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Location:
Gold Coast
Registered:
June 2004
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Power to wieght ratios , how relevant are they?
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Wed, 20 October 2004 10:47
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Ok so when I was a younger car enthusiest I thought the maximum power a car could make was the absolute mark of how fast it would be on the road. Now I realise how stupid I was (this was only 3yrs ago). Wieght is abviously just as important to a cars performance as the power.
I looked up specs on some of the most comman imports around and looked at there power and wieght and compared what i knew about each car in there relative performance.
http://www.carizma.com.au/Print/Details/ that is where all the specs were gotten from
here are the specs i got:
Engine Car Power Weight Torque Weight/Power
(Kw) (Kg) (Nm) (kg per Kw)
CA18-DETSilvia 131 1090 225 8.32
1JZ-GTE Soarer 208 1623 362 7.80
RB25-DETR33 Skyline 187 1530 296 8.18
SR20-DETSilvia 153 1090 274 7.12
2JZ-GTE Supra 208 1590 427 7.64
* power to wieght was gotten by dividing the weight by its power
so by these numbers an sr20 silvia should beat a supra, but that obviously isnt the case. How do these numbers apply to a cars performance in reality? I also realize factors such as transmission ratios, max torque and traction are major factors as well but how would these factor in?( I guess this would only be relevant in a straight line)
Any help or discussions would be appreciated 
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I supported Toymods
Location:
Epping, Sydney
Registered:
May 2002
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Re: Power to wieght ratios , how relevant are they?
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Wed, 20 October 2004 10:59
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supra will rock the silvia in traction, having bigger/better rubber standard, and more weight over the arse holding the wheels down.
plus uber torque (extra litre of displacement)
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Location:
Land of Oz
Registered:
June 2004
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Re: Power to wieght ratios , how relevant are they?
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Thu, 21 October 2004 00:21
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Hi,
You said the answer in your initial post. The transmission and diff ratios also make a huge impact. Finally, at top speed, the CD efficiency of the body may also come into effect by having less drag.
Also engine variations come into play. Some engines generate more HP at the hi end and less torque, some are the other way. The car designers must build a set of ratios that will allow that car to perfrom within a wide band. Some cars are faster off the mark, but lower top speed, and others are the other way around.
While pwr/wght ratios are indicative, there are many, many other variables that come into play.
seeyuzz
river
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I supported Toymods
Location:
Perth
Registered:
May 2002
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Re: Power to wieght ratios , how relevant are they?
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Thu, 21 October 2004 00:25
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Also, a Mk4 Supra weighs 1510kg, not 1590kg.
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Registered:
August 2002
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Re: Power to wieght ratios , how relevant are they?
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Thu, 21 October 2004 00:29
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and 208kw is the factory 'capped' figure.
its a fair bit more inflated in reality, thus throwing your equation out.
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Location:
mel
Registered:
March 2004
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Re: Power to wieght ratios , how relevant are they?
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Thu, 21 October 2004 00:53
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Remember the setting of the cars affects their performance alot and the setting you are getting is a balance point( or what they think the besting setting) of human comfort(such as ride quality,road noise etc...),performance (handling and power etc....) and reliability.
And the most important fact is, the power,torque and weight are also determined by the manufacturers in the design phase. Therefore, the performance is sometime affected by the convention and logic of the manufacturers , such as skyline need to be faster then silivia ,
supra need to be faster then celica etc.....
-
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Registered:
May 2002
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Re: Power to wieght ratios , how relevant are they?
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Thu, 21 October 2004 01:08
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Like has been said above, power to weight is just one of many factors to consider when looking at the performance of a vehicle.
A rough calculation on my own vehicle yields the following results:
Car weight (full tank of fuel) ~900kg (weigh bridge ticket)
Power (at the wheels) 170kw so we might assume approximately 200kw at the flywheel.
900kg/200kw = 4.5kg/kw (any values of 10kg/kw tend to indicate the car will go ok)
Now this little figure of 4.5kg/kw relates a little bit to the following characteristics that are present with my car:
*It's nice and quick off the line as it makes reasonable power and doesn't have much mass to move
*A good launch can be hard on some surfaces as traction (even with decent street rubber) can be difficult due to the low weight, especially over the rear end
*Acceleration from corners etc is great as it is not accelerating a great mass (not a great mass for a car anyway)
*at the drags when I race a larger capacity car that runs similar times if we both get off the line at about the same time mine will scream ahead till about half track then the other car will start making ground and we will cross the line almost together (got this on video a number of times).
Some performance figures are:
12.8 on street tyres over the quarter at WSID (2.2 second 60 foot for this particular run due to lots of wheel spin, so with a bit less of a retarded/lead footed start the times will drop a little ) it has previously run many 1.7 second 60 foot times (with less power so take offs were a tad easier).
