INDEX of Subjects: Any New Additions will be the TOP


  > > Operation of DEPAC C.F. Probe

  > > Comments on Atmospheric Correction Factors

  > > Drive Shafts in Dyno Testing

  > > Torque Strain Gauge Application

  > > INERTIA FACTOR and Corrections

  > > HOT TIP on Upgrading the Go-Power or DTS Dyno

    > > Disclaimer

 The Operation and Application of the DEPAC Correction Factor Probe:

The Remote Correction Factor (CF) Probe Senses Both Air Temperature and Absolute Humidity. The Barometer term in the CF equation is input manually using the BARO Dial. The Remote CF Probe uses a precision Sensor that measures the Thermal conductivity of the air. Moist air has a higher heat Conductivity than dry air and the sensor responds to this very accurately out to 50 grams per Cubic Meter, which corresponds to the highest levels of humidity that you can expect in practice. This Probe is accurate, Rugged, and Practical, But there are certain characteristics that need to be considered.

1. This probe requires 3 to 5 minutes to 'warm-up' after system turn on.

  Two super heated micro-bead thermistors form a balanced bridge where the cooling effect of the Humid air is compared with that of a sealed container of Dry air. Several minutes are required for the thermal balance of the thermistors to Stabilize.

2. This probe is very sensitive to motion.

  The hot thermistors (about 570 DegF) respond to the cooling affect of the surrounding air. Moving or Waving the probe causes the air near the thermistors to move and create an imbalance of the bridge.

3. This probe can be affected by very loud sound waves at certain pitches.

  In practice, very intense sound pressure waves can cause the Humid side thermistor to vibrate and cause a full scale imbalance of the bridge. The Humid thermistor is exposed to the open air but the dry reference thermistor is enclosed and protected from outside sounds.

4. This Probe can be knocked out of calibration by a shock (Like being dropped onto the floor).

  This shock can cause the location of the thermistors (supported by Fine Platinum wires) to be knocked to one side, and this affects the 'Zero' thermal balance. Fortunately the probe can be re calibrated. Of course, its not a good idea to drop any precision instrument.

5. This probe does not operate properly in still or dead air. (Same for ANY Probe)

  A volume of air that is still can can have wide variations in air quality. A good example is that the air near the ceiling is warmer than the air near the floor. Air must be stirred with a ceiling fan or similar device to mixed the air into a consistent mixture. The CF Probe's Air temperature and humidity sensors Require that Air be moving passed the sensors for accurate sensing.

With care in handling and observing proper operating conditions, this Humidity sensor will provide excellent long term accuracy.

Intense sounds at a frequency of about 480 Hz (or a V-8 running at 7200 RPM) can cause the resonant vibration affect. The problem is mostly in Dyno rooms with no Sound absorbing walls/ceilings and the engine is run without an air-filter and the probe is place very close to the Open engine intake. A good dyno cell should have Walls covered with sound absorbing materials and the Engine air inlet should also have Sound and Thermal insulation. Of course, if the engine operates with an air filter in practice it should also be dyno tested with the same air filter.

Engine Air intake ducting should have a large cross sectional area of 2 to 4 square feet and and NOT be sealed to the engine inlet. A high volume, Low velocity, Fan like a propeller fan should be use to force air through the duct. (Do not use a squirrel Cage type Fan, which produces a localized turbulent high velocity air stream. These fans are good for evacuating air, though). ANY Sharp bends Must have Turning vanes to help the air change directions with minimum turbulence and flow losses. See Fig 1. The air flow volume should be at least twice the CFM required by the engine at full power. Any excess air just floods over the open inlet around the engine. The velocity is low so there is no Ram charging effects. The CF Probe is place within this ducting, away from the engine inlet but so that it senses the air just seconds before it is ingested by the engine air intake. The probe should be mounted at a fixed point and not be allowed to swing or bob. this motion will upset the careful thermal balance of the bridge. The sensor can be subjected to some High frequency vibrations without effect but not intense enough to cause fatigue fracture of the internal wired component leads.



The Turning Vanes can be obtained, pre-fabricated, from a company called:

AERO/DYNE PO Box 861, Woodinville, WA 98072 1-800-522-2423 (Product: HEP Vanes)

The Dyno system is turned 'ON' at least 5 minutes before the test (and left 'on') to allow the humidity sensor probe time to stabilize. Air intake fans should be turned 'on' at least a minute before the engine test to stabilize the air quality within the duct and to allow the CF probe time to sense these changes.

