What Size Exhaust Cutout Do I Need?
Short answer: match the cutout to the outside diameter of the exhaust pipe you are cutting into. A 3" pipe takes a 3" cutout. If your pipe measures between two sizes, size down and use an adapter rather than up.
How to measure your pipe
Exhaust tubing is sold by outside diameter, and so are cutouts. The mistake that costs people a return is measuring the inside of the pipe, or measuring a tip or muffler inlet instead of the pipe itself.
- Get under the car at the point where the cutout will actually live — usually ahead of the muffler, on a straight section with room for the motor to hang.
- Measure across the outside of the pipe at its widest point. Calipers are ideal; a tape measure across the face works if you are careful to catch the true diameter rather than a chord.
- If you can only get a circumference, divide by 3.14. A pipe measuring 9.42" around is a 3" pipe.
- Check the same pipe in two places. Crush bends and previous clamps can distort tubing enough to read a quarter inch small.
Measuring a dual system? Measure both sides. Factory duals are usually matched, but aftermarket systems and one-side-replaced exhausts often are not, and a dual cutout kit assumes both pipes are the same size.
The sizes we build
Granatelli Motor Sports cutouts are made in five diameters, covering everything from a mild four-cylinder to a big-inch race car.
| Size | Metric | Typically fitted to |
|---|---|---|
| 2.25" | 57mm | Four-cylinder and small V6 systems, factory-diameter compacts |
| 2.50" | 63mm | Most factory V8 duals, mild builds, daily-driven muscle |
| 3.00" | 76mm | The most common performance size — aftermarket cat-backs, forced induction, LS and Coyote swaps |
| 3.50" | 89mm | Big power naturally aspirated, larger turbo and supercharged builds |
| 4.00" | 102mm | Race and heavy forced-induction applications |
Does a bigger cutout make more power?
Only if your exhaust was the restriction to begin with. A cutout's job is to give exhaust gas a shorter path to atmosphere, bypassing the muffler and everything behind it. On a system that was already flowing freely, opening a cutout mostly changes the sound.
Where it does pay is on a restrictive system — stock manifolds into a small-diameter pipe with a big muffler — and on forced-induction cars, where reducing back pressure ahead of the turbo or after the supercharger is worth real numbers on a dyno.
What a bigger cutout than your pipe will not do is help. Fitting a 3.5" cutout to a 3" pipe means the gas still has to get through 3" of tubing either side of it. You have bought a step, not a gain.
Slip-fit or weld-in?
This is the other decision that trips people up, and it is less about performance than about who is doing the install.
Slip-fit
Clamps over the pipe. No welder needed, and it can be moved or removed later. It is the right answer for a driveway install, a leased or resale-sensitive car, or anyone who wants the option to put the exhaust back to stock.
Weld-in
Welded directly into the pipe. It is a cleaner, more permanent install with no clamp to leak or shift, and it usually sits tighter to the floor. It is the right answer if you have access to a welder or a shop, and you are not planning to reverse it.
Stainless or mild steel?
Stainless costs more and lasts longer, particularly if the car sees winter roads, salt, or rain. Mild steel is the value option and is perfectly adequate on a dry-climate or trailered car. If the vehicle is a daily driver anywhere it snows, stainless is the one that is still working in five years.
When there is not room
Lowered cars, trucks with tight floor pans, and anything with a full belly pan often cannot take the height of a round cutout. That is what the oval design is for — same flow, flatter profile, more ground clearance. If you have measured your pipe and the answer is "there is no space down there," the oval range is where to look before giving up on the idea.
Reading a Granatelli part number
Every cutout part number tells you what it is. Once you can read it, the whole catalogue makes sense at a glance.
- 302 — stainless steel, slip-fit
- 303 — mild steel, slip-fit
- 307 — stainless steel, weld-in
- 313 — oval, weld-in
The two digits after that are the size: 25 is 2.50", 30 is 3.00", 35 is 3.50".
So 302530 is a stainless slip-fit 3.00", and 307525 is a stainless weld-in 2.50". A letter on the end marks a dual kit rather than a single.
Ready to pick one?
Every size, material and fitment is in one place: shop all exhaust cutouts. If you are tight on clearance, start with the oval range instead.
Still not sure what you are measuring? Call us on 1-805-486-6644, Monday to Friday, 8am to 5pm Pacific. We build these in Oxnard, California, and the person who answers has fitted them.
