Has anyone done spark plug gap tests?

[ziddey]

I've done some searching and see most people try to dabble in iridiums or other "upgrade" plugs. I've personally always stuck with plain old coppers; they've always worked best. It also helps I've got a simple 8v i4, so spark plugs are a quick 5 minute job.

For the 80's vw's, gap spec is 0.028 +/- 0.004. California spec is 0.028 + 0.004.

For pretty much as long as I've been changing out plugs, I've gone with bosch coppers, and kept the 0.032 gap, since in theory, that should be best?

But on a current car that I've been tuning up (1990 vw fox, ke-jetronic), I've killed off all vacuum leaks; pulling almost 20 inches at idle. Full ignition tuneup. MSD Blaster 2.

Before the tuneup, car had some old bosch platinums. I'm not sure of the gap. Don't have a tool at the moment.

But after swapping in the coppers, the idle is noticeably lumpier. Still trying to fix a lean running problem as well, so I'm not putting it specifically on the spark plugs (failed smog two times in a row now. insane NOx, but that's another story, not related to this topic..)


So taking this application as an example, has anyone tried say 0.024, 0.028, 0.032, 0.035, 0.040, 0.044?

I had a 1991 vw fox; pretty much the same car, albeit with digifant management. I've also used bosch coppers on that one, and after 20k miles or so, when I goto change the plugs, I'll often find the plugs have worn down / been blown out to 0.035-0.044 easily.

What's the response to different gapping? I haven't checked to see what color spark I get, but I can't imagine it being anything short of blue, since coil, wires, cap, rotor, and plugs are all new, and I added an additional ground strap to the valve cover.

[DifferentPointofView]

I don't know, I tried Bosche +2 platinums in my Jeep, but it seems that they don't like them. I'm gonna get new copper plugs soon and see if it helps my really rough Idle situation. I hope it does.

I'm guessing that you should use whatever type of plug your vehicle was designed for, (unless you have an aftermarket ignition system) because voltage sent to the plug might create a stronger/weaker spark because of the metals used have different conductions of electricity. The escort used platinum plugs, so I used those, and it worked out great, now I'm gonna switch back to copper on my Jeep and see if that works out great.

Never messed with gaps though. I go with the assumption that whatever the spec is that the manufacture made is what is best for the most reliable, best burn possible without misfire, or weak spark, as well as long plug life. Wide gaps have to be regapped more often.

[DonR]

If your new Blaster 2 has runs at a higher voltage than what you were running before, you should be able to take advantage of by using a larger plug gap. If this is the case you may see increased erosion on the plugs & may want to consider a different electrode material. If it runs at the same voltage you will probably need to use a similar gap as before.

I swapped in a GM HEI distributor into my Jeep. From 30,000 to 40,000 volts. Stock .035", now .042". I can notice a difference. But there are other differances between the two distributors also.

Don

[bgd73]

Originally Posted by DonR

If your new Blaster 2 has runs at a higher voltage than what you were running before, you should be able to take advantage of by using a larger plug gap. If this is the case you may see increased erosion on the plugs & may want to consider a different electrode material. If it runs at the same voltage you will probably need to use a similar gap as before.

I swapped in a GM HEI distributor into my Jeep. From 30,000 to 40,000 volts. Stock .035", now .042". I can notice a difference. But there are other differances between the two distributors also.

Don

I don't mess with gaps either when upgrading. It seems funny to read 40000 volts, I upgraded an old sube from <7000 to 11000.
In 2007, I swapped out the three remaining original NGK old school "crap keepers" after 20years (exactly the month it turned 20- Freakin amazing). the bosch platinums work nice on the lower coil at 11000.
I too run an 8v 4cyl engine, but its a boxer. Different world of physics for the fires. Inlines need special attention, as most long time users figure out. I would go with the coppers too if in the scenario, and wouldn't even step up the coil too far unless the head was known to be a bullet proof rock, as well as pistons. most 8v are anyway, but there is extremes there too from the past.(ie, 83 escort)

[ziddey]

I'm going to try gapping my plugs down to 0.025 soon. Definitely am getting god mileage right now though. I've got a real heavy foot, so take that into consideration. I need to source something cheap to replace my leaking filler neck hose. OEM part is $80 for 5" of rubber. No thanks.

Right now, the plugs are 0.032, and the idle is a bit lumpy. Back with 0.028 platinums and an aging stock coil, it was smooth as silk.

But on the last tank, I lost over 3 gallons due to the leak, and still managed 25mpg. That'd have been almost 36mpg if it weren't for the leak. Includes a lot of lead foot and maybe 50/50. Too bad the good mileage is also from running excessively lean. Somehow, the o2 ecu is still thinking everything is normal, even with a new o2 sensor. I'm pretty sure the noisy fuel pump isn't putting enough fuel pressure for the system. NOx was almost 3x the legal limit for the car, and about 9x higher than "normal"

It's so bad the back bumper by the tail is browned from heat, vs black from rich or nothing with neutral.



Come on, someone has to have tried different gaps. How does heat range come into play as well? Wider gap and higher heat range? Or lower? I'm not looking to lose any performance. If anything, gain performance, especially now since I want to be able to fully utilize the msd blaster 2. Hell, performance and economy should go hand in hand, discounting driving habits.

