OldDan

Right, confused.

I've got two rolling road print outs, one from Minisport, and one from Awesome GTI.

Awesome GTI use "Dynojet research" rollers, now my wheel hp figures, differ , MASSIVELY.


Minisport was 81whp and 109lbs/ft torque. (117bhp)

Awesome GTI was 97whp and 104lbs/ft. (114bhp)


Both same mods, and both around the same temperatures.

What's going on? and why do the wheel hp figures differ so much?

OldDan

And no "was it TUV approved" comments.

AlexF

wheel HP is absolutely meaningless !!!



Alex

OldDan

Why so? I thought it was a truer figure?

AlexF

well up the tyre pressure 10 psi and the BHP reading will go up! So its pointless to talk about WHP.


To measure work done (HP) you use a brake against the engine...

One of the methods above uses the tyres to transfer the power to the roller, where as the other directly bolts to the hub removing that link.

Hence the two figure must be different. but once you look at the calculated flywheel figure it all evens out

Alex

OldDan

I see! Why does the tyre pressure alter the reading though?

AlexF

the tyre sits in the rollers



hence there are TWO contact patches.

If you up the pressure the side walls can support a greater load and hence the tyre will defect less. This effects the run down calculation(where the engine is disconnected and the momentum of the drivetrains runs against the force of the rollers and slows down).

Alex

OldDan

Thanks Alex, perfect!

rapidcossie

thats right Alex...when i want more power i just pump my tyres up

AlexF

I won a RR shoot out before now like that - a quick dab on the brakes during run down also helps boost your curve or even a little wheel spin on the way up

jammy86

Why are you moaning over 3hp anyway? Every RR will be subject to hysteresis losses, especially if there has been a lack of maintainance. no two RR's will ever give the exact same output, unless they were calibrated using the exact same machinery at the exact same time and were both upto the same service standards.

JAmes.

cossie4i+

iTrader: (15)

What tyre pressure do you use then Euan ? 150psi

Steve.

OldDan

I'm not, I'm wondering why I've lost 16whp between dynos.

It seems every run at Awesome GTI is high up on WHP figures......

AlexF

IIRC Awesome GTi use a single roller setup...

arr look it is



As the car accelerates against the force of the RR it climbs up the roler (thus compressing the tyre hard against it as the vehicle is strapped down).

On overrun you let off and the car drops down the roll and releases some of the pressure on the tyres - thus changing the way you are measuring what is happening.


Alex

JamboRS

does ava use the coast down method,never seen them do it tbh,they usually just hold it at 500rpm intervals and load it then,anyway i was only runnin 26psi @ tyres last time i was there as i had a slow puncture so ill take it to 32 as its been modified to run that now see if i can crack 220@wheels

need to edit my sig to read 214@wheels@18psi@26psiTP(tyre pressure)

_DAN_

Is this what Dave Walker was trying to mean in PPC this month, Alex?

AlexF

LOL! I know the one you mean....

But I haven't bothered reading that article yet - but I would hope so. He's a knowledgeable guy and I've had some very good conversations with him about RRs in the past!

Alex

rapidcossie

The rollers i use measure BRAKED horsepower not calculated run down figs or intertia runs.

Each power point held for a minimum of three secs to stabalise then recorded.

This is the most accurate way IMO....

if all else fails i get the foot pump out

AlexF

so they only measure power at the wheels....

Alex

rapidcossie

yeah only power at the wheels...

everything else is a guess by a computer

TiB

iTrader: (1)

What was the flywheel figures?

I've had various wheel figures from various RR's but most tend to be similar once calculated into flywheel figures.

I got
414bhp whp
490bhp flyhp at nobles.

i then upped the power slightly which was guessed at 10-15bhp and then went on at awesome...

482bhp whp
502bhp flyhp

So not too far out overall.

AlexF

if your happy with a computer working out BHP on the way up - why are you unhappy about it measuring run down?!?!?!?!

After all you can't measure BHP - you calculate it!


BHP = (torque x rpm) / 5252

BHP is brake horse power

Alex

foreigneRS

computers can't guess, only calculate based on measurements and formulae

rapidcossie

i cant argue Alex as i cant put what i mean into words

sorry....

each to their own

OldDan

Lee, Flywheel HP is in bold.. 114 and 117.

AlexF

sorry matey I'm not trying to wind you up or anything - just passing the time and waffeling on my fave subject (engines!).

There are pros and cons to ANY measurment method and at the end of the day they are all fairly compairble! To get serious about it you need a dyno and FFS lets not get started on those or people with little knowledge will shout TUV TUV TUV TUV TUV TUV TUV TUV TUV TUV TUV

rapidcossie

its cool Alex i know your not trying to wind me up..we can only listen to the people that really know what they are on about and take from them what we will.

TiB

iTrader: (1)

Note to self... learn to read

Sort of makes my point valid though...


I also cannot believe why some rollers recon trans losses should be so high.

For instance... my car lost 76bhp at nobles
Why so much?? if a 2.0gls sierra went on with around 90bhp std with a 'similar' trans setup it would only have around 15bhp left to move the car... doesnt make sence.

Now i know its meant to be a percentage of the output type of thing but i just cant see that it should be to the extents it is.

A gearbox takes x amount of power to drive it.. Fine
why does it need more power to turn it if you put more power through it? and surely on that priniple it would mean you would actually lose more power the heavier your car was? (if you get what i mean)

If a gearbox was sealed and hydrolic i could understand it more but it isnt.

rapidcossie

alot of that make sense Lee

AlexF

the harder you turn the cogs... the more power they absorbe doing things such as making noise and pushing themselves harder into the bearings.

hence its a percentage thing rather than a simple loss figure.

Alex

TiB

iTrader: (1)

I'm glad. i know what i mean to say but just struggle to get the words across through a keyboard sometimes

Its a subject that has puzzled me for a long time and the best i can come up with is that trans losses have been worked out like this for alot of years... but with all the disbelief in rolling roads anyone who dares to question the workings would get shot down.

Lee











watch me get shot down now for suggesting such an outrage

TiB

iTrader: (1)

I do actually understand that but the actual figures all seem far too high to me.

for example... The shaft pushing onto a bearing so hard it collapses the bearing and digs itself into the alloy casing and starts gouging it away... how many bhp would that actually take?? a cheapo 12v cordless with a smooth round bar in place of a drill bit could chew up part of the casing without too much trouble etc... and what power would they make? 0.01bhp???


All the above is just an example btw,

TiB

iTrader: (1)

Ps... ive edited my original post to hopefully explain a little clearer what i was trying to say

Mike Gurney

Read the last paragraph from the first document and the first paragraph on the second document

http://www.factorypro.com/magazine/S...e/scan0002.pdf

http://www.factorypro.com/magazine/S...e/scan0003.pdf


that make of Dyno is regarded at 15% overoptimisitic