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Is your Dyno lying? This is a MUST read!!!!!!

1899 Views 26 Replies 11 Participants Last post by  nohsreg2000
I read this in my new Car and Driver mag and had to post the story here. Would a Viper respond to airflow like this car did? Should we re-think how we dyno our cars and how much power they really may have? What about that VIPAIR that elite sells , could it really help at speed? Read this and post your thoughts. (READ BOTH PAGES)
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Great read....But it does not really explain some of the other phenomena that are occurring. Anything that helps create smooth airflow or reduces the load on the engine will create power. The amount will vary from design to design.

For example; Say you are Dynoing a small block Chevy with a motor driven fan. The difference between a dyno graph with no airflow and putting a fan to blow a 100mph wind will be rather great(I don't know how much I am just pointing out a load situation.) The increased air stream on the fan will reduce engine load quite a bit.
viper spray said:
It just seems crazy that he picked up over 40 RWHP with a big fan blowing 38,000 cubic feet per minute of air at 75 mph down a narrow duct, right into the M5's radiator and intake on the dyno on top of the 37 RWHP picked up with a Home Depot fan.
In that particular case is makes sense. The computer is set up to manage the car based in real world parameters. An indicated 160MPH with 0 MPH airflow is not very real world. I imagine the computer compensated. Perhaps riching the hell out of the A/F ratio to adjust for what it perceived as insufficient airflow for cooling. Also just the lack of a positive pressure situation (or at least increased pressure) around the air intake can create additional load on the engine and it seems the complex BMW engine management system might have compensated for this as well.

These are shots in the dark but based on the article and what I know about airflow(Thats what I do) it adds up.

The last possibility is Mike Dinan could be pulling a fast one to make up for not making a number.
Their is another way of looking at this for understanding. The car is supposed to make the high Hp number. It did not "Find" that extra HP with the fan. It compensated various systems that reduced HP without the fan.
I am not a dyno expert, so I dont know how the tie down system works, but one other hypothesis to consider as a contributing factor might be aerodynamics.

By directing a very high speed wind over just certain parts of the car you will again create a non real world situation. Some of the byproducts could include unnatural chassis loading,(I am leaving out a text book worth of stuff here so those keeping score please realize I am going for the lowest common denominator on this one.) I.E. If you put this high speed wind on the front of the car The power it could exert over the surface area of just the front of the car could "Lighten" up the back end. This would reduce rolling resistance(among other things) perhaps it is of enough force that a change in the dyno reading is noticeable. wind coming of a fan is also turbulent for the most part. this non laminar flow could create a mild chassis cavitation. Their is nothing quite like a cavitation to give false readings on an instrument.

Just remember whenever you change the parameters of any set of known quantities their are a myriad of contributing factors that can explain away what seem like anomalies.
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Lucifer said:
viper spray said:
Will you try to replicate this with a Viper on your Dyno with the big fan?
It'll be installed by late August, after that I'm going to invite all Texas Vipers over for some free dyno time and playing on decommissioned Air Force runways. /images/graemlins/drive
That sounds great. I will bring my ghetto Aussie, It would do him some good to hob-nob with royalty.
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I have never seen a dyno with a strap like that, of coarse that is not saying much, I have only been to a few dynos. I also dont know if such a strap would be without a little stretch.
Lucifer said:
Well it's designed for Mustang dynamometers and for "high horsepower applications". It'll handle 1475rwhp so it must work somewhat.
Hell, if we can find out what it is made of, the length and thickness, I am sure we can google a stretch factor. Then figure out the load the fan causes, and a few other things we can calculate if their is any validity to this.
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