Holy Heat Range Batman, (spark plugs)
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Topic: Holy Heat Range Batman, (spark plugs)
Posted By: Capt Fiero
Subject: Holy Heat Range Batman, (spark plugs)
Date Posted: 01 July 2009 at 4:47pm
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OK just a quick preface to this, I am not the ubber mechanic, so if anyone of the more seasoned mechanics wants to chime in here feel free. OK I had this all written out and was about to hit submit and decided to delete "one" word, and hit backspace at the wrong point and IE decided to go back a full page, and I lost it all. So in frustration, here is the short version. OK this topic comes up every know and then, spark plugs. Most people are not even aware there are different heat ranges of spark plugs available for Fiero's. Well there are. Among dozens of different designs, there are heat ranges within those designs. Unless you are running a Turbo, Supercharger or Nitrous, for 99% of the 2.8's out there the best mix between longevity, anti fouling and pre-ignition is the NGK UR5's. They are fairly cheap at 3.50 each, and frankly work really well. SO unless you have.... Oh... the Fed Mogul 12.5-1 compression pistons in a 2.8/2.9 build, or are running a 3.4 using the 3.4TDC pistons. You are fine with the UR5 plugs. The owner if this car, had came to me a few times, and we had done a few rough things to get it run better and it had been improving, however he came in and asked me to dig a bit deeper into the motor. So first things first we needed to get plug out to see what general idea we had to work with. Knowing how to read plugs can be a life saver when working on engines. We pulled the plug out and frankly I was amazed at what I saw. I went grabbed my compression tester and wanted to find out what the cranking compression was. Now bear in mind, that cranking compression only gives you a "ballpark" at best, indication as to the engines compression ratio, but when combined with that information and other indications, you can put a fair guess it to. Stock compression is usually around, 140psi for a good stock engine. My 88GT as it sits now is only about 120psi. My old first "hotrod" 2.9 had 215psi cranking compression and, Fire451s aka Zanders red dragon was in the 215-220psi range for a performance engine. This motor came out with 155ish, I actually redid the test several times and then called to run the numbers by another mechanic friend of mine. Note that cranking compression again does not take into account, cam duration and overlap, so it is really only a ballpark. After seeing the first plug we were off to the store, for a new set of plugs, as I didn't even need to look at anymore to know where this was headed. Note only 5 plugs are shown as the 1st one I pulled I took straight to my wire wheel and cleaned it up, so I could read the markings on it. So its not pictured in the list. The ones with the rusty ring were on the firewall side. OK pics of the plugs. At first they don't look too bad, aside from the noticeable short electrode, (due to the colder heat range) 
Now when I put the camera into Macro mode and grab a few close up shots. 
Still not too bad, but a little questionable 
Gee I wonder why he had a bit of a misfire. I am just about betting 1 out of every 5 times that plug fired it was a parital misfire or not even firing at all. 
With the mods done to this car, it should be considerably faster than my 88GT and in a dead on street race, it was only barely creeping on my rather tired 88GT that I would estimate is only putting down 115hp at the crank due to age. So we knew there was something out of whack. Now in fairness his problem was not entirely bad plugs, as he also had poor wires, timing that was set on the conservative side, and even a bad ground from the engine to the chassie. I had fixed the ground for him a few days previous to this. Once we got the new plugs in, new wires on, and moved the timing to a place it was happy with, he brought the car back with one hell of a happy look on his face. There is still room to improve, but it just goes to show, that motor that should make XXX hp, doesn't necessarily mean that it is, if its not setup to what it needs. ------------- Capt Fiero 88 Fiero GT 5spd V6 Eight Fifty Seven GT V8 5spd. |
Replies:
Posted By: Patrick
Date Posted: 01 July 2009 at 5:02pm
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What am I missing here? I'm no expert, but those plugs don't look terribly bad to me. I expected to see an image of a spark plug with bridged electrodes. Great spark plug chart http://www.verrill.com/moto/sellingguide/sparkplugs/plugcolorchart.htm - Here . I'd say the plugs you removed are "ash fouled" according to this chart. Were the old plugs "colder" than what the factory called for? If so, it's possible the proper heat-range plugs would've burned that crap off before it started to accumulate.
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Capt Fiero wrote:
Posted By: Capt Fiero
Date Posted: 01 July 2009 at 5:19pm
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In those pics on that chart those are "worst case" scenarios of each problem. It doesn't take an ark welding experiment to shut down a motor, a simple non visible minor short in a system will give you the same effect. ------------- Capt Fiero 88 Fiero GT 5spd V6 Eight Fifty Seven GT V8 5spd. |