Sorry but I didnt know where else to post this, if you can think of a better place feel free to move it.

I've got a question for people who are good at this sort of thing. well, a few questions...

Let me start with these:
Torque X RPM / 5252 = Horsepower
1 Hp = 180 Lbs moved 181 Feet in 1 minute (right?)
(HP X BSFC) / Duty Cycle / #injectors

What Im gathering is a few things, correct me if im wrong:
-That horsepower is directly related to fuel consumption; i.e.
(A small block 350 making 500 horsepower @ 6000 rpms uses the same amount of fuel as a big block 455 making 500 horsepower @ 6000 rpms provided the BSFC is the same.)
-That the torque output of any motor is always higher than the horsepower output before 5252 RPMS...
-That the Torque and horsepower lines ALWAYS cross at exactly 5252 RPMS.
-That high cylinder pressure at high RPMS results in high torque at high rpms and that results in very high horsepower numbers at high rpms resulting in high fuel consumption at high rpms etc...
----
and now for some questions;
-Where does that 5252 come from? I know it has somthing to do with PI but not what.
-If horsepower is only a measure of work done over time, then why does a very high torque / very low horsepower figure still seem to push you back in your seat (i.e. TPI motors in stock trim)
-Since torque is directly related to Horsepower, and gearing is directly related to rear wheel torque (it seems?) then isnt gearing responsible for directly affecting RWHP or at the very least make it easier for the engine to do more "work" in less "time"?
-since torque converter multiply torque does this mean that the more a converter "multiplys" at high rpms the more RWHP we will see? thats still blurry...

and ive got more coming but right now its off to eat!

 

1 horsepower was actually measured by a gentleman named Watt; it's the amount of work a horse could do in 1 minute. He used a pulley arrangement, and discovered that a horse could lift 33,000 lbs 1 foot in 1 minute. That would be 33,000 ft-lbs/min. Since a 1-foot radius thing moves a distance of 2pi feet when rolling, that means that for a twisting force, 1 HP = 33,000 ft-lbs of torque / 2pi per minute. 33,000 divided by 2pi is 5252.

High tortque at low RPM, but low HP, such as TPI, means that you get a great grunt rush off the line, than it peters out to nothing as engine RPM increases... just like TPI. Although actually, TPI has nothing particularly special off the line, but has teh resonance reinforcement torque peak at around 3600 RPM, then trails off into oblivion.

RWHP is corrected for gearing; it measures engine RPM at the same time as wheel RPM, so it's very easy to divide that out.

A torque converter is not a perpetual motion machine. More energy does not come out than went in. Think of it more as a continuously variable fluid "gear". It can only multiply torque when the output RPM is lower than the input RPM. A chassis dyno will correct for this in its calculations, just the same as it does for gears.

 

Hmmm, I'll try going down your post in order and see what I can say about things.

Ok, I see now that RB beat me to it.


Horsepower is related to fuel consumption. The thing is efficiency. Assuming two motors produce the same power and have the same efficiency, they should use the same amount of fuel.

 

I always knew about the HP and ft-lb intersection at 5252 rpms. But I never knew WHY.

Too cool :hail:

 

okay that makes sense to me now.

SO technically rear gearing DOES add... power... its just that when we are measuring RWHP we want to find out how much is going through the engine to the ground, not how much acceleration or work there is being done or whatever.


is it true that every 2:1 gear reduction you double the amount of torque being applied? so if the motor makes 100 ft lbs and you have a 12:1 gear reduction (3:1 X 4.11:1) you actually have 1200 Ft lbs of force, minus drivetrain loss, at the wheels?

I know they divide the gearing out but if you DIDNT you would see an increase as the gear ratios went numerically higher wouldnt you?

 

oh and explain this one,

My friends 71 GTO chassis dynoed nearly 32 horsepower more @ 5500 rpms after a rear end gear swap. i always assumed they did somthing to factor out the gearing, to make it accurate, but it almost seems like adding gearing "helps" the motor along with less of a load to give it more "horsepower" Even though it doesnt actually put out any more power.

