TECH QUICKIE: Cool or Not–What is Blower Surge?

Blower surge is one of those hot-rod dynamics that presents a paradox to enthusiasts. In one ear it sounds like the engine is struggling if not defective. Then the other ear is treated to a throbbing cadence of performance pulses that help define a unique cool factor that not everyone understands. On one hand blower surge can be so smooth and melodic that it’ll put a crying baby to sleep. Or it can also drive a sane man into paranoia.

“It’s typical that the guys who have it, don’t want it,” says Ron Hayes of The Blower Shop. “And the guys that want it, can’t get it.”

By definition, blower surge is a rhythmic acceleration and deceleration of engine speed at idle. It’s prone mostly to V8s that are force-fed with a large Roots-style supercharger. Granted, an uneven idle condition can be found in many different engines. Most of those are simply out of tune or suffering from a major malfunction, like an improperly installed camshaft.

To be most effective in generating cool vibes, the blower surge should be attached to a wicked vehicle with plenty of blower case and hat sticking out of the hood. Check out the mid-’70s Chrysler Valiant CL ute in the top video. It’s part of the thunderous burnout movement in Australia, so watch the video to the end. This is blower surge in all its sonic glory: uneasy but powerful idle groundswells that bark the tires with every rev increase. Look up blower surge in the dictionary, and you should find this trick pickup.

Blower surge by definition

So what really is blower surge?

“It’s cycling between rich and lean conditions at idle,” explains Hayes. “You typically see it more with mechanically injected engines versus carburetors, but they both surge.”

On a mechanically injected engine, which is the fuel delivery on the Mopar engine in the above video, when the engine richens up, the idle drops. As the idle speed goes down, the fuel pump, which is driven by the engine, doesn’t send as much fuel to the engine. That means the motor leans out slightly, increasing the RPM.

“It’s that constant hunting back and forth between rich and lean that causes the blower surge,” adds Hayes. “Typically, it’s not going to hurt the engine.”

A number of factors can exaggerate or minimize the effect. Blower speed is one. Aggressive cam timing is another.

“A mild motor is not going to surge while a full blown alcohol engine is going to have a bunch of surge,” says Hayes. “If it’s underdriven and guy’s running only five pounds of boost, it’s not going to surge as much as guy spinning the blower 20 percent over and making 12 to 15 pounds of boost.”

Also, centrifugal superchargers are less likely candidates, but the video posted above shows such a blower acting a little uneasy during idle.

“With a centrifugal, you don’t have the same pull on idle,” says Hayes. “Typically you’re not going to see it as much.”

EFI engines with electric pumps and more precise fuel metering tend not to exhibit surge. Carburetors allow for more adjustment than mechanical injection, so there’s promise if in the right hands.

“You can adjust the air bleeds,” adds Hayes. “A good carb guy can adjust it out.”

But who really wants that??

EngineLabs has assembled a few of more sonic expressive videos demonstrating blower surge. Enjoy and tell us your favorite in the comments section.

About the author

Mike Magda

Mike Magda is a veteran automotive writer with credits in publications such as Racecar Engineering, Hot Rod, Engine Technology International, Motor Trend, Automobile, Automotive Testing Technology and Professional Motorsport World.
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