edmandsd Posted July 2, 2002 Share Posted July 2, 2002 One to wind Peter Carmichael up methinks ? www.pumaracing.co.uk/coastdwn.htm www.pumaracing.co.uk/trans.htm Link to comment Share on other sites More sharing options...
Peter Carmichael Posted July 2, 2002 Share Posted July 2, 2002 You shouldn't believe everything you read on the internet... Dave Baker and I debated in the past our difference of opinion on the subject, but he is entitled to publish what he likes on his own web site. Peterid=teal> 253 BHP K-seriesteeth.gif, no gearboxbum.gifid=red> Link to comment Share on other sites More sharing options...
Gridgway Posted July 2, 2002 Share Posted July 2, 2002 ooeee, can we have that debate again?? that sure was fun!! Graham Link to comment Share on other sites More sharing options...
oilyhands Posted July 2, 2002 Share Posted July 2, 2002 Dave Baker comprehensively and ungraciously lost the debate on power at the wheels vs power at the flywheel, unfortunately he was the only person involved who didnt see this. Dave Walker wrote a short article explaining why measuring power at the wheels on a rolling road and attempting comparisons is fundementally flawed, DBs reaction was to adopt an ostrich stance. Believe what you choose to believe when you have all the facts. Oily Link to comment Share on other sites More sharing options...
Graham Perry Posted July 3, 2002 Share Posted July 3, 2002 I appear to have missed this debate, so what is the opposite opinion ? Are we actually saying that at the wheels numbers are ok'ish as a back to back comparison on the day but never rely on the coastdown losses ? Edited by - Graham Perry on 3 Jul 2002 07:08:22 Link to comment Share on other sites More sharing options...
oilyhands Posted July 3, 2002 Share Posted July 3, 2002 The gist of the conclusion is that back to back comparisons of 'at the wheels' figures on the same vehicle on the same day at the same roller speed under indentical conditions are valid, otherwise variations in roller speed, tyre pressure , weight over axle etc. conspire to make the losses too unpredictable thereby invalidating the ATW figure as an absolute. A change in tyre pressure of a few PSI or an extra person on the back of the car or the choice of a different gear in which to make the run can have a dramatic affect on the losses thru the tyres. Some RR can *measure* these losses and add them back to the at the wheels figures to give a very close approximation of the flywheel figure, the point being that the most variable part of the equation.. the losses is taken out and you have a consistent baseline to work with. Although the car on the road only sees the power at the wheels, this is *not* the same as the power at the rollers for all sorts of reasons. Oily Link to comment Share on other sites More sharing options...
Roger King Posted July 3, 2002 Share Posted July 3, 2002 Ditto most of the above. Any 'measurement' of power at the flywheel by estimating transmission losses can only be an approximation. If I exaggerate to illustrate the point:- Put a Morris minor engine in your 7, developing, say, 35bhp and give the car a good thrashing. Then swap the engine for a 400bhp Dodge Viper unit (change nothing else on the car - I know it won't really fit) and give the car another good thrashing. Guess which engine will make the transmission red hot and which will barely warm the oil? A hot transmission has absorbed more power than a cold one, but put the car on the rollers and coast down figures will be identical for both engines, thus claiming that transmission loss is identical with both engines. Then there are the tyres to consider, the weather, etc, etc. I would never get excited about a 5bhp gain on rollers - even stopping to change jets can produce a power increase, not from the different jets, but the fact that the engine and air in the test cell will have cooled. Having a cup of tea has much the same effect. Rollers are great for setting cars up, but not for nth degree measurement. Link to comment Share on other sites More sharing options...
Simos Posted July 3, 2002 Share Posted July 3, 2002 Tea can give me another 5 HP ? Fantastic !! What's the mixture ratio in the tank and would Earl Grey or Lapsang give me say 10 HP or is it all the same and Typhoo or PG Tips is fine ? Presumably we'd steer clear of Sainsbury's or Tesco's own brand for obvious reasons ? Link to comment Share on other sites More sharing options...
Mike Bees Posted July 3, 2002 Share Posted July 3, 2002 I drink a lot of tea. This explains my car's 119mph through the speedtrap at MIRA. Mike Link to comment Share on other sites More sharing options...
Morls Posted July 3, 2002 Share Posted July 3, 2002 I'm not trying to fuel any contoversy, just thick, but since the transmission and tyres and things are part of the whole car surely these need to be matched to any engine developments (I like the idea of the 7 Litre Minor tho smile.gif). Assuming the transmission IS important then why are the tests done at the back wheels not the most valid to the real worldquestion.gif Apologies if I've missed something blatantly obvious here. Mark Link to comment Share on other sites More sharing options...
Roger King Posted July 3, 2002 Share Posted July 3, 2002 Lapsang can be a problem in the fuel because all the little bits can block your idle jets. Link to comment Share on other sites More sharing options...
Graham Perry Posted July 3, 2002 Share Posted July 3, 2002 My car went a whole lot better after tea as well. It was the suger that blocked the filters that was the problem. Link to comment Share on other sites More sharing options...
Tony Martyr Posted July 3, 2002 Share Posted July 3, 2002 I think this debate divides us up into two camps. 1. Those who are engineers and who understand the definition and concept of accuracy - which is not a trivial or trite thing to say. 2. Those who treat numbers like accountants where there is no right or wrong but a wide band of numbers that can be adjusted to suit that which you wish to prove. This is not meant to insult the accounting profession but to point out that they are allowed to move numbers around in different way I have spend the majority of my life in the testing of engines and vehicles and I have a lot of admiration for tuners who, using simple rolling roads and a huge amount of intuitive skill and experience, can map track car engines in hours rather than the months taken by OEMs. BUT the actual numbers they record at the rollers give only - in my terms- an approximate indication of the torque produced at the flywheel. If people wish to use these numbers in pub talk pissing contests then that is fine, because that is all they are good for Link to comment Share on other sites More sharing options...