And on this note, if a G-tech measures G-force, then when you change your gearing, the G-tech has no idea how much gearing you are adding, and it will automatically pick up the increased G-force of the gearing, and think you are making more horsepower right? How the heck is that thing supposed to be accurate anyways?

according to the G-tech my blown 355 had 220 horsepower! its got 2.77 gearing and runs a high 14 @ almost 109 MPH because it takes almost 9 seconds to get to 60 MPH!!!! I havnt G-teched it after the gear swap though....

 

The G-Tech doesn't need to know about gearing to calculate power, it needs to know the mass of the car. If you wanted it to tell you your rear wheel torque, it would need to know gearing/tire size info, etc.

 

Lower gears will decrease your E.T., but your trap speed will be relatively unchanged.

Rule of thumb: Rear-wheel torque determines how fast the car will be going after a certain time has elapsed. Rear-wheel horsepower determines how fast the car will be going after a certain distance has been travelled.

 

<b> it would need to know gearing/tire size info, etc.</b>

it doesnt ask that, just the weight. so you can see why it seems to be innacurate....


<b> but your trap speed will be relatively unchanged. </b>
I know thats usually the case, but i was trapping 98-99 MPH before the 3.73's, and with the 3.73s now i trap nearly 110 MPH without the blower.

with the blower, as i said before, i was trapping almost 108 MPH with 2.77 gears.

so if the blower added 100 horses, then removing it and adding gearing more than made up for the blower, it seems. or there could have been a tunining issue... it was in awful tune for a while.

 

It doesn't ask that because it doesn't calculate torque... only HP. Since HP is a measure of work over time it can do that.

 

Wait your telling me that the Gtech's horsepower number wont be affected at all by swapping around gearing?

Gearing affects torque, and torque affects horsepower. how can the Gtech possibly know when you swap gears? all it does is measure G's, and G's v.s. weight doesnt seem like an accurate way to determine horsepower.

 

Gearing doesn't "affect torque"... it affects both torque and RPM. One goes up, the other goes down, therefore HP remains the same.

The reason your friend's GTO picked up RWHP probably had to do with frictional losses changing.

 

Ahh, a guy named Isaac Newton worked this out about 300 years ago. I guess not everyone got the memo yet... so for future reference:

His equation is F = M x A (Force = Mass x Acceleration).


A G-Tech contains an accelerometer and a clock(time). If you give it the weight (mass) of the car:

1. from acceleration and mass it calculates force.

2. from acceleration and time it calculates distance.

3. from force and distance it calculates work.

4. from work and time it calculates power.

(as a bonus, from acceleration & time it also calculates speed.)


Note that gear ratios do not appear in any of the equations.

Have fun.

 

To complicate things more, and to get more accuracy, you need to introduce Volumetric Efficiency and fuel mixture.

A theoretical ideal fuel mixture is around 14.7:1 , meaning it takes 14.7 quarts of air to burn off completely 1 quart of fuel. This ratio is also known as lambda=1.0. However, for various reasons that's not when the engine makes most power, it's closer to low 14s, high13s (lambda=0.9). That's not even when the engine is the most fuel-efficient, which is closer to high 14s, low 15s (lambda=1.1) and which makes the engine run slightly lean.

Now, when pistons go down and pull air, the air doesn't get sucked in perfectly. It depends on piston speed and on the timing and duration of intake valve. If everything is just perfect, then when piston speed is just right and when the intake valve opens at just the right time and closes at just the right time, you have that perfect burn and highest torque. This would be 100% Volumetric Efficiency at a certain RPM -- for a given displacement volume, you've burned off 100% of normal fuel/air mixture that would get inside.

That's how your engine power depends on airflow (heads, valves, intake, exhaust, piston shape, compression) and your valve timing (camshaft).

