It's a bad idea because power/weight is not directly proportional to a cars lap time.

 

Doesn't that formula already work well for NASA's GTS series.

http://www.gtschallenge.com/rules.htm

I don't race with them so I don't know if they've got one car dominating over another. Anyone out there run with them?

 

It does work fairly well, but you've still got the issue of a car that's able to run a significatly higher top speed than another which messes up the whole equation.

 

I think the basic premise of power/weight is a decent idea, but that the other factors need to come into play as well. I would start with weight / rwhp to get a base number (need dyno chart to prove rwhp), but then modify that with a series of simple "adds" that help/hurt performance. Here's a couple of examples (Illustrative only...not real numbers!):

Trans type
4 speed: +0.75
5 speed: no change
6 speed: -1.0

Gearing
Stock trans, gears & R&P for this vehicle: no change
Non stock trans, or gears orR&P for this vehicle: -2.0

Absolute Weight
For every 100lbs over 2800lbs, add 0.25
For every 100lbs under 2800lbs, subtract 0.25

Front Suspension
Strust with torsion bars: +0.5
Struts w/coil-overs: no change
Multi-link: -0.5

Rear suspension
Swing Axle: +1.0
Semi trailing arm w/torsion bar: +0.5
Semi trailing arm w/coil-over: no change
Multilink: -0.5

Using an example, a E class C2 might be:

2900lbs / 235rwhp (they all make way more than 247hp) = 12.55
5 speed = +0.0
100lbs over 2800 = +0.25
Struts w/coil-overs: no change
Semi trailing arm w/coil-over: no change
TOTAL RATIO = 12.55 + 0.25 = 12.80

CLASSES
A 5.99 and under
B 6.50 to 6.00
C 7.5 to 6.49
etc


Classes would be decided based on this number. There would need to be more than I have just listed, but I would bet we could get this down to ~15 or so major things that impact performance. The other things I could think of would include size of rear wing, aero efficency/drag coefficient,
wheel/tire size, and tire type (DOTs vs. slicks).

I would think this would take the basic NASA GTS formula, but solve the few inadequacies. The other advantage is that you could really do any mod you wanted, without imediately going to a big $ GT class. So I could put motec on my turbo, which would increase power to maybe 425, but I would not be forced into GT1. Sure I might get bumped up a few classes, but it would not be that bad. It would also solve the issue of the S/R classes in GT, which dilutes the field.

 

Ive been racing PRC, PCA, SCCA, POC, and the NASA Chariot series which then became BMW/porsche challenge over a 10 year period.

what i have found , is exactly what i have noted . Cars that are the same Hp to weight ratio, perfom close to the same in a straight line, (equal drivers) If weights are compensated for in the ratio, the racing can be dead even because of the lighter car handling and braking advantages. Regulary, i spend time racing a euro M3 BMW in SCCA with less hp but lighter weight. our times are within tenths of each other, year after year. Im at 320rwhp, he is at 285rwhp. He is at 2450lbs and im at 2750lbs. Other cars with this kind of proportions run the same way. Again, look at grand am cup GS. same drivers month after month, proven tallent, yet their cars are always within a second or so for the top 10 times, and made up of over 4-5 totally different types and manufacturers of cars. (mustang, M3, porsche, 350z,etc)

GTS strictly looks at HP to weight, which is a great start, and far better than what PCA brings to the table. as good as their intentions are, the factory specs are usually way off, weight is never considered and is a HUGE factor. The dyno is a valuable tool and is readily available for most all racers.

I hope something is done to correct a set of rules that worked well when PCA got into racing a short time ago, and now that it has become so big and most club teams are actually faster and better prepared than the pro's 10 years ago, the time as come to level the playing field to bring the Racing back to the fate of the driver, prep, consistancy and dependability.

MK

 

good idea (formula)

I think there has to be some coorelation for the effects of weight vs the hp to weight ratio. for example. generally, 50hp can bring 2 seconds a lap, where usually 300lbs is required to help this much on a 2900llb car.
(ball park example)

a 6 speed porsche transmission can be a 8% advantae over a 5 speed with wider spacing based on the area under the hp curved used over the operational range. this should coorelate with the HP to weight ratio formula.

changing final drive is a rounding error for most all tracks. i would have no problem keeping that part open with no penalty

good ideas regarding the absolute weight.

PCA should take a serious look at this an revamp not only the race classes but the base car classes

MK

 

Only completely accurate in a perfect, frictionless, world. As the vehicle accelerates more and more power is required to overcome friction and maintain velocity. So the higher power vehicle eventually, with more excess power available, has the advantage.

