Tim Smith Posted December 8, 2002 Share Posted December 8, 2002 I'm new to this Sevens business having bought one in August after running around on lairy japanese motorbikes for many years. One thing that's puzzling me is the quoted power outputs from various engines is various states of tune: 1) Are they measured or calculated outputs? 2) If they are measured, where, at the back wheels or at the crank? Most bike power outputs in reviews are taken as measured at the back wheel which after all is where it actually counts! This seems to me to be a sensible approach in that any claims are fairly easily verified without having to drop the engine to check the tuner's/manufacturer's claims. One particular claim that seems highly suspicious to me is the 252hp claimed for the Radical SR3 running a tuned 1500cc Hayabusa engine. From my knowledge of bike motors this seems rather high as even a very quick Streetbike class drag racer is unlikely to have much more than 200hp at the back wheel running a normally aspirated motor running on pump petrol without the aid of nitrous. Anyone like to comment? Maybe the 252hp is calculated power output at the piston crowns! Link to comment Share on other sites More sharing options...
Gridgway Posted December 8, 2002 Share Posted December 8, 2002 The 252 will be brake horse power. In the case of this engine which is well tuned I would assume it is measured on a dyno as "flywheel output". Not sure what that would equate to as power at the wheels. In most cases with regard to 7's on this forum you will see bhp that is either manufacturer quoted (measured on dyno during engine development) for standard engines, bhp that is inferred from rolling road tuning (for most one-offs), or bhp measured on a dyno for more race developed engines. I'll leave it to others more knowlegable than me to argue, but I understand that power at the wheels is not that useful a measure as it differs with different measurement methods due to the ways that transmission losses work. That all being said, I have no idea how believable the 252 bhp is, but it's an oft quoted number of a race developed engine. Thus it may be quite believable! Graham Edited by - gridgway on 8 Dec 2002 14:44:40 Link to comment Share on other sites More sharing options...
Tim Smith Posted December 8, 2002 Author Share Posted December 8, 2002 Surely power output at the wheels is the most useful measure as this gives a better measure of what the performance of the vehicle is likely to be than power at the flywheel. Link to comment Share on other sites More sharing options...
Blatman Posted December 8, 2002 Share Posted December 8, 2002 Use the search fuction, 'cos the question about whether BHP at the weels or crank is more relevant to performance has been asked, argued, and resolved. BHP at the crank is all that matters.................. Link to comment Share on other sites More sharing options...
Blatman Posted December 8, 2002 Share Posted December 8, 2002 Searched meself. Have a look here Link to comment Share on other sites More sharing options...
Gridgway Posted December 8, 2002 Share Posted December 8, 2002 the problem with power at the wheels is it depends on what gear you are in for example. So from what I understand the engine power is the engine power. However at the wheels it depends what gear is used. Graham Link to comment Share on other sites More sharing options...
Graham Perry Posted December 9, 2002 Share Posted December 9, 2002 TimSmith, You are not the first person to question the flywheel figure on the Radical, we have debated that here as well. Link to comment Share on other sites More sharing options...
Tim Smith Posted December 9, 2002 Author Share Posted December 9, 2002 Blatman, I stand corrected having read the previous debate, power at the crank is indeed the best engineering measurement of an engine's output! However having read the Radical 252hp? thread I remain deeply sceptical about the claim. Maybe there was just a hint of avgas in the tank when they did the dyno runs. Link to comment Share on other sites More sharing options...
Peter Carmichael Posted December 9, 2002 Share Posted December 9, 2002 Powertec claim 252bhp@9500 and 152lbft @7000, incorporating a 4% ram air effect. They get the figures from their rolling road and then add 4%. So consider instead: 146@7000 - This is 97lbft/litre which is as much as is achievable. 242@9500 - This would be equivalent to 133lbft sustained to 9500 rpm which is 94% of peak, so this is very unlikely to happen. Much more likely is 235bhp power peak at 9500rpm. No more than that is believable. Either that or it runs to higher peak revs. Tim, I think someone is telling porkies. Link to comment Share on other sites More sharing options...
Jason Plato Posted December 9, 2002 Share Posted December 9, 2002 Maybe someone jumped off the back off car during run down on the rollers dave Link to comment Share on other sites More sharing options...
edmandsd Posted December 10, 2002 Share Posted December 10, 2002 The fact that radical quoted the peak power/prm and torque figures does as Peter indicates prove the claims to be false. I'm a big believer in the phrase 'When the flag drops the bull**** stops' and performance testing of the Radical would also support that their claims are well overstated - It really pi##es me off in fact, because people buy off the back of these claims - That said overstating power is unfortunately something that will remain a constant factor in the aftermarket tuning industry. Transmission/tyre losses on bikes are understandably far less than those experienced by cars, so wheel power in these instances may be more representative. Home of BDR700 Link to comment Share on other sites More sharing options...
oilyhands Posted December 10, 2002 Share Posted December 10, 2002 The issue with wheel power is whether what you are measuring on a RR is the same as that experienced on the road, in simple terms it isn't since the tyres are resting on small diameter rollers rather than the infinite radius of the road and there is therefore an unnatural compression of the tyre and hence unrepresentative losses. That's why I and a number of other prefere theses losses to be measured and added back to give the level playing field of the crank HP. Oily Link to comment Share on other sites More sharing options...
