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Discussion Starter #1
hey guys i have a question that i have been trying to figure out but its just kinda weird how i come up with the numbers i get. basically its just trying to figure out the numbers of horsepower readings ive seen. our cars are RWD, so we get rwhp readings and on a stock gxp i know that we have a bhp of 260 and rwhp of 216. which leads to 17% that we "deduct" when its transferred to the wheels.
260-46=216whp..am i right so far?lol

i also remember seeing a gm tune dyno (whether it was here or the sky forum) where the person got a rwhp reading of 242rwhp so..
290-49=241rwhp..basically im trying to establish that 17% is a reasonable enough "deduction" when trying to solve for horsepower readings, seeing that the numbers are pretty spot on

however what im seeing is that some people are getting about 270rwhp just off a tune!
325-55=270rwhp...if this is somewhat correct, given the dyno could be off a few depending if it is a high/low reading, thats still a 65hp increase over the stock 260bhp, its pretty crazy how our lil engines can do that (if this is true)...hoping you guys can fill me in on if im either right or wrong, and if im wrong my bad :lol:

258 Posts
As Rob mentioned so many variables like temp, barometric pressure, wheel weight/diameter(since some run aftermarket wheels), fuel etc. Not to mention people use different dyno's, Dynojet, Dynomat, Dynapack, Dynodynamics, Mustang dyno. They all have their way of reading a cars power. This is the reason you have varied results from different Kappa's. My first dyno was 319whp 412wtrq. That was from a stock run of 217whp 246wtrq. Focus on the gains instead from one dyno to the next and stick to one kind of dyno.

Super Moderator
16,393 Posts
Dyno's are notorously inaccurate

Some references and exerpts from a 2010 discussion on the same subject


Kenny's GMPP Turbo Upgrade Dyno Results - Saturn Sky Forums: Saturn Sky Forum


Drivetrain loss is a common topic of conversation in the tuner world because any time you strap your car to a chassis dyno, the output being measured is at the wheel, not at the crank like the published SAE net horsepower figures used by the auto industry. Strap your 298-bhp RevUp G35 Coupe to the dyno and you may be disappointed to see little more than 220-230 horses measured at the rear wheels. Where did that 60-plus horsepower go missing? It was used up in a variety of ways before it could reach the drive wheels, the primary source being what's broadly described as drivetrain loss.

What's interesting about this example is that when you do the math you'll see that the percent loss is much higher than the 15 percent "rule" you'll find in any number of online threads on the subject. For whatever reason, drivetrain loss seems to be one of the most poorly understood subjects discussed on online car forums, so despite my love of the Internet and the limitless pornography it makes available to me, when it comes to a fairly technical subject like this it's hard to find good information.

A few years ago, I needed to educate myself on drivetrain losses while heading a rulebook committee for a local racing series that wanted to use dyno tests to measure engine output and then convert the results to net horsepower. After fruitlessly Googling and sifting through endless car forum threads polluted with half-truths and misinformation, I turned to the same source that developed the current manufacturer horsepower standard, the Society of Automobile Engineers (SAE). On its website you can access brief summaries of technical papers published by some of the world's leading automotive engineers and download the complete documents for a relatively small fee (usually less than $10 per article). As luck would have it, in 2002 the SAE held a symposium on transmission and driveline systems, and the papers that came out of it covered drivetrain loss in great detail.

One of the first things I learned from reading these papers was to completely disregard the 15 percent drivetrain loss "rule" (or any other percent value) that so often comes up during online discussions of whp versus net horsepower. The fact of the matter is every vehicle experiences different levels of drivetrain loss as determined by the design of its transmission and driveline components. Simply put, the amount of horsepower lost to the forces of inertia, drag, windage, pumping and friction are different for every engine, transmission and driveline design.

