The Underdog

hi. how does a rolling road know your engine rpm?

maybe a basic question but....

COLEYST200

iTrader: (3)

Your car gets wired up to the rolling road computer

STeve

iTrader: (11)

best dyno in the country is Stu's @ MSD in my opinion and as Ian said its get wired up like a heart monitor

Caddyshack

When I watched Mark Shead mapping a couple of cars for their final power run his lap top is reading directly from the car ecu. He also listens for knock through earphones....but the rolling road computer is also reading the rpm and displaying on a graph.

Stu @ M Developments

iTrader: (12)

There are many ways.
Most reliable is a hook directly into your ignition coil. Thats the method I use where I can.

We can also monitor injector pulses, and if all else fails we just roll her up to a given speed and then tell the dyno that that roller speed = X RPM and it knows the gear ratio from there onwards, but thats the least reliable due to tyre flex and growth.

The Underdog

thanks for replies. i had a dyno session this week and no wires were hooked up so after it got me thinking how they measured rpm.

studabear

Stu@MSD Could you set your dyno to measure exactly 2 miles to calibrate a Stack speedo?

Stu @ M Developments

iTrader: (12)

Yes mate, the dyno has a trip meter and I can measure down to 0.001 miles.

Rsmat

iTrader: (2)

Love posts like these ..another thought in my head answered .. cheers guys.

studabear

Great I will book in when the car is back together.

burnzy

Do you also take the reading from OBD? I'm guessing ignition signal is best as it will be real time reading, is there too much lag from OBD and that's why it's not the best method or is refresh rate no good?

I'm guessing older stuff before OBD you have no other choice.

Stu @ M Developments

iTrader: (12)

OBD refresh rate is rubbish. We can do if need be, but the resolution is poor as the dyno graph can only be drawn with the same resolution as its RPM input because.

Because HP = Torque x RPM ÷ 5252, if you only got say, 20 RPM readings over a 10 second power run, you would then only have 20 torque data entries.
The power would then be interplated between those columns, giving a very smooth, but unrepresentative account of the power curve.

On some newer cars, the OBD refresh rate is great and can be used, but those seem to be pretty few and far between.
Probably due more to an incompatability with my dyno and newer protocols though admittedly.

Martin-Hadland

iTrader: (2)

You don't need a rolling road for that, a GPS speed reading at a constant speed is all you need. Get your Tomtom out!

studabear

The speedo I'm looking at is calibrated over 2 miles, the more accurate the 2 miles the more accurate the speedo is. It must count pulses over the miles. I will double check the instructions though.

Martin-Hadland

iTrader: (2)

I've never come across a Stack that's calibrated that way, which model is it?

Fullflush

It should count the pulses per complete wheel revolution.

So the only thing you have to measure is the circumference of the wheel

studabear

I will link it later when I'm on the pc.

burnzy

Thanks Stu, to be fair I guess on the newer stuff it takes a while for the makers of the dyno to suss out their programming to work with new ecu's, just like your R&D with the mk3 RS

studabear

Its a stack professional 85mm speedo unit.

https://www.merlinmotorsport.co.uk/f...structions.pdf

https://www.merlinmotorsport.co.uk/p...ce-st3801-3802

ajamesc

Tom Toms cause no end of arguments at my work lol dam things. We calibrate tachos at my work in trucks as we are a tacho agent. The rollers are calibrated every year and the amount of time I hear my tacho is wrong as my speedo says such and such but my sat nav says different