if you had a rear gear of 3.42. and turned 500hp on a chassis dyno. pulled off and went to a 4.56 gear would the dyno say you have more or less torque?power?

 

It would say you have the same.

 

not according to some.

the word on the street is that you lose hp by changing to a lower gear.

http://www.ls1.com/forums/showthread.php?t=742



http://www.ls1.com/forums/showthread.php?t=742

 

you'll lose HP on the dyno by swapping 3.42 to 4.56

 

There'll be a small change due to changes in friction and mass, but I wouldn't worry much about comparing different dyno readings taken at two different times under different conditions.

 

but the car runs faster and the motor wasn't touched. how can you lose hp? i think this is a good thing to know to prove how inaccuarate and useless dyno's are.

 

The car may have a faster ET after the swap because of the change in rear wheel torque, but the trap speed will be nearly the same because the HP won't change much.

Changing the rear gear ratio changes the torque multiplication at the rear wheels, which changes the rear wheel torque, but that's not what the dyno is indicating for its torque figure. The dyno compares the speed of the rear wheels with the speed of the engine to come up with an estimated final drive ratio to correct the measured rear wheel torque to give an approximation of engine torque, less the drivetrain loss.

It's not really that the dyno is inaccurate and useless, just that its numbers have to be interpreted in the right context. Just like flywheel HP and torque numbers aren't a perfectly accurate gauge of how the engine will perform when installed in a real-world car, the chassis numbers aren't a perfectly accurate indication of how the car will perform on the track.

 

ive read something on this before it has to do with x gear ratio its easier for the motor to turn the rear wheels than the other so the hp/tq readings would be diff. i dont remember the exact formulas and technical bs but it makes sense like riding a mountain bike with multiple gears

 

Dyno's do not measure hp, they measure tq, hp is a figure based on the amount of tq and the rate that the dyno wheels were spun up.

A lower(numerically higher) rear gear will show less hp because the wheel speed does not come up as fast. - Your e.t may improve with a shorter rear gear IF the engine's powerband is better suited to it. Your trap speed may also be lower, as you aren't bringing the wheel speed up as fast, thus it doesn't charge as well at the top end of the track.

Case in point. Most 10.5 outlaw cars are running 220+mph with a 3.90 or steeper rear gear. We're running tha same e.t's with the same hp at around 200 mph with a 4.56 gear and a higher revving engine.

all that said, I hope you're not running a 4.56 in a 10-bolt, as it won't live long.

 

Guy just did a 3.73 to 4.56 swap on his 454 SS truck

dynod 398whp and 430 ftlbs of torque with 3.73's


swapped gears and dynoed 397whp and 417 ftlbs of torque

so basically he lost torque

 

Did he have an automatic transmission?

 

i just think that it shouldn't change at all. but i don't anything anymore. i do know that physics says that it takes a certain amount of power to move certain amount of wieght a certain distance in a certain amount of time. if you change gear or motors and pick up time more hp must be getting to the ground or you would be defying physics. if you go from a 3.23 to a 4.10 in a mild performance vehicle you would surely pick up time. thats why it's one of the first thing people do to enhance performance.

 

All cars will accelerate faster with numerically higher gears, because the increase in torque multiplication increases the rear wheel torque, which increases the force that the tires apply to the pavement to accelerate the car (assuming traction isn't an issue, of course).

It's possible that the 1/4 mile E.T. could be slower though, because even though the car may accelerate faster, the change in gear ratio lowers the maximum speed of the car. (Although this only happens in fast race cars and generally isn't the case on streetable cars).

The idea is to pick the gears which will have the car at redline as it crosses the finish line. A car with numerically higher gears than this will cross the finish line later because it speed topped out and it wasn't able to accelerate any more. A car with numerically lower gears than this will cross the finish line later because it wasn't able to use the full potential of the engine, but interestingly it will be going the same speed when it crosses the line as the properly geared car.

 

nope its a 6 speed T56 i believe behind a 454 fuel injected big block

 

Still that's only about a 3% difference between the two runs. Not really all that much.

 

that could be a diffenrce in air temp or even bearing's being new and not loosend up yet.

 

maybe, but from what they were talking about, the dyno runs were in similar weather and not too long before this dyno run.

hp was the same but torque was down. all three runs were 416-417 lb ft. i dont see losing 13 lbft of tq consistantly being from bearings or anything else. I'm not sure what happened there but the difference in gearing had to make up for it.

 

Different gears have different mass and different friction.

If the dyno indicated actual rear wheel torque, you'd see a large increase.

 

No doubt you will recall, there's a hookup that the dyno uses to measure engine RPM. A spark plug wire pickup usually. The dyno software uses engine RPM to calculate out the effects of gearing and tire size.

