HP at the rear wheel

[JohnEd]

Now here is the real deal.  I own a device called the "GTech".  It has accelerometers in it, I am certain.   It is as big as a couple packs of cigarettes and has a nifty lcd screen.  Now it purpose is many fold:  Reads out your rear wheel HP, 0 to 60 time, 1/4 mile et, HP at rpm, cornering "G"s and braking Gs and a whole host of other crap any bus Knut would not be interested in.  0 to 60 indeed!!!! Lots of us should perk up at HP and braking, however and 0 to 30 would be an interesting.  I made a bunch of changes to the Lex....air filter upgrade, exhaust mods, spark plug upgrade, bigger brake rotors, pads.  I watched my stopping distance shrink a little and I saw my rear wheel hp creep up more than a few.  In short....the thing works.  No wires connect to the Gtech... you just tell it your weight and the number of cylinders and when you want to start your run and it does everything and reads out all sorts of stuff.  AND here's the rub: remember I said it was interested in the number of cylinders and that there are no wire connections?  It measures rpm by sampling the spark generated RF pulses.  We ain't got those so I don't know how that could be worked around.  I should think that acceleration over time and weight should yield rear wheel HP....right?  And braking "g"s shouldn't have rpm as a term in the equation.  I think the thing would need rpm to tell it you were up and running and might truncate the comps without it.  I'll contact them and see what they say as to how to get around that.  I'll bet that anybody at a rally with this system up and running will get a workout.  Ebay for a couple hundred and you could use it to eval your car at least.

FYI,

John

[Jeremy]

The device may be useful for comparative results over different runs in the same vehicle, but you cannot produce any definitive numbers by merely measuring speed (or acceleration) and weight, and then performing a mathematical calculation to produce a horsepower reading. It could theoretically be done if you took all the other variables into account as well - no doubt the device has some reasonable guesses for some of these variables for the sort of vehicle that it is likely to be used in, but I bet it's still at least 25% out on any figures that would be obtained empirically. And on a bus of course those guesses would be wildly inaccurate - for instance rolling resistance from 8 tyres and air resistance from the huge frontal area would mean that far more horsepower would need to be generated per ton to obtain a given speed than would be the case in a sporty car.

But as I say, for comparative testing the results might have some value

Jeremy

[JohnEd]

Jeramy,

I agree with what you said...mostly.  Any figure would have to the HP above all losses and that is rear wheel hp.  The hp of the engine is a guess as the losses in tires and diff and tx and cooling and alt, etc are only estimates.  Agreed many times over....so many unknowns.  Air resistance?  Run the test to 20 mph or 30....min factor for aerodynamics or certainly less.


Going back to my high school physics...50 years in the immediate past...if memory serves:  mass and acceleration and time would yield the energy needed and energy translates to hp.  Or have I forgotten something? Really, anything?

I agree that the think would be use full for determining the delta in power mainly and any other number would be a best guess or that famous, "other things being constant".

John

[Jeremy]

I agree with what you said...mostly.  Any figure would have to the HP above all losses and that is rear wheel hp. 

I don't think that is true though - the RWHP figure can only take account of drivetrain losses - all other losses (including air resistance and tyre rolling resistance) is extra, and without knowing these you cannot work backwards from 'real life' performance data. Looking at it another way, imagine the bus sitting on a rolling road with the throttle wide open. The speed indicated on your speedometer would be far, far higher than the speed you could actually acheive on the road.

Jeremy

[poppi]

 JohnEd,

   I think you also forgot ambient temp., humidity, elevation, slope of the road, wind, new moon or waining, price of petrol.

 Did they have moments of inertia back then (just funnin)
 Don't know about your device but some of the ones I have looked at wanted all sorts of vehicle info. (including front area)

 Skip

[JohnEd]

Skip,

Mine wants a lot of data and I didn't mention that.  I quit entering data and just made the run when it asked for my birthday   The people on the board use the Gtech to do 1/4 mile runs from a freeway on ramp.  I think they are speeding a mite at the end of it as some have 500 hp on the ground.  Oh, to be young again and be able to afford the tickets and ignore the hazard.  Mostly it is used for comparison and evaluation purposes and for that I am sure it is valid.  I have heard this system discussed many many times in the past but this is the first time I have ever heard it said that it "couldn't work".

All my docs and the G are packed away so I can't delineate what the thing asks for. Temp and rel hum have a serious affect on the engine performance so i can easily understand their interest there.  I found this device to be well thought thru and I suspect a accurate item.  One thing that will screw up your results is to not get it absolutely level and also to ignore the "level shift" under acceleration.  That one item will throw things way off and I rem,ember that you are supposed to go to the track and run with the unit set up and then input a correction factor to force the G to agree with the drag strip clocks and other data.  Once you got that correction factor input it would be accurate anywhere that the road was level. 

They use the thing to eval the g force in every turn on a race course.  It remembers every turn and will give you a profile of the exact g's through every corner.  really great for evaluating suspension performance and tires not to mention the driver.  I don't race any more and I was never serious but I do enjoy some of the more shallow science and engineering.

I think all the parameters and validity of output data is well presented on their site.  Check it out.  I am interested in your take.

John

[poppi]

John,

    Went to their sight and I like their different calculators but the drag coefficient approximator link seems to be a dropped link

     The manual says that the system measures the net HP. To me this is more useful than engine or @ wheel hp. It is the overall hp used
    to move the rig at a given speed etc. an interesting note that second gear is where the hp is measured. so drag may not be to great.
    for those slide rulers yes I realise the 40 to 80mph is not a 2x but ~8x drag. but if you can do 80 in second on a bus your top end is just
    to fast for me  .

    Nice unit and probably accurate enough to establish a good base to do over all diagnostics with. (did those new bigger injectors really
    net me a large enough performance boost to justify the cost etc. yadda yadda yadda) I love to have good statistics to sell my cost
    benifits............"look dear a 2.4% efficiency increase" 

    Skip