Dyno question? Experts please respond.

[Brad04Mamba]

Last weekend we had a dyno day and the I have a question for the dyno experts out there.

One of our members (you know who you are) gained 13 hp just by disconnecting the battery for a few minutes and reconnecting. Can someone give me their opinions on this ie which reading is more accurate before or after, reasons for this increase etc.

Thanks in advance.

Brad

 

[Bill/APEX Motorsports]

I'd have to see the dyno sheet and air/fuel ratio. My guess is that after the battery was disconnected, the car was running leaner than before. Leaner (to a point) = more power.

 

[Bob Woodhouse]

Two probabilities: When the battery is disconnected the engine computer will lose it's short term memory. It takes some run time to re-learn this and it is done as the car is driven over the next 50 miles or so. In the mean time the engine may not idle perfectly or **** around a little. It will also change the air-fuel map slightly. This could result in a change in the engine output. But not too likely as full throttle causes the computer to go to a already mapped closed loop circuit for fuel. So try this then:

Dyno runs within 10 hp is about as repeatable as it gets. At the Viper Days Nationals this last week end we found variances on the same race car as wide as 30 hp. No changes. On my car we found a 10 hp variance that we credited to the trans and rear end heat soak that came from sit time between pulls. Fluids lose friction when they get warm.

 

[Jack B]

See if you can pull out the coolant temp data (if collected) from the different pulls. My hp tends to vary with the coolant temp

 

[TOOOFST]

All the above true plus engine temp(major factor)and 5% dyno margine.Better test is the dragstrip(MPH)Shows the cars true power.

 

[Russ M]

You gentlemen are absolutly wrong on the dyno jet repeat pull comparisons. As long as the person running the dyno knows how to correct properly for weather changes the numbers are going to be almost identical. Maybe a varience of several hp but far from 10, as witnessed at the socal dyno day.

The only time any car pulled considerably different was when engine temp got too high, in which case the power dropped by over 5 hp.

Most likely the problem was that the dyno jet facility was not run by the properly skilled people, and you should probably find a better one.

 

[Jack B]

I have to agree - My pulls have been by the the builders of Dynojet. I have had a consistency between successful pulls, but, I have also had a consistency on a different dyno. They manufacture the dyno's and I have been on four separate machines and all have been within a few hp/ft-lbs. All these have been with the same operator. He is also the instructor for those that buy a dyno.

If the operator is knowledgeable the pulls will show consistency. If not? - with the Viper it is easy to alter the max torque if the operator is not consistent. The torque can be manipulated by starting the pull at diferent rpm's. True performance is measured by the an overall raise in the curve between 3500 and 5500, therefore, small anomolies are meaningless.

Because of the Vipers torque it is especially ******* the half shafts to start the pull at anything less than 3000 rpm's. This is exacerbated by any mod's such as nitrous or supercharging. Try starting a pull at 3000, versus, 3500 and you will see substantial difference in peak torque, however, the overall curves will fairly close. With the Viper a meaningful pull is 3000-5000, anything else a waste.

Another issue is the degree of smoothing utilized on the graph, and/or which correction factor is used. If little or no smoothing is selected the anomolies (spikes in hp or torque) will be measured and the measure quantity will be higher. It is unlikely that different selections would be used on succesive pulls, however, it could happen. The operator error can be with the actual pull, or with the familiarity of the software. Remember that coolant temp can also affect hp to some degree.

I don't understand why most of the curves posted don't show the a/f ratio below the power curves. This is as important as the hp/torque and we could all learn alot if the a/f curves were also posted. If your operator is not printing these, tell him to do so.

Can anyone beat 750 rw ft-lbs for a Gen I.