Opinions wanted for spark plug heat range on iron head V6

[pmbrunelle]

So I've done about 100 km so far on my turbo iron head V6.

The car is largely untuned, and often pig-rich; the wideband is pegged at 10 AFR during short WOT pulls.

I've done a lot of idling, and calm driving. This engine may have produced about 200 whp for a moment. Don't take this as a serious claim; it is a rough estimate for discussion purposes.

Anyway, so I installed spark plugs that were 1 heat range colder than stock. I used NGK UR6 rather than NGK UR5. The common doctrine goes that when you increase power, you increase the heating of the spark plug. To prevent the spark plug from overheating and becoming a hot spot, you should use a colder plug in a souped-up engine.

I haven't noticed any misfires, but I decided to pull the spark plugs for a look.

Holy carbon deposits!

Any opinions? I suspect Shaun might have something to say!

The tune is a work in progress; my plan is to increase the engine's power output, and to set the AFRs to more normal values.

I guess I'm wondering if the deposits are a problem that will solve itself by increasing the engine's power, or if I should go to the stock heat range.

I might also just clean these plugs, throw them back in, and see how things go. Gap is 0.028" for now.

[ericjon262]

if your current plugs were performing adequately, and you weren't detecting any KR, I would clean them and reinstall, then try and get the tune a bit more in the ballpark. if you pick up another set, maybe go a step colder, but it would probably be fine either way for now until you lean it out a bit.

you might keep an eye on the spark plug gap as you turn up the boost. I've seen more than one occasion of "spark blowout" because the gap was too big, once on a supercharged mustang, and another time on a turbo Saab (had like .060 gap! ) both cars picked up a significant amount of power (dynobutt 9000) by closing the gaps up, and also felt more responsive. you're probably fine at your current boost level with .028" gaps

[The Dark Side of Will]

Nice closeups!

0.028 is a cozy gap, so that shouldn't be a problem.

Definitely get your tune straightened out before you make any other changes. The extra fuel will cause that even if the plugs are the right heat range. Use of boost and WOT can keep them clean.

[pmbrunelle]

I received my Nord-Lock style exhaust manifold bolts this morning, so I'm looking to button up the car tonight after work.

I just burned off a bunch of the carbon with a propane torch, so I should be able to continue driving with these plugs. I want to troubleshoot other problems and fix other aspects of the tune without having the distraction of fouled spark plugs potentially causing misfires.

This discussion isn't over. I can foresee the plugs coming out a couple of times during my engine's infancy; they are my only window into its combustion chambers.

As for the camera, I found out about its "macro" feature only about a decade after getting it!

[The Dark Side of Will]

Also, if your AFR is hitting 10:1, you could lean it 15% and be at 11.8 or lean it 20% and be at 12.5... That might be an easy fix to get you into the ballpark.

A rich misfire shows up as lean on the O2 sensor because of the excess oxygen in the exhaust

Too bad you have a distributor... with DIS or COP you could use spark plug ion current sensing to detect knock

[pmbrunelle]

I don't know if a dizzy is an automatic showstopper for ion current sensing.

If you can't do it via the coil wire, surely you can do something down by each spark plug? I don't know, I haven't studied the problem in depth, nor do I know how ion current sensing works.

If you can do something with the coil wire, you'll be limited to taking measurements for the time that the rotor is facing a terminal, but you'll be able to spend 6x the amount on a cost-equivalent solution.

On my Fiero, the computer is always fully aware of which cylinder is firing, so it would be able to associate a coil-wire current waveform with the appropriate cylinder.

The constraint of having the rotor be close to the terminal in the cap led me to (somewhat arbitrarily) limit myself to a spark timing range of 40° BTDC to TDC. With the timing at 20° BTDC, I set the rotor centered with the terminal post.

[ericjon262]

0.028 is a cozy gap, so that shouldn't be a problem.

I wasn't trying to say it would be a problem, but I can see if it came off that way.

Too bad you have a distributor... with DIS or COP you could use spark plug ion current sensing to detect knock

What exactly would be the advantage here? does it sense knock more finitely/faster than a in block sensor? 6x knock sense?

[Shaun41178(2)]

On my 3.4 iron head I ran stock heat range ac delcos. R42ts.

With 10lbs of boost and a 90 shot of nitrous. Tuned Afrs low 12s or so.

Made 370 whp and 464 tq. You can go stock heat range no prob. Also had 8 to 10 degrees of base timing

What are afrs while normal driving? If you arent boosting much and your plugs are that black then its too cold.

