Indexing Plugs - Loop Charge, Carb

[mitch184]

If someone could easily index the plugs on a loop charged, carb 2 stroke....which way would you index the electrode? I know for DI motors you want the open gap facing towards the injector. And I'm not even talking about buying the shims. Just when you buy 6 plugs... the threads all seem to start at different places and each cylinder head seems to have different starting threads. Buy 6.. chances are pretty good you can index them without using any shims. Or buy 8, use 6... you get the point. I think having the right electrode height is more important anyways so personally I wouldn't use shims.

I've heard you want the open, gapped side facing both ways. Just curious on opinions. I realize any gains would be very small.

Going further, would you change the orientation for a motor that was ported, higher compression and tightened squish band.

[Motv18]

Me personally I screw them in and it doesn’t miss.

Lots of theory and hypotisis floats around about it. What I do know. Air has less resistance than air/fuel. The arm is an obstacle to the swirl and flow of atomized fuel (surface gap was one attempt to improve). Placing the spark sets up the pressure wave.

My instructer’s preference is arm to the head, and calls the direct injection orientation hocus pocus that just keeps the plug cleaner. Also states that it disrupts the pressure wave placing the arm in the path shorting plug life. With n itching to base it on other then his learned thoughts.

My drag racing friend who wins like the gap to the piston or exhaust valve claiming some thermography study I cannot find or source, but makes sense. States that fresh enters flows to piston then up to exhaust side like a U and gap should face greatest mass of fuel air. Two strokes are different but work like a L in the intake port low and cooler then up exhaust side as squeezed. and as stated never actually seen the data but man that’s believable.

Good of luck on the hunt.

[BOATMANBLUES]

On a DI the ground strap is turned away from the injector..since the fuel is blasting straight down to the piston, if the strap is turned to the injector it created droplets instead of staying atomized...critical on ETECs...not so critical on Optis in my opinion, since it also gets a blast of air to help atomization.....On HPDIs it doesn't matter because they never really run good anyway.....
So...in theory on a carb looper the fresh charge comes in the boost ports towards the far side of the combustion chamber,loops through the chamber,the down out through the exhaust ports...if you want the gap in the flow you would turn it towards the exhaust ports...
On a xflow you would turn the gap to the intake side...the fresh charge comes in, hits the air dam on the piston and crosses out through the exhaust...
In practise I don't think it makes any difference what so ever on carb motors...there is a lot of turbulence in the charge and it's all just theorizing in my opinion.....

[Merc 2.5]

It's never crossed my mind to index a plug, will this result in a cleaner burn or???, should this b sumthin I should look at?

[FMP]

I try and try to index my surface gaps but...lol

[Merc 2.5]

Lol. Just keep tryin , they'll eventually line up

[Dave Strong]

I have indexed plugs on carb sled motors not so much for performance as more consistent plug reading when jetting.

Dave

[coffeeguy]

On the sleds did you index them towards the intake (like a car) or exhaust?

[Dave Strong]

Open end 90° to the exhaust port.

Dave

[mitch184]

So even just trying to line the gap up more to the exhaust size is more desirable that the intake. In general

[Dave Strong]

Open end 90° to the exhaust port.

Dave

Sorry stated wrong should have said gap 180deg to exhaust port.

Dave

[mitch184]

Sorry stated wrong should have said gap 180deg to exhaust port.

Dave

So the gap facing the intake port? Same as the direct injection 2 stroke motors

[Dave Strong]

So the gap facing the intake port? Same as the direct injection 2 stroke motors

Direct injection lines the gap with the injector, 1 exhaust port, more than one intake port to deal with.

Dave

[mitch184]

ok thanks. I know the 2 stroke jet ski and snowmobile guys always talk about cutting back the ground strap and indexing the gap towards the exhaust port. Just wasn't sure if I was the same for outboards. thanks

[Motv18]

When it’s all said and done it just depends on the engine.

For instance my optimax the di is nowhere near the plug, and the plug is nearly flat to the piston (laying on top)

the 85 Yamaha v-4 I swear go in straight as a nipple on a teet and are also surface gap. Literally T shaped.

The 60 merc 4s on the pontoon you know I haven’t bothered to look at the valves spark orientation.

The 87 Nissan 50 I think was also a T but ran br8hs-10

on a t head I’m not sure any index would help as the electrodes 90* to the piston no matter what.