Chevy engines have VE (Volumetric Efficiency) in low 80% numbers (occuring, of course, at peak torque). You cannot hope to go much better on Natural Aspiration, especially on our engines that, mighty as they are, still use pretty old technology. There are racing engines out there that have a VE very close to a 100%, but they are ridiculously expensive. The higher displacement you go, the lower the VE is on a N/A engine.

Now there's several choices you can try to improve the output of your engine, given certain CID:
1) For streetability: make sure that VE is high across a wider range of RPM band (so that, perhaps, you have peak torque, but that the area around peak torque has a very similar torque output):
1.a) by better mixing fuel and air at various RPMs -- good TBI or TPI systems (which is why people say they have a flat torque curve).
1.b) by changing your cam profile so that the intake/exhaust valve timing is approximately OK no matter what RPM you're running at.
1.c) by changing your intake and exhaust valve timing depending on your RPM (VTEC -- most primitive, variable valve timing, etc.)

2) For more power: make sure that peak VE is higher than what you have.
2.a) better airflow (which will allow for more fuel to be put in and still have lambda at 0.9) by doing heads, intake, exhaust and having a cam that opens and closes at the right time and, perhaps, has a higher lift.
2.b) increase the number of valves (works, but with risky outcomes on non-peak RPMs) so that, again, more air can flow in and out.
2.c) add a nice supercharger or (better for peak VE) add a nice turbo... (or two nice turbos).

That's how it really actually makes sense to turbocharge high CID engines. Anything from 350 upwards (and even a 305) should theoretically be turbocharged, because these engines have gobs of low-end torque that they can donate to a turbo, which can help them increase their VE and increase their high-end torque. That's why twin-turbo 406 and 427s make around 20mpg -- the twin-turbos boost their VE well over 100%..

That's why people with super-high-displacement engines end up not making as much power as their displacement would make you think they are making -- the little imperfections add up as the size goes up and the airflow becomes imperfect, ruining much more horsepower than a slight imperfection on a, say, 283 engine. These imperfections are much less important if the air is forced into the engine instead of sucked into the engine (superchargers and turbochargers)..

That's also why in europe you see heaps and heaps of TDI engines -- a diesel with a turbo... mucho low-end torque donated to making high-end power otherwise unattainable in diesels. That's also why some of these euro-trash cars are 5 valve engines (2v intake, 3v exhaust), because though they cause way too much air inertia at non-peak RPM, VVT and turbo take care of that. Did I mention that putting all this in tune is a nightmare and a black art, costing these companies millions?

Does it also make sense now why they choose 4-bangers (or more recently inline 5s and inline 6s) for their passenger engines? Less heads to work with -- less cams to work with (if you work with OHC combos, which you have to for any configuration with more than 2 valves per cylinder) and less of a risk of mis-matched flow. Again, tuning these 'technologies' is so hard and sometimes impossible outside of a multi-million dollar facility, which is why I stick with my V8.

Allright, I am okay now

 

alright i get most of what was said, but the Gtech thing still bothers me.

maybe im thickheaded, ok i am.

<b>from work and time it calculates power.
</b>
I get that part. work done over time = horsepower. thats easy enough. the equation says so.


but heres where im getting stumped. if work done over time = horsepower, then why does swapping things like gearing around change the speed that your car accelerates, which seems to me like the engine is getting more work done in less time.

i mean realistically, if you go from 2.73 gears to 4.11 gears your car can accelerate faster right? wrong? am i missing somthing?

and if it CAN accelerate faster, doesnt that mean its doing more work in less time, even though the engine is still making the same amount of power?

I can look at it one other way. the G-tech measures G-force and time and looks at the vehicles weight to determine how much work is being done over time right?
If the G-tech sees a 3500 Lb. car pulling say .5 G's through first gear, (my 305 TBI) it gives me a readout of 165 horsepower.
now the same exact car, same exact motor, but different gearing is going to pull nearly .9 g's through first gear, instead of .5, because of the gearing... wont the g-tech see that as more work being done because of the increase of G force and less time involved?

hah answer that one!

 

Yes it will.

How's that?

