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bazhart
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PostPosted: Thu Oct 24, 2019 8:09 am    Post subject: reliability of wheel V engine power tests (Pwheel V Peng) Reply with quote

If there are opinions on the relative accuracy or reliability of the results of rolling road tests for "wheel power" (Pwheel) compared to "engine power" (Peng or P norm) I would appreciate their comments.

Many Thanks,

Baz
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996ttalot
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PostPosted: Thu Oct 24, 2019 9:42 am    Post subject: Re: reliability of wheel V engine power tests (Pwheel V Peng Reply with quote

bazhart wrote:
If there are opinions on the relative accuracy or reliability of the results of rolling road tests for "wheel power" (Pwheel) compared to "engine power" (Peng or P norm) I would appreciate their comments.

Many Thanks,

Baz


Baz,

I have never been an advocate of quoting hp. It really does open up a large debate between different shops on whether someone is quoting too high or in some instances too low.

What we have done is use a vbox dyno (performance box). So we are not talking about a rolling road but real world test, which can be an issue because you do need someone to do that testing.

However, what we do is take a stock car and measure the whp from the vbox data. This is generally accurate within 5ps. Furthermore even if it was say out by 2-3% it really does not matter because you have a true whp base figure before any changes. We find for example that from the Porsche quoted figures (flywheel) that modern PDK is around 10-11% to the wheels, manual is slightly more, tiptronic even more. We have done this on so many stock cars of different variants and it is always consistent.

So that is a base whp.

You make the changes that you want and you retest in the same conditions.

The biggest advantage we see is real world air flow and conditions. Turbo and NA cars just get too hot on dyno. The second advantage is that you clearly see a time difference between the performance measure - so you not only have a difference in whp but also in terms of a performance time.

We have seen in the past cars that have been modified that quote ridiculous hp numbers on a short dyno pull, but do not perform when the car is pushed on. So in simple terms too much timing and changes in boost etc, which gives an incorrect over inflated dyno reading, but when you get to a certain speed all that power is taken back.

For na cars the maximum increase in whp that we see generally depending on modifications is around 25whp tops. But you do get the performance time to prove the figures as well.

The bottom line is that it is true world data with actual performance measures.

One other point - we have seen modifications that have been tested on an engine dyno that did not show increase in hp. However, when you do a performance test it shows an improvement. Some engine dyno do not have airflow as such, so therefore intake changes do not always show.

Ken
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Mezgerite
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Joined: 02 Nov 2016
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PostPosted: Thu Oct 24, 2019 9:47 am    Post subject: Reply with quote

Hi Baz,

I know a fair amount on this subject.

It's very complicated. The first thing you often hear is it is impossible to measure engine power on a chassis dyno. While this is true, we can get a pretty good repeatable approximation. What most fail to realise, is that the wheel power will vary dramatically depending on the dyno type - i.e. the roller design (diameter, surface finish) and the vehicle speed. The latter of these means that the wheel power will vary depending on gear ratio.

It is impossible to compare wheel power between different dyno types. A larger single roller will show considerably more power to the wheels than a smaller twin roller. The calculated flywheel power, if the algorithm is good, should show similar results regardless of dyno type and gear.

Power at the wheels is also a misnomer, what you are directly measuring is power at the power absorption unit - e.g. the eddy brake. Not all the power at the tire/roller interface makes it to the absorption unit and load cell.

Total system loss, is made up of two components, one relating to power output, the other wheel speed. Power output is generally linear - you loose a fixed percentage regardless of power. On my twin roller dyno this is around 17% for a 2wd car and 19% for a 4wd.

The second component is the wheel speed. This can be approximated with a polynomial function. The percentage loss increases with wheel speed in an exponential fashion. It may be 5% at 50mph and 15% at 150mph. Dyno software can either use a coast down method or an algorithm to determine this part of the loss. Some dyno's do this much better than others.

M
 
  
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HSC911
Long Beach


Joined: 23 Jul 2014
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PostPosted: Thu Oct 24, 2019 11:42 am    Post subject: Reply with quote

worship

3 legends in one thread...well 4 I guess Floor

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jonttt
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Joined: 20 Aug 2012
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PostPosted: Thu Oct 24, 2019 12:01 pm    Post subject: Reply with quote

Hand

Its no fun when people know what they are talking about, I'll stick to the footy thread Grin

In all seriousbness its why I never ever take any note of dyno results as I've driven cars that too often in the real world simply do not back up the figures quoted.

nb the "other factor" I have seen used to manipulate results in terms of pre set adjustments is ambient temperature readings ie dynos are calibrated to a pre defined temperature (usually around 15 degrees) and so compensate results for the actual ambient temerature. I have seen ambient temps recorded as too high on purpose so the power is adjusted up on the readout Wink
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MaxA
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PostPosted: Thu Oct 24, 2019 12:33 pm    Post subject: Reply with quote

jonttt wrote:
Hand

Its no fun when people know what they are talking about, I'll stick to the footy thread Grin

