Does Gear ratio affect HP rating on a Dyno?

[92ghost]

This is a fundamental question that may be obvious to some. I've been following several topics on HP and rear gear ratio and have to wonder. Numerically, if you have a higher gear ratio you will accelerate faster when compared to a lower gear ratio number (i.e. 3.73 and 2.59 respectively) Anyone who has riden a 10 speed bike can attest to this fact. My question is: If on a dyno will a 300hp engine show the same HP from the 2.59 ratio as the 3.73 ratio?

 

[Ken]

The final drive ratio that dyno shops should use is 1:1, therefore you're rpm would still be the limiting, or indicating, factor. Let me think about how I could re-word this. :L

 

[Ken]

After some research...

I came up with this:

Normally all dyno runs are done in whatever gear provides a 1:1 ratio through the transmission (3rd in automatics, 4th in most manuals, or as in my case with the ROD 6-speed where fifth is the 1:1 ratio). On the dyno, work is being performed in the rearend: the speed of the axles is related to the speed of the driveline. Torque is a measurement of force, not work. Work is a measurement of force in motion. Horsepower is a measurement of work over time. A dyno measures the power produced by the vehicle as rotational force. The dyno knows the mass of the drum that the vehicle is strapped to. Knowing this it can calculate how much force (torque) was required to accelerate the drum in a given amount of time. Because the drum is also moving the dyno can calculate horsepower by using the rotational mass of the dyno wheel along with how far and how fast it was accelerated in a given amount of time. The point is the two values result in the same values when converted using the formula hp = torque * RPM / 5252 (see explanation). The only major difference being in either case what is actually being measured is work (not torque or horsepower). Work is torque in motion and hp is work over time.

Work is force through a distance. For linear motion it's simply Force X distance. For angular motion, the distance the force moves is again used, only now that distance will be the circumference of the circle that the force moves through, or 2 * PI * radius. If you assume an imaginary 1 foot long radius for a moment arm, the distance is simply 2*PI. So then for angular WORK, the Force X distance = Torque X 2*PI. If we're turning at 1 RPM, then the 2*PI distance is covered every 1 minute (1 revolution per minute). At 2 RPM for example, we're now doing the same work, but in half the time (1 revolution in 1/2 minute), and so on for higher speeds. So our angular Power, Work/time = T X 2*PI X RPM, which gives us power in units of lb-ft/minute. To get that into lb-ft/sec divide by 60, so Power = T X 2*PI X RPM/60, or T X RPM X 2*PI/60. Lastly, "Horse Power", as defined by Mr. Watt, is 550 lb-ft/second, so to get our angular power from lb-ft/second into HP, we divide by 550, so finally:

Power = T X RPM X 2*PI/60/550, or T X RPM X .0001904 or T X RPM/5252.11

For a lot more in-depth reading on this subject, see: Understanding Horsepower Loss Through Drivetrain (and no wise cracks about it being a Mustang site. :L)

Hmmm... still haven't answered your final drive ratio question though have I?

Let's see now, where was I? Hmmm... :Roll

 

[92ghost]

Ken, thanks... I'm still struggeling with the 1:1 ratio being the baseline. As you mentioned 1:1 in an auto is 3rd gear and in a stick is 4th. However, this still does not take into account the rear gear ratio which can be a variable. I guess my question should be: When dyno testing do you input the rear gear ratio and (for argument sake) the tire diameter into the test parameters? This will greatly affect the speed at which the Dyno drum, in your description, will accelerate etc.

 

[Ken]

With a 1:1 ratio, it would still remain constant, or rather, consistent, with the engine rpm. I don't recall entering gear ratio or tire diameter on my two (only) dyno pulls.

_ken

 

[*89x2*]

the short answer to the ? is, no effects on hp.

Dyno in 1:1 and you will be fine, whatever that gear slot is...

Enjoy

 

[vettered91]

I understand the 1:1 is in the auto in 3rd BUT out of the tranny and before the rear axle that changes this to 2.59:1, 3.07:1, 3.54:1, etc. so the dyno never gets the 1:1!

So, I have the same 92ghost question. How can the dyno grafics can be the same if the engine is going, let's say 2,000 rpm and
1.-with the 3.54 at that revs in 4th is moving at +- 60mph
2.-With the 2.59 at that revs in4th is moving at +- 80 maybe?

If the dyno measures the performance at the wheels the same engine at the same revs but with diferent rearend ratios I think it's making more effort in the number 2!

But then again, I DON'T KNOW!

Can you explain it to me again KEN? :crazy :eek :L

 

[Ken]

I give up.

 

[Ken]

Ok, let's try it once more...

