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View Full Version : A dyno story - Mustang vs Dynojet



Lowndex
04-12-2008, 07:16 PM
Dynojet
The Dynojet chassis dyno is referred to as an inertia-type dynamometer, because large drums provide an inertial load to the drivetrain instead of a friction brake. The working end of the Dynojet includes two 48-inch diameter drums that are mostly below the surface and driven by the vehicle's drive wheels. In the photos of the Dynojet, notice how the rear wheels are centered on the drums and there is one drum per wheel. This will become important later. The vehicle is typically run in the transmission gear closest to 1:1 (Forth gear for manuals and Third gear for automatics) to or a variable load that maintains a preset engine rpm or vehicle speed. This feature is ideal for forcing the vehicle to operate at certain loads for tuning. The Dynojet can also measure air/fuel ratio while testing.

Mustang
The Mustang chassis dyno uses an Inertia load as well as an eddycurrent brake load to simulate the "actual" load (combined aerodynamic plus rolling frictional load) that the vehicle would experience when in motion. Notice in the photos how the rear wheels sit between two smaller 10.7-inch diameter rollers. There has been some discussion about the tires getting "pinched" between the rollers and creating more rolling friction, but no substantial evidence of this could be found. However, Mustang has a dyno (MD-1750) with a single 50-inch diameter roller per wheel that alleviates the wheel-pinch concerns. The internals of the Mustang dyno are composed of an eddy current brake to provide a variable load and an inertial disc to provide a fixed load.

Mustang claims because its dyno loads the vehicle as it would be on the road, you can perform 0-60 mph, 0-100 mph, and quarter-mile measurements on its chassis dyno. Launch dynamics are simulated on the Mustang dyno, which includes weight transfer, acceleration, jerk (the derivative of acceleration - how fast the acceleration occurs) and some other variables. The Mustang dyno can also measure the air/fuel ratio while testing.

Correction Factors
Correction factors are used by both dynos to account for varying atmospheric conditions such as temperature, pressure, and humidity. The measured horsepower and torque are multiplied by the correction factor to obtain the corrected values. This is similar to the corrected times and speeds provided by some quarter mile tracks. Theoretically, you can dyno on a hot day in the high altitude of Denver and on some other cool day at sea level and produce the same corrected horsepower even though the observed horsepower you are producing at each location is different. Both dynos calculate a correction factor based on a Society of Automotive Engineering document (SAE-J1349).

When testing was performed on the Dynojet, the correction factor was 1.10, which means the observed numbers were multiplied by 1.10 (adding 10 percent) to get the corrected values. The correction factor for the day when testing was performed on the Mustang dyno was 0.9595 (removing 4.05 percent). The correction factor when road-testing at Keystone Raceway was 0.962, a correction reduction of 3.8 percent.

Why Is loading the Vehicle Important?
The answer to this Question is twofold. First, the engine produces horsepower at the flywheel (brake horsepower) that is reported by the automobile (http://g35driver.com/forums/showthread.php?t=26743#) manufacturers. Engine power is coupled to the rear wheels by a transmission and a rearend. But this is no free ride - there are losses in both the trans and the rearend. Therefore, the power to the rear wheels is equal to the flywheel horsepower minus the drivetrain power loss. The drivetrain losses are mainly composed of three loss areas: friction loss, inertia loss, and viscous loss. The friction loss is largely due to the surfaces of the gear teeth rubbing against each other. Gear friction is related to the torque being transmitted through the drivetrain. The gear power loss is related to the speed at which the torque is being transmitted.

Inertial loss is related to the rotational acceleration (i.e., angular acceleration) of the drivetrain components. The inertial loss does not result in a power loss (i.e., heat) but absorbs energy that can be coupled to the rear wheels. This energy actually gets stored in the drivetrain components. The stored inertial energy in the flywheel keeps the revs up while the clutch is pressed in during shifts. The inertia loss is more pronounced in lower gears (i.e., First or Second) when the acceleration is highest. The viscous loss is basically the pumping of lubrication fluid in the transmission and the rearend. This is one reason why you get better e.t's when the drivetrain is warm, because the oil is thinner and provides less "pumping loss."

Therefore, to measure the actual rear-wheel horsepower, the drivetrain must be properly loaded to obtain the correct drivetrain loss. If the dyno provides a lower drivetrain load, then the drivetrain losses will be lower and the resulting rear-wheel horsepower will be higher.

The second reason why vehicle loading is important is that the newer computer-controlled vehicles use engine load as a control parameter. For example, ignition timing is a function of engine load. You will see higher timing advance when revving the engine in Neutral than you will when the vehicle is fully loaded at wide-open throttle in Third gear. This engine loading factor (and airflow dynamics, which is beyond the scope of this article) can help explain why some people have dyno'd identical to a friend's engine on a Dynojet dyno but got different results on a Mustang dyno.

