Understanding dyno results after a final drive ratio change

[In-Tech]

I don't know F'bird personally and his posts are generally annoying as are some of mine. He was "johnny on the spot" with his reply though.
Your question answers itself, go to the same dyno before and after, assuming similar conditions, and compare. Did you pick up or lose due to frictional factors?
Ya, I know I'm the idiot

[rebelrouser]

The gear ratio does not materially affect chassis dyno numbers. If you were going from say a 3.08-1 to a 4.88-1 you might see a slight drop due to the higher friction of a higher ratio and the inertia effect of the faster RPM climb rate.

For the learning if you have a manual, you could do pulls in say 2nd, 3rd and then (direct-drive) 4th, where the former two would also show some slight loss due to the extra gear pair in play.

Previous runs were done in 3rd gear (auto), basically for roller speed as far as I know, with the previous diff 4th gear was 260kph at high rpms, and the ratio has gone from 3.133 to 4.363.

So do you think it is fine to basically look at my last run, and use it to compare the effect of new headers and exhaust vs the previous ones without things being radically different on the dyno chart?

I have used a mustang eddy current chassis dyno a little, and the main thing is to ask the operator how he is giving the computer the engine rpm. Mustang used a device called smart tach, it worked well but was tricky sometimes to get the rpm correct, MSD, coil on plug, etc. were all different setups. Or a lot of dyno operators simply bring up the rollers to a certain rpm hit a button and then the roller rpm is used instead of engine rpm, if you have it in the direct drive gear for the whole pull then that works OK, if you do not the results are not correct.
The dyno measures Torque and the needs RPM so HP can be calculated. If engine RPM is used it should not matter what gear you have selected.

[Rob-bb]

I have used a mustang eddy current chassis dyno a little, and the main thing is to ask the operator how he is giving the computer the engine rpm. Mustang used a device called smart tach, it worked well but was tricky sometimes to get the rpm correct, MSD, coil on plug, etc. were all different setups. Or a lot of dyno operators simply bring up the rollers to a certain rpm hit a button and then the roller rpm is used instead of engine rpm, if you have it in the direct drive gear for the whole pull then that works OK, if you do not the results are not correct.
The dyno measures Torque and the needs RPM so HP can be calculated. If engine RPM is used it should not matter what gear you have selected.

I am not sure what brand it is, never asked, it is burred in the concrete of the floor, I am pretty sure they hook to the spark lead to get engine rpm. thanks for the info

[427dart]

Let me throw this wrench in the gears of thinking....What would be the minimum amount of engine dyno power needed to see 450 HP at the wheels?
How does the old Clayton chassis dyno used in the 70's compare to a DynoJet? Going thru some of my old High Performance CARS magazines I saw where they were tuning a 1970 LS-6 Chevelle and a 1970 440 6bbl Challenger on I believe it was a Clayton roller dyno and 325-370HP at the wheels were some good numbers from the hot big cube muscle cars back then.

[Rob-bb]

Let me throw this wrench in the gears of thinking....What would be the minimum amount of engine dyno power needed to see 450 HP at the wheels?
How does the old Clayton chassis dyno used in the 70's compare to a DynoJet? Going thru some of my old High Performance CARS magazines I saw where they were tuning a 1970 LS-6 Chevelle and a 1970 440 6bbl Challenger on I believe it was a Clayton roller dyno and 325-370HP at the wheels were some good numbers from the hot big cube muscle cars back then.

I have no idea, not knowing anything about them. I am of the opinion (unpopular to some) that engine power is unimportant as a tool for comparing cars, a lot of people want to try and back calculate and say an engine makes X power because the car makes Y power at the wheels, but it is all a guess as far as I can tell, no one can look at a car and say "yea that drive drain loses 100hp, or i'll eat my hat"

And even if they could, it's the power at the wheels that gets you moving, it's the only power that matters, which is why I was asking the original question about whether or not a gear change will render pervious charts obsolete when comparing things like intakes, exhausts, and the like.

I have a question to ask you though if you happen to know the answer, how old is dyno tech, when did people first start using some form of dyno for power tuning?

[MadBill]

You can find dozens of S-T threads (also Google "Wheel HP vs. Engine HP") on the subject of wheel vs. engine power but in short there are way too many engine, driveline and dyno variables to allow an exact number to be stated. There isn't even consensus on whether the difference can be assumed to be a fixed percentage or not.

