How much difference does 1 passenger and a tank of fuel make?

[len_beach]

To cut a long story short. A mates girlfriend has a standard '03 172 FF which required a new exhaust. We picked it up from Powerflow with me in my 172 FF. On the way back home we had a lttle jaunt up a motorway slip road and down to the next junction. The performance of the cars was absolutely identical, I could not have tried to be more evenly matched.

Bearing in mind his car is standard barring the new cat back Powerflow, my car is approx 15kg lighter than standard and has a sports cat, cat back exhaust, piperX panel with acoustic valve removed with feed and remap. Last RR was 180bhp before the cat so optimistically 183bhp.

Phil was on his own and I had 1 passenger and 3/4 tank of fuel. Should my car have been quicker or is the extra weight really that noticeable?

 

[DebV6]

I can always tell the difference in speed when I have a passenger but I don't know how much difference it actually makes.

 

[miltage2]

i would be interested to know!

 

[len_beach]

For reference, I weigh 85kg, my passenger 72kg, fuel? and I don't know how much fuel Phil had. Phil weighs approx 75kg.

 

[Phill1978]

when two cars are really close in power it does come down to the weight more often than not. i remember crawling past an old rover turbo (long stretch ton+) the only reason i passed was that i had 1/4 tank and was on my own and he was a fatty by all accounts the power of the rover was around 180bhp so 10 more and probably a tiny bit more torque must have been his weigh and my lack of weight and 1/4 tank that edged it

 

[len_beach]

So the 50-75kg deficit would have been more than noticeable?

 

[domtheone]

I can always tell the difference in speed when I have a passenger but I don't know how much difference it actually makes.

Ditto that.

Readily noticable.

Same with a full tank.

Exactly what difference it would make on the 0-60 and zero to hero runs i wouldn't know for sure. Couple of 10'ths maybe.

 

[BrianR]

Look how much difference the ballast makes on the touring cars, and thats just a few kilos.
If your mates are Rick Waller and Johnny Vegas, you would never get away from the lights.
(not that anyone would own up to having those numpties as mates)

 

[azzer2000]

not much differance.

 

[len_beach]

If your mates are Rick Waller and Johnny Vegas, you would never get away from the lights.
(not that anyone would own up to having those numpties as mates)

LOL, thankfully we're all quite trim, i'm the heaviest of the lot unfortunately.
75kg would equate to just short of an extra 7% added to the weight of the car so I suppose it is quite alot.

 

[domtheone]

75kg would equate to just short of an extra 7% added to the weight of the car so I suppose it is quite alot.

Aye, was thinking along those lines.

Xtra person and a heap load more fuel could add near on 100 odd kilo's which is pushing 10% extra.

 

[Ads_172]

What does a standard FF 172 weigh?

 

[keben6]

gotta be around 1150-1200kg's

 

[len_beach]

1110 IIRC as the Cup is 1020.

 

[Ads_172]

So...

1110+75=1185
170bhp / 1.185 = 143.46bhp/tonne for the mate.

1110-15+85+72=1252
180bhp / 1.252 = 143.8bhp/tonne for you.

Ignoring petrol, it's a gnat's testical difference.

 

[len_beach]

Good way to show it Ad. So the old gal didn't do too badly then. I thought there would have been a slightly more noticeable difference.

 

[Ads_172]

Hardly scientific but it's interesting to know how something as small as having a passenger / spare wheel / full tank of fuel / a big sandwich can make a difference.

 

[keben6]

I once had a scotch egg in mine and really felt mine struggle...

 

[keben6]

Joking aside I can feel a big difference with someone else in my car.

 

[lee_cup]

damn! i weigh 95kg, i need to lose some weight

 

[keben6]

same here, I might be nearer 97/98...

 

[keben6]

ron in my car that is with V Power and all that. There's no way you could have imagined that was my weight!?

 

[GordonD]

As I've advised in another thread basic physics dictates certain relationships between various performance results and power or power to weight. The real world is more complicated than a physics equation, but you can estimate roughly how much extra power you've got from what performance figures you're getting, and vice versa.

For example top speed is related to power by a third power (n^3) relationship. It takes eight times as much power (and appropriate gearing changes) to go twice as fast in a given car.

Standing quarter mile (or 400 metre) time is related to power to weight by a fourth power (n^4) relationship, albeit with complications related to being able to get traction so front wheel drive cars do worse than they should and four wheel drives do better. So to halve your quarter mile time you need 16 times the power to weight ratio. On a more realistic scale a 4% improvement in power to weight produces about a 1% improvement in quarter mile time. So take two identical Clio Sports with identical weight drivers, have one somehow lose 45 kgs (100 lb, 7 stone), the weight of a full tank of fuel, and it covers the quarter mile time in 0.15 seconds less and crosses the line about a car length and a half ahead of the other one. And going in the other direction the average weight female in the passenger seat would add 0.2-0.25 seconds to the quarter mile time, and the average male 0.3 seconds.

Off the top of my head I think the 0-60 time is related to power to weight (with the same caveats about traction) by a second power (n^2) relationship. So that same 4% reduction in weight would cut the 0-60 time by 2%. For a Clio Sport that also happens to be about 0.15 seconds.

So the 0-60 time for a damn quick car can be 4 seconds, a third the time a pretty mediocre car can do (about 12 seconds), but there's less variation in top speeds, and even less in quarter mile times with a damn good car doing 12s and mediocre one doing 18s. 0-60 times are the most sensitive indicator of power and weight.

 

[Munson]

I live my life a 1/4 of a tank a time

Never put more than an Ayrton in if I'm just going to be rolling around local routes.

 

[bmh.01]

As petrols specific density at 60F is 0.7kg/cu.m a full tank (50l) weighs 35kg but obviously it changes with the temp, increases as it gets colder and decreases as it gets hotter. I can feel the extra weight of a full tank and passengers make a bigger difference again, not much but I can feel it.