slick Posted July 19, 2005 Share Posted July 19, 2005 My brother in law had his car on a brand new set of rollers and then not long after he went on Thors as well. The difference in wheels to hub figures was 5bhp which was literally a few % difference overall (2-3% ish?), barely anything in that instance really. It wasn't a supra not that it matters, and put down around 170 @ hubs iirc, but the difference was definately 5bhp. Quote Link to comment Share on other sites More sharing options...
Scoboblio Posted July 19, 2005 Share Posted July 19, 2005 I also had my standard soop on the rolling road at EA in Witham and had a bit of a strange result (nowhere near 390ft/lb though). The thread's here: http://www.mkivsupra.net/vbb/showthread.php?t=30240&page=1 Quote Link to comment Share on other sites More sharing options...
Bobbeh Posted July 19, 2005 Share Posted July 19, 2005 My brother in law had his car on a brand new set of rollers and then not long after he went on Thors as well. The difference in wheels to hub figures was 5bhp which was literally a few % difference overall (2-3% ish?), barely anything in that instance really. It wasn't a supra not that it matters, and put down around 170 @ hubs iirc, but the difference was definately 5bhp. Funny that, as I seemed to have gained about 40hp to the wheels/hubs on my dyno run with just a cat back exhaust as the difference, still with cats in and stock engine etc. I think the difference is more than you make suggest. Quote Link to comment Share on other sites More sharing options...
slick Posted July 19, 2005 Share Posted July 19, 2005 Funny that, as I seemed to have gained about 40hp to the wheels/hubs on my dyno run with just a cat back exhaust as the difference, still with cats in and stock engine etc. I think the difference is more than you make suggest. I dunno bobbeh, I'm no expert as I expertly showed already but I do know I saw both runs/results and it was true what I said... thats all I can say really as I don't know the hows, whys and therefores. Quote Link to comment Share on other sites More sharing options...
Class One Posted July 19, 2005 Share Posted July 19, 2005 Bobbeh, as I am stupid and only got a grade II CSE at maths can you work out my fwhp? It was 262bhp at wheels on a JDM TT Auto. I know its all been explained how to do it....but I'm crap at maths. Quote Link to comment Share on other sites More sharing options...
Bobbeh Posted July 19, 2005 Share Posted July 19, 2005 354 - ish for that. (if assumed 25% loss due to auto) Quote Link to comment Share on other sites More sharing options...
Class One Posted July 19, 2005 Share Posted July 19, 2005 354 - ish for that. (if assumed 25% loss due to auto) Thanks mate, that the result from the Surrey rolling Road day ( J Tuner mag issue 3 I'm the bloke with two chins!!!!!) So i reckon really is only about 330 ish? Quote Link to comment Share on other sites More sharing options...
Bobbeh Posted July 19, 2005 Share Posted July 19, 2005 I dunno bobbeh, I'm no expert as I expertly showed already but I do know I saw both runs/results and it was true what I said... thats all I can say really as I don't know the hows, whys and therefores. Yeah I understand what you're saying. I dont know the facts either, just trying to comprehend all the weird numbers a lot of us seem to be getting. Quote Link to comment Share on other sites More sharing options...
chilli Posted July 19, 2005 Share Posted July 19, 2005 Does anyone provide a "callibrated" rolling road? Otherwise, with all this variation it seems that the figures are a bit meaningless, certainly when comparing readings taken from different places at least. It seems the only way to make them useful is to use the readings in a relative way. i.e. use the same rolling road, measure before, make changes, measure after... (assuming that the rolling road spec hasn't moved in the mean time due to wear, servicing etc). Quote Link to comment Share on other sites More sharing options...
Digsy Posted July 19, 2005 Share Posted July 19, 2005 Just to add another variable into the melting pot, power figures from and engine dyno are corrected to account for variations in atmoshreic pressure and temperature - so that you can meaningfully compare readings taken anywhere and any time. Do the chassis dyno people do this? Quote Link to comment Share on other sites More sharing options...
Bobbeh Posted July 19, 2005 Share Posted July 19, 2005 I wouldnt be suprised if the calculations/ratios some configure the dyno software with differ so different results occur. Quote Link to comment Share on other sites More sharing options...
Dash Rendar Posted July 19, 2005 Share Posted July 19, 2005 220bhp at the wheels? Did you have your aircon on?? Only just read this. I'm so gonna hurt you Doug. BTW, I couldn't be bothered to read through masses of posts, but I thought I'd try and simplify the maths for you. (Sorry if this has already been done.) As someone has already said, when you take 25% off the FWHP to arrive at the RWHP through drivetrain losses, the difference is *not* the same as adding 25% to the RWHP to get to FWHP. What you really need to do is add a third! Why? 25% is 0.25 right? So taking off 25% is the same as multiplying a number by 0.75. (Since 1-0.25 = 0.75.) So the inverse calculation is a division by 0.75. When you divide by 0.75, you are in fact multiplying by 1/0.75 = 1.33. This is the same as adding 33% to the original figure. Try again with a 20% drivetrain loss figure. This is the same as dividing by 0.8 (since 100% - 20% gives us a multiplication factor of 0.8). 1/0.8 = 1.25. This is the same as adding a quarter. So, in a nutshell, if we assume 25% losses for an auto, you can always convert RWHP to FWHP by DIVIDING by 0.75. So, if I take my pitiful JTuner mag result of 244bhp RWHP... Add 10% for air con (oi, not a word you lot!), that's 244 x 1.1 = 268bhp RWHP. Now I divide by 0.75... That's 268 \ 0.75 = 358bhp FWHP. Sounds about right for stock tubbies, Apexi, one cat out and a Nurspec Type R, I think. Quote Link to comment Share on other sites More sharing options...