There is a 'CF HOLD' switch on DEPAC's front panel that will freeze the current CF Number and has been used in cases where the Humidity probe is affected by the intense sounds during the run . This switch should only be used in an emergency. The proper procedure is to reduce this sound noise below the 'threshold level' so the probe can respond to changes of the CF during the test run. OF course, if the CF changes more than a few 'tenths' during the run then the air system is inadequate, or you haven't allowed enough time to stabilize the air quality or warm-up of the CF probe. A proper air system will have only a small change in the CF before, during, and after the test run.



Coupling and Engine to a Dynamometer is no Trivial task and most Engine dynamometers are using improper drive shafting that can lead to serious engine damage, to say nothing about the damage to life and property.

It is Wrong to have a Stiff or Rigid Drive-Shaft connecting the Engine to the Dynamometer. This is a very common situation with stress levels, combined with high fatigue cycles, than will cause catastrofic failure. If you have broken a dyno drive shaft please do not look for an immediate cause. The damage started as soon as you first installed the drive shaft and it may have taken years for the stress cracks to finally get large enough to let go.

Its also Wrong to Bolt on a Dyno Flywheel that is Much Heavier than the normal engine Flywheel, even though several dyno manufacturers recommend this practice. The reason for all this is that the direct connection of an inertial mass (flywheel or Dyno) to a Spring (crankshaft) has a characteristic Natural Resonant Vibration (like a tuning fork). The frequency of this Vibration is Normally much Higher than the engine's Maximum RPM Range, which is good. A vibration Damper is usually bolted onto the Free, Front end of the Crankshaft to Dampen any whip-lash, from any High frequency 'Ringing'. BUT, when a much Heavier flywheel is bolted Directly (Solidly) to the Crankshaft, this Resonant frequency could be Lowered to the RPM Range you are Running the Dyno (This is VERY BAD!). No front-end Dampener is going to Help reduce this Destructive situation. If you Run your Dyno at this Resonant RPM for any length of time you will eventually have Cracked or broken Crankshafts, Twisted-off Water pump Drives, Broken Cam Gear Drives, etc. You may not have broken a crankshaft on your dyno simply because you have never run the engine long enough at the Critical Vibration point(s). Many just sweep an engine so fast that there is no accumulated Structural fatigue to cause any serious damage. Don't feel that your the exception and the above does not apply to you... it does. Too many people have put a new or rebuilt engine in their car and only to Break the crankshaft in Hot Laps. The REAL Cause of this Failure was from being Fatigue cracked on the Dyno!!

Consider a normal Vehicle Application. There is a lot of Torsional wind-up between the engine and the final drive (wheel, prop, chain, track, etc.). The Resonant frequency of the entire vehicle (Engine + Drive line) is thus normally VERY Low (from lots of Torsional compliance). Why is it then we put an engine on the dyno with little or no Torsional compliance, with a high Inertial load, and then Wonder why the Engine Breaks??

Understand that you Can Have a Heavy Flywheel on the dyno IF you place it at the end of a proper torsional drive shaft. So, you can have a Starter Flywheel on the dyno for testing Sprint and Midget engines.

Some very common water dynamometer Designs have little inertia when dry But can have Much More inertia when partially full of water and Spinning. Also the amount of water in the brake will change this flywheel effect. This type of dyno tries to run at a steady speed because of the flywheel effect. An engine, though, has instantaneous Speed changes every degree of rotation. This Causes an enormous Stress on the drive shaft if it is very stiff. The engine feels the reaction of this Stiff stressed drive shaft Causing (destructive) Internal Stress and Vibrations.

To help Solve this Problem, there is a Dyno Driveshaft made by DANA/Spicer, that is described later , but we feel that this not as practical a Solution as the following. (Jump to DANA)

DISCLAIMER: The following is a driveshaft application that can work Safe and Well, IF done properly. An Improper application can lead to damage to life and property. ONLY Those who are very knowledgeable and responsible should attempt these conversions. We can not be responsible for the action's of Idiots! (Idiots turn off your PC NOW and sit in the corner!)