[pmbrunelle]

With a warmed-up engine, the AFR in normal city driving ranges between 13 and stoich.

I think I'm going to continue with the same plugs (cleaned) for now, and work on my tune.

Later on (maybe next time I pull the plugs) when I'm tired of fiddlefucking with these NGK UR6s, I'll probably follow what you did and install AC Delco R42TS plugs.

Got a suggestion for gap? Did you have a stock HEI ignition coil like I have?

Since I have water injection (not yet active for my current boost level), that might change things with regards to a gap that works.

[Shaun41178(2)]

I used stock coil and distributor. I dont remember my plug gap. .030 where you are should be no problem.

[pmbrunelle]

Also, if your AFR is hitting 10:1, you could lean it 15% and be at 11.8 or lean it 20% and be at 12.5... That might be an easy fix to get you into the ballpark.

I've now enabled closed-loop fueling. It works with feedback from the wideband oxygen sensor.

The PI loop (I didn't use D) is doing a great job of taking an inaccurate open-loop tune and making it driveable. I've given the loop a long leash; +/- 15% adjustment to the calculated open-loop value.

Now that the car drives better, I'll be able to do a decent amount of datalogging. From (semi-automated) analysis of the datalogs, I can dial in my VE table. The VE table is the basis of the open-loop tune.

[pmbrunelle]

Now that the AFRs are more under control, the spark plugs are looking better.

In the photo they are arranged in order 1-to-6 from left-to-right.

The porcelain on 5 and 6 is a bit dirtier. 1 and 2 are the cleanest. I don't know if this has something to do with my Fiero log manifolds, which are not equal-length. With sequential injection, I can adjust the fuel to each cylinder in an effort to equalize things, but I'm not at that level of optimisation yet.

Last time I pulled the plugs 5 and 6 didn't clean up so good, so maybe it's just the deposits from last time.
I'm going to re-clean the plugs, then I think I'm going to swap:
1 <-> 5
2 <-> 6
I'll see what happens like this.

I have AC Delco R42TS plugs ready to go, but I'm not done playing with the NGK UR6s.

The threads on 6 appear to be a bit wet with fuel. Dunno if that's because I shut down the engine while that cylinder had an unburned air/fuel charge.

[The Dark Side of Will]

What do you do to clean them?

[pmbrunelle]

I'm using a propane torch to heat up the end of the spark plugs. The junk burns off.

When I do this, the ground electrode gets a definite red glow, while the porcelain has a faint glow. It's an accelerated version of what's supposed to happen inside the engine during operation.

From what I've read, other types of torches (MAPP, O/A) are not ideal for this job, as it's easy to go overboard with the heat and damage the plugs. Not so with the propane.

I wrap up with a wirebrush on the threads and tapered sealing seat.

[Honest Don]

UR6 is fine once your tune is worked out. A new set of 6 is like 9 bucks from rock.

[pmbrunelle]

My Fiero is down due to a broken air filter housing (of all things), so I decided to remove the AC Delco R42TS plugs that were in it.

This time, the front bank plugs are quite clean, but the rear bank plugs are darker.

Hmmmm... a consequence of unequal pipe lengths before both banks converge into the turbo Y?

Not so long ago, I was battling problems with my rocker arms (excessive wear requiring rocker replacement). In the last (hopefully) round of reassembly, I poured oil onto the rocker arms, so that they would be lubricated for their first engine start. Anyway, some oil spilled out of the rear head (but not front), and trickled down into the spark plug wells.

When I removed the rear bank plugs, the body + threads were wet with oil. I suspected that the oil filled the gaps between the threads, and improved heat transfer between the spark plug and the cylinder head.

So I wanted to see if my explanation made any sense at all; with some internet searches, I found that some engines (Rotax was mentioned) are to be serviced with thermal compound on their spark plug threads.

When I reinstall spark plugs this time, I suppose that I should make sure all six threads are in the same condition (either all dry, or all with anti-seize), if I want all the plugs to look the same next time I remove them for inspection.

It appears that the effective heat range of a spark plug can be tweaked by playing with heat transfer compound on the threads.

My tune is quite good now. My dad says that it drives like a normal car now, without bucking or other weirdness.

[The Dark Side of Will]

I always put anti-seize on spark plugs. I've had too many scares and close calls with old ones (that I didn't install...) breaking off.

[Shaun41178(2)]

Those plugs look fine to me.

[pmbrunelle]

Individually, each plug looked okay to me as well, but I didn't like that there was a difference between the banks, versus all six being the same.

[Honest Don]

First guess would be that the intake plays favorites with airflow, but it could be a number of things.