Marina Blue

Most of you are probably aware of the never ending debate on LS2 power that pops up from time to time. Some interesting figures were presented from Chevy charts in the last one from which I made some comparisons. Initially my intent was to prove the LS2 had more than 400 hp, but instead I ended up with some interesting figures concerning the effect of weight on quarter-mile speeds. My calculations actually worked, at least for the C6 Corvette. I may be approaching this all wrong and if I am the mathematicians here can tell me so, but don't let this turn into another horsepower debate. If you read this, take your time to digest the figures. I was surprised it worked out like it did and might end up being useful for those wondering what effect weight reduction will have on quarter-mile runs.

The effect of horsepower and weight on quarter-mile trap speeds for C6 Corvettes

My observation is that there is a one mile-per-hour trap speed difference in the quarter-mile for every ten flywheel horsepower if all other factors are equal. Variables that affect this correlation are weather and track conditions, driver skill and overall weight of the vehicle, which includes driver weight and gasoline volume. Therefore, getting a good comparison between vehicles is difficult without using controlled conditions. This is exactly what GM does when testing the performance of their cars. Corporate drivers make performance runs with all conditions, including overall weight and weather, adjusted to a standard and then the averages of numerous runs are used to come up with advertised performance figures. Using Chevrolet’s performance data for Corvettes is the only way to make a fair comparison between different models of the car.

Listed below are horsepower and weight figures as well as quarter-mile performance numbers for the C6 Z06, 2005 to 2007 model years of the base C6 and the enhanced 2008 Corvette C6. All horsepower ratings are derived using the SAE Certified standard, however Certified ratings for the 2005 to 2007 cars were never published. The 411 hp rating used here comes from a combination of insider information and calculations using the best information available. Accuracy cannot be verified but it should be very close to what the rating would be under the newer standard as it is the only rating that correlates to Chevrolets performance figures after calculating the effects of horsepower and weight on quarter-mile trap speeds.


C6 Z06 – 505 hp & 3132 lbs. (11.7 sec. @ 125 mph)

2005 to 2007 C6 Z51 – 411 hp & 3179 lbs. (12.5 sec. @ 115)

2008 Z51 with NPP exhaust – 436 hp & 3217 lbs. (12.4 sec. @ 117 mph)

Comparison 1 – Z06 vs. ’08 Z51

Quarter-mile speeds are 125 & 117 mph per Chevrolet. With a 69 hp difference (505 – 436) the Z06 should be 6.9 mph faster, but is actually 8 mph faster as a result of a 85 lb. weight advantage.

Comparison 2 – Z06 vs. ’05 to ’07 Z51

Quarter-mile speeds are 125 & 115 mph per Chevrolet. With a 94 hp difference (505 – 411) the Z06 should be 9.4 mph faster but is 10 mph faster as a result of a 47 lb. weight advantage.

Comparison 3 – ’08 Z51 vs. ’05 to ’07 Z51

Quarter-mile speeds are 117 & 115 mph per Chevrolet. With a 25 hp difference (436 – 411) the ’08 should be 2.5 mph faster but is only 2 mph faster as a result of a 38 lb. weight disadvantage.

These comparisons show the effect of weight and horsepower on speed. From my calculations, every ten pounds of weight affects quarter-mile trap speed by .12955* miles-per-hour. However, a pure mathematical formula will never be completely accurate because of variances in real world factors and the probable use of rounded horsepower and speed figures by Chevrolet. However, I believe the results are close enough to show a fairly accurate correlation between horsepower and weight on trap speeds for C6 Corvettes.

*Calculated by subtracting expected mph from actual mph and then dividing by the number of 10 lb. units of weight difference between cars being compared. The resulting figures for each comparison are then added together and divided by 3 to get the average miles-per-hour effect on quarter-mile speed from weight in 10 pound units.

Calculations:
From Comparison 1 – 8 mph minus 6.9 mph = 1.1 mph/8.5 lb.=.1294117
From Comparison 2 – 10 mph minus 9.4 mph = .6 mph/4.7 lb. = .1276595
From Comparison 3 – 2.5 mph minus 2.0 mph = .5 mph/3.8 lb. = .1315789.

.1294117 + .1276595 + .1315789 = .3886501/3 = .12955 average mph change per 10 lbs. of weight

Checking accuracy of speed difference per 10 pounds of weight (.12955)

Comparison 1 – 8.5 (10 lb. units) x .12955 = 1.101175 mph difference due to weight
Calculated speed difference of 6.9 mph + 1.1 mph for weight advantage = 8.0 mph calculated speed difference.
Actual difference using Chevy’s numbers is 8 mph.

Comparison 2 – 4.7 (10 lb. units) x .12955 = .608885 mph difference due to weight
Calculated speed difference of 9.4 mph + .6 mph for weight advantage = 10 mph calculated speed difference.
Actual difference using Chevy’s numbers is 10 mph.

Comparison 3 – 3.8 (10 lb. units) x .12955 = .49229 mph difference due to weight
Calculated speed difference of 2.5 mph minus .5 mph for weight disadvantage = 2 mph calculated speed difference. Actual difference using Chevy’s numbers is 2 mph.

GMuffley

Nifty! Thanks.

427Z

too much for my attention span

VET4LES

Interesting information.

k0bun

Are you calling me fat?!

'06 Quicksilver Z06

Very interesting information.

First time I have seen anyone go to this extent to demonstrate how much weight can affect trap speeds.

Thanks for the info.