From Simple Digital Systems EM-4:

Chassis dynos are great tuning aids but they only give a approximation of power output as some of the important variables are not accurately controlled. Certain magazines seem to think that results obtained from chassis dynos are the gospel. They are not. In one recent independent test, hp figures varied by 11% simply by doing the runs in different gears and in another test, results varied by almost 4 % by doing the runs with a different wheel/tire combination. Tire alignment has been shown to affect results up to 3% as well. Note that Engine hp DID NOT change here yet the dyno recorded an increase in hp at the wheels. A change in wheels/tires also does not affect true, wheel hp either, only the rate of change on the rollers and the vehicle acceleration on the road. One can only conclude that inaccurate moments of inertia and correction factors are being used.

On intercooled, turbo cars, there is usually insufficient airflow to ensure accurate results due to charge temperature variation which can be substantial. Even coolant temperatures may not stay down during the run which can affect power outputs considerably. The rate of acceleration is also important on turbo cars to be sure that the boost is not lagging the engine rpm. With RPM climbing too quickly, the boost has not reached a peak value so the hp figure is again inaccurate. Turbo cars should therefore be tested in top gear.

Without proper temperature stability and accurate moments of inertia on the rotating components, there CANNOT be accurate results as the scientific method is no longer being applied.

When all things are kept the same between runs and you get a tangible gain, it is a gain at least. How much, is open for discussion. It is important to note that as the oil temperatures in the engine, transmission and differential increase, friction usually decreases. This manifests itself as an increase in power at the rollers on each subsequent test. This factor should be accounted for when doing back to back runs. It may look like you are gaining some power on each run by making other changes when in fact this is due to reduced oil viscosity. When using a chassis dyno, always use the same gear and tires and wheels and start the runs from the same speed or RPM. Re-baseline periodically to see what temperature increases have done to power output.

Chassis dynos are quick and easy to hook up but have many of the above failings. They do not possess the accuracy of a properly calibrated engine dyno which has a more carefully controlled environment and condition set. Obviously, most non-professionals don't want to be yanking engines to use an engine dyno so chassis dynos do have their place.

Flywheel vs. Wheel HP

As most people know, there are power losses through the drivetrain so wheel hp is always lower than flywheel hp. Front wheel drive cars with transverse engines tend to be more efficient than most rear drive configurations due to the layout of components. However most publications overestimate these losses considerably.

Most rear drive cars have a 1 to 1, 4th gear which means that the power path goes directly through the mainshaft of the transmission. The only losses here are bearing drag which is less than 0.5% and the viscous drag of the gears running through the oil which is about 1% with hot oil. Indeed, published data indicates a transmission efficiency of 98 to 98.5% for conventional transmissions in 4th gear.

Losses within the driveshaft account for about 0.5% if they are properly aligned, balanced and with fresh U-joints.

Differential losses in the commonly used Hypoid type gearset is in the order of 6 to 10%.

The worst scenario case for a rear drive setup is on the order of 12.5% in 4th gear, not the 20 -25% often published. If 25% was being lost in the drivetrain, the oil would boil in the differential housing in short order and aluminum transmission cases would fatigue and break from the temperatures generated. On a 200 hp engine, something on the order of 37,000 watts would have to be dissipated out of the transmission and differential housings. Obviously, this is not the case.

Transverse, front drive transaxles usually have no direct lockup gears and no 1 to 1 ratio, however, since the torque path is never turned 90 degrees as in the rear drive setup and efficient helical gears are usually employed for the final drive set, losses are more on the order of 6 to 9 percent in the upper ratios.

Tire pressure and wheel alignment can have very significant effects on losses at the rollers. Tire pressures should be set the same between each test. Tire rolling resistance varies inversely with speed, another factor not taken into account by most chassis dynos when applying phantom flywheel hp formulas.

Comparing the Numbers

Many novices are quick to compare hp numbers between chassis and engine dynos and come up with all sorts of wild conclusions about drivetrain losses. These comparisons are essentially meaningless. Inertial dynos are based on the sound scientific priciple of accelerating a certain mass with a known moment(distance) over a given time. The rate of acceleration of that mass and moment is a result of the force applied (torque). If the RPM is known, HP can be calculated. On an inertial chassis dyno, it is virtually impossible to calculate the the moment of inertia of every tire, wheel, gear, joint , axle and shaft in the power train between the crankshaft and roller, therefore its results cannot offer an accurate HP figure. Even with coastdown drag measurements, these cannot be accurately calculated as different factors are affected in different ways. Some are proportional, some are inverse squared functions etc. Inertial engine dynos offer a very accurate figure if properly calibrated as only the flywheel's moment of inertia needs to be calculated and added to that of the billet. Water brake or eddy current dynos generally measure force (torque) directly through a ram or strain gauge so moments of inertia are not important on these in fully loaded tests.

Concluding that there was a 25% drivetrain loss by comparing HP achieved on an inertial chassis dyno and that obtained on an engine dyno is fundamentally flawed in that the chassis dyno numbers are highly suspect in the first place.

