Dyno numbers?

[dragracer300]

On back to back dyno pulls how much do the numbers change? What causes variation on a superflow 901?
Thanks

[dieselgeek]

On back to back dyno pulls how much do the numbers change? What causes variation on a superflow 901?
Thanks

LOTS of factors can cause "un-repeatability" on back to back dyno pulls. I've discovered that an "un-sorted" intake manifold can wreak all kinds of havoc on repeatability. But things like changing coolant temps, ignition system weakness, mechanical issues - tons of stuff can cause this.

Got any more specific details?

[MELWAY]

Also the dyno room design and fan arrangement,,,and also if the seals inside the absorber arnt in good condition can cause this.

[xenginebuilder]

On back to back dyno pulls how much do the numbers change? What causes variation on a superflow 901? Thanks

IIRC Superflow quotes 1% repeatability, all things being equal. I have observed variations less than that, but you have to have everything right in the room and in the engine. There are a host of things that can make your "numbers" vary even when the engine is running rock solid, which is why they started doing average sums to help average out the spikes. The data would be a LOT better if they would take more samples and display the average sample at each data point, (instead of single point data) something that I believe the Depac system has been doing for years, and sadly SF never implemented. But, with some diligence and care in your testing regime, you can get good repeatability.
1% of 700 HP is 7 HP, and you frequently are looking for gains and losses less than that, a well running engine on a sorted 901 can repeat within 1-3 HP. Having said that, a good engine is more repeatable than the dyno, if you analyze drag race times, for instance on a S/C or S/G type of race car, the engine has to reproduce power more like within .01% or your times would be so inconsistent that you might as well go home.

[dragracer300]

The other day we had a 496 alky engine on the dyno. First pull it made 800hp next pull it made 788? next pull was 799. Didn't change a thing between pulls coolant temp was within 5 degrees oil temp was about the same. we pull air from outside not in the room exhaust is sealed pretty good not tight but better then most i have seen. I had the brake apart and checked the seals and everything looks fine? Most pulls are within 5hp with no changes but every now and then it will throw a crazy number at you? I just wanted to know if this is something everyone deals with or if i'm developing a problem?
Thanks

[xenginebuilder]

Most pulls are within 5hp with no changes but every now and then it will throw a crazy number at you? Thanks

The data is collected at a pre-determined rpm data point, and is a single snapshot of the sensors, so anything can cause a funny reading or bounce. The absorber has a lot of water going through it and it is pretty turbulent, and since the load is being read directly through the torque link to the absorber housing, even a minor or momentary cavitation or other water turbulence can both add or subtract from the torque load reading for a brief instant. If that happens when the data is collected, you get a bounce. I feel that if your average between back to back pulls is within 5 hp on a 800+ hp injected alky engine, you're doing fine.

[dragracer300]

Thanks I was hoping that was the case. We are still learning this software. Now if i can just find something to put in the water to reduce corosion and not foam? I just keep draining and refilling every couple days of use now and its getting old. Has anyone tried the water soluble oil? I tried the synthetic tool coolant and it foamed real bad.
Thanks

[promachine]

You might look at your water pressure supply to the brake and your water feed line size. Also the water return to tank line size. Foaming water is not normal, sounds like this could be a problem

[dragracer300]

Water only foams after it comes out of the brake. The inlet side has no foam. Every water brake dyno i have ever seen in person has some foaming in the return side. We have 5hp pumps and have what superflow recommends for hose size. Inlet screen is clean also.

[promachine]

What is the water pressure going in?

[dragracer300]

I will check water pressure on the next pull the guage is on the back

[fastvette]

I have never seen any foaming in my water, with or with out the water soluble oil that I now use. That has stopped the water from getting nasty looking and smelling bad.

Randy

[dragracer300]

I checked water pressure going in. It was at 30psi i cranked the valve all the way open and the highest i could get was 55psi? Is this the problem to high or low of pressure?
Thanks