 

i like the HP vs weight ratio as laid out by Colin. I also feel a portable dyno, ala NASA would go a long way even without any change in the regs and would be an eye opener for many...
Finally the premise as it stands with allowing headers, open exhaust and airbox dilutes the current HP / Weight ratio as posted in the rules with some cars clearly benefiting more than others, so IMHO these should be banned or in the premise as laid out by Colin carry a point percentage of some short.

 

I should have been more clear. Net power (or net force at the rear wheels) Acceleration would be proportionate to net power and net force

remember we are talking about two identical cars here, same weight and at any speed. in comparing different cars, the hp to weight ratio is keeped constant, but yes, if the car has differerent areo and differnt friction, the negative forces would go up at different rates.

for our discussions, if two 911s have the same power to weight ratio, they will accelerate the same at all speeds. rolling friction would be the only other variable, but probaly equal to a rounding error in a calculation.

your right though, if the drag is different of two vehicles, you can calculate its effects, so as a secondary consideration, HP to drag ratio, could be looked at as a factor. a huge wing may produce 300lbs of drag at 100mph. that would be less than 30ft-lbs at the wheels (assuming a 12" radius tire) through a 5:1 gear box, at 6000rpm, at 100mph, that would be near 6ft-lbs of engine torque, or near 7hp. at 150mph, the same numbers run out to 675lbs of downforce or 67lbs of drag, 67ft-lbs acting on the tire, so 20ftlbs in a gear with 3.2:1 (ie 4th) and 6000rpm. so the hp cost there would be 22hp.

so, if someone chooses a huge wing over none at all, he sacrafices 7-22hp from 100mph to 150mph, but gains ???? in cornering ability. those are the trade offs you want to have the racers play with.

If someone has a real dirty car (ie drag), then that should be taken in consideration. otherwise, its really a non issue car to car. drag (c/d) are realatively close with all models of porsches.

Quick correction: yes, you are right, as far as aerodynamic drag, the higher hp car will have a better power/drag ratio and will assist it in the higher speed ranges. so yes, two cars, same model, different hp to weight, the higher hp car , even though the same hp to weight, will have a net power advantage at the higher speeds. ( lessened only by the increase of rolling friction that it has over the lighter car )

Another point about two cars with equal hp to weight, is that if the car is larger with higher hp, then its frontal area could be more. since total drag is the C/D x the total area, its quite concieveble that the larger car like a 928 could have much more drag than a 911 even if they started out with the same C/D. that, could off set the difference between the two cars as far as HP to drag ratios.


Mk

 

This topic has come up quite a bit lately. I'd say those of us with older cars or in lower groups don't really complain as much about the classes, as ours don't change much, meaning no new models in the lower classes. Maybe the performance differentials were bigger 20 years ago than they are now, so the classes are more obvious.

It would be interesting to compare the classes by track records, now that we are collecting them. It would be best to do this for each model, not just class. That way we can see if the differences are really the cars or the drivers' abilities. After a year or two of data, you would have real evidence of performance advantages across many tracks and driver's ability.


Anyone that doesn't buy the car that wins a class is either racing for other reasons, or is not doing everything they can do to win, either way, stop whining.

 

I don't think open is the right word. I think that like grand am, each specific model and year should have certain allowances to even the classes. For example...

3.4l 996's can run a chip, LWFW, cams, alum intake, etc.
3.6l 996s can run a chip and LWFW
3.6l X51s must run stock

all 996's can run GT3 brakes to even with the 993RSCSs

all cars runs at the same base weight and classes should be close, where factory weights are withing 200-300 lbs, not 700 or 800 lbs difference as they are now.

 

The difference comes at the end of the straight when the lighter car stops quicker and pulls more g's on the same tires

 

Yeah, just power to weight doesn't give a good comparison. For example, if I were at the NASA Nationals this week, I would be competing in GTS5 against a GT3RS. Now, we fall pretty close to the same power/weight ratio. But in reality, the performance of the 2 cars is nowhere near the same. As 38D and I have discussed before, I believe gear ratios are a very important part of the performance equation. My stock 930 4speed compared to a pretty well dialed in GT3RS 6 speed are quite a bit different.

So the power to weight thing alone really doesn't always classify cars well into comparable classes.

I think 38D is on the right track with his point system based on weight initially then factoring in tires/#of gears, etc.

Brian

 

I like 38D's proposal as well.

 

Not only does hp/weight makes, many other sanctioning bodies handle it that way and certainly makes the racing with a variety of cars an interest race. A dyno is a requirement at the beginning of the season. Check out the NASA GTS Challenge rules. Not say f course that PCA CR should follow NASA...