Pro's & Cons of Dyno's

[duncan]

I am in no way having a pop at APT.

I know.

Duncan, I see that you're local to me, happy to meet up and discuss rollers over a beer sometime. The problem that you're missing is that transmission losses differ in each gear. Adjusting RPM by gear ration gets you part way there but 4th is typically a straight though 1:1 gear so has substantially lower losses than 5th which is typically overdriven to say 0.8:1 - in 5th your transmission losses will be different but so will your tyre losses because your wheels are now spinning faster and thus are compressing a greater amount per engine revolution.

Absolutely - but this is what makes wheel figures - wheel figures. It is the net of transmission loss power that actually propels the car in the real world which is where I prefer to drive

The trick to coast-downs is to balance the clamping load, on a double roller setup this is comparatively easy as you can tie the car such that you compress agains the rearmost roller on coastdown and against the foward one as it tries to climb out under acceleration. Yes there is a still a certain scope to get it wrong but it's not hard to get runs repeatable within 1% irrespective of which gear you run in or what you tweak the tyre pressures to be.

From my limited experience of coastdown losses on rollers I wouldnt like to comment on how they set it up.

Personally I measure my modifications in the real world with in gear acceleration tests - cant argue with father time

[cng1]

Sorry, but 'at the wheels' is just that? At the wheels. How do any other factors hold any relevance?

At the wheels means at the wheels in a given gear. If you do a run in 4th and then one in 2nd you will necessarily have a substantial apparent increase in power! Unless all vehicles are tested in a 1:1 gear then you cannot even compare "at the wheels" figures.

I'm guessing that you have a 'home' dyno that uses the coastdown method, which is understandably why you fly the flag for it. God knows we spent quite some time looking at DD's methods from a constructively critical point-of-view, but in our hands-on experience, the results speak for themselves.

Nope, I rely on empirical evidence, mathematics and physics. I like gary and respect his opinions which on the whole agree, as I remember from having one of my cars on your rollers in the summer he believes in coastdowns and agrees that the DD fudging shouldn't work but says that for the vast majority of cases it works just fine. I'm happy to go along with that but in my own experience if you are testing a vehicle that is outside the norm then whilst they are very repeatable and have good resolvability the actual numbers generated may be quite substantially wrong, both by under and over-reading.

[duncan]

What do you drive Chris?

[Ross]

I am in no way having a pop at APT.

Personally I measure my modifications in the real world with in gear acceleration tests - cant argue with father time

LoL. Were you on a road dyno test when I saw that blue flash near Ely last week :lol:
And you can argue with your road dyno tests if you want to start another thread :-D

Personally I like DD rollers. WHP is a known yard stick between DD rollers providing the operators can run the cars correctly.
Incidentally Tyre pressures and in gear runs make virtually no difference providing you program the dyno with all the genuine info. But like any dyno it is open to operator error & banzai runs. Used correctly you will struggle to find a more consistent package.

[Gerry H]

Just a quick few words of caution. The chaps at APT are really good guys but do not under any circumstances view the numbers that come of their rollers with any significance or use them for comparison with anything else.

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I have no beef with APT at all, they are really nice chaps with a great facility who do superb work.
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To me, that sounds somewhat contradictory. Maybe you'd have received a warmer response if you'd left APT's name out of your opening post as it merely served as a distraction to the point you're trying to make and the subsequent discussion.

[duncan]

I am in no way having a pop at APT.

Personally I measure my modifications in the real world with in gear acceleration tests - cant argue with father time

LoL. Were you on a road dyno test when I saw that blue flash near Ely last week :lol:
And you can argue with your road dyno tests if you want to start another thread :-D

Personally I like DD rollers. WHP is a known yard stick between DD rollers providing the operators can run the cars correctly.
Incidentally Tyre pressures and in gear runs make virtually no difference providing you program the dyno with all the genuine info. But like any dyno it is open to operator error & banzai runs. Used correctly you will struggle to find a more consistent package.

Blue flash... me ?

Lets face it none of them are perfect because engine's real power output varies according to the prevailing condiitons - great isnt it ?

[cng1]

Ignoring the boring stuff the garage currently contains:

1992 mini cooper, injected (megasquirt management) and part carbon/composite panelled

Westfield, cosworth lump, 4x440 + 4x 220cc injectors, GT2871 ball bearing turbo, ported head, emerald ecu, pace chargecooler, water injection, -34 actuator. Despite being shaped like a brick the GPS logger says it hits the limiter in top at just under 170mph. Last time out at the ring it was 150 brake down on the current spec and had 20 stone of ballast in the passenger seat but still was doing a shade over 8 minutes B2G.

[duncan]

Ignoring the boring stuff the garage currently contains:

1992 mini cooper, injected (megasquirt management) and part carbon/composite panelled

Westfield, cosworth lump, 4x440 + 4x 220cc injectors, GT2871 ball bearing turbo, ported head, emerald ecu, pace chargecooler, water injection, -34 actuator. Despite being shaped like a brick the GPS logger says it hits the limiter in top at just under 170mph. Last time out at the ring it was 150 brake down on the current spec and had 20 stone of ballast in the passenger seat but still was doing a shade over 8 minutes B2G.

They sound great :thumb:

[cng1]

They sound great :thumb:

Indeed If Hannah is arround she'll give you the lowdown as I thnk she rather enjoyed the run at Marham

[duncan]

Get some pics and full spec up :thumb:

When were you over at the ring? Was there 3 times last year myself.