Stephen Fisher

I have "up-graded" my 993 to very large big reds. I am quite sure that I have increased my lap times as a result of the added unsprung weight. Does anyone know the weight dfference between stock rotors and the large big reds? Has anyone put a 993 on the dyno to compare the difference?

Thanks for your help.

Stephen

Mark in Baltimore

Hmmm, lots of variables here. Do you have a lap timer, what was the difference in times, were conditions exactly the same from one event to the next and do you drive with enough consistency to preclude you from being an unknown to the testing? I'm not sure if the difference in weight/rotational mass is the reason your lap times went up...

Jean

I agree with Mark. Other than track conditions, the only thing I can think of is if you did not get the right temperature on the brakes. Large rotors will take more time to get to optimal temperature. A 20-25lb difference will not be noticeable on a track as far as lap times are concerned.

I hope you did not install the 380mm rotors (I think translates to 15") as they will definitely impact your lap times during a short race.

Stephen Fisher

I have a hot laps timer and I participate in a lot of track events (instructor level). Conditions from day to day are never the same. However, I lost three or four seconds on a 2 min and 17 sec track. I am confident that my cornering speed is the same (if not better) due to my shift points.

I believe that I am down on hp and have attributed this to the rotors (I admit that there must be other contributing circumstances).

I have done some research on what people are saying about unsprung weight. In general, most people put a five or six multiple on that to convert it to additional "sprung" weight. Therefore, 20 additional unsprung pounds could be the equivalent of 120 additional "sprung" pounds. Many people use the rule of thumb that eliminating 10 lbs is the same as gaining 1 hp. Using that rule of thumb, I lost 10 to 12 hp if the additional rotor/caliper weight is 5lb per corner. On the positive side, the car stops like nothing else on the track and I can brake later with confidence.

I thought that someone did a dyno test on this, but I could not find it.

Any comments would be appreciated.

Thanks.

Stephen

Greg Fishman

I don't buy that you lost that much time due to the larger brakes. If you lost 3-4 seconds something else is up. Maybe old tires, strong winds on track, cold temps, engine issues, etc. Maybe a dead body in the trunk

Steve Weiner-Rennsport Systems

Stephen:

I'd concur with Greg on this; unless you installed the 993 EVO endurance brakes (380mm/355mm), your acceleration is not negatively impacted in a way that you can reasonably quantitize.

You did not say precisely which brakes you installed, but the EVO 355mm/355mm combo is 3-4 lbs per wheel lighter than the Twin-Turbo 322mm/322mm brakes and works far better,...

FWIW, when I assess the need for more thermal dissipation capacity and increased braking torque, weight is not at the top of the priority list. Thankfully, the makers of such brakes (Brembo-Alcon-AP) provides relatively lightweight componentry so one can add braking performance without unsprung weight penalties.

Needless to say, these solutions are not always inexpensive.

Jean

Just to clarify, I meant you will never build up enough heat in your 380 mm brakes to have them running at the right temps in a 15-20 minute race, therefore your lap times will be impacted. They weigh a ton too! Even the GT3 CUP runs 355mm Fr in FIA GT.

Flying Finn

Stephen,

Just in case if you want to get rid your big reds, I volunteer to "help".

For excange if you need, I have stock brakes with almost new front rotors & Pagid Orange pads (2 track days & about 300 miles) and still very useable rears with PF97 pads.

paradisenb

Aren't the Finns compassionate? How kind of you to volunteer to "help"

Flying Finn

Always ready to help a fellow Rennlister.

TheOtherEric

Why would you think that a dyno run would be affected by unsprung weight? That makes no sense whatsoever. There might be a tiny tiny influence due to larger rotors and thus more rotating inertia, but the affect would be less than the accuracy or repeatability of the dyno itself.

chris walrod

This is an interesting point.

Was watching a show on Speed called Dream Car Garage. One of the segments was on a Acura Integra (IIRC) and their weekly progress of its race prep for, I think, SCCA IT class. Anyhow, they chassis dyno'd this car throughout all of the mods they did, and noticed a loss of ~7 RWHP after just a brake upgrade. They attributed this to additional rotational mass. I was a bit surprised as well.. Then again, maybe we shouldnt believe everything we see on TV?

Bill Verburg

Look else where, the difference in caliper weights is minimal as is the difference in rotor weight.

A front 993 caliper w/ pads weighs ~12.7# a BigRed similarly loaded is ~12.8#

A front 993 1 piece 304x32mm rotor weighs ~17# the 2 piece 322x32mm 993tt rotor w/ al. hat weighs ~18#

Some dynos do measure inertial effects but it take s ~10# of wheel speed rotating weight delta to be measureable

Martin S.

One wonders why these Big, Big Reds were added? Running on the track with my neighbor, he has stock brakes, I have Big Reds from a 993 TT car, and we both run Pagid pads. The only possible difference, I suspect my Big Reds dissapate heat a little better that stock calipers with the bigger pads....as a result it takes me a little longer to cook the brakes.

Don't ever under-estimate the cabability of the standard 993 brakes to stop your car...they work. I oinly have Big Reds because the P.O. installed them, what a guy!

PedroNole

I've read all of the posts and wonder the following... Although you say your corners are faster/same, I wonder how you're determining that. I know with Big Reds you can probably go deeper before braking and you may be slowing the car down more than with your prior brakes which means that you are entering/leaving the curve with a bit less torque. On a 2 min. 17 sec run, a bit less torque here and there can easily add up to 3-4 seconds. JMHO