[RobertSmalls]

Originally Posted by ziddey

Hell, performance and economy should go hand in hand, discounting driving habits.

Mods that result in increased engine efficiency give you more horsepower OR better fuel economy: you choose with your right foot. Better driving habits are more effective than any mod.

When it was time for new plugs, I took a few measurements. My fuel consumption at idle and at 3000 rpm in neutral was unchanged when I swapped out the Autolite copper plugs with 40kmi on them for Denso copper plugs.

Denso came out with the Denso Platinium TT, and their ads boast about a 0.54% improvement in fuel economy, from 36.8 to 37.0mpg, versus "Competitor A". They don't say under what conditions, so you can't draw any conclusion other than that Plug A vs Plug B isn't going to make more than a 0.54% difference in power or economy. Utterly invisible to your butt-o-meter, and almost certain to be drowned out by noise in the data at the pump.

The manufacturer of your engine spent hundreds of engineering hours (I hope) optimizing the ignition system. If you believe it was optimized for performance/economy instead of cost, then I wouldn't mess with it.

The best place to test different gaps and different plugs is a bench dyno, but since I don't have one of those in my garage either, I'd be happy to read any data you collect on the road or in the driveway.

[eco86]

Everyone is talking about modifying the plug gaps across all cylinders in the engine, but has anyone tried using different plug gaps for different cylinders? It sounds crazy, but I was reading an article on Autospeed about cylinder-specific tuning, and the variance in AF ratios and other factors for different cylinders in the engine was reasonably large.

The Autospeed article
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For instance, one of the situations in which cylinder imbalance of Air/Fuel ratios would occur happens when an assymetrical intake is utilized:
http://www.autospeed.com/cms/gallery...0&a=109725&i=5
In this situation, the air is taken in through one side of the manifold, before being distributed through the various, equal length runners to the engine. From the flow diagram, it seems that the runner farthest from the intake port would be receiving the most air, while the runner closest to the port would be receiving the least.

Now, because the engine is running in closed loop (feedback from the O2 sensor and various other engine points determines fuel delivery), the amount of air reaching each cylinder plays a major role in determining whether the car runs as close to 14.7 to 1 as possible. Due to the fact that many cars run a single O2 sensor, that sensor is only seeing the average of AF ratios from the engine. Also, each cylinder recieves the same amount of fuel (because there is only one O2 sensor, fuel isn't automatically adjusted for each cylinder). This would mean that some cylinders are receiving more fuel than necessary, and others less than they need.

For instance, the cylinder farthest from the intake (I'll call it cylinder 1) is receiving much more air than the other cylinders, while also getting the same amount of fuel. This would cause it to run leaner than the other cylinders. The opposite cylinder, closest to the intake is receiving less air with the same amount of fuel, causing a rich condition in that cylinder. To protect the engine, most cars will err on the side of delivering too much fuel, rather than too little. This means, if the O2 sensor finds that the average cylinder AF ratio is too lean, it will immediately enrich the mixture, thus impacting fuel economy. While this will average out to approx. 14.7-to-1, the cylinder that is running lean is also influencing the knock sensor of the engine. Engine knock, or detonation is primarily seen when an engine is running too lean for the operating conditions. When knock is detected in fuel injected cars, the vehicle's ECU immediately responds by retarding the ignition timing (less power) and dumping more fuel into the engine to eliminate the lean condition.

So basically, cylinder imbalance can result in the car using more fuel than necessary to compensate for the differences across the cylinders.

What I'm wondering is whether it would make sense to counter this with variable plug gaps across a given engine. My 96 Accord uses an intake setup similar to the flow diagram pictured, so I figured that by using a narrower plug gap at the same position as cylinder 1, and a wider gap at cylinder 4 (in my case, the cylinder closest to the intake), the cylinder that is running rich would burn more fuel, while the cylinder running lean would burn less.

[pgfpro]

You might have something here?

I haven't tried different plug gaps to control lean/rich conditions, but I do use my management system to control individual cylinder fuel and timing trims.

This makes a big difference and you would be surprised how much there is to gain.

[tasdrouille]

pgfpro,

How do you tune individual cylinders? How do you get your feedback for individual cylinders? I guess you do this on your del sol.

On TDI engines, each power stroke is sensed by the crank sensor and fuel trims for individual injectors are adjusted to compensate for variations between cylinders and injector bodies and make the engine run smoother.

On a gas engine with perfectly matched injectors and cylinders I can see how you could compute individual cylinders AF ratio from the power they generate given an equal quantity of fuel injected in each cylinder. But as soon as there are mechanical variations between cylinders, say uneven compression for example, or something that throws an injector's flow off, that all goes out the window.

I think it would be easiest for manufacturers to ensure their intake manifolds evenly distribute air between cylinders, or just go with an o2 sensor per cylinder.

[eco86]

I'm going to go ahead and try it on my Accord. Don't know if I'll be able to do any A-B-A tests today, since everyone is going to be out christmas eve.
I'm planning to set the plugs as follows: (MFG recommended plug gap .039 to .043)
Cylinder 1 (farthest from intake): .035
Cylinder 2: .039
Cylinder 3: .043
Cylinder 4 (closest to intake): .045