Other things to watch are correction factors applied for altitude, barometric pressure and temperature. These factors are NOT the same for atmo and turbo engines. Using atmo factors inflates the true, corrected HP figures on a turbo engine. In fact, look at the correction factor applied on your dyno sheets and see if they make sense. Many shady dyno operators simply enter a phantom correction factor to make the customer happy. This is a case where the dyno sheet DOES lie. Chassis dynos are essentially for tuning purposes, they are not well suited to giving an accurate hp figure.

I'm still not getting any closer am I? Nobody seems to have addressed this issue before. Either that, or I just can't find the right combination of search words.

_ken

 

[vettered91]

Ok Ken, you win.

But before I read your book let me get some corn pops!

:crazy

I hope this time I don't get lost between that world of knowledge!

:Silly

 

[MARKSC4]

Motor HP does not change when you change gear ratios.

The only thing that changes is torque.

Torque equation (ft-lbs.): TORQUE = HP x 5252 ÷ RPM

Therefore: HP = Torque x RPM ÷ 5252

A dynamometer measures torque and RPM, and from those two values calculates horsepower.

When you change gear ratios both RPM and torque will change but horsepower will not.

 

[rrubel]

Ken, you may not be finding the RIGHT words, but you're finding fascinating ones non-the-less. Since I'm intending to do a number of chassis dyno tests over the next couple months, including one with a rear gear change (3.07 -> 3.45), I find the discussion useful (and maybe I'll be able to contribute some information).

Now if that bit about a 12.5% loss max is correct, then GM has some 'splanin' to do... I've seen LT1 dyno results on fairly stock cars be ~200-220 HP at the rear wheels. Adding even the WCS of 12.5% only yields 225-245 HP at the flywheel - a far cry from the 300 an LT1 should have...

[RICHR]

 

[motorman]

deeper gears will, use more HP to turn but how much is a question. chassis dynos are best use for comparing one setup to another but not true HP. if you watched one of the sunday TV car shows were they ran the same car on 2 different chassis dynos with different HP numbers.

 

[vettepilot]

Somebody will probably tell me I'm off base on this but, I think the rear gear ratios have more of an effect during acceleration than steady state operation. Since dyno pulls are established at a steady state (once the engine RPM tops out) then rear axle gear ratio has little to no effect at that time. The 1:1 transmission / engine speed is important because it establishes the actual engine RPM going through the drivetrain to the rear axle.
I'm no expert on dyno procedures, but that is my understanding.

vettepilot

 

[UB2 SLOW]

Dynos lie. They are best used to compare mods and thats about it. The only true way to find out what your engine is producing is to pull it and put it on a an engine dyno. Then you can put the engine back in, and run it on a regular dyno and see what the loss is.

 

[oldvet]

Somebody will probably tell me I'm off base on this but, I think the rear gear ratios have more of an effect during acceleration than steady state operation. Since dyno pulls are established at a steady state (once the engine RPM tops out) then rear axle gear ratio has little to no effect at that time. The 1:1 transmission / engine speed is important because it establishes the actual engine RPM going through the drivetrain to the rear axle.
I'm no expert on dyno procedures, but that is my understanding.

vettepilot

Yea, you right. The lower the gear the faster excelleration. Save money and put smaller tires and achieve the same results.Will not change the HP, but you will notice the torg.

 

[G Winter]

You say that having the transmission in 1 to 1 is the key but 1 to 1 with a rear ratio of 5 to 1 might be the same as another in third gear with a 2 to 1 rear ratio. Is the rear wheel hp going read the same? peak hp will still be at the same engine rpm but the actual reading would have to be diff with diff rear ratios the gearing is a multipier.:confused

Glenn

 

[NORTY]

I think the best way to describe the difference in gears is to use a dyno graph. The peak number is going to be the same, but the time it takes to get there takes longer. A higher speed will be attained, creating the same HP #'s. This is of course taking nothing else into consideration. (i.e. increased/decreased tranny/diff internal frictions,increased heat and possibly pulled timing because of the time duration under load.)

 

[MaineShark]

You say that having the transmission in 1 to 1 is the key but 1 to 1 with a rear ratio of 5 to 1 might be the same as another in third gear with a 2 to 1 rear ratio. Is the rear wheel hp going read the same? peak hp will still be at the same engine rpm but the actual reading would have to be diff with diff rear ratios the gearing is a multipier.:confused

Glenn

Okay, how about this:

The dyno doesn't care about the gear ratio, because the rpm number it gets is from the engine, not the drum. An inductive pickup, or somesuch, is attached to one spark plug wire, and the dyno computer uses that to determine the rpm. The drum is only measuring torque.

The reason to put the transmission into drive is that the transmission is most efficient in drive, not because of anything to do with the effect on overall drive ratios.

Clearer?

Joe

 

[Evolution1980]

Holy thread resurrection, batman! This thread has almost as many lives as a cat!

original post: 11-01-03 - 92ghost
last post: 11-03-03 - UB2 SLOW
Resurrected: 06-21-06 - oldvet
And again: 09-14-06 - NORTY
And again: Today - MaineShark