Bryan Watts

It's already been stated that the rest of the setup will be the same...the decision is whether to build a more powerful motor and add weight or to build a less powerful motor and lose weight.

Jean

Bryan, I saw that, my post is saying build a more powerful motor and add some weight, provided it goes towards improving braking and wider wheels/tires. Maybe I am unclear, same power to weight ratio as the lighter car, but with the right components that will offset the superior handling benefit of a lighter setup.

Higher HP due to higher rev limiter has more hidden benefits, like the ones I mentioned. Little to do with power under the curve or torque as such.

Flat Top

WEIGHT DIFFERENCE -

There is 200 kg (450 lb) difference between the two cars. The 180 hp car, IF we added 200 kg to this car would run slower at 1/10 of a second slower for every 10 kg that is added to it. (This 1/10 th per 10 kg is a rule of thumb that I have found generally works on TRACKS THAT WE RACE ON.) This figure may change from track to track and is best gauged by running with and without a passenger in the car and than making the analysis. Assuming I am correct with the 1/10 sec per 10 kg is correct the 2200 (at 180 hp) car would be slower by 2 second a lap if you added 200 kg.

By the same token the heavier car would be 2 seconds quicker if it discarded the 200 kg. This, in itself, does not give us an indication of which car would be quicker because of the power difference.
To get a closer feel for what will happen we have to factor in the horsepower advantage.

POWER DIFFERENCE -

Thie heavier car has a 40 hp advantage over the lighter car. Conventional wisdom (mine) says that for every 100 kg a car is heavier it would require 40 hp to run the same lap times. In this example the heavier car is 200 kg heavier but only has an 40 hp advantage. Hence the cars would be on equal footing, if we accept the 40 hp per 100kg rule, only if the 220 hp car increases it power by a further 40 hp to 260 hp or to reduce its weight by 100 kg (220 lb) to 2430 kg. This will equalise performance between the two cars.

CONCLUSION

In motorsport there are many unknowns and many accepted "facts" that may turn out to be false. In the same light my assesement is based on MY experience and may be totally inaccurate. Nevertheless l thought it worthwile to post my thoughts just to give an alternate perspective on this issue. The only way to accurately asses, would be if the two cars were actually setting lap times on a SPECIFIC track. (Results may vary from track to track, as has been pointed our earlier).

My assesment (thumbsuck) would be that the lighter car on a general road course with slow and fast corners mixed into the pot and with laptimes around 1:30 seconds would be 1 second per lap quiker that the heavier car. Someone out there, before you totally disregard my input, please test this and report back.

M758

Well,
Flat Top's analysis shows how the GTS rules come in place. On paper both cars could be just as fast. However in practice it is anyone's guess. Power/weight is only one factor. With respect to "all else being equal" that is not quite right since you would want a different set-up on heavier car to deal with the weight. Then you must figure in the costs. In general a lighter car is cheaper to run assuming is is not so light that it is fragile. (not factor for most production cars over 2000lbs).

So which is better? Ha... make one of each and bring both to the track. Pick and choose based on which track you run and who the competition is.

Bill935K3

All the way baby!!
Seriously though if they made a rule that said add 200lbs or give up 100HP. I lean to giving up the HP

Larry Herman

Spoken from the man who would miss losing 100 hp like most of us would miss losing a dollar.

Geoffrey

When I built my car, I went with the "less is more" strategy which is much different from the strategy I used with my turbo car. My car has less weight (2225 wet), less aero (stock RSR), less width (stock RSR), smaller width tires, and is not significantly lowered (soft springs and proper roll centers given the compromizes of a GT car), and a similar powered engine to a factory RSR. After spending a year with this car, I can say that I favor less weight over more power. At WGI and Mosport this car matches a later GT3 Cup in speed (mine is slightly faster), but will brake 100-200 feet later. The thing that I like about it is that the consumables are considerably less. I have yet to change brake pads with 2200km and the used rotors I put on the front of the car are not yet cracked. At LRP, I have turned a 56.5 which will match a similar driver in a GT3 Cup and with a good driver should be able to be 1.5 seconds faster than my best time, or similar to Dino Loles in a GT3 Cup. Different strategies, but I definately have proved to myself that "less is more" in this particular case with this particular car which was purposely built.

Bill935K3

The discussion of frontal area - Vmax - bhp vs drag etc is so true. Remember Daytona a couple of years ago when they were allowing GT3R's and teams were adapting and running Cups to the engine rules because of the better top end and overall times due to less drag.
Very noticeable to me RE GTA and 06-07 cups Vs my hi drag/downforce car. Catch them coming off a corner at the bottom of 3rd or 4th huge advantage but at on high speed straits 145+ very little closeure rate

Jean

Bill

That's a nice engine

Just for fun, I attempted to calculate your HP loss as a result of your aerodynamic drag at 145mph..
Assuming you have the same CD= 0.397 as the 962 (instead of a 935 K3), due to the elongated body work, and an frontal area of 24.64 Sq. ft. , your aerodynamic drag at 145mph represents a loss of about 200HP.

