're "torque" motors, and naturally aspirated to boot. High rpm? Check. High horsepower? Check. High torque? Hell yeah, probably better than 600 lb-ft. even at idle. So what went wrong? Aren't big cube, big torque engines only for towing boats and hauling logs?

No, they can run 6.90s @ 200+ mph without a power adder, too. Torque is a given with larger displacement and stroke, but that doesn't mean you can't build the top end to make power at higher rpm and allow the mechanical advantage of the increased stroke to fill in on the low end. Why do you think I built a 396 stroker?

I'm not ignoring horsepower, I'm just focusing on the people who seem intent on dismissing the important of torque.

SPOautos

Right, we all understand that more tq at the same rpm is more hp. What we (or at least I) was talking about is where you were comparing high tq low rpm to low tq high rpm, even when they come out to the same hp. Everyone understand it takes tq to make hp however you can also make hp with a small amount of tq at higher rpms.

Its not only about the tq, like you said the big blocks are also making a huge amount of hp. And like F1 they wouldnt have to make so much tq if they turned 18K rpms. Its just 2 ways of achieving the same thing.

I guess its just that your examples would leave one to thing that tq is the determining factor of acceleration but its tq and rpm (which is hp) that determines acceleration.

STEPHEN

jimlab

If you consider two cars with identical gearing and tires, let's say that each has a 2nd gear that starts at about 20 mph @ 2,000 engine rpm, and ends at 70 mph @ 8,000 rpm. Both have a 3rd gear that starts at 30 mph @ 2,000 rpm and ends at 100 mph @ 8,000 rpm. Both have engines capable of an 8,000 rpm redline.

Both cars are travelling at 50 mph in 3rd gear, at about 4,000 engine rpm. Car A has an engine that makes 300 lb-ft. of torque at 4,000 rpm. Car B has an engine that makes less, but after the downshift to 2nd, engine rpm jumps to about 6,000 rpm where it also makes 300 lb-ft. With me so far?

Both are sending the same amount of torque to the axles, right? If the engines accelerate at an equivalent, controlled rate (assume that engine acceleration is limited to 1,000 rpm every 10 seconds), Car A is going to end up gaining wheel speed at a faster rate than Car B.

Time zero...
Car A - 4,000 rpm/50 mph (3rd gear)
Car B - 6,000 rpm/50 mph (2nd gear)

10 seconds later...
Car A - 5,000 rpm/58 mph
Car B - 7,000 rpm/63 mph

10 seconds later...
Car A - 6,000 rpm/78 mph
Car B - 8,000 rpm/70 mph (shift to 3rd)

10 seconds later...
Car A - 7,000 rpm/91 mph
Car B - 6,000 rpm/78 mph (3rd gear)

Finish...
Car A - 8,000 rpm/100 mph
Car B - 7,000 rpm/91 mph

It's a *very* simplified example, but hopefully you get the idea of what I'm talking about. Not taking into consideration time lost for shifting, if the engine in Car A can pull every bit as hard with less gear multiplication one gear higher than Car B, then it will build wheel speed more quickly over the same period of time and cover more ground, outdistancing Car B.

I feel like I'm trying to explain what should be blatantly obvious.

Assuming identical tire circumferences, if you rolled one tire through 600 revolutions and marked the distance covered and rolled another through 700 revolutions and marked the distance covered, it would be obvious which had travelled farther, correct? So ignoring the fact that two engines wouldn't accelerate at the same controlled rate of speed, you can still see that the car in a higher gear would build wheel speed at a faster rate and therefore cover more ground over the same period of time, correct?

Or am I wrong?

DamonB

That explains it. You didn't need all that other stuff, but thank you

wanklin

iTrader: (2)

What's the bottom line gentlemen? HP is a product of revs and displacement (oversimplified of coarse). Now if GM can build a 5.7ltr LS* reving to ten grand efficiently.....

KevinK2

My point is that hypothetical is not just unlikely, it's impossible, and confused the valid points you had made.

I understand the joy of torque. I helped build a 454 for a friend's 68 vette. Roller rockers, P&P, doug nash trans ...

Hit 150 at summit point (with a poor exit speed from turn 10) and was pulling hard at 150. Any rpm and any speed, bam! it just mashed you against the seat.

SPOautos

But, then there is the whole thing of being mashed against the seat eventhough your not really accelerating fast. I've got a Ford F-350 with 650lbs of tq that feels fast as hell and pushes you in the seat but it ran head to head with my brothers Miata.

But yea, that feeling is pretty cool.

STEPHEN