Ian C Posted July 19, 2005 Share Posted July 19, 2005 10% for your aircon?! Surely not... -Ian Quote Link to comment Share on other sites More sharing options...
chilli Posted July 19, 2005 Share Posted July 19, 2005 yeah surely the air con can't be consuming 25 bhp odd lol, thats more than some small engines put out! Quote Link to comment Share on other sites More sharing options...
Digsy Posted July 19, 2005 Share Posted July 19, 2005 You can't add 10% on for aircon at the rear wheela and then bump it up again by including it in the drivetrain losses factor! Aircon losses are quite high, but not quite high. 10hp at full chat is closer to the mark. Quote Link to comment Share on other sites More sharing options...
Dash Rendar Posted July 20, 2005 Share Posted July 20, 2005 Yeah, I've heard various figures. I even heard some tuners saying it could take as much as 45 off a Supra! I was sceptical, so I picked a number on the more conservative side of what I'd been told. But you guys are talking even lower numbers. In which case my engine is just a bit poo and my lightly modded supe is way slower than stock. (Although one tuner that's driven it says it goes like a rocket. Very confused...) And I'm thinking 25% drivetrain losses on the auto might be a bit optimistic too. I'm thinking that if a completely stock J-spec dynos at around 260 at the wheels, then 20% sounds closer. But no two engines are the same and all that... Quote Link to comment Share on other sites More sharing options...
Digsy Posted July 20, 2005 Share Posted July 20, 2005 Assuming my NA really has 220hp at the flywheel, it dynoed at (no giggling at the back) 163rwhp, which is a loss of 24%. Quote Link to comment Share on other sites More sharing options...
Digsy Posted July 20, 2005 Share Posted July 20, 2005 Aircon losses are quite high, but not quite high. 10hp at full chat is closer to the mark. Bugger. LOL @ Jake's new sig...! Quote Link to comment Share on other sites More sharing options...
slick Posted July 20, 2005 Share Posted July 20, 2005 Plus ofcourse not all of the variables which make up transmission losses rise in relation to engine output so cannot be expressed as a percentage For example if you modded your engine and gained 100bhp over stock and you used 25% as a loss would you be right in adding on 25% of that new 100bhp you'd gained as additional losses? Certain losses would increase along with output but surely not all of them(?), therefore adding further confusion to the mix Is that an agreed assumption? Quote Link to comment Share on other sites More sharing options...
Keith C Posted July 20, 2005 Share Posted July 20, 2005 I thought the aircon disengaged at WOT. Quote Link to comment Share on other sites More sharing options...
Digsy Posted July 20, 2005 Share Posted July 20, 2005 Plus ofcourse not all of the variables which make up transmission losses rise in relation to engine output so cannot be expressed as a percentage For example if you modded your engine and gained 100bhp over stock and you used 25% as a loss would you be right in adding on 25% of that new 100bhp you'd gained as additional losses? Certain losses would increase along with output but surely not all of them(?), therefore adding further confusion to the mix Is that an agreed assumption? Anything extra you make at the flywheel due to mods you have to add a % loss to in order to get back to the rwhp because all the losses from the flyheel to rear wheels are drivetrain losses an are a %. In fact expressing drivetrain losses as a straight % is probably an over-simplification because they are unlikely to be linear with fwhp, but it isn't a bad rule of thumb. Quote Link to comment Share on other sites More sharing options...
chilli Posted July 20, 2005 Share Posted July 20, 2005 Plus ofcourse not all of the variables which make up transmission losses rise in relation to engine output so cannot be expressed as a percentage I have to say that you have a point there. I don't think the losses are really as simple as a linear relationship to bhp. This may work ok as an approximation when the bhp changes are small. I expect it's quite far out when bhp changes significantly. I would expect in fact that its more a function of the rpm (of the part being measured) plus some fixed component due to other losses. After all, a gearbox say (with everything else being equal) doesn't really "know" its passing 300bhp or 400bhp (at the same rpm) apart from the additional torque being placed on it. That torque may raise losses slightly due to increased friction (because of lateral forces generated on the parts internally, such as bearings and thrust washers and between the teeth of the gears) but I'd be almost certain that relationship is not linear and the dominant factor must be the angular velocity of the part(s) concerned. So, in this case, a straight percentage is only a very rough approximation! Quote Link to comment Share on other sites More sharing options...
chilli Posted July 20, 2005 Share Posted July 20, 2005 *** accidently double posted (damn wireless) but not sure how to delete this posting - so ignore it lol *** Quote Link to comment Share on other sites More sharing options...
Dash Rendar Posted July 20, 2005 Share Posted July 20, 2005 Totally agree! (I reserve the right to inflate my estimates depending on the knowledge of the audience.) Quote Link to comment Share on other sites More sharing options...
slick Posted July 20, 2005 Share Posted July 20, 2005 Totally agree! (I reserve the right to inflate my estimates depending on the knowledge of the audience.) lol, I like your thinking Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.