At what point does it not make sense?
The Z06 is like 7 to 8 lbs/HP, what is the magic ratio?

 

Please clarify your question...

Not make sense for what use?

magic ratio?


MD

 

You can never be too light or have too much power.

 

Couple things that have been made pretty evident to me in several years of Solo2 type time trialing.

HP/weight ONLY affects straight line performance. X amount of power moving an X amount of mass.

Weight (by itself) affects everything, acceleration, braking, corner grip (speed), mpg, etc.

3200 lb car with 400 hp, has a 8:1 hp/weight ratio. So does a 2800 lb car with 350 hp. Around a road course all else being equal, the 2800 lb car is gonna be quicker. Why? Same basic straight line performance, but better in the braking zones, more speed in the corners, more exit speed (and thus faster down the straights).

Higher hp DOES have the advantage of once you get into fairly fast speeds and aero starts to come into play (more than weight), more hp is gonna go faster. Still though, hp/weight is mostly an acceleration figure only, low weight affects virtually all parameters other than aero related stuff (top speed, etc)

The C5 Z06 for it's size/footprint, is pretty light, compared to other cars in it's class/size/hp range.

 

Couple things that have been made pretty evident to me in several years of Solo1 type road course time trialing.

HP/weight ONLY affects straight line performance. X amount of power moving an X amount of mass.

Weight (by itself) affects everything, acceleration, braking, corner grip (speed), mpg, etc.

3200 lb car with 400 hp, has a 8:1 hp/weight ratio. So does a 2800 lb car with 350 hp. Around a road course all else being equal, the 2800 lb car is gonna be quicker. Why? Same basic straight line performance, but better in the braking zones, more speed in the corners, more exit speed (and thus faster down the straights).

Higher hp DOES have the advantage of once you get into fairly fast speeds and aero starts to come into play (more than weight), more hp is gonna go faster. Still though, hp/weight is mostly an acceleration figure only, low weight affects virtually all parameters other than aero related stuff (top speed, etc)

The C5 Z06 for it's size/footprint, is pretty light, compared to other cars in it's class/size/hp range.

 

There is no "magic ratio" of power to weight. The more power you have, and the less weight you carry, the faster you will (generally speaking) go.

For a given platform it is usually easier to add big bunches of horsepower than it is to remove a lot of weight. While you could maybe remove 300 pounds from a car if you started stripping and getting agressive on the weight side, you would only improve acceleration by 10%. Adding 40 hp (similar 10% in acceleration improvemnt) is a lot easier, so most folks thinking to improve performance go the power route.

The downside of adding power is that you get to the point where the driven tires can't put any more power down. Our Z06's (even the C5's) are pretty much there in first gear. That is why, when the did the C6Z they increased the torque of the engine by essentially the same amount they increased the first gear ratio (they made first gear taller). The net effect is to put about the same amount of torque to the rear wheels, because that is about all you can put down. The very similar 60 ft times that C5Z's and C6Z's get shows how traction limited we are.

To get our cars to go any faster in first gear the only thing you can do is reduce weight on the front end of the car. If you do that, the car will accelerate faster and a higher percentage of the weight will shift to the rear wheels, and you will accelerate faster still...

In second and higher gears, you can add a lot more power (and put it down) than you could reasonably remove weight from the car, so it makes sense to add power if you are looking to improve acceleration at higher speed.

For cornering, yes weight is a big deal, but balance is important too, and to a large extent, you can make up for most of the weight by adding tire. If you look at a typical road course, most of the time on the track is spent on the straights. The SCCA regulates many of their classes by weight, not because of the effects on cornering, but because the added weight effects the acceleration all the way down the straight, and they can put the car on the scales and police the class pretty easily.

Don't get me wrong, as a road racer and autocrosser, I believe that getting all the weight out that you can is a good thing, and we set up our cars to get every pound that we can out of them since on the autocross track we are mostly traction limited and straights non-existant, and adding power will do nothing for us, but on longer tracks and in terms of pure acceleration, it is simply going to be more productive to add more power than it is going to be to attack the weight side of the equation.

As Don Gartlits used to say, "it gets hard to add in any more lightness"...

 

My weight to HP ratio is about 5.9 (3100lbs/525HP)

 

NASA ( National AUTO SPORTS Association) use WT/HP ratio for car classifications. ( road racing)

the lower the ratio the faster, in theory, the car will go.


The car can not drive itself, so dont for get to add the driver wt and fuel into your ratios.


Edited.

 

Good point ! I assumed MFG weight from the factory with no driver (04 Z06=3100lbs). I wonder how much weight my car lost with the heavy cast iron manifolds i replaced with the LG pro long tubes. One cast iron manifold weighed more than my entire LG pro header setup ! It certainly felt that way anyway when lifting them.

I think you made a typo... Its WT/HP...
If you ratio goes down using HP/WT ratio your getting slower in thoery

Toque

 

DOH Yes WT/HP ratios.

PPl tend to forget about looking at the dunlap effect. and I dont mean the Dunlap tires. The belly dun lap over the belt.

Cars can weigh the same, but if one driver is 280 lbs vs another driver at 180 lbs.

 

Very good analysis!
Thanks!

 

Always an insightful topic, and definitely something endearing about the Z06 Corvette (C5 or C6)...the focus on not only useable horsepower but also light weight.

 

I'm at 6.5 with 435 guessed flywheel hp, makes it tempting to really go with heads or forced induction to drop it into the 5's...