So the total power lost between combustion and forward motion is specific to each vehicle and therefore no single rule, percentage or fixed number, could possibly apply to all vehicles. Even on the most superficial level, this is easy enough to understand because an all-wheel-drive Subaru obviously has a lot more driveline components to spin (front, middle and rear differentials along with front and rear driveshafts and two prop shafts) and a beefier transmission to hold all that turbocharged torque, so it's naturally going to suffer from greater drivetrain losses than a Honda Fit with its much smaller and less robust transmission, smaller and lighter driveshafts (and no prop shaft) and single differential.

Breaking down the different types of losses that occur within a vehicle's drivetrain, steady-state losses occur while the vehicle is cruising at a steady or constant speed, where average angular acceleration is zero because no additional torque is being called upon to accelerate the drivetrain's rotational mass. Within the drivetrain, steady-state power losses occur from the following components: the transmission torque converter (in the case of automatic transmissions), the transmission oil pump, clutch pack drag, one-way clutch drag, seal and bearing drag, gear windage and friction, and final drive losses.

Dynamic drivetrain losses, on the other hand, include the rotational inertial losses from angular acceleration occurring within the drivetrain while accelerating. In fact, during acceleration there are losses from the rotational inertia of spinning transmission and differential internals as well as driveline components like driveshafts and prop shafts, but also from the increased load and friction being generated between the gears within the transmission and differential(s). And as you already know, with increased friction comes increased heat (more on that later).

It's important to understand the difference between steady-state and dynamic losses because SAE net horsepower, as reported by the auto industry, is measured in a steady-state condition. What this means is that the horsepower rating for your vehicle doesn't take into account dynamic losses that occur during acceleration. However, when you strap your car to a chassis dyno to measure its engine's output, the test is conducted at wide-open throttle and power is measured by the speed at which the dyno's rollers are accelerated. This means that drivetrain losses from rotational inertia and increasing friction, drag and windage are at work and will reduce the peak horsepower reading at the wheels.

Within the drivetrain itself, the primary loss sources are the differential and final drive, with further losses stemming from within the transmission, and in the case of AWD vehicles, from the transfer case. Within the transmission, as much as 30 to 40 percent of power loss can be attributed to the pump, with the clutch contributing another 20 to 25 percent. The rest of the loss within the transmission comes from seal drag, gear meshing, bearings, bushings and windage (drag on the gears caused by the gear oil). However, when dyno testing in the direct drive (1:1) gear, power is delivered directly through the mainshaft of the transmission, so the only loss sources are windage, friction and drag, resulting in total at-the-wheel losses as low as 1.5 to 2 percent, according to the published data.

Differential losses tend to be considerably larger, especially in the case of RWD and AWD vehicles where the torque path is turned 90 degrees as it enters the rear diff and exits it toward the rear wheels. In the case of hypoid-type gearsets (where the gear tooth profile is both curved and oblique) that are commonly used in RWD differentials, losses in the 6 to 10 percent range are the norm, while loss from the driveshaft(s) and prop shaft(s) tend to account for about 0.5 to 1 percent of total loss, depending on how well they're balanced and how many the vehicle is equipped with. In the case of FWD vehicles, the torque path is more direct to the front wheels and the use of efficient helical final drive gears means that drivetrain losses can be as much as 50 percent lower than on RWD and AWD vehicles.

In any drivetrain component with meshing gearsets, heat generated by contact friction between the gears is a significant contributor to drivetrain loss. This is true during steady-state driving, but is far more of an issue when the throttle is mashed to the floor and the resulting thrust force and angular acceleration builds up in these drivetrain components. The heat generated by this dynamic friction is absorbed by the transmission and differential fluid as well as radiated to the atmosphere through the transmission and differential housing(s), and in some cases, via a heat exchanger or oil cooler. This absorbed and radiated heat is literally the conversion of engine torque into thermal energy because you can't technically "lose" power, but can only convert it into other things (some of our favorites being forward motion and tire smoke).

