Brake duct sizing question…

[A930Rocket]

I’m installing brake ducting on the E46 and have a question on duct size.

I’m using Corvette Z06 brake ducting from the front bumper to the wheel well, which makes a 90° turn with a 2.5 inch outlet at the end. This is a good solution in the wheel well, because it’s flat and thin and somewhat durable compared to brake ducting.

My brake duct backer has a 3 inch inlet.

Provided everything fits either way, would it be better to run 2 1/2 inch duct to the brake backer or run 3 inch duct to the brake backer? If I run the 2 1/2 inch duct, I’ll have to use my transition to 3 inch there.

I don’t know if the Corvette uses 2 1/2 inch duct all the way, but if they do, it seems like that’s the answer and would keep the airflow at the same rate after the 90°.

Thoughts?

[Scott Douglas]

After watching an Engine Master's show on TV that dealt with intakes (I'm assuming the same would hold true for this application), they found that bigger was better and the less bends the better too. If you have the room, use the 3" duct work.

[A930Rocket]

My idea was a bust. While it works on the driver side, the AC receiver/dryer interferes on the passenger side. There’s no way I could finagle it in there.

So it’s 3 inch duct for me.

[doug_porsche]

It depends.

What are you doing with the car?
If you are doing mainly track days, lots of 100+ to 40 mph braking with short straights, bigger is better.
If you are doing mainly street, going smaller (or none) may be better.

Again depending on what you are doing with the car, and the relative temperature you are doing at, and is it raining, and, and, and
You may not even want to cool the brake calipers.
You may want to cool the wheel, which helps cools the tire, which helps keep the rubber in their optimal temp range.

Everything has an "optimal temperature rage", and you want to optimize to target that temp range.

[A930Rocket]

Originally Posted by doug_porsche

It depends.

What are you doing with the car?
If you are doing mainly track days, lots of 100+ to 40 mph braking with short straights, bigger is better.
If you are doing mainly street, going smaller (or none) may be better.

Again depending on what you are doing with the car, and the relative temperature you are doing at, and is it raining, and, and, and
You may not even want to cool the brake calipers.
You may want to cool the wheel, which helps cools the tire, which helps keep the rubber in their optimal temp range.

Everything has an "optimal temperature rage", and you want to optimize to target that temp range.

It’s a street/track car. It might be overkill, but material cost is low and my labor is cheap. The ambient temperature can be cool to hot depending on the season. No winter track days though.