In all seriousbness its why I never ever take any note of dyno results as I've driven cars that too often in the real world simply do not back up the figures quoted.

nb the "other factor" I have seen used to manipulate results in terms of pre set adjustments is ambient temperature readings ie dynos are calibrated to a pre defined temperature (usually around 15 degrees) and so compensate results for the actual ambient temerature. I have seen ambient temps recorded as too high on purpose so the power is adjusted up on the readout Wink


We've used a so-called heartbreaker dyno to set baselines. I imagine that it is simply that the truth hurts. I have had my cars road tested 'before' and 'after' as I think the road is more likely to provide some potentially useful information.
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jonttt
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PostPosted: Thu Oct 24, 2019 2:48 pm    Post subject: Reply with quote

Yes, you have to assume for many dyno operators there is a tendency to provide exaggerated BHP results as no one wants a rep of having one that under gives “low” figures Rolling Eyes .....

However its the “improvement” they can show that people want to see just as much thats why the ambient temp “trick” is so tempting as you can manipulate even an apparent direct comparison on the same dyno on the same day.

People don’t like it when you point out that their “before” dyno was at 5 degrees and there “after” dyno was at 30 degrees (some have it on the printout) ......thats one strange day weather wise Grin
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MisterCorn
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PostPosted: Thu Oct 24, 2019 4:34 pm    Post subject: Reply with quote

I consider chassis dynos to be useful for making tweaks and seeing if the output goes up or down, but that is it. Not to get an absolute power figure 'at the wheels' or an estimated engine power. If you want to know engine power then put it on an engine dyno.
My problem with 'real world' tuning is the variables you can't control, chiefly wind and temperature. I have some data which shows a 16 degrees C drop in inlet temperature 69 vs 53, a handy drop, I know which intercooler I want Smile Except it was exactly the same one, one was driving in to the wind and one with the wind. Looking for small changes in performance by testing on the road is tricky, but you can get a pretty good estimate of power at the wheels. We supply equipment to do exactly this for a number of race series for scrutineering purposes.

MC
 
  
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bazhart
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PostPosted: Fri Oct 25, 2019 8:32 am    Post subject: Reply with quote

Thanks guys, I was trying hard not to influence the replies in order to obtain different unbiased opinions but obviously left the request too vague.

I have actually used engine, road and roller dynos (air, water brake and eddy current) for nearly 50 years now (and we have one of the most reliable roller dyno set ups possible here at Hartech), but I have come across an issue in which someone else is using wheel power to validate performance set against manufacturers engine power which I am investigating.

I didn't want to say why as this might influence opinion and responses - so will still not for now - sorry.

My interest is the comparison with manufacturers stated engine power and wheel power measured on a roller dyno (not rear wheel fixed dyno).

I do appreciate that for measuring change with identical set-ups - it doesn't matter too much what method is used and I also agree that an engine dyno will measure engine output. However there are numerous pitfalls in all systems and I wondered how accurately others thought a measure of wheel power on a rolling road dyno would be to compare with an empirical manufacturers engine output and how reliable others thought the results and repeatabillity might be?

Baz
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Mezgerite
Monza


Joined: 02 Nov 2016
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PostPosted: Fri Oct 25, 2019 9:23 am    Post subject: Reply with quote

Ok,

So you are asking how wheel power compares to OEM?

Well as mentioned it depends which dyno. But if it's an American Dynojet than their figures are rubbish and can only be compared to another American Dynojet. There are two reasons for this, one is a fudge factor and the other that it is an inertia dyno that cannot load the car. Remember OEM measure steady state.

It should however, once a few runs have been done to stabilise tire and fluid temps be repeatable, especially on a NA car.

M
 
  
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murph7355
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Joined: 20 Mar 2012
Posts: 67



PostPosted: Fri Oct 25, 2019 9:48 am    Post subject: Reply with quote

bazhart wrote:
...but I have come across an issue in which someone else is using wheel power to validate performance set against manufacturers engine power which I am investigating....

Why are they doing this? What do they hope to achieve?

Wasting their time IMO.

If it's against Porsche's stock figures, manufacturers have a fair degree of latitude. And if a customer approached Porsche moaning about a lack of power, and was convincing enough (unlikely!) Porsche would simply tell them to give them the car and they'd no doubt take the engine out and test it against their own rigs.

If it's for 3rd party tuning claims, I can't imagine third party contracts are much different. Nobody can guarantee absolute gains.

I would hope for any serious mods a third party would have taken reference readings before work started (and/or that the customer had insisted on it) and then took readings on the same dyno with the same parameters afterwards. Prints of both. Bish, bash, bosh, see you in court Smile

As others have noted, use the same dyno for before and after relative testing. Don't get hung up on absolutes as you cannot get them on a rolling road. It's not what they are for.

Road testing is all well and good too, but on cars with near 400bhp and weighing what a 911 does, another 15bhp will be difficult to spot in outright terms. Any difference will be more down to the shape of the curves having changed and/or you being sat 10mm closer to the floor as your wallet in your back pocket is now much thinner Very Happy
 
  
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