Poor results from chassis dyno

[bigjoe1]

Unlike the results with the chassis dyno, every time I was able to find more power on my engine dyno, the car would run quicker and faster at the drag strip. Only on one occasion did more horsepower on the dyno not make the car run better. I went to the drag strip with this guy, and the new found power made his car spin the tires on the starting like, where as before, it would hook up better ( better 60 foot times proved it ) After several outings to the track, he finally began to get faster time slips, On my sons race car, more HP always made his MPH get better, even though the E T did not always improve.


JOE SHERMAN RACING

[Orr89rocz]

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.

On high stalls, i have seen some false spikes on the hit but after stall rpm is reached, torque seems in line

[MadBill]

Unless a car has a lock-up converter, any slippage means it is acting like a crude absorbtion unit, just like the one attached to the crank on a water brake dyno...

[cv67]

Have any of you seen or heard things like this too ?

A number of times

[Rizzle]

It seems mentioning the type of chassis dyno is important in discussions like these. I'd think absorption type dyno's would reflect the track and engine dynos more consistently than the inertia dynos, especially for turbo'd vehicles.

[cr480r]

During my one and only chassis dyno experience we added 4 degrees timing and found 25rwhp. The next day I tagged along to a test and tune day at the track. There was a small turnout so I decided to run my car too.. Ended up taking those 4 degrees back out and picking up 3mph..

[MadBill]

It seems mentioning the type of chassis dyno is important in discussions like these. I'd think absorption type dyno's would reflect the track and engine dynos more consistently than the inertia dynos, especially for turbo'd vehicles.

The biggest variable with roller type chassis dynos in general is the considerable variation in rolling resistance depending on tire choice, single/double roller and roller diameter. Hub dynos, e.g. Dynapack have close to zero inherent losses. The difference from crank HP (~10% in my experience) is all up to driveline friction plus installation losses, e.g. exhaust system, water pump, etc.

[Ken_Parkman]

Be careful of when stall is reached. The drivetrain may have not caught up to the engine, in which case the converter may still be working and providing some level of multiplication. I understand the dynojet calculates a ratio between wheel and engine which becomes fixed for the pull, and that does not work for a race converter. I've seen some torque curves that flat do not make sense at all. You can't be reading a proper torque or power below stall.

But when you are well above stall the converter should be "relatively" locked up, so then the torque numbers should be OK.

[les327]

During my one and only chassis dyno experience we added 4 degrees timing and found 25rwhp. The next day I tagged along to a test and tune day at the track. There was a small turnout so I decided to run my car too.. Ended up taking those 4 degrees back out and picking up 3mph..

What type of dyno was used?

[turboivo]

You guys should check this dyno manufacturer:

http://mainlinedyno.com.au/index.php/pr ... axle-shaft

..nothing in common with the outdated chassis dynos you're talking about.

[RL]

I think you are looking at it too one dimensionally. It might be the dyno, or not.

HP is not the only thing that matters, torque also plays a part.

We had a 427 with 1¾" headers that ran 10.3. We switched to 2" headers and picked up 60hp on the same chassis dyno, but at the track it still ran 10.3. It did run 5mph higher, but you could feel the lack of torque at launch because now you couldn't spin the sticky slicks any more. Changing tyre pressures and gearing might have made it go faster, but that's something you only see at the track.

Also, if you change a part and peak HP point moves to a different rpm point, you now have to shift at a different rpm to utilise it.

[Orr89rocz]

I think you are looking at it too one dimensionally. It might be the dyno, or not.

HP is not the only thing that matters, torque also plays a part.

We had a 427 with 1¾" headers that ran 10.3. We switched to 2" headers and picked up 60hp on the same chassis dyno, but at the track it still ran 10.3. It did run 5mph higher, but you could feel the lack of torque at launch because now you couldn't spin the sticky slicks any more. Changing tyre pressures and gearing might have made it go faster, but that's something you only see at the track.

Also, if you change a part and peak HP point moves to a different rpm point, you now have to shift at a different rpm to utilise it.

I think that just proves the combo was there, but the car wasnt. Restall and regear to take advantage of the new found power. 5mph is huge

[289nate]

I think you are looking at it too one dimensionally. It might be the dyno, or not.

HP is not the only thing that matters, torque also plays a part.

We had a 427 with 1¾" headers that ran 10.3. We switched to 2" headers and picked up 60hp on the same chassis dyno, but at the track it still ran 10.3. It did run 5mph higher, but you could feel the lack of torque at launch because now you couldn't spin the sticky slicks any more. Changing tyre pressures and gearing might have made it go faster, but that's something you only see at the track.

Also, if you change a part and peak HP point moves to a different rpm point, you now have to shift at a different rpm to utilise it.

I think that just proves the combo was there, but the car wasnt. Restall and regear to take advantage of the new found power. 5mph is huge

I agree completely. The headers gave the car the ability to run significantly quicker ET's. The car was simply not set up for the new rpm range. What did the numbers look like to the 1/8 mile? What did they look like from the 1/8 to the 1/4 mile? Bet it was a turd in the front half and accelerating like a bat out of hell in the back half.

I personally have only done two engine dyno sessions. I chose the engine dyno for several reasons. First being you don't want to find a problem or have your engine come apart when the engine is already in the car. The other main reason was to eliminate variables. You come out with a very good understanding of what you're dropping in your car.

I believe the engine dyno is to find out how much power you're making and to get in the ballpark with the tune. The drag strip is the chassis dyno, tells you how well your engine and vehicle are working together, and where the fine tuning is done.

Your average chassis dyno stays in business because of all the dyno sheet racers. They would care less and less about their RWHP and more about the breakdown on their time slip if they went to the track more.

[Chris1]

A steady state step test will negate inertial variability. A properly calibrated CycleDyn with a capable operator is very repeatable. I'd be reluctant to blame the tool for the track result inconsistency...

[289nate]

My opinion only. If I'm selling parts, I want a verification on an engine dyno. If want to make money I want a chassis dyno because it's easier and cheaper which will get me more business over the engine dyno operation and actually have a chance of making me money. If I want my car to go fast, I want track time after getting in the ball park on the engine dyno. This is not including the high end specialized racing dyno set ups.