It has been G-teched in the high 12's and doing 0-100 km/h in 4.3 seconds (2 people in the car, flat as a tack road. As a note a couple of GTR's and WRX's were G-teched as well as many other cars (mostly imports) and the nearest were nearly a full second slower 0 - 100 and the GTR's were closest over the 1/4 but were still 0.5 seconds slower ) **The G-tech times are indicative only but were a great comparison between almost 50 cars that ran (mine taking the quickest 0-100 and 1/4 miles times ). Also my car has very solid stiff suspension set up for cornering rather than drags so it does not squat off the line and therefore does not "trick" the G-tech into thinking it is seeing greater g-forces than the acceleration is actually producing (a common occurrence in many g-tech runs for softer suspension vehicles).
I think the best thing is to think about a car as a package and what it is to be used for. I wanted an "all-rounder" that will provide fun in many different events and situations and I think I have got pretty close to that now.
Cheers.
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Location:
Perth
Registered:
October 2003
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Re: Power to wieght ratios , how relevant are they?
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Thu, 21 October 2004 02:14
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Remember peak power is not the be all and end all. Whats more important is the total area under the power curve. The 2JZ supra would have alot more power throughout the rev range than the silvia. An indication of if a car has alot of power througout the rev range is usually a large RPM split between peak power and peak torque.
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Registered:
May 2002
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Re: Power to wieght ratios , how relevant are they?
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Thu, 21 October 2004 02:45
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Remember peak power is not the be all and end all. Whats more important is the total area under the power curve.
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Too true!!
The biggest single improvement in the performance of my car was made when we went from factory ECU (air flow meter) to after market ECU (a few other minor mods were done at the same time), top end gain was minimal (up about 15rwkw) but right smack bang in the middle of the power curve we had gained almost 45rwkw. Power was coming on earlier and right through the mid range there was a sizeable increase in power. The difference in the way the car drove was awesome, much more low down pull and more importantly much more fun.
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Location:
Melbourne
Registered:
November 2003
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Re: Power to wieght ratios , how relevant are they?
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Thu, 21 October 2004 02:51
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As has been said. Firstly, you must measure power at the wheels, not flywheel to get a more accurate result. The power to weight can be measured.
I hear heavy cars are more stable at high speed too.
I was gonna post more, but everyone else has prittymuch covered it.
The only way to truly see how quick a car is going to be is to drive it.
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Location:
Ballarat, Victoria
Registered:
March 2003
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Re: Power to wieght ratios , how relevant are they?
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Thu, 21 October 2004 02:54
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| BlackSupra wrote on Thu, 21 October 2004 10:29 |
and 208kw is the factory 'capped' figure.
its a fair bit more inflated in reality, thus throwing your equation out.
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they're like 240kw or something aren't they?
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Location:
Melbourne
Registered:
November 2003
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Location:
Campbelltown
Registered:
November 2003
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Re: Power to wieght ratios , how relevant are they?
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Thu, 21 October 2004 07:45
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would that equate to closer to like 300kw assuming the dyno reading is correct and that 30% is lost through drive train. When 300kw GTS first released motor got it dynoed and was only putting 209kw at wheels
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I supported Toymods
Location:
Perth
Registered:
May 2002
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Re: Power to wieght ratios , how relevant are they?
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Thu, 21 October 2004 07:47
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There isn't just a direct correlation between engine and rear-wheel figure. As it stands though, I highly doubt the Supra's Getrag box and Torsen diff sucks as much power as that god awful Tremec.
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Location:
Campbelltown
Registered:
November 2003
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Re: Power to wieght ratios , how relevant are they?
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Thu, 21 October 2004 07:49
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yeah I know but most mags quote around 30 % loss although most cars these days lose less ie recent test of gts coupe gave 224rwkw.
POwer to weight ratio I feel is one of the most important performance indicators, ie no point having 500kw if its in a 3 tonne car as it still wont be ripping fast
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I supported Toymods
Location:
Perth
Registered:
May 2002
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Re: Power to wieght ratios , how relevant are they?
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Thu, 21 October 2004 07:56
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What reputable mag quotes 30%?
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Location:
mel
Registered:
March 2004
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Re: Power to wieght ratios , how relevant are they?
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Fri, 22 October 2004 22:47
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| ke382TG wrote on Thu, 21 October 2004 11:08 |
I think the best thing is to think about a car as a package and what it is to be used for. I wanted an "all-rounder" that will provide fun in many different events and situations and I think I have got pretty close to that now.
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yes the package is really important, brakes are really important too. If you go rally or touring car racing the braking ability will affect your lap time more than the ability to accelerate, because if you can't barke you will keep missing out of the racing line or running out of the track. This rule apply to the daily driving too, if you can't brake you will hit other cars....
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Location:
Campbelltown
Registered:
November 2003
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Re: Power to wieght ratios , how relevant are they?
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Sat, 23 October 2004 00:09
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Motor street commodores wheels etc
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Location:
sunny coast, qld
Registered:
October 2002
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Re: Power to wieght ratios , how relevant are they?
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Sat, 23 October 2004 05:38
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From all figures I have seen for supras (or 1jz powered cars), it has been 17% loss for manual and 24% for auto.
maybe 30% if you whack in a 9" diff 
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