We're just making sure you understand that the G-Tech doesn't calculate torque. It can't unless it knew at what RPM the work (HP) is reached. But even then, it could only tell you what the torque applied is at that RPM (like it does with the HP). You'd have to be able to give it your torque curve, which defeats the purpose of using the G-Tech in the first place. If you knew what the torque curve was, you wouldn't need a G-Tech.

FWIW, the NEW G-Tech is supposed to be able to tell you all that stuff AFTER you input all the info(cam, gears, etc.) You can hook it up to a PC and swap info with it. Or at least that's what I understood by reading about it.

Put it this way... I have a G-Tech. I'm going to change to a 3.42 rear end soon (from my 2.73). I'll be sure to run it the same day to tell you what it says... OK?

 

Marin,

14.7 to 1, I believe this is a molar mass ratio, not a volume ratio as you stated by using quarts.

As for Kingtal0ns post, try averaging work done over a longer period of time, (not just off the line). Sure the lower gear is faster off the line, but what about when the two cars (same execpt for gearing) are at faster speeds (say drive), the higher geared car will be able to apply its peak HP for a longer time than that of the lower geared car, maybe the lower geared car will pull away until it starts loosing power from over reving, but the higher geared car will be able to accel to a larger top speed.

What I'm trying to say is that the two cars will be equal (for work done) at some equal point in time. Marin may have already said something like this in his post.

Also friction loss is greater in in the higher geared rear end.

I know I guy who had 2.73's that went to 3.73's (he also had a 4000 stall) . He ended up running the exact same 1/4 mile times before and after.

The Gtech is just guess at HP of the motor, It does an Ok job of telling HP applied to the rear wheels.

A true Dyno uses the tranny gearing, as well as rear gearing to tell HP.

 

Already answered. Read the equations again.

[Here's a hint: "3. from force and distance it calculates work. "]

If you change the gear ratio as described you change the force at the wheels - that is correct. However you also change the distance the car travels in a period of time.

The net effect of the two changes is 0. In other words, changing gears does not change the (horse)power of the engine. (You can get the same answer by solving for kinetic energy vs. time, but I don't think you're ready to go there yet.)

To learn more about this and other useful topics, see if your town has a library. That would probably be the best place to work at proving ol' Sir Isaac was wrong about acceleration. Let me know how you make out...

Enjoy

 

This is all simple physics, available to small children in high school. I'm sorry you have missed it thus far in your life. If you're still in school (HS or college), take physics; if you're not, use the library.

Physics in general is one of the most useful of all branches of knowledge; it is the study of motion and force and all that (mechanics), heat, light, sound, electricity and magnetism, gravity, subatomic particle interactions, cosmlogy, etc..... in other words, physical phenomena. It is the most basic of sciences.

You will rapidly discover as you study physics that it requires good math skills. In fact, most of it is nothing but math; it's just real-world quantities superimposed on top of familiar algebra and geometry and calculus and differential equations. Which is why those of us who learned these simple things in school keep trying to explain that power is the product of torque and RPM, and that as you use gearing to increase one, the other decreases, and as a result, anything that measures their product doesn't care what the gearing is because it's also measuring the speed. It is not necessary to try to "stump" reality; reality is always right.

I was a math and physics major in college. That's why today I have no marketable skills, and like many other such ill-equipped individuals, have drifted inevitably into a life in broadcasting.

 

It will calculate more HP at the rear wheels. "Why" you ask? Simple.

HP is also measured in TIME (something you left out Doctor J).

Let's scratch this "G-TECH" crap...... that thing is just making it more confusing.

Let's use an ACTUAL 1/4 mile.

I'm running 2.73 gears right now. That puts me through the finish line at ~3200 RPMs in 3rd gear. I know because I just went to the drag strip last Sunday.

If I were to change to 3.42 gears it would put my RPMs right around 4000 RPMs in 3rd gear. That's closer to my peak HP (170 @ 4000 RPMs). Since I'll be applying my peak HP through every gear, it will decrease my ET and increase my MPH. That would be an increase in HP at the rear wheels based on time, distance and weight.

