Indexing spark plugs ??

[Grits & Gravy]

What kind of actual gains will you see on the dyno with indexed plugs vs non indexed?

While I do index my plugs I've never actually did an A/B test on the dyno, any actual data would be appreciated.

[Windsor377]

In all honesty zip...unless without indexing the piston crushes the ground strap.

[Grits & Gravy]

In all honesty zip...unless without indexing the piston crushes the ground strap.

About what I figured.

Way back in the day on open modified briggs engine we ran a NGK "FN" plug which had a diiferent orientation/design ground strap. Does anyone know if they still have these or if there are any pros/cons to running them in a strip engine?

[#84Dave]

Grits et al..... I respectfully disagree on the plug indexing matter. There's a very good SAE paper on the subject of mis-fire and plug orientation. It's within the SAE race engine book that has the pictures of flame kernals on the cover. I'd have to dig it out of my library, but can if you'd like the paper number. I was also very fortunate, one time, to look over the shoulders of a couple of gents who were testing a injected 4-valve 4-cylinder with pressure transducers in the plug shells and a hardware/software package that recorded 77 consecutive cylinder firings. The '77' the apparent limit due to memory space in the hardware. This was a few years ago without the benefit of the current electronics technology. Their tests, at that time, confirmed what I read later in the SAE paper. If memory serves, the worst-case mis-fire on the 4-cylinder was 6 mis-fires out of 77 consecutive firings. That's a 7.8% mis-fire rate. The thing that amazed me most? The gents were testing full-load on the engine dyno @ 8300 rpm and the engine 'sounded' just fine to me, listening to the acoustic exhaust note! Now..... what is a 'mis-fire'? I don't remember their engineering definition but you could actually watch the pressure traces on the 4 vertical-raster o'scope and watch those cylinders that often had reduced/varying chamber pressures at the plug area. And there was always ignition voltage at the plugs. So it wasn't the ignition system causing the mis-fire. If I remember, peak cylinder pressure was taking place about 8° ATDC. In that particular 4-cylinder engine, the least mis-fire condition was when the ground electrodes on the plugs were oriented front-rear on all 4 cylinders. Then the average misfire rate went down to 2 per 77 consecutive firings. About 2.6%. They contributed their findings to the tumble and turbulance character within the cylinder near the plug area, affecting kernal development/displacement as a function of the ground electrode position, and subsequent 'lighting the mixture fire'. The SAE paper, in many ways, substantiates these gents findings. They were at least a mile over my head, at the time, but it was some darned interesting stuff!

Dave

[Windsor377]

I would be interested in the book number if you run across it.

Thanks.

[automotive breath]

I'm currently reading "Design of Racing and High Performance Engines 1998 - 2003" . It's a group of papers one being 2000-01-1207 " Cyclic Variations of Initial Flame Kernel Growth in Honda VTEC_E Lean-Burn SI Engines.

It's some really interesting stuff with great pictures of different spark plug orientations and its effects. #84Dave is this what your talking about?

[Cobra]

Wet-flow benches are giving new insight on spark plug wetting problems. Spark plug indexing may substantially reduce wetting problems.

[Windsor377]

Good stuff guys. If you find any other documents please pass along.

Grits. I couldn't see you picture last night, but now that I do that plug remeninds me of the the last T/F plug i've seen. They were Autolites.

[#84Dave]

Auto breath....... that's the book and the paper! Edited by Dan Holt. They were working with a lean-burn engine but due to tumble, swirl and turbulance factors, I suspect the same affects could also take place in a richer running race engine? There has got to be one 'violent storm' inside a cylinder/chamber when we're trying to get these things to run and make power! Even normally-aspirated. And I can't imagine the havoc taking place inside a blown Top Fuel engine.

Dave

[Windsor377]

What I'm thinking and the direction I'm pursuing this is from a "safety cushion" perspective.

If you go lean for some reason I wonder how thick a safety cushion you might have if you apply some "lessons learned" from the study?

In all honesty I will strive to have the open end of the ground strap facing the 90* arc between the exhaust valve and the piston. But in all practicality when rushed to swap out a set of plugs and dropping a shim or two and getting some plugs in "wrong" I've never seen a difference in on track performance. That's in a drag race application.

[#84Dave]

Windsor..... there's obviously not much collected, statistical data out here in the real world, that I know of. Either oval track or drag racing. With most of today's heads, particularly V-8's, the plug position(4-valve excepted) tends to favor the exhaust side of the chamber. Intuitively, with no engineering data at all to substantiate my preference, I try to position the open end of the ground electrode within a 45° arc front or rear horizontal, hoping that decent tumble within the chamber will permit good kernal development and subsequent spread to the dense, vaporized mixture. The SAE paper mentioned in this thread talks about the initial kernal being 'blown' away from any 'nested' position about the center/ground electrodes for good start of the flame front. That is, if the kernal spends too much time nestled around the electrodes, kernal energy can be absorbed by the local metal of the electrodes. That isn't ituitive to my small mind. But apparently it has proven to be true via actual observation and measurement. It's these kinds of things that can drive us nuts if we think too much about it. We wouldn't be required to think about these matters if we were racing diesel's, would we?

Dave

[Windsor377]

Dave, thanks for the feedback. I think the problem with the initial kernal possibly going away might have to do with how the cylinder is filled. I don't know? I need to read the article too.

I always find it kind of interesting how the different forms of the sport might lead us in slightly different directions. But when you finally get a chance to step back and look at the big picture, there is always something to be learned that can be applied to another part of the sport.

[#84Dave]

To expand a bit more, the SAE book referenced in this thread is titled "Design of Racing and High-Performance Engines - 1998-2003". Published by SAE and edited by Daniel J. Holt. With ISBN 0-7680-1298-8 . The book also contains a very good paper authored by GM Engineering and Shaver Specialties on the development of the Turbo 2.0L EcoTec for Sport Compact drag racing. Another good paper on a 'swirl' experiment. Loads of material throughout the book to digest if one is a hard-core 'gear head'!

Dave

[blown265]

There appears to be supporting evidence at a cylinder level, but not at the wheels/track.
Is there is an 'acceptable percentage' of misfire that doesn't affect the hp?
How much does chamber design influence the effect?
One engine we assemble uses a late 1960's open hemi style chamber and we 'feel' that indexing helps the cause. It's a case of 'if it helps -sweet, if it doesn't - no loss'. Not the most scientific approach, but neither is that particular engine.

[#84Dave]

265......... I have not seen any quantifiable data to indicate power loss as a function of 'mis-fire'. I think the true IC engineers call it 'cyclic variation'? With the standard engine dyno potential measurement error, it may not be accurately measurable on a low HP engine? It might be measurable on a 800 HP ground-pounder? Either way, I'd feel there is power loss if any cylinder is not working up to its full potential, burn and pressure-wise. Can that be translated to the race track? It might be only hundredths of a second in lost time on a drag strip? Or 1-2 seconds loss on a 5km road course? I have no idea. Chamber design is an influence. Including the piston tops. We've all seen a race car lose by inches at the finish line. But I personally have never heard a losing driver exclaim, " If I could just get rid of the cyclic variation in this engine, maybe I could win a race!"

Dave