Think about it.... do you HONESTLY think the dyno would have read a statistically significant difference if the trans was in some other gear? Or if the car had 28" tires instead of 26" or whatever? Well the rear gear doesn't matter either. Beyond, as said, the slight difference in individual gears and setups.

 

those guys have noticed different hp/tq readings for different trans gears as well. there was different numbers for 3rd gear than 4th gear in the guys S14.

and yes they did use a plug wire pickup to get engine rpm.

I'm not sure why the numbers were different, and would agree it shouldnt matter, but the results speak for themselves.

 

thats what i want to know is why. it's got to be the way the dyno translates it's information.

 

Why what? Why the readings are different?

 

yeah. why does it read false like that. you figure a gear is a torque multiplier. your engine puts out 500 ftlbs. if you have a 3.42 gear thats 1710 lbs of force on the rear axle. change to a 4.56 gear and you have 2280lbs of torque. obviously more. at least thats how moser calculates rear torque. the dyno changes all that up and put's it into a reasonable number for everyone to understand.

 

Yes, that's how the rear gears multiply torque. Why the dyno doesn't just report that directly, I don't know. The tire size would have to be entered for that, which would be kind of annoying. Maybe they're just making the numbers easier to compare for bragging rights. For tuning purposes, it doesn't make all that much difference, I suppose.

 

Gearing multiplies torque; but it also decreases RPM. Double the gear ratio, and torque doubles, but RPM drops by half.

Power = torque x RPM x a constant. (The constant happens to be 1/5252.11, which is 2pi/33000)

It doesn't take a rocket scientist to see that if you multiply 2 numbers together and get one number, and then you double one of the numbers (torque let's say) and half the other one (RPM) and then multiply them again, the answer (power) stays the same.

The dyno measures both engine RPM and the RPM of its roller, so it "knows" what the gearing is. All it has to do, is to calculate the ratio of engine RPM to roller RPM, and divide the "measured" torque by that number; and now gearing, tire size, transmission gear, and EVERY other such variable, is taken out of the picture.

Which is why dyno readings DO NOT CHANGE based on the gear ratio; as long as the final drive ratio puts the roller RPM within the physical range that the roller's inertial control system can work within, the number will stay the same.

Any variation is purely random, just like driving the same car up onto a dyno 2 times and getting 2 slightly different measurements even if the gearing stays the same.

Threads like this make me glad I paid attention in high-school physics class. For those of you who don't understand physics too well,

http://www.splung.com/content/sid/2/page/circular
http://www.howstuffworks.com/pulley.htm
http://www.physics.umd.edu/perg/abp/...ech/mechda.htm
http://hypertextbook.com/physics/mechanics/
http://www.secondchancegarage.com/public/201.cfm
http://www.shotimes.com/dyno.html

Incidentally, some dyno softwares WILL report the "gearing" that they observe; only thing is, the dyno has no way of knowing whether it's the gear the transmission is in, the rear gear, or the tire size.

 

thats what i would think. but i have read and the only ones i found are all on a dyno jet. it has to be some kinda flaw with the software. i wanna see someone do this with a mustang dyno.

 

yes we used a dynojet and got different readings so it has to be a software error or some type of miscalculation or calibration error

 

I couldn't have posted any better myself ^^^^.

The only thing I can really think of is the gear types. When I say that I mean did he go from a stock style gear to an aftermarket one? The aftermarket strong units have a lot more surface area, and are louder so they use more face of the gear. This makes you loose more power as more friction is introduced.

That combined with random error and maybe bearings breaking in as well and I am sure your disrecpincy is accounted for.

 

one of the biggest problems with chassis dynos is ideal world vs real world. You get a different number just by changing the tension on the straps or by changing where they are tied to. Bottom line being that they are a great source for tuning and for an estimation of power. If you want to know exactly how much power your car has, pull the motor and strap it to an engine dyno, and even that will vary some pending testing conditions. - then you can do the math to figure rear-wheel power, but that's still theoretical due to the frictional losses involved with tunring the driveline.

There's a reason we don't race dynos.......

 

well a 4.56 gear should have less mass than a 3.73 gear due to more teeth of less material. I mean you can only fit so many teeth on a certain size ring gear. the more teeth means a weaker gear since there is less material per tooth, so i think it would be lighter. then again, it could be the same weight and distributed out over the number of teeth.

either way i guess that could have some differences from run to run. he's planning on gettin a tune since its running abit rich all over. we'll see what it does.

 

Who knows, maybe there's more preload on the pinion. Whatever it's due to, it's not worth thinking about.

 

Most of the tooth count change is done with the pinion, not the ring. Many lower(numerically higher) gears actually have a heavier ring gear as it is thicker to make up for the smaller pinion size. The only way a lower ratio ring gear is lighter is if it's back cut to lighten it.

...and yes, a different pinion pre-load could skew the numbers some more.