Engine FAQ

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What is Fuel Cut and how is it different from Detonation?

The difference between fuel cut and detonation is that with fuel cut, the fuel (and ignition) are completely cut whereas with detonation, the ignition timing is retarded.

Fuel cut occurs when the ECU has seen a condition of too high of a concentration of incoming oxygen for too long -- something like 1.5 seconds. It issues a fuel and ignition cut for a duration of about .5 seconds. Remember that in the wintertime, the concentration of oxygen is higher since the same volume of cool air contains more oxygen than warmer air. The threshold is easily reached if you run much more than stock boost.

What does fuel cut feel like? On the very first occurence, it is the most pronounced, occuring as low as 3000 RPM. You will be under heavy acceleration when suddenly, it feels as though something jammed your brakes on for a split second and then released them. On subsequent cuts, it appears as though the ECU lowers the oxygen level treshhold while shortening the length of duration of cut. This results in less pronounced but more frequent fuel cuts. Resetting the ECU clears this and the first one will again be very pronounced.

Detonation itself isn't really felt; it can be heard. It's that pinging sound most often associated with cheap gas, in cars other than our 3/S. Anybody ever drive one of those "smog" cars built in the mid- to late-70's? Who ever bothered to put premium fuel in one of those? As a result, you'd get that pinging sound when you started out or tried to accelerate out of a turn. That's detonation.

On our cars, detonation (or knocking) can be extremely harmful if it occurs under high boost, high RPM situations. Higher boost requires higher octane. Substantially higher boost requires higher octane fuel than what is offered at most gas pumps. Our cars have a sensor perched on a bracket which has 4 bolts anchoring it into both cylinder banks. The bracket straddles the valleys between the two banks and resonates at a frequency that makes it easy for the sensor to sense knock. The ECU monitors the output of the sensor and, when knock is detected, retards the timing which has the net effect of slowing combustion and subsequently, acceleration. These adjustments to timing sometimes go unnoticed and at other times are felt as mild hesitations, usually at RPMs higher than 5000.