Engine Tuning Using the Main Bypass

When you’re thrashing in the pits getting ready for the next run, a quick way to update your jetting is to adjust the main bypass while leaving the rest of the jets alone. If you have a really well dialed-in jetting combination with good fuel system pressure, this form of tuning simply controls how much fuel is going to the engine and how much is being diverted back to the fuel tank.

Tuning Steps

The larger the main bypass jet, the more fuel is diverted away from the engine. This can sometimes feel counterintuitive.

When the air density goes up, there is more oxygen in the air. In order to maintain the ideal air/fuel ratio, a smaller main bypass is necessary in order to send more fuel to the engine. Conversely, when the air density goes down, there is less oxygen in the air. This means a larger main bypass is needed so that less fuel is sent to the engine.

Density Altitude vs Air Density

Some racers monitor density altitude instead of air density. These are similar measurements but presented in different terms. Less oxygen in the air translates to higher density altitudes and lower air density. More oxygen in the air translates to lower density altitudes and higher air density.

Fuel Pressure

Fuel injection manufacturers such as Hilborn Fuel Injection recommend maintaining fuel pressures between 50-150 psi. We’ve seen quite a few setups that were dialed in for a higher air density such as in the morning when the temperatures are cooler. As the day goes on, temperatures increase and air densities go down. Racers will put a larger main bypass in the engine to compensate but if they don’t have the right size, fuel pressures can drop below 50 psi and cause sluggish engine performance.

Keep a large inventory of nozzle sizes

When guys do tuning with the mbp, it's really important that they have an adequate nubmer of mbp sizes so they can zero in on the right size. If they don't have a larger enough variation in sizes, they might end up grabbing one that's too big or too small to maintain their afr. We had good success maintaining jets in .005 increments. We put those in a jetting holder so we could see the sizes in sequence and the only one missing is the one that's in the engine. This made it easy to see what was in the engine and what sizes we had available.

We can help!

Our online jetting calculator provides a main bypass calculator which can determine the exact mbp size for any change in air density or density altitude. Click here for a free trial.