An exhaust brake slows a diesel truck by restricting the exhaust, which builds back pressure that the engine has to push against, so the engine itself holds the truck back on descents and when slowing down. It works either with a valve in the exhaust or, on many modern diesels such as the 6.7 Cummins in Ram trucks, by closing the vanes of the variable geometry turbo. A Jake brake is different: it releases compressed air from the cylinders and is stronger and louder.
How an exhaust brake works
- Restriction: a butterfly valve in the exhaust, or the VGT vanes, closes off most of the exhaust flow when you lift off the throttle.
- Back pressure: the pistons have to push exhaust against that pressure on every exhaust stroke, which absorbs energy and slows the engine.
- Control: the driver switches it on, and it engages automatically when the throttle is released; many trucks also downshift the transmission to keep engine speed up for more braking.
Jake brake vs exhaust brake
| Exhaust brake | Compression release (Jake) brake | |
|---|---|---|
| How it works | Restricts exhaust flow to build back pressure | Opens the exhaust valves near the top of the compression stroke, releasing the compressed air |
| Strength | Moderate | Stronger |
| Noise | Relatively quiet | Loud, especially with unmuffled exhausts |
| Typical use | Pickups and light trucks | Heavy trucks |
“Jake Brake” is a trademark of Jacobs Vehicle Systems, though it is often used for any compression release brake. Some towns restrict loud engine braking, which is why you see signs about it.
Why use an exhaust brake
- Less heat and wear on the service brakes when towing downhill; see how long do brake pads last.
- More control with a heavy trailer on long grades.
- Lower brake temperatures leave more braking in reserve for emergencies.
Using an exhaust brake well
- Switch it on before the descent rather than after the brakes are already hot.
- Use tow/haul mode or a lower gear so engine speed stays up; exhaust brakes are strongest at higher engine speed.
- Let the exhaust brake hold a steady speed and use the service brakes in short, firm applications rather than riding them.
Does an exhaust brake hurt the engine?
A factory exhaust brake is designed into the engine and its controls manage the back pressure, so normal use is not harmful. Problems come from sticking VGT vanes or valves, which show up as poor braking or low boost.
VGT exhaust brakes and replacement turbos
Because the 6.7 Cummins uses its VGT as the exhaust brake, a replacement turbo needs to keep that function. The Fleece Performance HE351VE replacement for 2007.5-2012 is listed as retaining the factory exhaust brake function and ships with a new pre-calibrated actuator. See variable geometry turbo explained and 6.7 Cummins turbo.
More guides: diesel losing power · big brake kits for towing · 6.7 Cummins turbo actuator.
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