It's also worth noting that the more powerful you make your engine, the greater the thrust force and angular acceleration it's able to exert on the drivetrain, generating even more friction and heat in the process. But because both steady-state and dynamic friction vary depending on engine speed, engine load and the efficiency of the engine and drivetrain's design (how well they limit friction and the associated thermal conversion of torque to heat), there's no way to apply a universal percent loss to it. Nor is it possible to apply a fixed drivetrain loss figure to your car (say 60 whp from my RevUp G35 example), because as you modify the engine and increase its output its ability to generate thrust force and angular acceleration also increases (though not in a linear fashion).

In the end, there's no easy way to estimate the drivetrain loss your vehicle experiences on the road or even on the dyno. Coast-down tests are sometimes used on a dyno to attempt to measure frictional losses, but because this test is not dynamic (meaning they're not done while accelerating, but rather while coasting to a stop with the direct drive gear engaged but the clutch depressed so that the engine and transmission aren't linked) it really only captures steady-state drivetrain losses as well as rolling resistance. So rather than attempting to convert your vehicle's dyno-measured wheel horsepower to a SAE net horsepower figure using a percentage or a fixed horsepower value, you're far better off accepting the fact that these two types of horsepower measurements aren't easily correlated and forego any attempt at doing so.

used to foolishly believe dyno numbers. Until I got to know a dyno owner and went through a lenghty engine rebuild and tuning session that lasted several months. He taught me that all dyno's are not created equal, and that unless you know that a particular machine is both calibrated, operating at sea level and has a great , honest operator, do not believe anything you see.
I offer the following I have gleaned from various places.

Dyno design

Article found via google

While this article is ten years old and the technology has changed, it is pretty interesting and physics have not changed.


I have a nice little side business repairing SuperFlow dynomometers, the
overwhelmingly dominant dyno in the US. Every magazine article I've
ever read used a SuperFlow. The standard SuperFlow is rated at 1000 HP,
10,000 rpm and 800 ft-lbs of torque. The RPM signal is converted to a
voltage by a tach chip before being submitted to an A/D converter. The
torque signal is derived from a strain gauge attached to the absorber.
This signal is also applied to the same A/D converter through an analog
mux. Horsepower before SAE correction is the simple calculation:

(torque (ft-lb) * RPM ) / 5252

This computation is done in an analog multiplier for the analog readout
and by the CPU for the digital readout. So good, so far. But here's
the kicker. The A/D converter is an 8 bit unit. That is, it digitizes
the incoming signal into one of 256 binary values. For torque, that is
800 ft-lbs / 256 = 3.13 ft-lbs per bit. For RPM, 10,000/256 = 39 rpm
per bit. At a constant 6000 RPM, the best HP resolution is 3.5 hp. At
a constant 500 ft-lbs of torque, the best HP resolution is 3.7 HP. This
lack of precision results in the best theoretical HP measurement at 6000
RPM being +- 3.5 hp. Worst case is 3.5 + 3.7 = 7.2 hp. The
root-sum-square (much more representative of the real world) is 5.0 hp.
The precision varies, of course, with RPM. The important point is any
horsepower variation less than about 5 hp is meaningless and is more
likely attributable to quantitizing error in the electronics. Understand
that this does NOT include other systematic error terms such as the
errors associated with the analog electronics or the torque sensor
calibration. I personally attribute no credibility to differences
less than 10 hp.

The other thing to keep in mind when viewing published figures is that
the most frequently published numbers are corrected to SAE Net. This
correction for ambient temperature, humidity and barometric pressure
is only approximate and is really suitable for generating numbers for
ad copy where they are legally required. We have conclusively proved
that the correction is only approximate using a client's dyno cell
that is equipped to control temperature, humidity and baro pressure.

