wheel HP or flywheel

[ratsnotmice]

Please comment on this excerpt from another forum regarding the use of a chassis dyno to measure power at the wheel vs flywheel. I am of the opinion that trying to gauge flywheel power on a chassis dyno will, in most cases, result in erroneous data.



... But for the life of me, I REALLY do not understand why most people Stateside still set their chassis dyno´s up for only WHP. I mean, like I wrote earlier, it shows what you get to the ground, but the hp loss throught the system can be ANYTHING from approx 15 to 30%. There are WAY too many variables to be able to take the numbers serious. But the dial in part is naturally about the same.

Is there an easy way to determine drivetrain losses for each vehicle? Other that, what really matters is the actual HP getting to the wheels.

No, that´s kinda my point.
Yes it matters what you get to the ground. But MAYBE it was interesting
1, to know what the engine really pulls, and where, because when you measure only WHP you do not get the factual point where the engine peaks. You get the point where the drivetrain losses sort of takes off. So the engine peak hp can easily be anywhere from 300 to 600 rpm further up. And
2. If you have excessive drivetrain losses due to the transmission, too heavy oil, bad alignment, wrong tyre pressure big tyres etc. you might want to know where you can do some changes to make it better.


Check out the PDF at the link below, talks about WHP vs power at the flywheel among other topics. I thought it was pretty good
https://www.rbracing-rsr.com/downloads/dynojet.pdf



Hmm. Apart from the explanation on varius testing methods most of the rest is just sales BS and padding themselves on the shoulder. His statement in general with using wheel power is EXACTLY why you have problems getting numbers that are within +/-1,5% one dyno to the next. It all relates to what I wrote earlier.
And another thing which I forgot to methoon last. When youre on a chassis dyno that measures WHP only, you can start in the morning and get "X" hp. Then you do some tweax, then you get "Y" hp then you do some more tweaks and the day gets warmer, then you get "Z" hp. WOW, now we are getting somewhere. That final adjustment really worked well...... Or did it(?) Problem is, now your transmission oil is getting warm, meaning less drag, so - some - of the improvements you think you have found is actually just less drag in the transmission.

WRT the accuracy one chassis dyno to the next. - Over here the most used chassis dyno software is Optima 2000 and Dynomet. A few use MAG. Most of them are calibrated via JTI (A technical institute that works with this stuff) We have tried several times to try varius dynos on the same day with the same car, same tank of fuel. The Optima´s with the newest 7,39 software vary 4 hp Flywheel hp on a 147 hp Honda civic 1,8 and 5 hp on a 220 hp Passat TSI. 2 years ago a customer of mine got his 2165 type 1 dialed in on the chassis dyno I use (Dynomet) to 169 hp and 215 Nm torque. About a month later he attended a dyno day at another site, but also with a Dynomet unit. That day it pulled 170 hp and 218 Nm torque. Another customer got a 1600 std plus engine. Had it tested at JP Motorsport to 74 hp. Came by me later in the summer and we tried it on "our" dyno. It pulled - 74 hp. Sooo...
- In general If you take the same car on an Optima 2000 and then on a Dynomet, the Optima typically measures a hairswidth less than the Dynomet on turbocars. N/A cars are very close to the same. On serius hp cars 300 hp & up it has more to todo with what type and size of rolls the floor unit has.
Another way to do a "self control" of the system so you know that youre measurement is correct is to take 2 maybe even 3 measurements in a row. If the 2 or 3 lines is on top of each other or within 1 to 2 hp you have good repeatability and there is a very high chance that the measurement is correct. Unless i spot somehting that is not right I always make 2 tests on top of each other for exactly that, verification. That is actually something JPM taught me about 10 years ago.
Once again and then I will cease. IF the dynos are calibrated the same they MUST show the same flywheel hp within +/- 1,5%. If they don´t something is wrong. And again, it is correct that if you measure wheel hp alone you cannot get the same reading on two different dyno´s UNLESS you KNOW the engine temp, the oil temp in the transmission, that your tyre pressure is absolutely the same, that you have the same amount of down force on the rolls via both the car and the straps that holds the car in place and that your tyre temperature is the same. a lot of things to control.
And last. If you have a really powerfull engine, like say 5-600 hp & up there is really no way to do an accurate test on a chassis dyno. They will always vary a great deal. To much risk of tyre spin, buildup of kinetic energy in the rolls and a couple of other issues. In those cases the Axle hub dyno´s is the way if it is an in the car test. With Axle hub dyno´s there is no foolin´around. What you see is what you get.
The one roll chassis dyno´s definitely have their advantages. For instance the "on site" dragrace tests through the gears. That´s one thing the Europeian chassis dyno´s are not so good at.
On a side note. I did´nt know Disney was getting into that market

[Truckedup]

For me, motorcycles, the chassis wheel power is the only real option.It's done on a Superflow..Three things come out of my dyno sessions, first is a clue to HP and torque and at what RPM.. Second is a bit of durability testing on modified driveline components . Third and possibility most important is engine tuning ...

[gmc406]

Here’s my story. Years ago when I had my EFI 406 built by Wester’s Garage, I had to get a custom chip made by the builder himself just to get it running “good enough” just to be able to drive it to his shop to get the final tune via the chassis dyno. No way it would even run on the stock TBI calibration. With 12.2:1 compression, cam and heads, it would’ve been a ticking time bomb.

Putting his “ guesstimate” chip in I was surprised how well it ran. I drove the truck to his garage to get dyno tuned. After 3 chips to get things perfect, I was rewarded with a whopping 9 rwhp increase, LOL. The final power came out at 284hp and 303ft/lbs. The next stop was the track the very next day.

Sitting in the staging lanes I began to have a conversation with a nice fellow. He had a nice late 80’s Mustang with a built 302. He told me his car made 360 rwhp on the chassis dyno( a different speed shop). After running down the track, he ran 1 tenth faster than me. An 80 hp difference to go 1 tenth in a car. I’m sure I had 700lbs on him and the aerodynamics of a brick to boot.

Moral of the story......chassis dynos are a tuning tool. They show what the combination likes and dislikes. The 20% rule of thumb still doesn’t jive with my numbers