The phenomenon known as fuel standoff, or just “standoff” can be a troublesome gremlin. When it comes to tuning specialty racing engines of the carbureted variety it occasionally rolls in like a front of bad weather. Extremely poor throttle response in harmonic frequencies, and aversion to rapid throttle position changes are the primary indicators something may be up.
Standoff is a haze of fuel above the intake that, rather than being pulled into the engine, hovers and back-flows the natural direction. Often characterized by fuel soaked air filter elements — which pose a clear fire hazard, and by a stuttering sound and feel under acceleration. Standoff can be a serious issue that runs deeper than just the fuel delivery system.
What Causes Standoff?
Fuel standoff can be the result of a number of factors, most of which are determined during the engine build process. As experimentation is always a role player in building power, a few hiccups along the way are inevitable.
Aggressive camshafts generally feature high lift, long duration, and narrow lobe centers. These characteristics make for compromised low-end power in the name of better peak power and scavenging. The valve overlap that a narrow lobe center generates is what makes scavenging possible but also introduces problems.
In instances of standoff the air/fuel charge is drawn down the intake runner and past the open intake valve. When the valve slams shut a reversion pulse like a shock wave is reflected back through the stream. This harmonic pulse pushes back the charge, and back-flows the venturi. Because this pulse has inertia and velocity it forces the atomized fuel the wrong way out the throat of the carburetor.
This harmonic is exacerbated by aggressive cams, short length intake runners, individual runners (manifolds without a shared plenum), and small or no velocity stacks. Velocity stacks serve to smooth and direct the incoming air and can dampen the effects of standoff by containing the fog before it just escapes to the atmosphere — but ultimately they are a bandaid-fix.
Think of this problem like a compressor stall in a turbocharger, the back-flow of charge air is damaging not only to performance but potentially to the surrounding systems.
Generally the problem is isolated to a specific rpm-range, and may only surface under a load like driving on the road or on a dyno. Free revving may not induce the problem. Once the harmonic zone is escaped, the problem begins to dissipate. In some instances, like the video, the problems appears to manifest indiscriminate of rpm or load.
There’s no simple fix for this issue, longer intake runners, carb spacers and taller velocity stacks may offer some relief from the problem but rarely solve it. The camshaft profile is the root of the problem and leaves the builder with the classic choice. Does drivability outweigh all-out performance or can the application tolerate some quirks?
A racing camshaft may have a cool lopey idle and amazing lightswitch power delivery, but that’s rarely a street-friendly combo. A potential engine bay fire won’t be cool, if a soaked filters catch a spark.