rolling road... now i'm confused
Discussion
my mate passed his driving test yesterday and some of his mates were meeting up at a rolling road in northampton today to test their cars...
so me and my mate go along for a laugh in his 1986 polo 1.0, which according to the VW handbook should develop 45bhp at the flywheel, my mate was hoping for 30bhp at the wheels...
so we stick the car on the rollers and when we get the printout back it says it develops 62bhp at the flywheel and 27bhp at the wheels, as well as 70ft/lb of torque
this seems an outrageous high figure for such an old, standard VW engine... especially considering it was almost 20bhp over what VW themselves quote...
can anyone explain why the car supposidly develops much more power than it should!?!
also btw, there was an euro look polo mk 3 coupe G40 with a 'PistonHeads.com - Speed Matters' sticker there, anyone from here???
>>> Edited by littlegearl on Saturday 8th March 15:10
so me and my mate go along for a laugh in his 1986 polo 1.0, which according to the VW handbook should develop 45bhp at the flywheel, my mate was hoping for 30bhp at the wheels...
so we stick the car on the rollers and when we get the printout back it says it develops 62bhp at the flywheel and 27bhp at the wheels, as well as 70ft/lb of torque
this seems an outrageous high figure for such an old, standard VW engine... especially considering it was almost 20bhp over what VW themselves quote...
can anyone explain why the car supposidly develops much more power than it should!?!
also btw, there was an euro look polo mk 3 coupe G40 with a 'PistonHeads.com - Speed Matters' sticker there, anyone from here???
>>> Edited by littlegearl on Saturday 8th March 15:10
Its all down to smoke and mirrors.
The operator of the rolling road can "adjust" the figures by inputting incorrect information such as temperature and barometric pressure.
When the run is computed the figures will be higer or lower depending on this.
Also 62 at the flywheel and only 27 at the wheels seems a bit supect to me, that a big transmission loss!
The operator of the rolling road can "adjust" the figures by inputting incorrect information such as temperature and barometric pressure.
When the run is computed the figures will be higer or lower depending on this.
Also 62 at the flywheel and only 27 at the wheels seems a bit supect to me, that a big transmission loss!
Aaah dyno printouts - not worth the paper etc...!
There are so many variables that can affect bhp readings, not least of which is the gear the car was in. Fourth is the usual as it is supposed to be a "straight through" ie 1:1 ratio. Was the car figured in third, or on the car in question is fourth geared low?
Air temp and as previously mentioned the dyno settings also make a big difference.
IMHO dynos are invaluable for setting up a tuned engine to run correctly (timing, mixture etc) but can provide no more than a rough comparison between the cars that were figured on the same day, with the same operator, in the same gear etc... Great fun though, when I had my Mini on the rollers showing 0 on the speedo (all the way round) and seeing 129mph on the roller speedo - excellent!
How did your mate's car compare to the others?
By the way my Mini is 85bhp (so there!!!!!)
Chris.
There are so many variables that can affect bhp readings, not least of which is the gear the car was in. Fourth is the usual as it is supposed to be a "straight through" ie 1:1 ratio. Was the car figured in third, or on the car in question is fourth geared low?
Air temp and as previously mentioned the dyno settings also make a big difference.
IMHO dynos are invaluable for setting up a tuned engine to run correctly (timing, mixture etc) but can provide no more than a rough comparison between the cars that were figured on the same day, with the same operator, in the same gear etc... Great fun though, when I had my Mini on the rollers showing 0 on the speedo (all the way round) and seeing 129mph on the roller speedo - excellent!
How did your mate's car compare to the others?
By the way my Mini is 85bhp (so there!!!!!)
Chris.
erm, his polo was run in fourth gear up to 100mph and supposidly produced 62bhp +70 ft/lb torque
can't really compare as we were the last ones to go on at that time, i'll see if i can find some other posts on the net to compare...
edited 'cos i put "10ft/lb of torque.. again!!!
apparntly the guys in their lupo's said their power/torque figures were accurate compared to standard outputs and/or previous rolling road testing...
>> Edited by littlegearl on Saturday 8th March 21:39
can't really compare as we were the last ones to go on at that time, i'll see if i can find some other posts on the net to compare...
edited 'cos i put "10ft/lb of torque.. again!!!
apparntly the guys in their lupo's said their power/torque figures were accurate compared to standard outputs and/or previous rolling road testing...
>> Edited by littlegearl on Saturday 8th March 21:39
see this
www.pumaracing.co.uk/setup01.htm
for some good info on rolling road stuff
and the main page for other good technical info
www.pumaracing.co.uk/mainmenu.htm
Sparks
www.pumaracing.co.uk/setup01.htm
for some good info on rolling road stuff
and the main page for other good technical info
www.pumaracing.co.uk/mainmenu.htm
Sparks
Sparks, I cannot access www.pumaracing.co.uk any more. It stopped working for me about 6 months ago, have you tried it recently?
kevinday said: Sparks, I cannot access www.pumaracing.co.uk any more. It stopped working for me about 6 months ago, have you tried it recently?
Looked at it this morning (checked before I posted). You may have some cache problem with IE/NS.
