Hey guys just curious what the recommendation is on clearance between inner and outer pipes for tails? Is there a rule of thumb for water flow ( single, dual, three stage water pumps) that feed the tails? Also does diameter affect clearance? TIA .

When I had my CMI tailpipes cut for O2 sensors, the gap between the inner and outer walls was about an eighth of an inch, maybe a little more. The inner pipe is 4-inches in diameter outside and the outer pipe adds to the total diameter of the tailpipe.

Stainless tubing used for tailpipes is usually measured by outside diameter and the inner tube is usually 1/2" smaller than the outer; 4.00" outside diameter outer tube, 3.50" outside diameter inner tube for example. The exact gap would depend on the wall thickness, but usually .065" or .085". So the typical gap would be .165" to .185", if everything is perfect, and it usually isn't. The gap can vary some for out of round, where fittings are welded in, and at bends. On tailpipes, the jumper fittings and the gap around the jumper fitting at the tube are a much bigger factor in flow than the gap around the full tube. The circumference gap between the fitting and the inner tube is usually the limit on flow. The main reason for large jumper hoses is to get a bigger circumference at the gap around the welded in fitting.

Not to get off topic but I am looking to run Holley on my new build, I was told not to run the system looking at the O2's all the time just keep them in for tuning purposes. Where did you locate your O2 sensors and do you leave them in all the time or run in open loop after tuning? I want to run dry tip tails on my setup since I want to get more aggressive on cam profile.

What gauge pipe is recommend for making the double wall tips?

CMI e-tops... I put them at about the 10 o'clock position a couple of inches aft of the flange. I leave mine in but I also have a heat sink that protects the sensor. I've only just begun to really run it so we'll see how long it lasts.