Conclusions
Realize that both the Dynojet and Mustang chassis dynamometers are useful tools that have excellent repeatability. Both dynos measure the correct horsepower and torque for the load that they apply. Both dynos will show losses or gains from modifications. It is recommended that you pick a dyno for your baseline testing and stick with that dyno type and dyno location (and dyno operator) for subsequent testing.

The bottom line:
dyno numbers are for show, and track times are for the dough!

-author unknown

sailsmen
04-12-2008, 08:13 PM
Mustang Dyno results w/ large differences on similarly equipped Marauders have been posted.

The only diiferences I have seen on Dyno Jet posted results are corrected vs non-corrected.

A critical factor for Mustang Dyno is a consistant vehicle & driver weight.

In theory and practice I can bring my MM from Dyno Jet to Dyno Jet around the country using the same fuel and the peak HP/TQ will be the virtually same.

20+K miles and my Dyno Jet run was w/in 3 HP/TQ.

The advantage of the Mustang Dyno is the operator can vary the load.

The advantage of the Dyno Jet is that the operator cannot vary the load.

There is a shop nearby w/ a Mustang Dyno. When I get an accurate weight on the MM I will have them do a pull and posts both a Mustang and Dyno Jet.

Raudermaster
04-12-2008, 08:32 PM
What'd you copy and paste that from this site? lol
http://g35driver.com/forums/showthread.php?t=26743

Lowndex
04-12-2008, 11:04 PM
:nworthy:
Mustang Dyno results w/ large differences on similarly equipped Marauders have been posted.

The only differences I have seen on Dyno Jet posted results are corrected vs non-corrected.

A critical factor for Mustang Dyno is a consistent vehicle & driver weight.

In theory and practice I can bring my MM from Dyno Jet to Dyno Jet around the country using the same fuel and the peak HP/TQ will be the virtually same.

20+K miles and my Dyno Jet run was w/in 3 HP/TQ.

The advantage of the Mustang Dyno is the operator can vary the load.

The advantage of the Dyno Jet is that the operator cannot vary the load.

There is a shop nearby w/ a Mustang Dyno. When I get an accurate weight on the MM I will have them do a pull and posts both a Mustang and Dyno Jet.

First, I want to acknowledge my appreciation of your input. I have received sound advice from you in the past. You have been recommended by other members. I went on my first dyno run today. All my past dyno experience is with my race bike. So, I feel my dyno knowledge is quite low. I have only been sharing what I read on the Internet and hear from you guys.

I say all of this to ensure you do not feel I am challenging anyone's dyno knowledge or any other aspect of the Marauder performance picture. I only mean to share information and benefit from dialog.

Thank you for all the help you have provided Sailsmen.

Sincerely,
Lowndex

Lowndex
04-12-2008, 11:07 PM
What'd you copy and paste that from this site? lol
http://g35driver.com/forums/showthread.php?t=26743

from a search. I left the "-author unknown" signature to show the information was not written by me. I would have posted the whole article, but the web site stated it was too long and the graphs mentioned in the article were all missing.

Anyway, the information was only meant for consideration; not trying to impress anyone.

red
04-12-2008, 11:59 PM
As I mention here (http://www.mercurymarauder.net/forums/showthread.php?p=605366):

Dynojet also offers a brake (eddy current/loading) dynamometer (http://www.dynojet.com/automotive_dyno/224xLC_dyno/default.aspx), like the Mustang. So, if the goal is to be accurate, then the type of chassis dynamometer (brake or inertia) should be specified, not the brand of dyno. Brake dynos will provide "real world" numbers, which will be lower than what is read on an inertia dyno. Though the inertia Dynojet has a fixed mass, the operator can apply several corrections. For example, they can set the dyno on SAE or UNCORRECTED (depending on environment) for a before tune run and then switch to STD for the after tune run, yielding higher numbers. Also, smoothing can be used to manipulate the output; jagged "curves" will show higher peak numbers. Generally, the operator can make a before tune run while the car's heat soaked (say after driving it to the shop) and an after tune run on a cool motor, giving higher numbers. Another variation is running the before in the middle of the afternoon and the after in the evening. Other tricks include increasing the load on the car for before runs (e.g., turning on accessories or tightening the straps) and further software adjustments.

Big House
04-14-2008, 04:30 PM
I would not be so concerned about the numbers. If you are evaluating modifications, look at the before and after number change. That is the real use of the dyno...that is to me.

red
04-14-2008, 06:55 PM
I would not be so concerned about the numbers.
No one likes a dyno queen.

If you are evaluating modifications, look at the before and after number change. That is the real use of the dyno...that is to me.
I believe the point here is to make sure the dyno configurations and setup are identical. Before and after number changes can be skewed (as mentioned) if the operator wants to play tricks.