Power/Weight Ratio...Someone Check My Math

[ahurst]

I am a bit anal when researching for a car purchase.

I like to figure power-to-weight ratios using HP/weight.

The tweak this time for me is I am living in Colorado at 6,500 feet. Most drivers here buy turbocharged engines to compensate for engine power loss above sea level.

Turbo engines also lose power at high altitude but to a much lesser degree than NA engines.

The chart below lists the power-to-weight ratios for cars I am looking at -- some with NA engines and some turbocharged.

The Genesis 4.6 surprisingly finished highest.

HP @ 6,000 ft--Model--------Weight--HP/Weight
316------------Genesis 4.6---4012----0.0787
296------------Audi S4-------3924----0.0755
325------------Taurus SHO---4368----0.0744
269------------Infiniti G37----3615----0.0744
263------------335i----------3593----0.0731
263------------335xi---------3814----0.0688
316------------Ford Flex-----4839----0.0653
238------------Genesis 3.8---3748----0.0634

The formula I used for NA engines was 6,000ft * .03 * HP / 1000.
The formula for turbo engines was max boost at 6,000 ft / max boost at sea level. This ratio was applied to the sea level HP of the engine to come up with HP at altitude.

For example, even though the SHO is rated at 365 HP - at 6,000 feet it will only put out 325. The Genesis drops to 316. But because of the 300 pound weight difference, the Genesis has a better power-to-weight ratio.

Does this sound right?

 

[Sayantsi]

P/W is half the story, now look at mechanical advantage from the gearing, or just skip to 0-60 times adjusted for elevation?

 

[ahurst]

P/W is half the story, now look at mechanical advantage from the gearing, or just skip to 0-60 times adjusted for elevation?

I know that. I was more interested in uncovering any errors in the math.

 

[CanukV6]

The math looks good, but your formula should be stated as:
Horsepower @ 6000ft = HP - (6000/1000 * 0.03 * HP)

Also the conventional way of expressing Power/Weight is in LBS per HP.
It doesn't change the listing, but is a more easily understood performance value. The fastest car (ignoring gearing) will be the one with lowest number of lbs/HP since each pony has the least amount of weight to accelerate.

 

[scottdk]

Wow, great calculations That's pretty cool to learn about in high elevation. Power vs. weight ratio is important to know. Also good to know regarding engine perforamance is HP per Liter. Genesis for a NA engine is amazing HP per Liter @ 81.52. The 385 HP Genesis is 83.70

 

[ahurst]

The math looks good, but your formula should be stated as:
Horsepower @ 6000ft = HP - (6000/1000 * 0.03 * HP)

Also the conventional way of expressing Power/Weight is in LBS per HP.
It doesn't change the listing, but is a more easily understood performance value. The fastest car (ignoring gearing) will be the one with lowest number of lbs/HP since each pony has the least amount of weight to accelerate.

Thanks for the feedback. The Genesis is looking better and better to me as my next purchase...quite frankly, I assumed the SHO would have a higher ratio, but when factoring the turbo's loss at altitude and the SHO's heavy weight, the Genny wins.

 

[muisui]

well, do we normally use like 1HP can push how much weight in today's world?
Also to speak of NA engine, I believe Honda S2000 has the best ration per Liter engine displacement.
To speak of Fastest cars, # of gears and gear ratio will pay a big role in here. You can speak of the best power to weight ratio car but may not say on the "fastest".
Just my 0.02

 

[CanukV6]

well, do we normally use like 1HP can push how much weight in today's world?
Also to speak of NA engine, I believe Honda S2000 has the best ration per Liter engine displacement.
To speak of Fastest cars, # of gears and gear ratio will pay a big role in here. You can speak of the best power to weight ratio car but may not say on the "fastest".
Just my 0.02

Not to be picky, but I did state "ignoring gearing" in my statement.
Maybe I should have also said 'all other things being equal' (temperature, air pressure, coefficeint of drag, tires, colour of the upholstery.....) but I stand by my comment that the car with the lowest lbs/HP will be the fastest to accelerate - not neccessarily the fastest in terms of top speed.

Honda engines do have very high HP/litre figures, but that is no indicator of actual vehicle performance. A Honda S2000 engine in a Crown Victoria would still be a slow car - although maybe faster than most. Also last time I looked, the Honda S2000 was made in Japan not NA.

 

[dataguy]

Also last time I looked, the Honda S2000 was made in Japan not NA.

Pretty sure that NA in this context means "naturally-aspirated" -- i.e. not turbo or supercharged.

 

[CanukV6]

Pretty sure that NA in this context means "naturally-aspirated" -- i.e. not turbo or supercharged.

I guess my attempt at subtle humour was just a bit too subtle..... Oh Well

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[dataguy]

I guess my attempt at subtle humour was just a bit too subtle..... Oh Well

You had the smiley in the wrong place

 

[muisui]

just want to be more funny....
the "fastest" is still not a correct word in here...
You actually mean, a better power/weight ratio car will be pick up quicker than a less favorite power/ratio car. And may not necessary be the fastest car regardless...

Anyhow, you are 99% right and I am just make fun of you. Happy new year.

And thanks for data guy for the remarks on NA engine. ;-)

 

[CanukV6]

I can live with only being 1% wrong. Now if I could just convince my wife of that....
Happy New Year