For what it's worth, one shop I'm familiar with has tested a number of engines on their SuperFlow engine dyno then dropped them into the cars with the same exhaust (open headers), accessories etc. and done a number of pulls on an eddy current chassis dyno. For a typical American V-8 4 speed with performance street tires and a mid to high threes rear gear, the numbers they came up with averaged around 15%* in the peak torque to peak HP range, but I didn't se any lower RPM data. *(Note that when correcting from WHP to flywheel HP, the WHP number must be divided by one plus the correction factor, i.e. 115.)

Oh yes, and the dynamometer was invented ~ 1800, a couple of decades after James Watt invented a much-improved steam engine and came up with the concept of horsepower, to equate the engine's work capability compared to that of the work ponies that would otherwise be walking a treadmill or some such to power a mill, mine carts, etc.

[427dart]

Let me throw this wrench in the gears of thinking....What would be the minimum amount of engine dyno power needed to see 450 HP at the wheels?
How does the old Clayton chassis dyno used in the 70's compare to a DynoJet? Going thru some of my old High Performance CARS magazines I saw where they were tuning a 1970 LS-6 Chevelle and a 1970 440 6bbl Challenger on I believe it was a Clayton roller dyno and 325-370HP at the wheels were some good numbers from the hot big cube muscle cars back then.

I have no idea, not knowing anything about them. I am of the opinion (unpopular to some) that engine power is unimportant as a tool for comparing cars, a lot of people want to try and back calculate and say an engine makes X power because the car makes Y power at the wheels, but it is all a guess as far as I can tell, no one can look at a car and say "yea that drive drain loses 100hp, or i'll eat my hat"

And even if they could, it's the power at the wheels that gets you moving, it's the only power that matters, which is why I was asking the original question about whether or not a gear change will render pervious charts obsolete when comparing things like intakes, exhausts, and the like.

I have a question to ask you though if you happen to know the answer, how old is dyno tech, when did people first start using some form of dyno for power tuning?

Some shops were using chassis dynos back in the mid 60's. One of the East Coast shops was Motion Performance owned by Joel Rosen and he had one in 1963 which was his primary business at that time.

[Cris]

I am not a chassis dyno expert, but I have some background in physics and engineering. A chassis dyno must know the RPM to make a calculation of engine torque and then HP (which is just proportional to torque x RPM).

Take two engines that both have 300lb-ft at the flywheel. One goes into a car with a 3.08 rear gear, the other goes into a car with 4.56 gears. You then put both on a chassis dyno. The uncorrected measurement on the chassis dyno shows the car that has the 4.56 gears puts almost 1.5 times the amount of torque to the rollers. But, the guys making chassis dynos aren't nimrods, and they realize that if they check the RPM of the two engines, the one with the 4.56 gears is turning almost 1.5 times the RPM for the same roller speed. By backing out this factor, they can make a calculation of the torque at the flywheel independent of gearing.

And here's the fly in the ointment. A chassis dyno can be fooled by tire slippage, converter slippage, and frictional losses.....and these might be dependent on roller speed which, tested over the same RPM range, would change with gearing. Chassis dynos that bolt onto the rear axle directly help mitigate the tire issues, but there is always a more uncertainty than with an engine dyno.

[Rob-bb]

I am not a chassis dyno expert, but I have some background in physics and engineering. A chassis dyno must know the RPM to make a calculation of engine torque and then HP (which is just proportional to torque x RPM).

This all started when I watched a video by an otherwise respectable 'expert' who said you have to use 1:1 gears to get the right reading, and I thought that made no sense, but I wanted to get input from people who have knowledge because I don't have the experience myself. Of course, with more thought it is obvious that the "must use 1:1" statement is bogus, 1:1 in a trans is not 1:1 from engine to the rollers unless the final drive is also 1:1

[MadBill]

...I am not a chassis dyno expert, but I have some background in physics and engineering. A chassis dyno must know the RPM to make a calculation of engine torque and then HP (which is just proportional to torque x RPM)...

Not if it's a inertia chassis dyno such as a base model Dynojet. They work from the RPM of the heavy drum(s) and its/their instantaneous acceleration rate to determine HP and then calculate torque. In the absence of a tach signal, they can graph HP vs MPH. For a couple of deeper dives on the subject: https://www.porscheboost.com/content.ph ... ary-wildly.
https://www.motortrend.com/how-to/hrdp- ... yno-guide/

PS: I muffed the math in my previous post: "*(Note that when correcting from WHP to flywheel HP, the WHP number must be divided by one plus the correction factor, i.e. 115.)" WHP should be divided by one minus the correction factor, i.e. 1.00-.15 =0.85, to arrive at flywheel HP.