I'm gonna guess that some people who change to 3.73's are putting their engine at redline BEFORE they reach the finish line. That's why they're not decreasing their ETs. I don't know that for sure, but it's a possibility.

The other possibility is the car is shifting into 4th gear just before the finish line, which as we all know, puts more strain on the engine when trying to accelerate.

 

<b>What I'm trying to say is that the two cars will be equal (for work done) at some equal point in time. </b>

i never took physics in high school maybe thats why im such a dope...

<b>If I were to change to 3.42 gears it would put my RPMs right around 4000 RPMs in 3rd gear. That's closer to my peak HP (170 @ 4000 RPMs). Since I'll be applying my peak HP through every gear, it will decrease my ET and increase my MPH. That would be an increase in HP at the rear wheels based on time, distance and weight.</b>

thats exactly my point. At a track you will be moving faster, with better gearing. if changing gears did nothing, no one would do it.

<b> In other words, changing gears does not change the (horse)power of the engine.</b>

SO if horsepower is measured in distance, weight, and time... then if we are moving a heavy car farther in less time, thats not considered horsepower? isnt that what a shot of nitrous does? move a car farther faster in less time?

it may not be "real" horsepower, and i see what you are saying when you move gearing up you move RPM down,

so maybe you are telling me that while gearing does not add HP, it does make the engine reach a higher RPM faster, therefore the engine is making more HP sooner, so the car moves farther faster in less time, although the actual RWHP does not change? is that it?

and if that IS it, then why does it become so much easier to say, spin the tires with 3.73 instead of 2.73's, does the torque actually get multiplied by the gears but not affect the RWHP? so we actually have more RWTQ but the "extra" torque does not get added into that HP equation?

 

<b>If I were to change to 3.42 gears it would put my RPMs right around 4000 RPMs in 3rd gear. That's closer to my peak HP (170 @ 4000 RPMs). Since I'll be applying my peak HP through every gear, it will decrease my ET and increase my MPH. That would be an increase in HP at the rear wheels based on time, distance and weight.</b>

Actually, you're talking about average HP. What changing gears does is let you stay closer to your peak HP by decreasing the RPM drop when you shift. The peak HP stays the same but the average HP over the whole run is what will affect the final ET and MPH. On a dyno the only RWHP difference would be from different mechanical losses in the gears and maybe some effects from rotational inertia depending on what kind of chassis dyno it is.

 

Okay, through these posts you see that we talk a lot about torque and RPM.

Torque is a product of force and length.

Torque (footpounds) = force (pounds) * length (feet)

When you take a socket wrench to tighten a nut/bolt, you apply torque to make it rotate. If you use a small wrench it might be hard to unscrew the starter bolt. If you use a long long wrench, it's very easy to unscrew it. That's because torque is a product of force and length and you just increased the length, thus you make more torque. You didn't become stronger, you just used a longer wrench.

That's also why it's harder to peel out if you have 25 inch rims with 80 profile tires -- it takes a lot more torque to get them to move, because their radius is so big, so the (inertial) countertorque they exert on a differential is so much bigger.

How do our engines make torque?

When air and fuel mix inside the combustion chamber and then get lit up by a spark, the mixture burns and expands rapidly, exerting a decompressing force on a piston top. Since the piston is connected to a crankshaft through a connecting rod, the connecting rod acts as length (like a long wrench), while piston forces it down and causes the crankshaft to rotate.

How do you alter the torque? What does it have to do with horsepower?

Gears... gears at the transmission, the final gear at the differential, gear at the flywheel, etc.

If you have a gear that turns at 5 RPM and 200 pound-feet of torque, you can connect it to another gear that is half its radius. That gear will turn at 100 pound-feet of torque and 10 RPM.

You decreased torque, but increased RPM.
It works the other way around too. Connect it to gear twice as big, and it will turn at 2.5 RPM and 400 pound-feet of torque.

