[ds968]

A while back I think i posted a question on whether the percentage of weight savings would provide a proportional percentage improvement in the car's performance / acceleration times. As I recall, there was a reasonably close relationship but not quite all
that simple ; lots of other dynamics at play influencing that outcome.

So how about HP/Torque ? Does an increase in those translate proportionately to a decrease in times ? For example, would a 10% increase in HP reduce the 0-60 time by 10% ? Seems like there would be a diminishing returns scale there, I doubt a 50% increase in HP would cut our car's 1/4 mile time in half ( he, that would be great, wouldn't it ?! ) or give it 50% more top end ( even a greater result .. oh man, if that was only so.. [img]style_emoticons/<#EMO_DIR#>/rolleyes.gif[/img] ) , but is there a formula that can be applied to the HP/Torque gain to determine just what exactly all that gives you, beyond the seat of the pants .. ?

Partial Post: Please Login or Register to read the full post.

[rhudeboye]

I have watched a lot of tuner shows, read a lot of the mags and followed certain projects over the web. A common thread in all of these is that as they change one thing to increase HP/torque they are met with a boat load of challenges to translate that new power to the wheels and decrease lap times.

There was a good episode of Top Gear where they were doing this in a Subaru (I think it was Subaru). With every change the lap times got worse???? They add a wing to give the FR more down force and the time lagged. They added lighter wheels, it added time. etc.

I don't see how there can be a simple equation. Cars are just to complex.

Partial Post: Please Login or Register to read the full post.

[flash]

i actually have a program to calculate this

bottom line is that it's not a 1:1 kind of relationship, but it is proportional - you can figure that a 10% increase in power will affect the power to weight ratio - then you compare the old ratio to the new ratio and have a third figure with which you can calculate the resultant performance numbers - similarly, a 10% reduction in weight will also affect that ratio in the same manner, and you can calculate a new tertiary ratio

the physics are constants, but there are factors that play into things, and what may seem obvious may not be

Partial Post: Please Login or Register to read the full post.

[rhudeboye]

I know of a few cases where the 0-60 suffered. The added power hurt the low end but had incredible effects up top.

My neighbors Supra is a prime example. Run his beast against a stock twin turbo and he'll loose a couple car lengths off the jump. He will catch up and over take but I'm thinking thats going to be pass 60MPH

A quicker 0 - 60 is best achieved by reducing weight. That's pretty straight forward as opposed to adding power but then having that power effect the way something spins or how something else is now over worked and what have you. Actually, when I asked Flash a while back what contributed to his quick 0-60 his #1 factor was weight loss.

Partial Post: Please Login or Register to read the full post.

[flash]

yup - there is software out there though to tell you what the effects will be based on changes made - it's pretty complicated stuff, but i've been having fun with the packages i've got - there are even more complicated ones out there too

it is important to note that when talking about power increases and such, acceleration related to 0-60 is not about peak horsepower numbers - it is really about torque and where it is in the power curve - a car with more torque down low will almost always beat the same car with high numbers up top, but less down low - that first second is the critical component

but yes, weight reduction will make the most noticeable difference

Partial Post: Please Login or Register to read the full post.

[wjk_glynn]

Lets say we lived in some airless idealized world.

If you recall your high school physics equations of motion, they are:



Then if you apply those to the 1/4 mile, you get some interesting results. To hit <b>100 mph</b> by the end of the 1/4 mile, you're accelerating at an average of roughly <b>0.25 G</b> over the length.

Now if you doubled your power to weight ratio (and ignored factors like aerodynamic drag, tire traction limitations, etc.) thus incresing your average acceleration to <b>0.5 G</b>, you'd find your trap speed would be roughly <b>140 mph</b>.

So increasing your power by 100% yields a 40% increase in trap speed. I attached a simple spreadsheet to show how it works (and if someone notices an error in the spreadsheet, please let me know).

Now of course in the real world there are a gazillion other things that affect time/velocity.

But I hope those equations (partially) show why the following have the 1/4 mile times they have:

996 C4S - low 13 sec range at 105-108 mph
996 TT - low 12 sec range at 112-116 mph (~7% faster trap speed then the 996 C4S, thought it has a ~25% bump in power to weight ratio)

Karl.

PS: I picked the 996 C4S and 996 TT as examples because they have similar drivelines, tires and diffs... but have a decent difference in power to weight rations.

Partial Post: Please Login or Register to read the full post.

[stewhatch]

Colin Chapman made a career out of beating higher HP machines with small, light / nimble racers. I've seen old Lotus 11's and Super 7's destroy high-powered cars.

Partial Post: Please Login or Register to read the full post.

[flash]

lol - yup - i used to do a lot of that in the MGB too - not all that much power, but the gearing and weight of the car, coupled with really big grippy tires, got me off the line quicker than a lot of cars with a lot more power

Partial Post: Please Login or Register to read the full post.