To illustrate the problems involved, I've spent considerable time with a
client because his dyno isn't "producing the numbers he wants". His
engines, which he sells to racers who make buying decisions largely on
dyno sheets, are considerably down on power compared to what his
competition claims. His dyno is spot-on calibrated. He has carried an
engine around to two other shops, one of which is Bill Elliot's shop in
Dawsonville, GA. The span of readings on this engine among the three
dynos is over 80 HP on a 500 hp engine! I have personally checked two
of the dynos and know them to be properly calibrated. The difference is
in the buildup of error terms in this inherently inprecise measurement
system and in the SAE net compensation between Florida at sea level and
here in Atlanta at about 1000 ft elevation.

Bottom line - take any claims of small increases in HP due to "tricks"
with a LARGE grain of salt.

Super Moderator
16,393 Posts
Dyno operator comments

from the Corvette forum

You are correct.. Mustang Dynos do read lower than the dynojet.. Dynos can vary quite a bit depending on what D/A calibrations they use,, if any.. I've seen bone stock LS1 numbers from 275-360rwhp on the Dynojet.. Its all about how the technicians run the dyno... The dyno jet or any dyno is just a tool for measuring baseline rwhp and monitoring the performance of future mods.. Always use the same dyno and technician. List your mods and we can probably give you A better idea.. Looks like you have a 01 Z06 from the profile

We have a Mustang unit... Yes they read lower, this is because they read accuratly. Eddy current load cell units actually compensate for TRUE vehicle weight, unlike Inertia type units, like MOST dynojets that use a set 3000lb load.

Technically you cannot even properly tune a car on a dynojet since it doesnt load the wheels correctly. If the wheels are not loaded to actual vehicle weight, or atleast very close, your A/F ratio will be totally different when the car is on the street.

Ive found that the mustang typically reads between 10-12% lower then a inertia unit.

Here's how I like to explain the difference. . .

DynoJets are inertia dynos, and have been around for years, much longer than any type of load cell dyno. Inertia dyno's work on the principle of the acceleration of a known mass over time. Their rollers are the known mass. Weighing in at over 2500lbs or so. Your car gets strapped down to the machine, and the dyno collects it's data. It is able to calculate horsepower by measuring the acceleration in rpm of the rollers in regards to RPM. This is why gearing can affect the dyno results, more on that in a bit. Now that the dyno has recorded the horsepower curve, it can take the integral of that curve and get the torque curve. Since the dyno’s power calculations are based on the acceleration of mass over time in regards to RPM, gearing is very important. Since a vehicle with a lower gear ratio can accelerate the mass to a higher speed using less engine RPM, it will show a higher horsepower number than a car with a higher gear ratio. If a car is able to accelerate the dyno’s rollers from 200rpm (roller) to 300rpm (roller)in 1500rpm (engine), then the dyno is going to record more power than a car that did that in 2000rpm (engine).

Now we go to Mustang dyno’s and other loaded dyno’s. Our Mustang MD-1100SE dyno’s rollers weigh 2560lbs. That is the actual mass of the rollers, much like the DynoJet. That’s about where all the similarities end. When we get a car on our dyno, we enter two constants for the dyno’s algorithms. One being the vehicle weight, the other being what’s called “Horsepower At 50mph”. This is a number that represents how much horsepower it takes for the vehicle to push the air to maintain 50mph. This is used as the aerodynamic force. Mustang dyno’s are also equipped with a eddy currant load cell. Think of a magnetic brake from a freight train. This magnetic brake can apply enough resistance to stall a big rig. Off one side of the eddy currant load cell, there is a cantilever with a 5volt reference load sensor (strain gage). As the rollers are spinning this load sensor is measuring the actual torque being applied. So as the rollers spin, the load sensor is measuring the force being applied, sending that information to the dyno computer, taking into account the two constants entered earlier, computing the amount of resistance needed to be applied to the rollers to load the car so that the force of the rollers resistance is as close to the force the car sees on the street. The dyno is then able to calculate the total force being applied to the rollers in torque, and then taking the derivative of that torque curve to arrive at the horsepower curve. Since torque is an actual force of nature, like gravity and electricity, it can be directly measured. Horsepower is an idea that was thought up by man, and cannot be directly measured, only calculated.