[In-Tech]

Correct Bill, I fook up math all the time and have to check myself frequently, especially as I age I have taken plenty from the engine dyno straight to the chassis dyno. 15% might be a tad low. Most manual trans stuff ends up around 18% loss(divide by .82) and auto trans are all over the place, sometimes higher than 30% depending on the convertor and/or tire. The key with all of that is to make improvements to the whp via oils and other lubricants as well as unsprung weight and etc etc etc. The accelerometer chassis dyno seems to work better for drag racing than an eddy current(although eddy is the king). Every tool has its' own quality and it's important to distinguish.
Carry on

[Stan Weiss]

I am not a chassis dyno expert, but I have some background in physics and engineering. A chassis dyno must know the RPM to make a calculation of engine torque and then HP (which is just proportional to torque x RPM).

This all started when I watched a video by an otherwise respectable 'expert' who said you have to use 1:1 gears to get the right reading, and I thought that made no sense, but I wanted to get input from people who have knowledge because I don't have the experience myself. Of course, with more thought it is obvious that the "must use 1:1" statement is bogus, 1:1 in a trans is not 1:1 from engine to the rollers unless the final drive is also 1:1

I think you missed the point. I will talk about a manual trans. All of the ones that I have worked on when 1:1 have "locked" the input shaft to the output shaft. For any other gear selection the power has to go thought the cluster gear. This means the 1:1 will see less HP loss from the trans.

Stan

[Cris]

As I understand it, inertia dynos also measure torque, not horsepower. They measure the acceleration of a drum of known mass to determine the force (torque) that is being applied. It all comes back to Force = mass x acceleration. They still need engine RPM to determine the engine torque. Gearing is a torque multiplier, and they need to back out the gear ratio to determine engine torque.

I did a little reading on the Dynojet website, this is from one of their white papers:

“The torque on the dyno’s drum can be calculated by multiplying the force applied by the drum’s radius. However, engine torque is not equal to drum torque because the gearing through the drive train changes the moment arm. The change in the moment arm is proportional to the ratio of engine speed to drum speed. Therefore, tachometer readings are necessary to calculate and display engine torque.”

[MadBill]

Here's a quote from the second link in my reply above:

"Inertia dynos extrapolate horsepower output by analyzing the dyno drum's acceleration rate using a sophisticated accelerometer and computer software. An inertia dyno works only when the car is accelerating. It uses heavy roller drums of known mass mounted on bearings that allow them to freely rotate. A vehicle is placed in position on the dyno with the drive wheels sitting on the rollers. The car is placed in gear and accelerated at wide-open throttle. It takes a certain amount of time and force for the tires to accelerate the heavy rollers. The laws of physics state that acceleration rate is directly proportional to how much power the tires place on the heavy roller to get it to rotate. The dyno software monitors roller velocity and the time it takes to arrive at a rate of acceleration and estimates power at the rear wheels. Using data from an engine-mounted inductive probe, the software then graphs the power and gear-compensated engine torque against engine rpm. Some inertia dynos also attempt to estimate flywheel power and torque numbers based on mathematical models and data from additional sensors."

As I mentioned previously, if desired, power (but not torque) on an inertia dyno can be be graphed against road speed instead of engine RPM input by calculating same from drum diameter and RPM. Why would you want to do that you ask? I was visiting a motorcycle shop a few years back when they were attempting to tune a Kawasaki ZX10 on their recently acquired Dynojet. The resultant 'curves' were looking like the seismograph traces of a major earthquake.

I diagnosed it as a problem with the inductive RPM pickup and after their unsuccessful effort at repair, suggested they reconfigure for HP vs road speed. After thumbing through the manual a bit they came up with the right settings and proceeded with their tuning efforts. AIR, peak power was ~190 HP @~ 180 MPH.

[Stan Weiss]

OK I don't have any thing from a chassis dyno I use. Here are 2 pictures. 1 is from the Carb shootout on BLP inertia engine dyno. The Other is from my program. The information in the red box I had to play with to get my output to match up. (The are a number of reasons that my output is not exact.) The information in the 2 green boxes came from the 1st dyno sheet. All of the rest of the data on that dyno sheet were calculated.

Stan

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