However you didn't gain or lose power.

Cough! Cough! Did I say power? I meant horsepower!

So why do we care about horsepower, isn't torque the most important?

Imagine an engine (engine BEEF) that can rotate at only 10 RPM, but has 400 footpounds of torque while rotating at that speed. It's really hard to stop that engine from turning at 10 RPM. However the engine produces only 50 footpounds of torque at 20 RPM.

However, imagine an engine that turns at 10 RPM, but has 50 footpounds of torque. However, imagine that the same engine has 250 footpounds of torque and 20 RPM. We'll call this engine RICEO.

BEEF peak power = 10 * 400 / 5252
RICEO peak power = 20 * 250 / 5252

RICEO has lower torque, but we can use gears to produce higher torque at 10 RPM than BEEF can possibly have at 10 RPM. In fact, given the proper gears, RICEO can produce higher torque at wheels than BEEF, you just have to rev the engine higher.

Horsepower is work done over time. Which means, how much torque and how many revolutions can we pull off in a minute?

The engine that does 400 ft-lbs of torque at 1000 RPM produces as much HP at that point as the engine that does 200 ft-lbs of torque at 2000 RPM.

This also answer the question of whether you increase power with a higher gear. You increase torque at that RPM. The gear is bigger so you have to go through more teeth to make a rotation. This means you decrease RPM. Guess what, power remains the same.

Why do people often gain in a 1/4 mile with a higher gear?

To reach the same speed, they need to rev their engine higher. Most of performance thirdgens have peak horsepower at 4000 or more RPM. Higher gears allow you to reach 4000 or more RPM sooner, making your car produce more power for a longer amount of time during a 1/4 mile.

What's 5252?

This post takes things for granted. It tells you only as much as you need to know to understand what torque is. The torque equation given is correct only when force applied is perpendicular to torque arm. The 1/5252 is a constant that takes into account that we are talking about rotation and horsepower is defined as distance that an average coal-mine horse can travel in a second. It keeps units matching. We could theorize a lot more, but this is the stuff that's important for you to understand what horsepower and torque mean.

Why don't our cars produce the same torque across all RPM?

Piston speed (when they move up-down in the engine) is different and air gets sucked in differently. A camshaft is just a rod with bumps on it, intake and exhaust valves always open at the same crankshaft angle in our cars. For every camshaft for your engine, there is a specific RPM at which they will work the best, i.e. let the best amount of air in and out, so you get the best boom, which makes the most force, which makes the most torque.

Why don't we produce peak horsepower at the same place as peak torque?

Torque drops after peak torque RPM, but doesn't drop insanely fast.

HP = torque * RPM / 5252

What we don't have in torque, we make up in RPM. This could be the motto of our beloved Asian engines.

 

I am going to pin that on my wall. thank you so much.

 

Marin

I got a feeling you consulted the old physics book, but great post man.

 

One thing overlooked with rear gears was optimizing. An increase from 2.73 to 4.11 OPTIMISES the car for the 1/4 mile. So the car gets a quicker et (torque) but the MPH (horsepower) stays the same --- roughly.

But say one puts in a 9:1 read end ratio, the car would be topped out at the 330' foot mark and hittin' the limiter down the entire track.

 

<b>Put it this way... I have a G-Tech. I'm going to change to a 3.42 rear end soon (from my 2.73). I'll be sure to run it the same day to tell you what it says... OK? </b>

do it yet? im putting 3.73 in my TBI 305 car this week too. I will G-tech before and after on video to see the results.

 

Cosmology? K...