Another interesting point about your writeup is, that if you were not using 411 for the '07 Z51 and were instead using 395 for the '07s horsepower rating, the '08 would have a 41 horsepower advantage, and from what you say above, should be 4.1 MPH faster in trap speed.

So following you here:

Comparison 3, heavier '08 Z51 vs '07 Z51 – 3.8 (10 lb. units) x .12955 = .49229 mph difference due to weight
Calculated speed difference of 4.1 mph (436-395hp) minus .49 mph for weight disadvantage = 3.6 mph calculated speed difference........ However the actual difference using Chevy’s numbers is 2 mph.

Of course it is now known that the claim by some here of 395hp for the '05-'07 Corvette under the new SAE standard was

bunk22

I take it you're retired? That's a whole lot of effort for a thread

welcome2try

I weigh, 150lbs, I win... Great write up...

johnodrake

Nothing wrong with being retired Keep the shiny side up.

johnodrake

Dayum, someone gotta start feeding you

Evilways

I'd have to get paid if I did that much work.....nice write up though, interesting. Now it's time to go on a diet....

welcome2try

Damn eat 5 times a day, damn metabolism...

blue racer c6

I have ran the 1/4 with a passenger at aprox 205 lb and have ran the same et and mph as with out .

Marina Blue

Your right. My wife is after me to get a job as I write this. I think she is right.

Marina Blue

It may also be the first time we had enough information on three similar cars to make such a comparison. Of course it's still possible that some of the info is no good which would make all of this wasted time.

jschindler

First of all, let me applaud you on the work you put into this. I think there is some very good information to be gleaned from your work.

A couple of thoughts...Trap speed is not likely a flat mph per hp. The reason is that the faster you go, the more aerodynamics come into play. And the Z06 has to have far worse dyanmics and rolling resistance due to the wide body and wide tires. Interestingly, the Z06's trap speeds seem to be at odds with this theory as it's trap speed is actually faster than would be expected - not slower.

We do not know if GMs performance figures are "corrected" or not. In fact, we don't know a damn thing about how they are obtained. We assume they are a representative difference based on exhaustive testing, but we really don't know.

We are using "raw" horsepower peak figures, not taking into account how the peak numbers are achieved. I tend to think that the reason the Z06 is so much faster is because it carries it's horsepower higher in the rpm band - which in my opinion helps in all out racing situations where the car is run to redline in each gear. I admit this is a very fuzzy theory on my part, but I believe there could be something to it.

One of the reasons I believe this is because of my experience at the drag strip pitting a 400 hp C6 against a 405 hp C5 - Z06. I've watched the C6 stay pretty even with the Z06 until higher speeds where the Z06's hp at higher rpms seems to allow it to pull away towards the end of the 1/4 mile. I think this theory can be argued both ways, so feel free.

Marina Blue

Some good points, Jim. Power curves and how long you are in the peak of the curve certainly may have an effect, but all of that is too much for my very limited analysis skills. These were just rough calculations that most likely resulted in figures that are close enough to be used as a rough estimate of the effect of horsepower and weight on potential performance in the quarter-mile.

Your experience at the track is a valued source of information. I remember a post by TTRotary that said the same thing about his C5 Z06 pulling on his C6 Z51 in the higher rpm ranges but the C6 pulled on the Z in the lower end of the scale. He also mentioned that when he swapped wheels between his Z06 and C6 the Z could no longer pull, so there are many variables that can be taken into account. However, I question whether they affect trap speed. Someone more knowledgeable than me will have to answer that.

xs650

Marina Blue, there are several MPH and ET calculators on the web that you could use to compare with your numbers. They have the advantage of having been tweaked based on data from hundreds of cars instead of performance from three models.

Your results are quite close to what you get using equations and data based on hundreds of runs with some smart people mashing the data together.

A good web page to see the actual equations instead of a mysterious web page calculator is:

http://www.stealth316.com/2-calc-hp-et-mph.htm

A good example is:

MPH = 234 (hp/weight)1/3 and ET = 5.825 (weight/hp)1/3.


Those equations are by Patrick Hale (see the web above page for a bit on his background).

I’m just using ¼ mile speed like you did, since that’s a better indication of power than ET (traction and driver skill has more effect on ET than trap speed).

I used the MPH equation above and also added 200 lb to each cars weight for driver and some fluids.

It's enough of a PITA to format tables in this forum that I'm not going to do it, but if you use the above equation, it gives trap speeds less than 1 mph faster than the GM data. For all practical purposes, that's identical.

If you add 100 lb to each car, then divide the speed difference by 10 to get MPH/10 lb, the Z06 is 0.122 MPH/10 lb and the Z51s are 0.112.

All considered, your number for mph/10 lb was pretty close.

As you can see from the equation and from the difference in MPH/10lb for the Z51s and the Z06, the MPH/10 lb is dependent on the power to weight ratio you started with.

Marina Blue

Appreciate your information and good to know I was in the ballpark. Thanks.

xs650

You are absolutely correct, trap speed vs power to weight ratio isn't a linear relationship. It's a function of the cube root of the power to weight ratio.

The relationship holds remarkable close for a wide variety of cars and small differnaces in aerodynamics don't make much difference on cars with the performance of a Corvette. That's because their aerodynamics are reasonably close and they are still using far more power to accelerate at 115MPH than they are using to fight air resistance. The link I gave Marina Blue above has a boat load of data points from road tests. It's amazing how closely the data follows the calculated line considering how different the tested cars were and that they were driven under different conditions by different drivers.