I like to state it like this. . . I start by asking how much your car weighs, lets say 3500lbs. Now you take your car and you make a make a WOT rip in your tallest non overdrive gear, how much mass is your engine working against? 3500lbs right? Now you strap your car on a DynoJet and you make a WOT in the same gear, how much mass is your engine working against? 2500lbs right? Now you strap your car on a Mustang dyno, how much mass is your engine working against? 2500lbs. Plus the resistance being applied by the eddy current generator. We’ve seen anywhere for 470lbs of resistance to over 700lbs of resistance as measured in PAU force in the data logs. So which one is more accurate? Well they their both accurate. If a DynoJet dyno says you made 460rwhp, then you made 460rwhp. If a Mustang dyno says you made 460rwhp, you also made 460rwhp. Now which one of those numbers best represents what your car is doing when its on the street. That’s a different question.

The most important thing to remember is that a dyno is a testing tool. If the numbers keep increasing, then you’re doing the right thing. We try to look over at NET gain, instead of Peak HP numbers. A 30rwhp increase is a 30rwhp increase regardless of what dyno it is on.

Now I can address how to calculate the difference between one type of dyno and another. Simply put, you can’t. Because Mustang dyno’s have so many more variables, it’s not a simple percentage difference. We’ve had cars that made 422rwhp on our Dyno, two days later make 458rwhp on a DynoJet the next day. We’ve also had cars that made 550rwhp on our dyno, make 650+rwhp on a DynoJet a few days later at another shops Dyno Day. For instance, my 2002 Z28 with a forged internal LS6 Heads/Cam/Intake, makes 460rwhp on our dyno. I thought that was a little low, since I’ve had cam only LS6 Z06 vettes make 450rwhp. So I overlaid the dyno graphs. Guess what, the PAU force for my car was almost 200lbs more than the C5Z06 that made 450rwhp with cam only. So I entered the weight and horsepower at 50 number for a C5Z06 and did another horsepower rip with my car. The only reason I did that was to compare Apples to Apples. This time my car made 490rwhp, no other changes. Now I don’t go around saying my car made 490rwhp, I say what it actually did with the correct information entered into the computer. It made 460rwhp. Now if I ever get a chance to take it on a DynoJet (which I plan to in the spring), I have no doubts it’ll be over 500rwhp. I know this based on airflow and fuel consumption on the data logs.

But since we’re asked this question constantly we're fairly conservative, and hence tell our customers that the difference is closer to 6-7%, but as you make more power, and the more your car weighs, the difference increases as well. You must remember, Dyno's regardless of the type are tuning tools, and are in no means meant to tell people how fast their car is. Now which one is more "real world" is a totally different question. I like to explain it like this..... If you drive your car in a situation in which you have no mass and you're in a vacuum, so basically if you do intergalactic racing in space, use a DynoJet. If your car sees gravity, and has an aerodynamic coefficient, and you race on a planet called Earth, then use a Mustang Dyno

6,944 Posts
The reason you can get such a large improvement from a tune on the GXP is simple. While you would expect a stock tune to optimize power while being safe, the stock tune used on the Ecotec in the GXP goes way beyond that. It artificially limits torque to 260 Tq and if you look at a dyno graph it looks like a table top between 2000-5000.

So you don't start from an optimized position, you start from an artificially low position and the after market tunes seem to have an unbelievably large effect.

Even the factory GMPP tune bumps the torque figure from 260 to 340, which is huge (power goes from 260 to 290)!

258 Posts

Bravo Rob, enjoyed this very much!!!! I know you had once explained this before but it was good reading it again. Job well done Sir.:thumbs:

272 Posts
Discussion Starter #8
very good read rob, thanks :thumbs:

Super Moderator
16,393 Posts

Bravo Rob, enjoyed this very much!!!! I know you had once explained this before but it was good reading it again. Job well done Sir.:thumbs:
Ok, that made me laugh. :thumbs:
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