What is the best vacuum hose material for a Mazda RX-7 FD?

Difficulty Rating: Moderate to Hard (Depending on how much of the "rat's nest" you are replacing)

For the Mazda RX-7 FD, the best vacuum hose materials are high-quality, thick-walled silicone and Viton. Each has its specific strengths, but modern, thick-walled silicone is generally the preferred choice for a complete vacuum system overhaul.

Here is the breakdown of the best materials and where to use them:

1. Thick-Walled Silicone (Highly Recommended for Vacuum Routing)

Modern silicone is excellent because it resists under-hood temperatures and stays pliable over time, meaning it won't harden and break the fragile plastic nipples on your solenoids or check valves if you ever need to remove a line ².

The most critical factor when using silicone is purchasing thick-walled hose to prevent kinking in tight bends, and selecting the exact metric inside diameter (ID):

• 3.5mm ID: This is required for the vast majority of the vacuum lines and solenoids. When you use a true 3.5mm hose, it fits extremely tight and naturally grips the nipples over time, meaning zip ties are not strictly necessary ¹. Avoid using 4mm hose for these connections; it will fit too loosely and easily pop off.
• 6mm ID: This size is used for the larger lines, primarily for boost control routing ².

While correctly sized silicone doesn't inherently need securing, zip-tying the lines that see direct boost pressure is still highly recommended as cheap insurance ².

A complete replacement on an FD typically requires about 20 to 40 feet of 3.5mm hose and 5 to 10 feet of 6mm hose, depending on your exact setup and whether you have deleted emissions components ¹.

2. Viton (Best for Oil and Vapor Lines)

Viton is an exceptionally durable, OEM-plus material that is impervious to oil degradation and high heat.

• It is the absolute best material to use for lines carrying pure oil and vapors, such as routing to an oil catch can ².
• The primary drawbacks of using Viton for the entire vacuum system are that it is much stiffer (making routing through tight bends difficult), more expensive, and extremely difficult to find in metric sizes like 3.5mm. Substituting with standard/inch sizes often results in hoses that fit either too tight or slightly loose ¹.

3. OEM Butyl Rubber

While OEM rubber survived well from the factory, it is not ideal for replacements. It bakes in the extreme heat of the FD engine bay, hardening and becoming brittle over time. This makes future maintenance very risky, as pulling off an old, hardened rubber hose frequently breaks expensive solenoid nipples ².

The GarageBot Recommendation: Plumb the majority of your FD's "rat's nest" with 3.5mm and 6mm thick-walled silicone for ease of installation, longevity, and flexibility. Reserve Viton specifically for lines exposed directly to heavy oil vapor, like your catch can routing.