spark plug gap question

[280zgod]

Hey everybody. Ive got an L28 that I'm setting up for high end power. I've got a stage3 cam on the way and some minor mods and i was wondering if i needed to change my spark plug gap to accomodate these new changes with the cam and all. At the moment I have the gap at .040mm and the car seems to run smooth at low rpms but backfires like mad at high rpms and sometimes it lags when i try to accel so should i be running my gap lower say .025 or what? Or if anyone could explain to me the science of gapping I'd be very happy. Thanks everyone.

[gramercyjam]

This is from NGK's web site - I don't have anything to add. Check out their tuner section for additional info.

Gap Settings

A spark plugs’ tip temperature and the voltage necessary to fire the plug are directly affected by the gap setting. Most manufacturers set the gap from the factory for that plugs most popular application. Unfortunately, that plug may have hundreds of applications from automobiles to golf carts. Setting the gap for your particular engine is important as insufficient spark plug gap can cause pre-ignition, detonation and even engine damage. Whereas too much gap can result in a higher rate of misfires, loss of power, plug fouling and poor fuel economy. Even if the preset gap is supposed to match your motor, it is always best to physically check that the gap is adjusted properly for your motor prior to installation.

For modified motors, proper gapping is essential; gap settings are affected by increased compression, fuel type, turbos, nitrous and high output ignition system. Most experienced tuners know that opening the gap up to present a larger spark to the air/fuel mixture maximizes burn efficiency, however, after they have raised compression and installed a turbo, they have to lower the gap (to ensure ignitability in the denser air/fuel mixtures). It is for this reason that most racers add high power ignition systems. The added power allows them to reopen the gap without misfire.

Compression ratio

An increase in the compression ratio raises both the pressure and the temperature within the combustion chamber. This will raise the spark plug tip temperature increasing the chances of pre-ignition. Adjustment in spark plug heat range is likely necessary, in addition, whenever compression ratios are altered, it may be necessary to modify the spark advance curve and recalibrate fuel system.

[1971rb26dett]

Generally you would only change the gap if you increased the compresson, ran alternative fuels or ran forced induction.

It's hard to tell what you mean by 'high end power' is it a track car 6k - 10k or just a weekend warrior.

Most often you will find that if your mods make it necessary to change gap then you probably need an upgraded ignition system.

[280zgod]

Well then guess i dont need to change the gap any i should probably lower it. That would explain the crappy running of my car lately and maybe the sudden gas mileage drop. But yeah its not a track car just a weekend warrior 4-7k power is what im aiming for. Thanks guys.