There are many dynos in use that have only a short and stiff drive shaft. This can result in large destructive stresses between dyno brake and engine with potential damage. To help solve this Problem we have found an Elastomer coupling made by LORD that can be directly adapted without major modifications to your dyno frame. This coupling can be used especially with Diesels and 4 Cylinder race engines (which have torque pulses 2 to 3 times the average engine torque). The Coupling Size, Model LCD-0150-25R-C, can be used on a V-8 and other smooth torque engines out to 850 BHP and 650 Ft-Lbs. A size larger -0200 can be used with higher BHP engines. This Coupling can 'Twist' up to 40 degrees or more. The Elastomer is not Bonded to the outer shell and at High Torque (well above the rating) it will just Slip, like a loaded clutch.

These LCD-0150 Couplings are now Available through DEPAC. The total cost to lower 48 States is $650.00, which includes UPS Ground Shipping.

 We also have complete upgrade kits. (click here)

SPECIAL NOTE: The intended application for these couplings was for low speed PTO and motor drives. But, the design can be used to much higher speeds (9000 RPM+) . Each coupling may require 'balancing' or have the rubber donut pressed into a Machined housing. Shafts may require an additional centerline support to prevent 'whipping'. (We, and the OEM of these couplings, assume no liability for improper application. Proceed at your own risk and enjoy the positive benefits)

A special splined Adapter is made as shown which can use the standard Stuska® splined drive shaft. If you have a Go-Power or SuperFlow(R) dyno that uses a Clutch pilot spline, then make a coupling similar to the drawing that accepts your pilot shaft. This Coupling does have some Inertia but not nearly as much as the heavy flywheels being used. The coupling's mass is Mostly Rubber and is self Damping. The coupling's Inertia will help simulate the inertia of a normal clutch pack. The LORD Bushing is supplied with an SAE #10 Bolt pattern ( 8ea .406" holes @ 11.625" dia.). You can add the hole pattern for your flywheel (Chevy Shown). If you test Sprint or Midget engines that use No Flywheel we highly recommend that you use a standard Sprint or Midget Drive shaft using the coupling used in the car (no Flywheel).

Notes on Fabrication: The Female spline for a Stuska® can be made from the existing U-Joint Yoke as Shown. The Adapter Plate has a 1/32" step to center the plate onto the 2.500" Coupling. The welding method is Important. We highly recommend: 1. Press fit spline to plate. 2. Make a fixture so that someone can rotate this assembly while welding. 3. Use a TIG Welder with a matching Alloy filler rod. 4. Pre-Heat the assembly to a dull red using a broad flame acetylene torch. 5. Use a lower current with the TIG and make a penetrating bead while someone smoothly rotates the assembly. 6. Do the same to the Flip side. 7. Allow the assembly to cool slowly.

 (Legal Stuff) We assume no responsibility for if you do not follow cautions and instructions. (Shafts can 'explode' at a critical Whip Speed and result in injury and property damage). Only an idiot would mount the LORD coupling on a flex plate and/or use Articulating joints on the coupling. If you have to ask why then you shouldn't even be thinking of doing this..

This will result in a Strong, Stress-Free joint with little distortion. Make certain that Engine is In-Line with Drive Shaft. The Rubber is normally runs cool and only gets hot if you have a strong driveline oscillation (torsional resonance), usuually at idle RPMs and no load. IF the Rubber gets hot, just avoid running at the critical oscillation speed.

VERY IMPORTANT Note: Elastomer coupling flange is normally cut down and Needs to be mounted on a ridgid Drive/Starter plate on the dyno input shaft. (Do NOT use a flex plate.. Why?). You need to make a spacer (shown in Red) with enough thickness to Crush, or Pre-Load, the rubber on the open back-side to about 1/8". This is needed to keep the rubber in place without shifting (actually best to pilot the splined shaft). Use only a splined driveshaft and Certainly do not put a U-joint or CV joint on the rubber coupling. (Why?). Best to use a light weight shaft splined at both ends, like those used in Sprints and Midgets. Do not use a large, heavy shaft (Why?). You just can't ignore the Physics and why things fail.


DownLoad Adobe PDF file of above..

