Rolling roads compared

[McBreadhead]

Following on from an earlier thread this year re comparing rolling roads, I thought I would lob this into the mix.

 

Had my car set up carbs and fuelling wise by Tom Airey and got a very optimistic 165bhp (adjusted) at flywheel earlier this month. It was a crisp, dry morning so optimum conditions for high reading.

 

Then took car to Sussex Performance Cars who recorded 118bhp (adjusted) at flywheel with no changes to set up. After they'd adjusted timing map only, they recorded 142bhp today in much warmer humid conditions. Just goes to show that you simply cannot compare readings.

 

Why the huge disparity between 165 and 118 with no change to set up? It must be due to a) the dissimilarities between rolling roads and b) the impact of weather. Real test is subjectively how quick the car feels which I can't do at the moment due to sh1t3 weather but can honestly say there's doesn't seem to be any difference in power, but car easier to start, idle and pick up from just timing map improvements.

 

McB

[Pezky]

It depends how they apply the load to the driven wheels when measuring the power. I believe that if the load is added quickly, power will be higher. Not entirely sure about this but it mentioned in a article I read many years ago.

[andy_h]

47BHP is a big difference between the two rolling roads ?

 

Slight tangent, but would I need to get Tom (carb setup) and SPC (mapping) to work on my car or could SPC do both carbs and maping set ups?

 

Cheers,

Andy

PS Still no email from you

 

White road legal 2.0 HPC VX race car 😬

[McBreadhead]

Andy, YHM

[Klunk]

McB,

 

I have just had a quick look at Dave Walker's Engine management book and an article he wrote in PPC magazine. In short the following will affect the figures anywhere from slightly to dramatically.

 

1) Ambient air temperature / pressure

2) Engine temperature at point of testing

3) Acceleration rate (as Q102 says faster = bigger figures)

4) Gear/Speed at which measurement is taken. A lower gear = less power loss therefore higher a BHP. He says the difference between 3rd and 4th at 6000rpm can be 10BHP.

5) Air pressure in the tyres which reduces tyre wall distortion. He states he did a back to back test and put 10psi in the tyres and gained 5BHP on a Golf GTI at 3000rpm. Tyre pressures should therefore be repeated for accurate repeat testing

6) Strapping down of the car. Over zealous strapping increases the tyres contact area with the rollers and increases the losses

7) Assumption of transmission losses. He says you can not assume this figure of say 15%, but have to work it out using experience of known/similar cars and applying it to each case (or something to that effect - my brain's starting to hurt )

8)Experience of the operator in interpreting all the above is paramount.

 

So to sum up in Dave's words:

 

...to get a really impressive BHP at the wheels figure you need to run in as low a gear as possible and put 90psi in the tyres.

 

That's takes care of the upgrades then 😬 😬

 

Regards

 

Giles

 

[McBreadhead]

SPC:

Tyre pressures 30 psi

4th gear (nearly 1:1 ratio)

Slow acceleration

 

No info on Tom's runs.

[Bob Simon]

Just out of curiosity, were both dynamometers of the absorption type or was one of them an inertial type? A large single contact roller or dual contact paired rollers?

 

 

 

 

-Bob

94 HPC VX Evo III

 

[neil.cavanagh]

"He says you can not assume this figure of say 15%"

 

BUt I thought in previous threads we had concluded that a loss figure if 28% can pretty accurately be applied to all caterhams??

 

*wavey*

[Klunk]

Hi Neil ,

 

The 15% I stated was only an example out of Dave's book and not specifically related to a Caterham's losses. The actual range he states is typically 15 to 25% depending on the vehicle. I'm sure if the general consensus is 28% from everybody's experience then that's what it is - After all, he seems to map a lot of people's Caterhams anyway .

 

The point I think he was trying make in his book is that one cannot just assume a set of losses applicable to all cars and that the interpretation of the BHP at the wheels back to flywheel BHP is complex and very much depends on the skill of the operator.

 

Regards,

 

Giles

[neil.cavanagh]

Giles,

I was trying to get a rise out of Peter Carmichael! He passionatly, and correctly, says that you should not talk about % power losses on rolling roads. It is not a % figure. It is an absolute figure that is affected by many factors and is unique to that particular run on the rolling road.

 

Generally on 7s it seems be be around 30-45bhp.. which happens to be around 20-30% for most. But talking % doesn't help you. If you did a power run with a 150bhp enigne, and then swapped the engine in the same car for a 300bhp one and did the run again you would not double the transmission losses value, as quoting a % would suggest.

 

Anyway... I think we should keep talking % to give PC something to rant about! 😬 😬 😬

[Klunk]

Neil,

 

I really should get the hang of spotting the Blatchat wind up 😬 😬. As you and Peter (and DW) say it is a definitive figure not a % - Having not met you I was trying not to rock the boat .

 

The interesting thing about this thread was I was only just talking to my engine builder yesterday about quoted BHP figures. He told me the story about one builder who was only getting 380BHP (only! )from a V8 on a dyno. However, at 5 in the morning he was getting 440BHP . Quite an "improvement" 😬

 

On the subject of windups maybe we should start up the old BHP vs Torque chesnut - always good for a laugh 😳 😬

 

Regards,

 

Giles

[caterhamnut]

Neil - nice try 😬

Peter is up a mountain somewhere for a few days - entirely unfair training for CLIC IMO, when my training is consisting of work....

 

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