Sparks
Interesting Pume Racing article.
I was talking to Dave Walker (CCC, Emerald Cams) about this once. He was adament that its the other way round - the wheels BHP figure is variable, dependent on tyre pressure, tyre temperature, tyre width, etc etc. But a good rolling road should accurately measure the transmission losses when you lift off the power and let the wheels slow down in 4th gear. Then you get an accurate FLYWHEEL figure.
Dave reckoned he could alter the 'at the wheels figure' anytime by just pumping up the tyres or fitting narrower ones. (or was it wider ones - can't remember).
Anyway the sentiments about a rolling road's main purpose to tune a car on the day are definitely true, my Golf was at Southern Carburettors last Thursday.
On the first run (and as I raced it last year) power was 125BHP, torque was 105lbs/ft.
By the 8th run, purely by adjusting the timing and replacing a dodgy G&S rotor arm, power was 140BHP and torque was 125lbs/ft. All flywheel figures - note!
Should make a massive difference on track.
If I can keep the thing on the track that is.
I was talking to Dave Walker (CCC, Emerald Cams) about this once. He was adament that its the other way round - the wheels BHP figure is variable, dependent on tyre pressure, tyre temperature, tyre width, etc etc. But a good rolling road should accurately measure the transmission losses when you lift off the power and let the wheels slow down in 4th gear. Then you get an accurate FLYWHEEL figure.
Dave reckoned he could alter the 'at the wheels figure' anytime by just pumping up the tyres or fitting narrower ones. (or was it wider ones - can't remember).
Anyway the sentiments about a rolling road's main purpose to tune a car on the day are definitely true, my Golf was at Southern Carburettors last Thursday.
On the first run (and as I raced it last year) power was 125BHP, torque was 105lbs/ft.
By the 8th run, purely by adjusting the timing and replacing a dodgy G&S rotor arm, power was 140BHP and torque was 125lbs/ft. All flywheel figures - note!
Should make a massive difference on track.
If I can keep the thing on the track that is.
I remember going to a rolling road day in mine once and the first thing the operator asked me was what power should it have as standard. Why?
I commented that I was there to find out what power it did have and as his dyno was supposedly accurate he would be able to tell me in 2 minutes.
After two attempts he said he couldn't get a figure because a) 4th gear was hitting the rev limiter before the max power was reached, b) 3rd gear was spinning the wheels on the rollers, c) didn't know what the transmission loss should be for my car so couldn't calculate the 'flywheel' figure.
The dyno print-out I got looked like something a monkey had drawn with lines starting and stopping in seemingly random places (i.e. plot starting at 1500rpm going to 2500rpm then disappearing until 4000rpm and continuing to 5000rpm, then disappearing again and finally reappearing at 6000rpm.
I commented that I was there to find out what power it did have and as his dyno was supposedly accurate he would be able to tell me in 2 minutes.
After two attempts he said he couldn't get a figure because a) 4th gear was hitting the rev limiter before the max power was reached, b) 3rd gear was spinning the wheels on the rollers, c) didn't know what the transmission loss should be for my car so couldn't calculate the 'flywheel' figure.
The dyno print-out I got looked like something a monkey had drawn with lines starting and stopping in seemingly random places (i.e. plot starting at 1500rpm going to 2500rpm then disappearing until 4000rpm and continuing to 5000rpm, then disappearing again and finally reappearing at 6000rpm.
DanH said:
Only way to get a properly accurate figure is to take the engine out and bench test it.
Rolling roads are good for ECU maps and a laugh if its a shootout, but not a lot else.
A common misconception.
Older style rollers were good for comparisons and on-the-day performance gain readings but modern computer controlled rollers and booths are repeatable to within 2%. The biggest variable is in the calibration of the unit, if it isn'tr calibrated every few months then it possibly won't be terribly accurate. The rollers i use can measure to half a bhp, record a multitude of parameters and overlay them for the whole run, a power run is over in less than 30 seconds. if you pick a good outfit there's no reason to suggest it's any less accurate than a bench dyno run by a numpty..
The comments about the variation in wheels figures with bigger / smaller tyres and pressures etc is true .. you can vary the power at the wheels by doing the run in 3rd gear or 5th gear, but when the coast down test is done that's taken into account since the computer can work it out from the roller (ie road) speed and the engine revs. As long as you tell it what it is in 4th gear by doing a short run at a coulpe of speeds it'll all work out fine at the flywheel.
The last point I would raise is who would want to know what their engine made on a bench dyno anyway? .. it's the power it makes installed in the car that's important .. not seen many bench dynos at santa pod putting in low 12second quarter mile times lately ...!
It is true.... Engine power figures are only good for pub talk.
The main use is for comparison (before and after) and as such tuning.
I've raced other cossies (in my cossie) in the past that were supposed to be over 100bhp more powerfull and yet couldn't match my car.
Oh my Ultima's engine produces 526BHP
The main use is for comparison (before and after) and as such tuning.
I've raced other cossies (in my cossie) in the past that were supposed to be over 100bhp more powerfull and yet couldn't match my car.
Oh my Ultima's engine produces 526BHP
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