 

hey guys think of it like this.



gearing is nothing but a torque multiplier. thats why you see an adverse gain by the seat of the pants. but trap speeds show true hp.


for example like traction with a better 60 ft the car has a better et but with a bad 60 ft the car has the same MPH why cause the car is still traveling the same distance.
for example take my car track times
run 1 60ft 3.20 1/8 11.73 mph 65.417

run2 60ft 2.32 1/8 10.51 mph 66.323

run 3 60ft 2.43 1/8 10.76 mph 66.007

run 4 60ft 2.46 1/8 10.81 mph 65.877

as you can see the mph is about the same throughout all the runs. the run with the fastest mph was # 2 . also the run with the faster 60 ft,means the car made more use of the same measured 1/8 mile to do the work. also look at the run 1 and 2 see the diffrence in ET run2 was so much faster in time but the MPH is about the same as run 1 witch was slower

so knowing that with gears with will make car car acclerate faster to it's top speed bill will taper off with rpm's cause it rev's faster.

but with a taller gears it keep the motor in the torque/hp band longer thus giving it more time to do the work.

with gear changes the grunt is better because of the torque being multiplied.

 

ok so lets say my car has 200 horsepower and weighs 4000 lbs.

I run down the track and run a 15.5 @ 98 MPH or whatever this is just an example.

Now say I run the car again, but this time i remove 2000 Lbs from it.

Im guessing the ET will drop alot, but MPH will remain the same because its still only got 200 horses?

 

That's only true (as I said earlier) if the engine is put higher in it's RPM band than redline when it crosses the finish line.

Evidently that's what happened to your car, or it allowed it to run into 3rd gear.

My car goes through the lights at ~3200 RPMs with my 2.73 gear. If I were to change to a 3.42 gear (which I'm going to when I put the 350 in), then it would put the RPMs around 4100. As we all know, a stock LO3 will only make power up to that RPM, or slightly higher. Heck... 4500 RPMs is redline.

So by you going to higher (numerical) gears, you put your RPMs higher, OR caused the transmission to shift earlier. For instance (since you haven't said what gear you changed to) if you were to put in 3.73's, you will put your RPM range from ~4400 RPMs in 2nd, to ~3600 in 3rd at the finish line. And you're correct. That won't change the MPH of when the shift takes place, it will only change the RPMs of the shift. Although changing to 3rd gear will lower your effective torque output at the rear wheels because of the 1:1 drive ratio in the tranny.

If your goal is to maximize the 1/8 mile, then you'd need 4.56 gears to put your car at max RPM (or at least closer) at the finish line. 4.56 gears would put the RPMs ~4400 RPMs in 3rd @ 70 mph. That's good for 1/8 mile track with an LO3.

 

No, mph will not remain the same, because you're changing the mass of the vehicle by subtracting 2000 lbs., which would alter the calculations of the following: force, which would alter work, which would alter power, which would alter mph.

1. from acceleration and mass it calculates force.

2. from acceleration and time it calculates distance.

3. from force and distance it calculates work.

4. from work and time it calculates power.

 

<b> force, which would alter work, which would alter power, which would alter mph.
</b>

so wait your telling me vehicle weight affect RWHP?

You said force alters work alters power. that means force alters power, and since force is altered by weight, weight alters power. you just said it. so if weight alters power then removing weight adds more power?

Edit: I dont think your right camarojoe. Force is Mass X acceleration, and when you alter the weight the acceleration may go up, but the mass is going down, so force remains the same. the only thing i could see altering MPH where weight is concerned is distance and time. you go farther in less time and since thats apparently part of work (distance and force) it may have an effect on work, but since horsepower is work done over time i think the time going down and work going up may artificially make a car have a higher MPH...

or i could be daydreaming. again.

 

You're ignoring one little detail in real-world application of all this theory. After a point, vehicle mass becomes inconsequential as a factor in power calculations, and aeropdynamic efficiency becomes an increasingly greater factor (and in an exponential way). Mass is only a factor in acceleration calculations (or, in the revcerse, kinetics). A 2,000 lb. vehicle with 18 ft² of frontal area will require the same power to maintain 300 MPH as a 5,000 lb. vehicle with 18 ft² of frontal area, all else being equal. The 2,000 lb. vehicle can be 16' long, and the 5,000 lb. vehicle can be 40' long, and it just doesn't matter if the cD is the same.

