2.5 Inch Exhaust Cutout Guide: Electric, Vacuum or Manual, and How to Choose

A 2.5 inch exhaust cutout gives you a quiet exhaust for the street and an open exhaust for the track, on the pipe size found on a huge number of cars and trucks. Before you buy, though, you have three decisions to make: whether 2.5" is really your size, whether you want electric, vacuum or manual control, and whether you need a single or dual kit.

This guide walks through each one, then covers mounting, the legal picture and every 2.5" kit Granatelli Motor Sports makes.

2.5" or 3": Which Size Do You Need?

Match the pipe you already have

Size the cutout to your exhaust tubing, not to your engine. Measure the outside diameter of the pipe at the spot where the cutout will go. If it measures 2.5", you need a 2.5" cutout. Putting a 3" cutout on 2.5" pipe means fabricating reducers, which adds cost and leak points. A smaller cutout, on the other hand, creates a restriction right where you want flow.

Is a 2.5 or 3 inch exhaust better?

Neither is better on its own. Larger tubing flows more, which matters most on high-output engines. However, the cutout should simply match the tubing your system already uses. If you're building a new exhaust from scratch, choose the pipe size for your engine first, then match the cutout to it.

Is a 2.5 inch exhaust loud?

Pipe diameter alone doesn't make an exhaust loud. Your mufflers do most of that work. With the cutout closed, a 2.5" system sounds the same as it did before. With it open, exhaust exits before the mufflers, so the car gets much louder at any pipe size.

Running smaller tubing? Granatelli Motor Sports also makes 2.25" kits. On 3" pipe, see our 3 Inch Electric Exhaust Cutout Guide.

Electric, Vacuum or Manual?

All three types bypass the mufflers when open. The difference is how you open and close them.

Why choose vacuum on a boosted or big-cam engine?

The Granatelli vacuum kits include their own vacuum pump, so they don't depend on engine vacuum. As a result, they work the same whether the engine makes boost, runs a big cam, or idles with very little vacuum.

Single or Dual?

Match the kit to your exhaust layout. A true dual exhaust needs one cutout per side so both banks open together and the sound stays balanced; that's what the dual kits (part numbers ending in K or D) are for. A single exhaust, or a system downstream of a merge, needs only one cutout.

Weld-In vs. Slip-Fit, Stainless vs. Mild Steel

Where to Mount a 2.5" Cutout

• Keep your distance. Mount at least 18" from the cylinder head and any O2 sensor (Speedway Motors).
• Always after the cats. Never install a cutout upstream of a catalytic converter.
• After the crossover for sound. Mounting after the X-pipe or H-pipe gives the best tone.
• Dump outward. Aim the turndown or opening toward the rocker panel, away from fuel, brake and transmission lines, so fumes don't collect under the car.
• Plan for the control type. Keep electric wiring and vacuum lines away from the pipe, and put manual cutouts where you can reach the block-off plate.

For step-by-step install notes and dyno results, see the 3 Inch Electric Exhaust Cutout Guide. The process is the same at 2.5".

Are 2.5" Exhaust Cutouts Legal?

Size doesn't change the law. SEMA describes U.S. exhaust noise rules as a 50-state patchwork, and many state statutes specifically name "cutout, bypass, or similar device." California Vehicle Code 27150, for example, prohibits cutouts on registered vehicles but exempts off-highway use at sanctioned racing events. In practice, keep the cutout closed on public roads, use it at sanctioned events, and check your own state's statute. This is general information, not legal advice.

Granatelli Motor Sports 2.5" Exhaust Cutout Kits

Electric and vacuum kits carry a 90-day warranty with optional 1-, 2- or 3-year extended coverage, and every order over $100 ships free.

Frequently Asked Questions

What size exhaust cutout do I need?

Measure the outside diameter of your exhaust pipe where the cutout will mount, and buy that size.

Can I put a 2.5" cutout on 3" pipe?

You can with reducers, but it creates a restriction exactly where you want flow. A 3" cutout is the better choice.

Do exhaust cutouts work on turbo or supercharged engines?

Yes. Mount the cutout after the turbo or downpipe and after the catalytic converters. If you want remote control on a boosted engine, the vacuum kits include their own pump, so boost doesn't affect them.

Which is easier to install, electric or manual?

The cutout itself installs the same way. Electric kits add a switch and wiring, and vacuum kits add a pump, control box and lines. Manual kits need neither.

Will a cutout add horsepower?

Opening the cutout reduces backpressure. Gains depend on your engine, headers and tune.

Keep reading: Shop all Granatelli exhaust cutouts · What size exhaust cutout do I need? · Exhaust Cutout FAQ

Shop Granatelli Motor Sports

View the Granatelli 302525 2.5" Stainless Steel Slip-Fit Electric Exhaust Cutout Kit

View the Granatelli 309525 2.5" Vacuum/Pressure-Controlled Exhaust Cutout Kit

Need help choosing a cutout for your build? Call 1-805-486-6644.