DANA/Spicer makes a line of Dynamometer driveshafts by bonding two Telescoping tubes together with different Durometer Urethanes. Due to this construction, they are much Stiffer than the LORD coupling. When given details on the Exact Application they can formulate a Driveshaft that will work. Unfortunately, if you change the Engine/Dyno combination, this stiff driveshaft may Not provide the required margin of protection. They say that they will not sell a driveshaft without knowing about the intended application. It is seldom that someone tests Only One type of engine, Always. One Dyno maker supplies these drive shafts with their dyno but these dynos are used to test a wide range of engine types. You Must make sure that this driveshaft is properly 'Tuned' for your particular application, even though you may need to test different engine types. The Best solution is to use a coupling which is much more Torsionally Resilient so that it is safe to use with a wide range of engine types without concern. Cost is another factor with a Spicer drive shaft, off the shelf, costing 15 times that of the LORD Coupling and you may need several to cover more than one type engine.
(Resume Above)

BELOW is a Suggested Application of the LORD Coupling to Existing Dyno Installations. The Stuska® Dyno is Shown with the typical short Driveshaft. A heavy drive-shaft may require centering support to prevent 'whipping'. (IMPORTANT: Do Not use a CV or U-joint at the Coupling. The reason should be Obvious, but we must caution anyway.)


Recommended Application: With or Without Engine Flywheel. This Setup is the Most versatile as you can test Engines that run a fuel pump off the back side of the Cam. Also important is that the Elastomer coupling provides kick-back protection when starting. Hookup is simpler by having the dyno start the engine.

Below is the LORD 150 coupling on a Froude F249 dyno. The engine is started through this coupling.LORD 150 Coupling used on a Froude F249 Dyno


TORQUE Strain Gauge MOUNT for Stuska® Dynos:

  Below is a Drawing for a STRAIN Gauge Mount for a Stuska® Dyno with 15" between the Torque Arm Clevis and the Stationary Base. The S-Type strain gauge shown is very accurate and linear BUT, like any precision sensor, it Must be Applied properly. This type strain gauge is sensitive to off-axis forces which will show up as errors. This strain gauge requires that all forces be along the center axis defined by the center of the mounting studs. There should be No Twisting, or side deflections and the Stud mounting surface should remain perpendicular to the force axis.

TORQUE Strain Gauge

  Problems arise when trying to use the strain gauge with an existing Stuska® Torque Pressure Sender. Most installations have caused the Strain Gauge to lose its accuracy. If one Must use both, then the best way is to Place the Strain Gauge as close as possible to the clevis. The Rod end stud will have to be cut shorter so there is space only for the Jam nut and a little adjustment. A 1" diameter Rod is machined Square in a lathe which then connects the strain gauge to the Stuska® Pressure sender Shaft (like shown below, but shorter). Since the pressure sender has some 'spring' it will provide some shock protection, but not as much as the urethane bushings shown.

It is recommended that the Strain Gauge be used as Shown. NOTE the Solid 'Square' bond between the Strain Gauge and the extension rod. The S type load cell Requires the force be perfectly in-line!

  Strain Gauges only 'Move' a few thousands of an inch under full range load. A common Mis-application is to mount the Strain Gauge solidly with no 'springing' to absorb Shock loads that can Stress the strain gauge, the Torque Arm, and the Trunnion bearings. We provide Urethane bushings (shown) and a Stainless stud with washers and jam nuts to make the conversion simple. The stud is cut back in the middle to allow more misalignment clearance to the mounting hole. The Urethane bushings are pre-loaded such that under Full Load the slack side is still snug. These bushings will 'give' more than 1/8" to absorb shock loads. In effect these bushings also allow for small mis-alignments, which is good.

A note on Long term effects. Most dynos use Ball or Roller bearings to support the dyno and provide the reference axis for torque measurement. These bearings move very little and, over long use, the contact points of the Balls and Rollers against the Bearing race begin to form little divots. What little bearing movement there is will now tend to bind under load although it will feel 'free' under no-load. Using 'Dead-Weight' Calibration, the torque measured by the strain gauge will 'Fall-off' as you increase the dead-weight load. Some of the Torque is now Shunted by the need for the bearings to climb up out of these divots. Sometimes Rotating these bearings to a new location will help otherwise these bearings need to be replaced periodically. This is an Improper application of a ball or roller bearing anyway. A better choice would be using a polished shaft and a solid space-age Dry plastic bushing. The torque reaction forces are now spread over a Large area and this bushing would be maintenance free for the life of the Dyno.

  Another common mistake in using strain gauges on dynamometers is to use a short distance between the arm and the base support, using Solid Rod Ends, for example. Consider that the:

TORQUE = FORCE x ARM x Sine (of the angle between the Arm and the strain gauge Force Axis).

