Hi Guys , new to the forum ,
Thought this was an intresting thread to post on first 
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Been reading with interest , i have a MK2 Leon Cupra , and its running 267 bhp standard according to rolling roads , claimed output is 240 bhp.
On seatcupra.net most Cupras which are rolling roaded do make above stock figures.
With regards to manufactures claims of bhp , i am led to believe they give an average account of the power figure they produce ,very similarly to fuel consumption figures , and btw i can get no where near SEATs claim either , so this shows there are inaccuracies and its not a complete science.
Tolerances on engine components and build standards , materials and process will have a marked influence on what a cars final bhp figures will be , unless we know what tolerances are involved in all the components we can not even begin to understand what figures will actually be able to be achieved.
Concessions on parts happen all the time ,for instance if a turbo charger vains are cast or machined slightly wrong but give better airflow they may be concessed and be deemed within limits , but inreality might add 1% performance , which might not sound much but how will that 1% affect other components in the engine ? by the time the bhp is measured it could produce 5% more power , this is purely an example.
oh and btw i have been an engineer for 21 years but i certainly dont know everything about everything and i dont watch discovery lol
Here's what I know about it and here's a bit of resume as to where my info originates.
I've toured 3 OEM engine building plants, have friends and associates that work at OEM final assembly and R&D centers, employ several mechanical engineers, most with OEM experience, and watch Discovery to boot!
Mass produced, modularly manufactured engines are assembled with a range of tolerances and the public info is usually given as 1-3% depending on the OEM for variance in power output.
The tolerances that are most important to power delivery are weight of the internals and clearances of the moving and sealing parts.
I'm sure everyone's heard of "Wednesday motors" as being these factory freaks that make 10 to 20% more than advertised. This is true to an extent but the real maximum difference is more like 5%. This is gained by comparing an average engine build to a completely balanced and blueprinted engine. Balanced and blueprinted means every weight is perfectly matched and toleranced and every clearance of moving and sealing assemblies is perfect and is usually accomplished by hand. Statistically, it is possible for a mass produced engine to be perfectly assembled and it has certainly happened at some point. So, yes, you could get lucky but not anywhere near as much as people romanticize.
Regarding hand built engines, as long as the guy doing the assembly hits all of his assembly specs each one should be less than 1% every time. Our raceteam builds quite a few engine and heads by hand and of course is looking for every last ounce of power and full blueprinting and specific balancing is performed with each one and the power numbers on our dyno are amazingly consistent. I've watched 2 different engines in the same chassis dyno within 2 hp of each other over and over again.
Regarding the differences between OEM power ratings and what we see on the RR:
OEM's collect their dyno numbers at the engine on an engine dyno. The differences between a chassis dyno and engine dyno alone are enough to make it impossible to determine by a RR if an OEM has over or under rated an engine or not.
Most do underrate their results however, based on their own RR testing because the Marketing Dept's know that if they don't hit their adverted numbers by independent RR testing, there will be bad press. Most OEM's also throw a sample of each vehicle or every vehicle depending on model on a RR at the final assembly plant before release to the dealerships as part of their own quality control processes.
Hope this helps.