Mike Bees Posted July 3, 2002 Share Posted July 3, 2002 Mark - if the numbers measured at the wheel can be vary significantly without making any changes to the engine (e.g. by changing tyre pressures etc.) then they're not very useful. What you want from a rolling road is to obtain repeatable consistent figures (the units themselves don't really matter that much), so that when you go back after your next set of engine tweaks you know that any differences you see are down to those tweaks, and not down to any other variables. Roger - if your jets are idle then I suggest you sack them and get some harder working ones. Mike Link to comment Share on other sites More sharing options...
Graham Perry Posted July 3, 2002 Share Posted July 3, 2002 Interesting stuff. So, Have I been barking up the wrong tree then when I have been attempting to reduce the mechanical losses on my car ? Car in current spec has been on the rollers with current engine 3 times, each successive time it has produced more at the wheels and apparently less losses, with no real engine spec changes. On all of the results when I have added the coastdown losses to the at the wheels figure they add up to the same total number to within 3 BHP. But the losses have apparently fallen from 43 BHP to 31 BHP after changes successively to the clutch, gearbox and then diff and Brakes . Have I been wasting my time as each occasion was done on a different set of rollers ? Car subjectively does feel quicker than it used to. Link to comment Share on other sites More sharing options...
Bilbo Posted July 3, 2002 Share Posted July 3, 2002 Hi All Well having read the articles I personally would love to have only 15% or 0.88 of the flywheel at the wheel. The original 170 bhp engine which was dyno at about 2 bhp more only ever gave between 126 bhp and on one 132 bhp it read a little more but the owner of the RR admitted it was a bit optimistic. ( must say it was a while ago and these may be a couple of bhp out but no more than that) So say its 170/130 bhp thats 30%. One think I can say is with the new spec the tunnel gets MUCH hotter as the losses are greater. Now I now lots have claimed the max power off a BDR is only 165 bhp but that's still near 27%. If you take the 15% loss its as low as 150 bhp! If I could get mine down to 15% I love to know how it would be money well spent. I not going to comment on the up-rated engine as this was never dyno so its pointless. However if you wish have a look at the long discussion between me and Oilyhands on the 7ns list archives. Its has to be accepted that a RR session should really only be power on the day on arrival verses power when you leave thats what your really paying for at the end of the day. If the RR is 20% out it does not really matter what's important is you left with a fine running car. If your mates car did better on his RR so who to say which is correct. Having been to a couple of the Dave Walker RR days with the 7ns list I am really impressed with his coast down figures and I dont think anyone had transmission losses as low as 15% but I might be wrong. If they were most of the R500 owner had better sue Caterham for misrepresentation. Most cars there had a lot of power and the more power much higher losses. So why does David Baker insist they are always this low! 1988 200 bhp, 146 ft lbs, 1700cc Cosworth BD? with Brooklands and Clamshell wings, Freestyle Motorsport suspension. Q 979 CGY Edited by - Bilbo on 3 Jul 2002 21:02:47 Link to comment Share on other sites More sharing options...
happylarry Posted July 3, 2002 Share Posted July 3, 2002 Anyone tried instant coffee?? This is rivetting stuff guys - but shouldn't it be on Tech Talk? I'll duck now!! Edited by - happylarry on 3 Jul 2002 21:50:14 Link to comment Share on other sites More sharing options...
oilyhands Posted July 3, 2002 Share Posted July 3, 2002 Bilbo, The losses thru the transmission may well be 15% or so, the point is that on the rolling orad you are not measuring the losses through the transmission you are measuring the losses at the rollers, these are very different from the losses you would see on the road. On the rollers you have two sets of compressions on the tyres by rollers of a fairly small radius, these absorb a lot more power than the normal compression of the tyre on the road surface which has an infinite radius. The losses due to the tyres are disproportionately high on the rolling road and vary greatly with tyre pressure, roller speed and weight over the driven axle, hence the large variances seen between similar cars. Oily Link to comment Share on other sites More sharing options...
scooby dooby doo Posted July 4, 2002 Share Posted July 4, 2002 cheers oily - that explains a few things! I'd not thought/realised how unroadlike the rollers are... yeah maybe it should be in techtalk but AFAIK you can't move an entire topic to another forum so I guess we'll just chitter here in a slightly technical fashion.... Dave Hooper - dmch2@lineone.net Link to comment Share on other sites More sharing options...
Bilbo Posted July 4, 2002 Share Posted July 4, 2002 Hi Oilyhands Well I read it again, may be its me, but it still seems says the losses are only 17% on a power run. Well I did make a small error last night its 17% rather than 15%. http://www.pumaracing.co.uk/coastdwn.htm Think it may be thats its just badly written. Thats to say the power run figures in 4th seem to match the true road figures. I was quoting the original engines performance for a power run on 3 different RR sessions not related to a Dave Walker day. Thats why I avoided quoting the rebuilt engine and coastdown losses. 1988 200 bhp, 146 ft lbs, 1700cc Cosworth BD? with Brooklands and Clamshell wings, Freestyle Motorsport suspension. Q 979 CGY Link to comment Share on other sites More sharing options...
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