Dynos, Gears and RWHP?

[ViperRichRT10]

OK, all you gear heads and math experts...I have 3.55 gears and several people have said it will result in lower RWHP numbers on the dyno. What is the truth and why? I look forward to hearing from those of you who are smarter than I explain what difference this will make if any.

Thanks!

 

[IEATVETS]

Changing the rear end gear to a numerically higher gear, like a 3.73 ratio, will increase the TQ multiplication of the motor.

Rear wheel TQ equals engine TQ x transmission ratio x rear gear ratio. For purposes of illustration, driveline losses are assumed to be zero. So, for a stock GTS, peak RWTQ is: (490 ft-lb)x(2.60 first gear ratio)x(3.07 rear end gear)=3900 ft-lb.

So, if you swap in a 3.73 rear end gear, the peak TQ at the rear wheels is now: (490 ft-lb)x(2.60)x(3.73)=4750 ft-lb. That is right, a 21% increase in RWTQ without touching the motor!

That's the same as keeping the stock gear and modifying the motor to increase TQ output to 570 ft-lb!
(570 ft-lb x 2.60 x 3.07 = 4750 ft-lb.

Regardless of accounting for driveline losses that differentiate flywheel and RWTQ, you will gain 21% more "pull" in every gear with a 3.73 gear swap. What is the trade off? You have to shift sooner in each gear. Now the redline in first gear comes at 48mph instead of 59mph. Second gear maxes out at 63mph instead of 76mph. The trade off is also worse fuel economy. Cruising 70mph in 6th gear now spins the motor at 1700rpm instead of 1390rpm. Of course we didn't buy a Viper for the fuel economy anyways so who cares about that?

Hope this helps.

 

[Mccarlin]

Great info IEATVETS. With my Roe going on the car i would think the 3.55 gear would be too much for the instant power of the blower, thoughts? I'd like to keep the tires on the ground.

 

[Joseph Dell]

But when the power is put to the dyno, a dyno shows a _lower_ HP and TQ number than it would see w/ a stock gear ratio. What is the formula to calculate this?

 

[GR8_ASP]

You may have some change in losses due to the differential itself. However I believe the majority effect is the measurement method. That is using an inertial dyno (dynojet). A lower gear means faster acceleration of the complete drivetrain due to the higher applied torque. That means all the system inertia from the crankshaft and rods, flywheel, to the wheels and tires come into play. The "lost" horsepower is actually going into stored energy at a faster rate.

 

[Whoaa GTS]

. The trade off is also worse fuel economy. Cruising 70mph in 6th gear now spins the motor at 1700rpm instead of 1390rpm. Of course we didn't buy a Viper for the fuel economy anyways so who cares about that?

If you are cruising at 70 or so you are obviously going to get worse miles per gallon, but when you are in other gears you can generally be in one higher gear than stock thus saving gas, and also can use 6th gear more often.

 

[Steve 00RT/10]

I have Unitrax 3:45s, light flywheel, and a 5 pound Roe blower on the 00 car. I love it. I can still get almost 21 MPG at 75-80 on the freeway. Our new ride is similar, but with a stock rear end and 19" wheels, equating to a 2.92 gear ratio(thanks for that GR8 ASP). This car is a torque monster for a 5 pounder and will tear the tires up at 20 MPH at 3/4 throttle.

That being said, my initial observations are that the 3:45s pull a little harder once we get going. More experimentation to follow

Steve

 

[ViperRichRT10]

Frankly, I could care less about gas mileage or the slight difference in MPH in different gears. What I am interested in knowing is the effect on the dyno. Does the dyno read RWHP and torque higher or lower than stock gears and how do you calculate the difference.

Example: 3.55 gears and dynojet reading of 700RWHP.....3.07 gears = ?RWHP on the same dynojet, same car?

Thanks,

 

[99 R/T 10]

It is suppose to be a loss shown on the dyno of about 2-3%. There was a discussion a LONG time ago, and there were many different opinions.

 

[carguy07]

Dynos are "smart" and compensate for the gear change. You will lose a small amount of RWHP with numerically higher gears due to friction. The END.

 

[VIPER GTSR 91]

I have the 3:45 gears and 600 dyno rwhp,and are fine for short tracks, but I had my ass handed to me by a stock Porsche twin turbo after 130 mph last year on our club road tour in Houston. Anything other than the stock 3:08s sure hurts on the top end side.

 

[bluestreak]

I have the 3:45 gears and 600 dyno rwhp,and are fine for short tracks, but I had my ass handed to me by a stock Porsche twin turbo after 130 mph last year on our club road tour in Houston. Anything other than the stock 3:08s sure hurts on the top end side.

You are kidding right? 600 rwhp and left by a stock AWD Porsche at 130? I say to check your dyno because I refuse to beleive that.