Aerodynamics start getting pretty important at around 80-85 MPH for most factory bodies. Relative speeds below that reduce the factor to almost insignificance, and speeds above that should consider drag as a factor in power calculations.

 

KingtalOn,

You're right, I'm wrong about when I said power would be altered by weight removal. I meant that in real-world acceleration less weight adds as you mention "artificial mph" or hp. I'm assuming that when mass goes down and acceleration goes up that force is altered in real-world situations. I'm sure it's physics related, although I never "cough" studied the subject formally. Then again maybe and probably I'm wrong completely about the force being altered, but I also would like to know why mph goes up for sure when weight is reduced like in your hypothetical situation.

Vader,

What you said makes sense, but I'm wondering what it has to do with the question KingTalOn raised. You are accelerating in the 1/4, so what does maintaining 300mph in two equally powered differently dimensioned cars have to do do with the question. I'm not well-versed in physics at all and I was wondering how the aerodynmanic theory is applied to King's question. I've already confused myself now and I'm by no means trying to be a smarta$$.

 

well i suppose the first thing to ask is "does weight affect MPH over a set distance" if the answer is yes then its a factor when considering horsepower at the wheels.

but how can a chassis dyno know how much your car weighs? it obviouselly doesnt matter.

lets use this question. say i buy a brand new Ls-1 camaro off the show room floor. everyone knows these things make about 350 RWHP stock, and run low 13's easy with smooth shifts.

I take it to get it dynoed, whoa and behold 350 RWHP SAE right?

So now we can take an educated "guess" at the MPH this thing will run. say, 112, 115 in the 1/4 mile right?
It was quoted earlier,

Apeiron:
"Rear-wheel horsepower determines how fast the car will be going after a certain distance has been travelled."

So our estimate would be correct considering RWHP can give us a general MPH at the end of a set distance.

But lets say, right before I run the thing down the track, I remove about 1000 Lbs.

Does the thing still run its 112-115 MPH? IT should, with everything else being the same. even though weve removed 1000 Lbs... that shouldnt matter from what was stated so far.

 

play with these for a while, you will figure it out.

1/4 mile Elapsed Time and MPH from Horsepower and Weight
» ET = ((Weight / HP).333) * 5.825
» MPH = ((HP / Weight).333) * 234
Horsepower From Elapsed Time and Weight
» HP = (Weight / ((ET / 5.825)3))
Horsepower From MPH and Weight
» HP = (((MPH / 234)3) * Weight)

 

rusty its not working for me.

MPH = 350 / 3500 * .333 * 234 gives me like 7. somthing

unless im doing it wrong.

same thing here:

HP = 115 / 234 * 3 * 3500 gives like 1500

i must be doing it wrong. explain?

 

sorry about that, i just grabbed some off the net to show how weight will affect mph in the quarter
try these

Calculate 1/4 mile ET and MPH from HP and Weight
ET = ((Weight / HP)^.333) * 5.825
MPH = ((HP / Weight)^.333) * 234
Calculate HP From ET and Weight
HP = (Weight / ((ET/5.825)^3))
Calculate HP From MPH and Weight
HP = (((MPH / 234)^3) * Weight)

those other formulas were missing the ^

for the mph formula
if you go into windows calc enter
350/3500=
hit the x^y button and then type .333 and enter
then multiply by 234
in this case it works out to 108.696 and some change

 

Right and with 1000 Lbs less its 121 MPH instead of 108.

SO weight affects MPH, just like RWHP. Artificial HP, camarojoe was right.

so technically a car weighing in at 2500 Lbs and 280 horsepower could run the same MPH as a car weighing 3500 Lbs and (LS1)350 horsepower.

but the ET would be 12.07 for the 2500 Lb car and 12.5 for the 3500 Lb car, so not only would the car with less horsepower run the same MPH it would accelerate faster and beat the other car with the extra 70 horsepower by .5 in the 1/4 mile.

I guess that explains why bikes with 80 horsepower can run 10's in near stock trim.

So its not just horsepower that determines MPH at the end, its also weight.