spark plug indexing

[dmatulion]

Can someone please comfirm that the open gap should face up towards the valves and not down on a small block chevy like my friend says?

[larrycoyle]

You have obviously never ran a motor with a bunch of piston crammed into the combustion chamber. If you have the gap facing up toward the valves, the electrode is going to get smashed by the piston dome coming up. This is the reason we started indexing plugs in the first place. You want the gap facing down toward the deck of the piston.

Larry

[Cobra]

Use just a single washer when indexing plugs. Heat is transferred through this area and multiple washers can interfere with this action. Larry is correct in his post. I would add that specific applications call for unique plug orientation to obtain best results.

[dmatulion]

Doesn't this open the spark to the cylinder wall instead of the center of the cylinder thereby closing off the spark to the flow of the fuel? I have seen other threads that specify gap side up. Is gap side down just for high domed, high compression engines? I will admit I am confused!

[beth]

The confusion comes from there being 2 different reasons to index plugs. One reason is because high compression engines with high piston domes will hit the spark plug ground electrodes unless they are indexed toward the chamber floor. Another reason to index plugs is to attempt to gain a more efficient burn. In this case the gap goes where ever the engine builder prefers. Engines without high domed pistons can be indexed this way. I prefer indexing toward the exhaust valve in most cases when there is room. The difference in power is hard to qualify.

[andyf]

I've been doing a little survey on spark plug indexing and what I've found so far is about a 50/50 split between those who say the gap should point towards the exhaust valve and those who say it should point towards the intake valve.

I'm working on a big block Mopar with flat top pistons so I have plenty of room to try either method. It is a 13.5 compression motor that makes 720+ hp. I originally set it up with the gap facing down since that is the way I usually do high compression motors, but then I realized that I have the freedom to point the gap anywhere I want to on this motor.

The next time I'm on the dyno I'm going to point the plug gap towards the intake valve just to see if it makes any difference.

[bigjoe1]

I used to run a BB Chevy, where I had to index the plug just to clear. I still get dyno test customers who want to try this. I have NEVER seen any change what so ever. It is a waste of time, unless you have a piston to spark plug problem. JOE SHERMAN RACING

[andyf]

I would assume it to be a waste of time for most engines also since the neither the mixture nor the flame front should have any trouble spreding around the ground strap of the plug.

One other thing I'm going to play with next dyno session is projected plugs vs. non-projected. I picked a few boxes of NGK projected plugs and the corresponding non-projected part numbers. This motor happens to have enough space in the combustion chamber that I can run either so it give me an opportunity to play around a bit. I doubt I'll see any difference but it is a cheap date.

[bigjoe1]

I have seen a 5 HP increase by using the long reach plugs over the shorter type. I have found that you can run LESS total timing with the extended tips,for the same ,or better power. I have only seen where the extra large sparkplug gap made any more HP ONCE in 25 years of dyno testing. Many people claim better drag strip times with a larger gap, but I never see it on the dyno. JOE SHERMAN RACING

[Cobra]

An important consideration is to minimize excess spark plug threads within the chamber.

[MadBill]

A projected tip will start the fire closer to the center of the chamber, so by rights it should promote a faster burn, thus less SA required, resulting in reduced negative work on the compression stroke = More Power...

[#84Dave]

I highly recommend for reading, SAE paper #2000-01-1207 on the subject of 'cyclic variations of initial flame kernal growth'. The paper is about 40 pages worth but has some very good pictures of the flame kernal growth and subsequent combustion ignition. Spark plug orientation is a big part of the writings. Along with AFR and spark energy. The subject engine is a VTEC 4-valve Honda. Obviously with the plug in the middle of the chamber. I suspect many aspects of the experiment would apply to a 2-valve racing engine. My only personal experience was a single .5-day test with a full-race dual sidedraft Weber-equipped dry-sumped 2-valve 4-cylinder. With the engine tuned, AFR and timing, for MBT/HP between 5000-8500, the engine 'mis-fired' an average of 6 cylinder cycles out of 77 cylinder cycles sequentially monitored/stored (memory limitation). That's about a 7.8% cyclic variation. By mis-fire, I mean the flame front never started or the flame front began but was not completed. IE, the 'fire went out'. The cyclic variation monitored via pressure-transducer plugs and the pressure trace presented on an 8-raster 'scope. The most amazing thing this relatively untrained gearhead got out of the exercise? The exhaust note out of the engine 'sounded' just fine with steady-state 8000 and 8500 'pulls' on the dyno. I could not discern that the engine was misfiring. The data showed otherwise. I'm sure the same would be found on a 2-valve.

Dave

[MadBill]

There was a paper presented at the first SAE Motorsport convention ('94? '96?) about the sources of cycle-to-cycle cylinder pressure variation on a Cup engine. As I recall the finger pointed at induction turbulence originating in the plenum...
I suppose you'd drive yourself mad by looking at a few hundred sequential traces and playing the "what if" game re every cycle on every cylinder being a clone of the single best trace...

[putztastics]

Dave, were you able to correct the misfiring? If so, did it show up on the dyno?

[#84Dave]

MadBill.... you're correct. I believe we went through [7] total test series that .5-day. At [77] sequential cylinder firings per series to analyze. My brain was fried by noon! Putz.... the [6] 'mis-fires' average out of [77] sequential firings was the best-case. I was using AutoLite/MotorCraft plugs at the time and the best case was for a projected-nose, J-gap plug. We ran [2] series each with non-projected nose and projected nose with the ground electrode covering the center electrode. Those [4] series revealed a higher % cyclic variation. Up to about 13% with the non-projected nose. We didn't try any 'indexing' tests at all. My dyno time and $$$ ended after .5-day. And yes...... the best-case test series showed [5] additional ft/lb of torque and [4] HP on the high end. This on a 2.0L ~232HP/171-torque assembly. Even with the best-case plug and a slam-bang(I believe!) crank-triggered ignition, the engine still mis-fired during the last [3] test series. With what the SAE paper had to offer, and MadBill's comment, I'm currently convinced cylinder/chamber swirl and tumble is a majority definer/contributor to cycle-cycle variations. Or lack thereof. Mixture motion near/around the developing flame kernal. And in the case of the Honda 4-valve used in the SAE paper, plug index/orientation was also a moderate 'fix' for cycle-cycle firing variations. I'm also convinced(no engineering data!) that there are induction/head/piston combinations that are so terrible with regard to swirl and tumble, particularly tumble, that no plug, orientation, or ignition system on the planet could completely prevent some amount of cycle-cycle variations in cylinder firing. Although you gents with 'wet-flow' hardware are getting us much closer, without mega-$$$ test hardware, than we were even a very few years ago. Even if it is 'steady-state'.

Dave