 

[KepRght]

I have the 3:45 gears and 600 dyno rwhp,and are fine for short tracks, but I had my ass handed to me by a stock Porsche twin turbo after 130 mph last year on our club road tour in Houston. Anything other than the stock 3:08s sure hurts on the top end side.

You are kidding right? 600 rwhp and left by a stock AWD Porsche at 130? I say to check your dyno because I refuse to beleive that.

you need to check the driver not the dyno. there is mustang drivers out there making 1000hp cars look dumb

 

[GR8_ASP]

Dynos are "smart" and compensate for the gear change. You will lose a small amount of RWHP with numerically higher gears due to friction. The END.

So the dyno knows the amount of inertia in the vehicle? Possible, yes. As a coastdown test can derive that information. However, the Dynojet system does not use coastdown information to derive driveline friction and system inertia. If it did it would be possible to provide a realistic hp curve at the crankshaft.

What the Dynojet measures is the velocity of the drum (really the angular position versus time). Using the velocity it calculates the acceleration (or velocity change per unit of time as it does it in small time steps). It also records engine speed so it can translate the information into engine speed rather than vehicle speed.

Now what does measuring the drum speed mean? It means that anything that reduces the engines power translation into drum speed increase affects the measured power. The more inertia you have in the system (crankshaft, rods, flywheel, transmission, driveshaft, differential, halfshafts and tires/wheels) the less the power translation into the drum, and the more power stored in the rotation of those components. The inertia components can be divided into those that are driven via road speed (tires/wheels and half shafts) and those driven at crankshaft speed (crank, rods, trans, flywheel and driveshaft). The reflected inertial components (upstream of the diff) will store energy at a faster rate de to the engine revving quicker. The difference reduces the hp reading accordingly. This is why a lighter flywheel will provide a power increase, as measured on an inertial dyno, and not on a steady state dyno. The driven inertia will be at a lower overall speed, but will be accelerated at a higher rate. Their impact on the measurement is pretty much negated as a result (that due to a different gear ratio). But, just like a lighter flywheel increases the measured power, so will lighter wheels.

What happens with a reduces gear ratio is the drum is accelerated faster, but at slower overall speeds. While doing so the reflected inertia components are storing energy at a higher rate and reduce the measured power.

Note the differential will also have higher friction with increasing ratio. that is because the reaction forces go up, and thus the bearing loads go up. My expectation is that this is a fairly small percentage though.

 

[Tom F&L GoR]

It is worth reading GR8 ASP's explanation until it sinks in. The engine is always producing the same amount of power and I hope everyone realizes that where that power goes may change depending on what's attached to the end of the crank.

And for reference, here is a graph from someone in a west coast VCA club whom I forgot to get the name of.

Steeper rear gears result in increased "thrust" (the push of the tire tread tangentially along the road surface) and not power. It also means at 130 you are in 5th and the combination of that engine speed in 5th with 3.45 gears does not have the same thrust as a higher RPM in 4th with 3.07 gears. Of course, the real question is why the Porsche was so close before that point that he could leave you behind...

 

[GR8_ASP]

Tom, very similar to a study I made 3 years ago when contemplating a gear ratio change (when stock ). The 2 graphs are made from my measured dyno information with 2 different gear ratios (3.07 and 3.55). Both in terms of thrust, which can be equated to acceleration forces (from the engine as there are decelerating forces on the car from aero and rolling friction as well).

 

[ROCKET62]

Were those previous 3 posts written in English?

 

[IEATVETS]

You must be registered for see images attach

 

[bluestreak]

I have the 3:45 gears and 600 dyno rwhp,and are fine for short tracks, but I had my ass handed to me by a stock Porsche twin turbo after 130 mph last year on our club road tour in Houston. Anything other than the stock 3:08s sure hurts on the top end side.

You are kidding right? 600 rwhp and left by a stock AWD Porsche at 130? I say to check your dyno because I refuse to beleive that.

you need to check the driver not the dyno. there is mustang drivers out there making 1000hp cars look dumb

How much skill is required at 130mph? The only thing you can do is misshift, let off, over rev or hit the brakes

 

[Whoaa GTS]

Since we are on the topic of rearends, what kind of top speed will a GTS get that will theoretically do 185 or so with stock gears do for a top speed with 3.55's?? Havent found out yet

 

[Tom F&L GoR]

It's predicted on the graphs - top speed drops from 190 to 181, both in 5th.

 

[ruckdr]

Calculate yourself here:

http://www.xse.com/leres/bin/gearratio?title=SS+%23670+with+MM6+manual+and+GU6+3.42+gears&rpm=6000&mph=55&gear1=2.66&gear2=1.78&gear3=1.30&gear4=1.00&gear5=0.74&gear6=0.50&reverse=2.90&axle=4.10&tire=diameter&diameter=25.91&circumference=81.40

 

[joe117]

We have been over this.

