Ok, I decided to make some threads that could teach us noobs some tech info so we can sound intelligent when talking to friends. I always hear suggestions on what we should do on this board, but hardly ever WHY we should do.... Here is the topic.

How does spark plug gap affect performance of the FI engine? Why do some people recommend .030 and some people .034? How do you determine the gap necessary for the application?

 

When tuning I had my BCPR7ES11 gapped at 30. When we went up in boost my tuner ended up gapping them at 25. It was a bitch to idle though!
I would also like to know what's the theroy behind this?

 

The combustion chamber is under pressure by the forced air from the turbocharger/supercharger. The spark plug essentially creates a burn of the air/fuel mixture in a controlled manner (if not then you have detonating or pre-igniting the mixture). The spark plug gap is critical to allow the spark to jump the right amount to ignite the mixture. With higher boost pressures, the cylinder is under higher pressures. You run a smaller gap on the spark plug to ignite the mixture. You are essentially peircing the air/fuel mixture through the high pressure.

Aftermarket ignitions are used to create a hotter spark that can penetrate the higher pressure without decreasing the plug gap. Running a longer gap on the spark plug allows for better gas mileage from a more complete combustion, as well as better start ups. Even with a MSD 6A unit, a .026~.028 is what I would recommend if you are boosting over 15lbs. You only really need aftermarket ignitions when the flame is getting blown out, which doesnt always happen.

 

Good stuff. Im getting the picture now.

So what gap would you recommend me running if im boosting 15 pounds with a 9:1 compression (or does compression figures matter)?

Stock ignition, NGK #6097

Let me get this correct:

Larger gap = better performance, but harder to ignite(flame blowing out)
Smaller gap = easier to ignite, and ocasional idle/startup problems)

 

Compression doesnt matter per say, but it does matter when selecting the heat range for your plugs. When i do high compression, I like at least 2-3 heat ranges colder just for extra insurance.

Plug gap on that set-up should be around .028. Its just a guess, you have to have trial and error. If you are blowing out the flame, then decrease it a bit.

 

blowing out the flame meaning......misfire?

 

Well to me blowing out the flame means that on the dyno you can visually see the drop in power, and hear that the engine is not firing 100% when in boost. Gap the plugs tighter and it almost always goes away.

 

i'm running a jrsc on my ls motor with one step colder plugs than stock. where should i gap them at? i think i gapped them at 33, don't remember, but i'm about to buy some more.

 

good stuff !!!!

 

I gap my plugs in all FI applications .030. I go tighter from there if need be.

 

then why do people say that at stock boost(5.5psi), stock plugs are fine at there normal gap? what plugs should i be running on my y8 with a greddy kit and what gap? thanks

rage

 

This thread is really intended at a "theory" "learning" thread. Not to post all of your specific application questions.

 

The air/fuel charge acts as an insulator. Stick a piece of plastic in the spark pluyg gap and it prolyl wont go off unless theres a shitload of power). The plastic insulated it, requirign more power to get the spark off. Same hsit wiht the air, when the air is compressed, its more dense, thus harder for the spark to jump cuz the desnser charge is more of an insulator. You have to Shroten the gap so there are less air particles in between the electrode and the tip so the spark can jump easier. More pwoer = can jump farthur which is why aftermarket igniton systems dont need to be gapped as low. In theory, you need to shorten the gap on a super high compression n/a motor as well.

This may or may not be 100% accurate, those were jsu the thoughts floating thru my mind


edit: I cant type

 

Makes sense to me