I did a little research on this because it is such a dynamic subject (sorry). I had problems with dynoing on my '99 Cobra before it was traded for my 2k L, so I've seen some of this before. Also, there is a lot more info out there now on what is going on (Corral has Ford's '99 Cobra dynoing procedure; some of which is recapped below).

As to the Dynojet units, there isn't much that can go wrong. They need to be checked from time to time to be certain that air temp and pressure & humidity that the sw perceives are real, and that the sw can properly compute time. (All the dyno sw is doing is measuring the amount of time it takes to accelerate a dead weight drum from speed "A" to speed "B".) The RPM input from your local spark plug isn't critical to this, but it does help identify where the engine is making power.

So... if the above is ok, then here's a list of the variables that can affect measured hp:

1. All the drivetrain rotating machinery and bearings must be at full operating temp. This includes tranny, diff, wheel bearings, u-joints, etc. Correct lubricants must be in use.

2. Unit must be broken in; for Cobras this is >3k miles on odo.

3. Engine oil level must be between the marks. Engine oil must be manufacturer's recommended viscosity for the temp environment where the vehicle normally operates.

4. Fuel octane must match manufacturer's requirements for max perf.

5. Tire pressure must be set correctly (35 psi for Cobras).

6. Strap tie downs must be done correctly; i.e. no "excessive" downforce applied... and ties s/b made to control arms. This is tricky, it can significantly affect measured hp.

7. Vehicle driving wheels must be centered on dyno roller; even a slight angle will reduce hp.

8. Testing is recommended with hood open and externally supplied cooling fans active. It is important that the vehicle charge air temp be the same as that viewed by the Dynojet hw as "ambient", else the correction factors will not work. It may be necessary to fuss with the cooling fans and cool down between runs to get the heat soak effect on air charge temp to cooperate.

9. Traction control and all electrical accessories s/b off.

10. Exhaust s/b open to atmosphere, or have slight vacuum (as in some shops which connect an evacuation hose system to exhausts). Too great a vacuum on the exhaust will cause incorrect hp readings.

11. Repeated runs s/b made with engine temps as close to the same as practicable. No runs s/b made on cold engines. For Cobras the ideal engine temp is 185 degrees in the coolant. If the engine cooling fan comes on during a run the run s/b discarded.

12. Wheel bearing friction can vary depending on how the wheels were installed (for those vehicles that use a torque technique to set the bearing load).

13. Hp s/b measured with tranny gear set as close as possible to 1:1 input-to-output ratio.

This is a lot of ways that dyno runs can differ between like units and even for the same unit twice in a row.

Now, ain't science fun?

-tvw


[This message has been edited by tvw (edited 05-22-2000).]

 

Very insightful, TVW. Thanks!

I guess what a lot of these variables boil down to is dyno operator error. I can honestly say that just about everything in your list has been done correctly every time I've had one of my vehicles or viewed other vehicles at the shops I've visited.

Obviously, things like tire pressure, engine oil level, oil temp (I agree not to make pulls on a stone cold engine), engine coolant temps, etc are the responsibility of the vehicle owner.

Good info here. Thanks again.

------------------
Wes Tarbox
90 LX 5.0 (10.69 @ 134.7)--597rwhp/590rwtq
96 Cobra (12.63 @ 114.5)--390rwhp/441rwtq
99 Lightning (13.20 @ 103.3)--362rwhp/444rwtq
00 Expedition XLT 5.4
http://members.aol.com/Wa2fst/index.html