Your dyno will show a slight loss of hp because of more gear contact, friction.

However,
Your horsepower will not be different due to a change in rear ratio.

The horsepower is calculated using torque and rpm.

When you multiply torque with your gears, you give up rpm.

You could develop lots of torque on any shaft, by using a lever with a big weight on it.

A 10 foot lever with 50 pounds hanging on it would give 500 ft/lb of torque.
How much horsepower?

Zero horsepower, because the rpm is zero.

My point here is that rpm and torque are what goes into the hp formula. Gears can multiply torque,
but gears can't multiply horsepower.

 

[DEADEYE]

Speed is measured by the amount of distance over time. When I had my 3:73 gears, I reduced the time but I also reduced the amount of distance each gear would cover.

Adding 3:73's would be like shoving your favorite food down your throat. Since you have more food in your mouth, the taste (rear wheel torque) is more intense but, the same flavor (flywheel power). The drawback is you finish the food faster and loose enjoyment(rear wheel horsepower) of the moment.

Your torque and horsepower will be read differently. Higher on torque and lower on the horsepower due to a change in wheel RPM. Kinda like dynoing in 3rd gear. It would show less hp.

Sean Roe told me that My 3:73's were the reason my 10 lb S/C was putting down only 604 rwhp. He said it should put down closer to 650. I think it's about 640ish.

 

[DEADEYE]

Lifted from another post...

Rear Gear 1st ********** 2nd ********* 3rd ********** 4th *********** 5th ********* 6th

3.07 **** 58 ********** 76 ********** 114 ********** 151 ********** 204 ********* 302 mph

3.31 **** 54 ********** 70 ********** 105 ********** 140 ********** 189 ********* 280 mph

3.55 **** 50 ********** 66 ********** 99 *********** 131 ********** 177 ********* 262 mph

3.73 **** 48 ********** 63 ********** 94 *********** 124 ********** 168 ********* 249 mph

4.10 **** 44 ********** 57 ********** 85 *********** 113 ********** 153 ********* 226 mph

 

[VIPER GTSR 91]

KepRght: Maybe someday you too will be Unfortunate to try and keep up with a LATE model 911 twin turbo on the sraights AFTER 130 mph. Keep us posted.

 

[KepRght]

yellow/red lining in 4th at 130mph should pull the Porsche, no? most drivers who also own the car don't like to keep the motor running at 4k-5k for extended time, that's why i stated the above. it was a really small jab, I'm sure the driver was trying to be conservative with his investment. What i was trying to say, any car can be bested if the driver isnt willing to lay it all on the line.

 

[bluestreak]

yellow/red lining in 4th at 130mph should pull the Porsche, no? most drivers who also own the car don't like to keep the motor running at 4k-5k for extended time, that's why i stated the above. it was a really small jab, I'm sure the driver was trying to be conservative with his investment. What i was trying to say, any car can be bested if the driver isnt willing to lay it all on the line.

I agree if your not driving it to its full potential or at least as close as you an get to it, then you will get beat. PERIOD.

 

[GR8_ASP]

We have been over this.

Your dyno will show a slight loss of hp because of more gear contact, friction.

However,
Your horsepower will not be different due to a change in rear ratio.

The horsepower is calculated using torque and rpm.

When you multiply torque with your gears, you give up rpm.

You could develop lots of torque on any shaft, by using a lever with a big weight on it.

A 10 foot lever with 50 pounds hanging on it would give 500 ft/lb of torque.
How much horsepower?

Zero horsepower, because the rpm is zero.

My point here is that rpm and torque are what goes into the hp formula. Gears can multiply torque,
but gears can't multiply horsepower.

Joe, I hate to say this but you are not correct. The Dynojet measures the speed and the rate of speed increase of a drum of a known inertia. It also measures the engine speed via spark plug wire inputs. That is all it measures. It does not measure hp or torque directly. It calculates it.

[Conversely, a true chassis dynamometer measures the beam or torque applied and the speed, and thus calculates the hp, and can do so steady state or at a slow enough ramp up such that inertia does not affect the readings]

Because the Dynojet works on the basis of inertia it requires the measurements to be taken over a speed range in a relatively fast manner (the more rear wheel torque the quicker the test). The quicker the test the more driveline inertia plays a part in affecting the measurement. Therefor anything that increases the applied torque will magnify the driveline inertias affect on the stated power or torque. Well an increase in gear ratio does just that.

That is about as simple as I can describe it. If you would like a more in depth analysis I could provide all of the equations and indicate the gear ratio impact on measurement inaccuracy in a clear, concise and technical manner.

 

[Tom F&L GoR]

I apologize for the Porsche remark. It was meant to be sarcastic fun and I hope we don't get carried away about the size of someone's balls or brains. They're probably inversely proportional. Mine are, anyway.