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Short answer: the dielectric strength of the dense air-fuel mixture encountered at the top of the compression stroke is much higher compared to free air (it will have a higher 'strike voltage'.) The higher spark voltage overcomes that.

The strike voltage will vary based on the engine's operating state. At high load (high BMEP) that voltage increases considerably: an engine that misfires will usually do so under wide-open throttle if the spark is too weak.

Knowing this, tuners building high-performance engines will thus use even higher spark voltages - 50kV or more - to avoid misfires. Aftermarket ignition suppliers like Mallory, MSD, Accel and others offer whole systems based on this need.

On the other hand, is there a downside to having less (just-adequate) spark voltage? I once worked on a system for V-twins that varied the spark energy based on engine load. The idea was to use a lighter spark at light load and thus use less energy. In retrospect it didn't make much sense since having a hot spark at light load does no harm (this observation, and other reasons led me to not waste a lot of time on it and instead walk away from the project.) Nonetheless you can buy aftermarket systems that do this from an outfit called Jacobs, who make lots of claims about increased performance / mileage / time between tune-ups, etc. Personally I think it's a lot of baloney.

Short answer: the dielectric strength of the dense air-fuel mixture encountered at the top of the compression stroke is much higher compared to free air (it will have a higher 'strike voltage'.) The higher spark voltage overcomes that.

The strike voltage will vary based on the engine's operating state. At high load (high BMEP) that voltage increases considerably: an engine that misfires will usually do so under wide-open throttle if the spark is too weak.

Knowing this, tuners building high-performance engines will thus use even higher spark voltages - 50kV or more - to avoid misfires. Aftermarket ignition suppliers like Mallory, MSD, Accel and others offer whole systems based on this need.

On the other hand, is there a downside to having less (just-adequate) spark voltage? I once worked on a system for V-twins that varied the spark energy based on engine load. The idea was to use a lighter spark at light load and thus use less energy. In retrospect it didn't make much sense since having a hot spark at light load does no harm (this observation, and other reasons led me to not waste a lot of time on it and instead walk away from the project.) 

Nonetheless you can buy aftermarket systems that do this from an outfit called Jacobs Electronics, who make lots of claims about increased performance / mileage / time between tune-ups, etc. Personally I think it's a lot of baloney.

Short answer: the higher voltage overcomes the denser mixture encountered at the top of the compression stroke.

The strike voltage will vary based on the engine's operating state. At higher load (high BMEP) the voltage increases considerably: an engine that misfires will usually do so under wide-open throttle. High-performance engines will thus use even higher voltages - 50kV or more - to avoid misfires (MSD, Accel and others offer a whole aftermarket based on this.)

I once worked on a system for V-twins that varied the voltage based on engine load. The idea was to use a lighter spark at light load and thus use less energy. In retrospect it didn't make much sense since having a hot spark at light load does no harm (for this, and other reasons I didn't waste a lot of time on it and walked away from the project.) Nonetheless you can buy aftermarket systems that do this from an outfit called Jacobs, who make lots of claims about increased performance / mileage / time between tune-ups, etc. I think it's a lot of baloney.

Short answer: the dielectric strength of the dense air-fuel mixture encountered at the top of the compression stroke is much higher compared to free air (it will have a higher 'strike voltage'.) The higher spark voltage overcomes that.

The strike voltage will vary based on the engine's operating state. At high load (high BMEP) that voltage increases considerably: an engine that misfires will usually do so under wide-open throttle if the spark is too weak.

Knowing this, tuners building high-performance engines will thus use even higher spark voltages - 50kV or more - to avoid misfires. Aftermarket ignition suppliers like Mallory, MSD, Accel and others offer whole systems based on this need.

On the other hand, is there a downside to having less (just-adequate) spark voltage? I once worked on a system for V-twins that varied the spark energy based on engine load. The idea was to use a lighter spark at light load and thus use less energy. In retrospect it didn't make much sense since having a hot spark at light load does no harm (this observation, and other reasons led me to not waste a lot of time on it and instead walk away from the project.) Nonetheless you can buy aftermarket systems that do this from an outfit called Jacobs, who make lots of claims about increased performance / mileage / time between tune-ups, etc. Personally I think it's a lot of baloney.

Short answer: the higher voltage overcomes the denser mixture encountered at the top of the compression stroke.

The strike voltage will vary based on the engine's operating state. At higher load (high BMEP) the voltage increases considerably: an engine that misfires will usually do so under wide-open throttle. High-performance engines will thus use even higher voltages - 50kV or more - to avoid misfires (MSD, Accel and others offer a whole aftermarket based on this.)

I once worked on a system for V-twins that varied the voltage based on engine load. The idea was to use a lighter spark at light load and thus use less energy. In retrospect it didn't make much sense since having a hot spark at light load does no harm (for this, and other reasons I didn't waste a lot of time on it and walked away from the project.) Nonetheless you can buy aftermarket systems that do this from an output called Jacobs, who make lots of claims about increased performance / mileage / time between tune-ups, etc. I think it's a lot of baloney.

Short answer: the higher voltage overcomes the denser mixture encountered at the top of the compression stroke.

The strike voltage will vary based on the engine's operating state. At higher load (high BMEP) the voltage increases considerably: an engine that misfires will usually do so under wide-open throttle. High-performance engines will thus use even higher voltages - 50kV or more - to avoid misfires (MSD, Accel and others offer a whole aftermarket based on this.)

I once worked on a system for V-twins that varied the voltage based on engine load. The idea was to use a lighter spark at light load and thus use less energy. In retrospect it didn't make much sense since having a hot spark at light load does no harm (for this, and other reasons I didn't waste a lot of time on it and walked away from the project.) Nonetheless you can buy aftermarket systems that do this from an outfit called Jacobs, who make lots of claims about increased performance / mileage / time between tune-ups, etc. I think it's a lot of baloney.