Horsepower figures; How close to manufacturers claim?

kbannon wrote: I had my old 320i on the rolling road in Enfield a few years back. Apart from a K&N panel filter the car was pretty much standard. It did However, need a good service. Factory bhp is 129, I got 128!

Hmmm, sounds like a dodgy rolling-road to me!

Dave I've heard that some rolling roads are a bit dodgy but good ones will give accurate readings. The thing is the temperature and atmospheric pressure on the day of the test.

I know airplane manufacturers use a thing called the International Standard Day for quoting performance figures. Can't remember exactly but it's like 18 degrees C and 1013 pressure. So if they say their engine puts out 150HP at 2600rpm it will be exactly that in those conditions. Higher temp and lower pressure will reduce horse power but lower temp and higher pressure will increase horse power. Maybe it's the same for car manufacturers.

thepits wrote: flyingalexf68 wrote:  Higher temp and lower pressure will reduce horse power but lower temp and higher pressure will increase horse power. So what we need - if we want a graph showing wonderful BHP figures - is to find a rolling-road located in a very deep cave?

Yes. If you can't find one in a cave try go to one when the pressure is high (watch the weather forcast). Also this time of year is the best, ie cold.

I can help on the new M6 in this respect....

BMW quote 507bhp....yet a hell of lot of rolling roads are quoting stock output of 530-535bhp. 

Understatement.  Now thats cool.

On the other hand I know of someone who bought an E46 M3 Coupe and its 343bhp was actually 323bhp when they had it tested (they had suspicions something was wrong).

Then there is the Audi S4 owner with his 344bhp car...that was putting out 306bhp!!!!!

Most of the time they're accurate, but the definitely lose power with age unless you keep on top of them. 

My 6 came out at 339bhp as opposed to its 333bhp quoted - and I don't think I'll bother complaining about that - but they reckon its because I run it on V-Power (or formerly Optimax) - so it proves it works.

flyingalexf68 wrote: Dave I've heard that some rolling roads are a bit dodgy but good ones will give accurate readings. The thing is the temperature and atmospheric pressure on the day of the test. I know airplane manufacturers use a thing called the International Standard Day for quoting performance figures. Can't remember exactly but it's like 18 degrees C and 1013 pressure. So if they say their engine puts out 150HP at 2600rpm it will be exactly that in those conditions. Higher temp and lower pressure will reduce horse power but lower temp and higher pressure will increase horse power. Maybe it's the same for car manufacturers.

Any decent rolling road should correct the figures for pressure and temperature. I have a link somewhere that explains...

Edit: here you go:

http://www.dynamometer.fsnet.co.uk/automotive-dyno-correctio n-factors.htm

276 bhp for mine (standard apart from a k&n panel and always ran on optimax/v-power) on Falkland Performance dyno, Alpina says 280 bhp

My old e46 323ci got 191 bhp, book figure was 170, something wrong methinks

funnily enough that's what the man said at the time (same rolling road)

thepits wrote: kbannon wrote: I had my old 320i on the rolling road in Enfield a few years back. Apart from a K&N panel filter the car was pretty much standard. It did However, need a good service. Factory bhp is 129, I got 128! Hmmm, sounds like a dodgy rolling-road to me!

Westward Engineering, Its probably the most accurate rolling road in Ireland and the UK.