This should be very close to 90 degrees so that the Sine(90) = 1 and so the effective arm length will equal the measured physical arm length. Under torque loads, there WILL be Deflections and Distortions of the Dyno case and Frame. That rigid Frame isn't as rigid as you think, under full load, and the Angle will Change slightly. If the Angle is close to 90 degrees, a movement of one degree (+/-) will not affect the Torque reading very much. If an angle other than 90 degrees is used, the arm length is now ARM x SINE(angle) and be Less than the physical arm length. What's worst, is that under load, when the angle changes due to distortions, the error caused by the changes in 'effective' arm length will be very noticeable! A longer Rod length (L), set at 90 degrees, will minimize the angle changes and errors caused by dyno frame deflections.



It always seemed to me that Stuska® Dynos were easier to 'Drive' than GO-POWER or DTS Look-Alikes. It didn't take much wondering to see why. The Stuska® has better water Through-Flow AND the Stuska® has the Outlet Water Ramps (intended for CW and CCW Rotation) Meeting Together into a COMMON Central Water OUT. This is important. (Ya done Good, Harv).

Below is shown a 2 Stage Modification for GO-POWER Type Dynos that GREATLY Improves the DRIVABILITY and Control ability. I don't know if these Dyno makers even know about this. It certainly beats the pitch to Buy a newer Model Dyno! I have recommended this procedure For years and those that listened were very surprised. Try It, You'll Like it! STEP 1 is Easy to do and will produce immediate Gratification. STEP 2 requires that the Dyno be disassembled some to properly make 2 ea 1/2" NPT Taps, but doing this will further Improve the Dyno by Doubling the Water Flow Through Capability.

STEP #1: The reason this works might have something to do with the effect of having a Dead Space at the Ramp of the PLUGGED-UP Outlet. The water in this area may develop high energy vortexes that could cause this water to flash into a vapor and follow the tips of the Rotor. Whatever,..? Taking the water Out from both 'Ports' DOES WORK and increases the Water Through-Put. Remove any Restrictor Plugs inside the 3/4" Pipe Outlets. A 'Y' is fabricated as shown to connect the 2 water outlets together into a Common Water Out. TIG Weld two steel hose fittings to a 3/4" pipe outlet to the new restrictor, as shown. It might be convenient to put a multiturn Flow Control Valve here (enlarged for illustration). For this DUAL ROTOR Dyno, there will be 2 such outlets, one for each Rotor assembly. These 2 valves allow an easy adjustment of the Outlet Flow Restriction, which needs to be changed when you test different BHP Engines. Close each valve and then open the Same number of turns and fractions. You will get used to what 'Turns' are needed for what Engine. Please note that If the Dyno housing is too Hot to touch after a Pull, that you NEED More Water Flow and need to open these valves more. Do NOT Add Any length of Hose other than what is shown.


Note: The original (BEFORE) setup also causes a false torque. Anyone who has ever Hung onto a high pressure fire hose 'Knows' That moving water creates Force. This outlet is Offset from the Center Line of the dyno. This Offset Arm times the water Flow Force creates a torque error. The New setup does NOT have this Torque error since the Water-Out is In-Line with the dyno Center line.

STEP 2: The Water-in Flow from the bottom 3/4" Pipe adaptor is restricted by the design (see internal flows). Adding 2 more (Direct) Water Inlets as shown will Double the Water Flow. We want the flow to be distributed evenly to both sides of the Dual Rotors. Tap two 1/2" NPT as close as practical to the center of the Dyno. The Flow is SPLIT according to the Ratios of Areas of the rubber Hoses. The added two Side lines each should carry about 25% of the Total Flow. The Size is either an AN-8 or -10. The Original Water-In carries about 1/2 the Total flow. Its size is that of a 3/4" Pipe adaptor and Hose. The short 3/4" Rubber Hose is needed to isolate the destructive Dyno vibrations so there will be no fatigue fractures in the plumbing. The Original water-in hose now feeds the (X) assembly which is made, not being a standard plumbing item. You could go Crazy with Aeroquip which would Look Great or just use Heater type rubber hose and Brass barbed fittings. The Dyno will now be Much more Responsive to Water Flow changes. Any Automatic Control systems will work much better after these Improvements.


Drawing ABOVE is just a suggested application. Plumbing here is not as critical as for Step #1. You can have Three hoses come from a more distant manifold. The two side hoses should have 1/2 the Inside Area as the Larger center hose, so that the Water flow splits 25% to each side and 50% up through the center.