At the same speed, a 997GT3 would loose the equivalent of about 130 HP. That is 70HP less losses than your car.

kurt M

True, If I gave up 100 HP I would have none and still OWE 10 hp.

One thing about HP it makes for easer passes and you can still get a little wide to protect the corners where you lose advantage.

Bill935K3

Hi Jean,
Thanks for the motor comment. We are working hard to solve some cooling issues with it.

I would doubt CD of 962 would be very close, they are a lot lower and of course used ground effects for much better down force with a lower drag price. Do you have any figures for 935’s??

I would think we are significantly higher CD than the original K3. My wing is up much higher in the air flow and has two elements. Also we are running dive plains on the nose. We are playing with some areo changes now with the hope of producing more efficient down force. Boy would it be fun to be able to play with a air tunnel!! (it’s nice to dream) Unfortunately have to improve the charge air temps so we have to pay the air flow price to really ram the intercooler (I’ll try to find a pic of that project)

We are really looking forward to getting up to Watkins Glen so stretch her legs and see if the changes we made do what we hope they will. Here are some good shots of how we raced last year

A.Wayne

Bill,
that's a pretty small core for a twin turbo application , seeing that you have the room , could you have not gone L/R for the intercooler setup, using 2 individual setups. If memory serves me right you have a lot of room with your current body work , I would have gone with a much bigger and thicker core, than the one currently onthe car.

Also the turbo's can saturate the intercooler fast if their are running out of there efficiency range are you running KKK 's?

they will saturate a small core like that fast ...

Jean

Bill

Your car is a true monster. The 935 configuration had different aerodynamic drag coefficients depending on the race it was involved with, the reason I chose a CD of 0.4 is because it was middle ground and also your car’s extended and steep raked rear will improve drag conditions..


The frontal area of the 935 was 22.6 Sq. ft however were limited by regulations to keep a lower rear aerofoil, in your case I bumped the area to 24.6, about a 10% more due to the higher rear wing. I don't have K3 data other than all the measurements, engine, performance etc..

Anyway FYI, here are the different drag coefficients and how they would compare in terms of HP loss at 145mph:

935/76 : Cd 0.435 – HP Loss 217HP
935/77: Cd 0.393 (Le Mans configuration)- HP Loss 197 HP
935/78: Cd 0.358- HP Loss 179 HP

I would suggest you contact Richard Chamberlain from the UK, who is probably the most experienced 935 Club -racer in the world, and has done extensive research on the topic with top notch aerodynamicists, as well as engine thermodynamics. He is on Rennlist under “935Racer”. His also has a shop called Chimp Tune Racing.
Here is a shot of his car in action.



Your IC looks like the Bob Holcomb setup, and same core size as mine. I might agree with Andy, you could go larger and thicker with your bodywork and air intakes, as long as the turbos are properly sized for it and you don't loose pressure in the core. I am guessing you have been through all this before

Alternatively a thinner and much more efficient Secan. If I recall correctly, Richard uses twin fender mounted ICs with openings in the fenders flowing directly through them, and some other tricks.

Sorry this has gone OT.

Bill935K3

Jean - A Wayne
We will soon find out if the intercooler is the right size we have been feeding it with nacca's off the bottom of the rear window. What really sucks is my engineer's laptop was stolen over the winter and with it went all of last years data. We know have dropped the rear of the rear windox aprox 2.5" the width of the intercooler and mounted the leading edge of an old wing at the top edge. (pic tonight). Our cuttent settup is two oil coolers V-mounted in the nose, oil return from the Garrett turbos goes throught it's own cooler in LR fender, and transmission oil cooler G50/50 is in the RR fender. My competitors will soon find out anyway- we just flat bottomed the car; the splitter front tray used to end at the fuel cell. We have know extended to the rear sway bar. We are hoping this will gain us some even down force so we cut wing-winglet drag. At the very least loose the top eliment on the R-wing and run at an angle with gurny. We have 5 20 min sessions to test before WG race weekend so I'm going to have to be on my toes to get the car sorted. It is a lot of fun making seat of the pants improvements over the winter and then make them work, so far NOCK ON WOOD we have made her quicker each year. here is the old way we fed the intercooler and the frt oil cooler setup

A.Wayne

Hello Bill,

Pic of a top mounted fan setup , provides excellent cooling for the intercooler and associate used to make this conversion ,not sure if it is still available , i can find out if you are interested ...