Starting to fab up some brake ducts for my new Pettit BBK kit. I am going to basically copy GooRoo as in this thread https://www.rx7club.com/showthread.p...ght=brake+duct

It seems to be pretty standard that everything is 3" but from the looks of everything, I think that 2-1/2" would fit better in every aspect. (Backing plate, inside fender well and under car)

My only question is, how much less flow will there be? My inclination is that there wouldn't be that noticeable of a difference. I plan on making the backing plate a very tight fit like GooRoo so all the air is going to be directed into the rotor.

Any thoughts?

 

a 3" diameter opening has ~45% more area than a 2.5" opening.

 

I estimated the volume of 12" lengths and the 3" has 30% more volume, but that doesn't tell me if there will really be that much different in cooling. The more I think about it, I think I am just going to go with 2-1/2" just cause it will fit better in all aspects. I might not get as much cooling but then again I am only running 20-30min sessions.

 

3 in has 44% more area (this equals to 44% more volume (which has no relevance) for same length tube) and thus 44% more air flow.

or the other way to look at it a 2.5 tube has only 70% of air flow of a 3 inch tube

 

I calculated it. 44% (or ~45% as I said above).

Are you serious? More airflow, less restriction. Or do you just not understand the problem?

If you are limited by what will fit then just live with it. Be sure to pick a hose color that will impress everyone as well.

You think your brakes won't overheat in a 20 minute session? okaaaaaaaay.....

 

Awesome! Thanks for the kind help!!!

You are right 2-1/2" will not be big enough, I think I am going to run 4" carbon fiber brake ducts to go with my bling BBK kit! This will make me mad fast on the the track!

On a serious note, are you running brake ducts? Do you track your car? If you are running brake ducts, how much did they help with your brake life, fade ect? What kind of events do you do?

P.S. Great use of icons, they really add a lot to the knowledge you have contributed to post!

 

Thanks for the info. Now that we have established there will only be 70% of the flow of a 3" tube, do you think this will have a quantitative effect on the track? I see you run Nasa TT, do you run brake ducts?

 

Do you need brake ducts - absolutely. How big they have to be is an unknown until you get on track and run the car.

The EP car below has 2.5" hoses connected to a backing plate cools the rotor and hub assy. The brakes get hot but the driver doesn't complain about fade issues. This car is braking at 135 to 140 (track dependent) and weighs around 2325. A typical session - race - is 45 minutes for this car.

Beyond cooling ducts, brake temp affects pad material and brake fluid. Are you running high temp fluid and racing brake pads? The EP car has Hawk HT10 pads and Motul fluid.

 

So far I have run stock brakes with hawk blues and Motul or now super blue. With stock brakes I get some fad but it really isn't terrible. I am also not being as aggressive with the brakes as I would like to be. The big thing with the stock brakes is I am going through rotors.

As I mentioned, I picked up a Pettit BBK with plans to run Hawk blues or HT70's. I would like to make them as efficient as possible and actually like fabbing things so wanted to fab up the brake ducts. I am leaning towards the 2-1/2 just cause fitment will be better and even when doing 3", they have to reduce down. Check out GooRoo's thread.

Tanks for the info.

 

Go 3"

 

yes and yes

 

Look at the Hawk DTC pads. Blues are old tech.

 

My thought, place of greatest restriction will determine the efficiency of the duct....in my case, nose cone inlet worked out at less than the 3" area of the hose....but as there were several metres of 3" silicone about, still went with that. Most likely place of drama appears to be where the hose has to drop down below the rail exiting the inlet, if using the standard plastics underneath - so culverts and speed bumps might present problems on the street, so an extra 1/2" might help the packaging.

Shamelessly stole the idea from GooRoo too...moo moo .....using 16g alloy instead, which made the small area of the lip easy to overheat welding - this is for the standard 294mm system, also made some up for the larger system once the wheel and tyre supply is used up in 16s.

 

Yea, I have been looking at these as well. Seems like DTC 70's front and DTC 60's in the rear would be a good combo.



I am not sure if mine will narrow down that much but it certainly will narrow some. I am going to make the backing plates first and see how much before I decide which route I am going. If the opening is roughly the same, flow will be also. Makes my decision pretty easy.

 

The split compound depends solely on your choice of calipers and more specifically piston/rotor swept area/pad area size. The two compounds feel roughly the same from a pedal feel standpoint. Run the numbers and see what the spreadsheet says and adjust compound from there.

You have it figured out!

There are ways to get creative with the duct opening area, but most would require some sort of composite layup (or alot of really creative welding) to complete. My 993 rotors (Coleman custom) and pads (DTC's) do not overheat with 30 minute sessions (340 WHP) anymore.