Poor results from chassis dyno

[bigjoe1]

I have a Super Flow engine dyno that I have lots of confidence in its results.. Several times now, an engine customer will put his car on a chassis dyno, to see if he might find more power ( this is with a drag race car setup ) After finding a big increase on the chassis dyno, when he goes the the drag strip, it shows NO GAIN AT ALL. I am beginning to think the chassis dyno operator is giving them false readings, to make their shop look better, Have any of you seen or heard things like this too ? I( will have more to say after I see a few responses.


JOE SHERMAN RACING

[needforspeed66gt]

Any dyno can be influenced with correction factors, the easiest way with our Dynojet would be to blow hot air over the temp sensor in order to make it think you're running the car on a much hotter day than you really are. We've never done that, never would consider it - if we make a few pulls and the fuel and spark are correct, maybe a couple small adjustments that see no gain, we don't sell them on a tune - just charge for a baseline dyno and move onto the next one.

[banjo]

The couple of times I have tried to find power on a chassis dyno, I have slowed down at the track. Chassis dyno doesn't load the engine the same way it does when you are going down the track.

[DaveMcLain]

I don't have much experience with having an engine that I dyno tested later being tested on a chassis dyno except for one of my friends. His 557 Ford made 870 horsepower on my dyno and when he ran it in his drag racing pickup on a chassis dyno they told him it made 870 horsepower. What I don't know is weather or not they were giving a wheel horsepower or if they were giving numbers after some sort of driveline "correction". Either way this was running through a Powerglide with about a 5500 converter and a 9 inch with a 4.86 gear and about a 32 inch slick so it seems like the losses would have been considerable.

[RevTheory]

Could there be a fresh air intake issue where incoming air from running down the track is different than stagnant air from the chassis dyno cell?

[427dart]

Back in 2001 while testing cold air/ram air setups on my 2001 Bullitt Mustang the chassis dyno showed a 5 HP gain with an electric leaf blower blowing air into the scoop inlet.

[F-BIRD'88]

Back in 2001 while testing cold air/ram air setups on my 2001 Bullitt Mustang the chassis dyno showed a 5 HP gain with an electric leaf blower blowing air into the scoop inlet.

HA hA HA HA Thats priceless.

[DaveMcLain]

It seems like running a chassis dyno would really be tough to do well. There are so many extra places where variability could creep into the data vs an engine dyno.

[F-BIRD'88]

I like the Chassis dyno design that the dyno absorbers bolt right to the axles of the car with the wheels removed.

Eliminates the Tires and Roller variables from the equation. I believe these are Eddy dyno type absorbers.
They even have 4 wheel versions.

A racing high stall torque converter is a BIG FACTOR. By rights the converter should be replaced with a 1:1 coupler link (dummy coupler)
to dyno the car.

Once my bud chassis dynoed his 11sec sbc monte carlo bracket racer on the DYno JET and run it on their 1/4 mile run simulator.
It said that the car that always ran 11sec ET would run a 14 sec ET.

[Dave Koehler]

Chassis dyno guy near me is also a racer. He said he does get guys looking for magic bullets and they are usually disappointed. Kind of like all the 9 sec 4000 lb street machines that seem to exist....somewhere.
However the veteran racers will use it primarily to eliminate the first day blues.
Making sure the ignition and associated gadgets work as expected under load, etc. Sometimes they tinker with the timing and fuel little bit but not much.
Most of them don't particularly care what the numbers say and tend to take them with a grain of salt.

[77cruiser]

Wouldn't you think if you shows up at the tires it should show at the track?
Dynojet just times the rate you can spin up a 5200 lbs drum, correct?

[Dave Koehler]

Redneck inertia chassis dyno?
I was looking at my dragster one day and had a hmmmm.
Buy one of those whizbang driveline torque sensors.
Mount Large steel flywheels in place of the rear tires. Size and weight to be determined.
Jack it up, tie it down and hit the button.........from the next county.

The hmmmm passed after I considered that there is some question as to the centering accuracy of the axle hubs and studs.
Whomp, wobble, flop, boom. Honey, we need a new roof...and walls.

[Orr89rocz]

Chassis dynos dont always load the car up like a pull down the track
And if the engine is setup with a scoop or ram air, it wont show power in the dyno room but will work at the track

I've gained tenths with ram air setups that hurt on dynos.

Forced induction turbo cars are notorious for requiring alot more fuel to run right AFTER the dyno tune
Seen my car require 5-8% extra fuel vs the chassis dynojets to maintain same afr.

Dyno is a tool. Sometimes it makes more power on dyno and works at the track. Sometimes it dont

[Ken_Parkman]

I've spent numerous sessions on the chassis dyno with my own car and a few others. Whenever I've made small gains on the chassis dyno it has been reflected at the track with small gains in ET. I made a large change in the engine, and the chassis dyno has matched well with the engine dyno, and again this has been seen with a large improvement at the track. My car is a stick, so that's one variable less on the chassis dyno.

Race converters are a real issue on the chassis dyno, and (at least the dynos I've been on) the dyno software has no clue how to deal with the variable wheel/rpm ratio. Don't even look at the torque number - it's wrong and irrelevant. When the software is back calculating torque it does not seem to understand a converter, freaks out, and gives bogus numbers based on ratios that don't compute.

But the power number has still been useful to figure out shift points, and once again small improvements found on the dyno have been reflected at the track. Watch the rpm against time and it help understanding what the converter is doing at the track.

[Ken_Parkman]

When you think about it the quasi steady state engine rpm while the drums are accelerating completely messes with assigning a formula to back calculate the torque. Take some fairly educated software to have a variable gear ratio.