Rolling roads and autoboxes.

[Timberwolf]

Something I've been wondering...

On a rolling road, you stick the car in gear, take an RPM measurement at a known speed, then run it up through the revs, let it coast back down, and these various things end up giving you curves for power and torque vs. engine speed, right?

So what happens if you rock up with a bog-standard autobox, no tiptronic, in fact no way of holding it in a specific gear at all? Can you get a sensible dyno graph for an auto?

[Cotty]

Don't you even have 123D?

[This Side Down]

Surely most autoboxes nowadays have some kind of lower hold/1st hold?

[Timberwolf]

Not all autos have a first-gear-only lockout. Besides which, wouldn't wheelspin be a problem in the lower gears?

[Overhaulin]

Vixpy

Get your arse out of Costco. We need answers.

[HD Adam]

You would never dyno in 1st gear.
As Gazboy says, you need a 1:1 ratio which is 3rd gear on most 4 speed auto boxes.
On mine, I can turn the OD off (4th gear) and make a run in 3rd as long as I start the run at about 3200rpm so it doesn't shift down into 2nd.
I can also lock the trans in 3rd via the hand held tuner that I have.

[eliot]

It can be done - but I wouldn't trust the figures that much. I've had very different figures out of the same dyno and same operator.

[anonymous-user]

Essentially it's a waste of time with an auto.

[Mars]

We had a fun day at Emerald when they were still in Brixton. Dave Walker did a "shoot-out" day for all the Se7ens owners. One of the chaps turned up with a drag-racing Westfield with an autobox. It was quite powerful (RV8 engine) but it unnerved DW because it kept kicking-down at most inopportune moments, lifting the car bodily off the rollers.

I think he said he wouldn't put another on the rollers.

[Rockatansky]

5 USA said:

Essentially it's a waste of time with an auto.

Care to give an explanation for that?

I'm not saying you're right or wrong, I'd just like to know why.

Thanks.

[Stu R]

on the 7 series where there's no '1/2/3' option the only way you'll get a half decent reading is by using the manual sequential and trying to avoid it changing up automatically (if you redline it it'll change on your behalf whether you like it or not).

[anonymous-user]

Rockatansky said:

5 USA said:

Essentially it's a waste of time with an auto.

Care to give an explanation for that? I'd just like to know why. Thanks.

On the rolling road test a car is typically accelerated at full throttle in third gear against a load. Under these conditions an auto box will generally kick down to second gear rendering the test useless. In addition, when a big car kicks down it will almost invariably spin its wheels on the rollers spraying anyone and anything behind the car with black rubber "sawdust" as the tyres are "sanded" against the grooved tread on the roller. On older cars it used to be possible to disconnect the old-fashioned kickdown cable from the throttle pedal but a modern auto makes its own decisions by computer in any event.

[Rockatansky]

Thanks for the reply, points taken.

I guess I'm fortunate that mine can be held in 3 without possibility of up or downshift.

[Silent1]

Not really, when Vixpy tried to RR Balmoral Green's bentley all manner of bdisation was going on.

[Vixpy1]

Rolling Roading most auto boxes is most of the time a bloody nightmare. They kickdown and you never really know what the flywheel figs are because of the slip in the torque converter.

However, many of the latest boxes are really easy to do, and have losses very similar to the eq manual box. Any of the more modern BMW's or Audi's are really easy to run, simply select gear and off you go. The Audi RS6 and 535D both come to mind as easy cars to run

[catso]

Silent1 said:

Not really, when Vixpy tried to RR Balmoral Green's bentley all manner of bdisation was going on.

But surely a Gentleman doesn't need to know how much power his Bentley makes, only that it is 'sufficent'?

[Vixpy1]

catso said:

Silent1 said:

Not really, when Vixpy tried to RR Balmoral Green's bentley all manner of bdisation was going on.

But surely a Gentleman doesn't need to know how much power his Bentley makes, only that it is 'sufficent'?

You've not met BG have you

[Rockatansky]

Vixpy1 said:

....you never really know what the flywheel figs are...

I thought that was a universal truth anyway? (unless you actually have the engine on a testbed)

Surely 'flywheel' figures derived from a rolling road dyno are no better than a good guess at best.

[Vixpy1]

Rockatansky said:

Vixpy1 said:

....you never really know what the flywheel figs are...

I thought that was a universal truth anyway? (unless you actually have the engine on a testbed)

Surely 'flywheel' figures derived from a rolling road dyno are no better than a good guess at best.

True, I like to use the word 'estimate'

But as a dyno operator I can be more sure of estimates on manuals than Auto's

[LHDisbest]

Off topic totally, but how did you get into the rolling road stuff Charlie?