Richard Miles Posted December 10, 2002 Share Posted December 10, 2002 As well as over stating power the under quoting of weight is the thing that winds me up. I've put a lot of effort into building a Striker that has been corner-weighted at 390Kg with all the fluids (empty tank), 2 seats etc and road legal. If you believe what you read/hear this is not much less than quoted for cars that are actually 100+Kg heavier than this which is not an insignificant amount. Being able to over-state the power and under-quote a stripped out dry weight is what produces nicer bhp/ton figures which is what sells cars. Most people don't notice until after they have built and weighed it... Richard Link to comment Share on other sites More sharing options...
Highlander Posted December 10, 2002 Share Posted December 10, 2002 Peter, You state a 'maximum acheivable' torque /litre figure of 97lb-ft. I've seen you quote this before. I realise that you've probably explained this before, but indulge me. Is this related to the efficiency of the engine, and is the figure different for a forced induction engine? Link to comment Share on other sites More sharing options...
Peter Carmichael Posted December 10, 2002 Share Posted December 10, 2002 It is related to volumetric efficiency and will be different for forced induction. Maximum achievable also varies with vlave config, with 2 valve per cylinder generally being capable of 86lbft/litre, 4 valve capable of ~96lbft/litre and 5 valve getting a bit more, up to 100 and a bit. It is a handy rule of thumb. Link to comment Share on other sites More sharing options...
Highlander Posted December 10, 2002 Share Posted December 10, 2002 Ta. Explains why WRC engines produce so much torque with blowers that look like they've been nicked of a Scania! Link to comment Share on other sites More sharing options...
scooby dooby doo Posted December 10, 2002 Share Posted December 10, 2002 The weight question has been raised before - Caterham claim 540 for a K series. With no heater, windscreen, or spare and with mag wheels and tillet seats (although with an apollo so an extra few kg of oil) I weighed 520 kg. You'll see references to "Caterham kilograms" in acknowledgement of this.... HOOPY 500 kg R706KGU Link to comment Share on other sites More sharing options...
oilyhands Posted December 10, 2002 Share Posted December 10, 2002 I have a little spreadsheet which will calculate maximum acheivable horsepower from a particular engine capacity and will give an indication of the RPM needed to acheive a given BHP figure, it has tailorable torque per litre figures according to the type/spec. of the engine. It is a good bull**** detector for quoted power outputs. Oily Link to comment Share on other sites More sharing options...
Jason Plato Posted December 10, 2002 Share Posted December 10, 2002 Hoopy - I think you want to change your signature then - or check your kilos 🤔 How about calculating the BMEP for this engine ? , if its more than 235 then it may be beef byproduct ?? .................... bmep = 240 !! "Bmep is a useful comparitor of engine performance and state of tune, although, care must be taken to consider the rpm at which the engine is operating to determine the power potential. Different engines can be compared directly as swept volume has been removed from the power or torque figures to calculate bmep". BMEP = Torque x 2473 / cc ps - my engine is at 93lbft/l at 1598cc and 230.5 BMEP Dave Edited by - Dave J on 11 Dec 2002 19:29:45 Link to comment Share on other sites More sharing options...
Swiss_Tony Posted December 10, 2002 Share Posted December 10, 2002 540 for a k makes my vx seem not quite so bad. iirc it was 580 with a screen fuel etc. All I need to do is make the engine lighter 😳 Oily Is it possible for you to send me a copy of your spreadsheet? James Edited by - james on 10 Dec 2002 19:00:57 Link to comment Share on other sites More sharing options...
edmandsd Posted December 10, 2002 Share Posted December 10, 2002 James - Quicker than Arnie......and with a 50kg weight penalty eh Home of BDR700 Link to comment Share on other sites More sharing options...
Paul Ranson Posted December 10, 2002 Share Posted December 10, 2002 My old stock Superlight (with Apollo and 1/2 tank of fuel) was under 500kg on a proper weighbridge. (The weighbridge wouldn't weigh below 500, so that's how I know. The operators stuck a forklift on with my car and then I removed my car. The difference was between 480 and 500.) The wheels and tyres make quite a difference. Swapping my Pilbeam from crossply to radial and aluminium centre wheels onto magnesium knocked nearly 10kg off. Paul Link to comment Share on other sites More sharing options...
Swiss_Tony Posted December 10, 2002 Share Posted December 10, 2002 Dave Its a power thing 😳 James Link to comment Share on other sites More sharing options...
scooby dooby doo Posted December 11, 2002 Share Posted December 11, 2002 Dave - its been further lightened since then. I predicted it would be 501 kg (based on what had come off) and it was weighed at 500 kg HOOPY 500 kg R706KGU Link to comment Share on other sites More sharing options...
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