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I know there are other threads that relate to this topic but I wanted to be clear of all relations to other products. This does not relate to just one type of tuning set up it is universal.
Spark plug heat range is confusing in that a hoter plug is for a more mild engine and a cooler one for higher perormance engines. The temp reference is the ability to reject the heat from the spark firing tip. The plug must reject the heat in order to keep the plug within a range where it does not get so hot that it glows red hot and causes detonation or preignition. Also it will fail if allowed to continue at high temps.
If it is to cold it will allow build up and foul to the point that the resistance to spark travel will cause a missfire. There is a wider range in the modern plug than was in the past. Hotter sparks allow a plug to function even slightly fouled.
Putting a hotter or colder plug in an engine will by itself not give you more or less power. I have noticed people wanting colder plugs for their turbo set ups. This may not be nessasary and probably will cause you to loose power down the road. The theory being bantered about is that you will avoid detonation with the cooler plug. This is true if the plug were to be glowing red hot. Thiss is a condition that will happen if you are doing continued full throtle running, as in racing. On the street this is not possable to do. Consider that most of the time you are not under boost, you are cruising or even idling. This does not happen racing or on the dyno. There you need the older plugs.
Heat ranges are different for each type engine, it depends how well the heat is transfered to the water jacket. It is done by designing the plug with different alloys and heat paths. Longer heat path is a hotter plug. Considering the cost of the rotary plugs you should be carefull about choice of heat range.
The NGK plugs have always been resistant to fouling by using a wide range design somehow. The trouble with this has been an inability to "read" it when tuning.
Those of you who buy the colder plug may not notice the performance start to fall off as the plug has a harder time firing. You may get a CEL that refers to a missfire and you will not believe it. In the rotary you have two plugs that appear to have different ranges. This to me means that the secondary plug is not seeing as much heat as the first. So therefore it must be a hotter range. I may not be seeing this right but if not someone will clue me in.
Remembering that street heat ranges are broader than race plugs. In racing they require the range to be spot on and don't have the wide range of engine operation. So you will find a lot of plugs in the colder ranges that split the function down to where a tuner will have a particular plug that he uses with his special set up. You may find this plug fouls in your engine.
It is not incorrect to say that a supercharged or turbo engine needs a cooler plug. However this figures that you are using that added chamber pressure. I will say that added compression ratio would require a colder plug before a turbo would. That is because you are always using that added pressure.
After all that I'll break it down to this, don't jump on the bandwagon so fast. Just a colder plug will not stop detonation. In fact if you knew the difference between preignition and detonation you would realize that the glowing plug causes preignition. You are not experencing preignition. How do you tell the difference? Well if the piston has melted away it was preignition, if it looks like it was hammered to death it was detonation. So how does that help you know since you have no pistons? It only shows you that I know the difference and I'm telling you not to worry about preignition.
I'm going to think about this for awhile as it never occured to me before to figure if there is a difference to a piston engine with this function.
Have you ever shut off an engine and it kept going? That was preignition. It fired by the "glow plug" as soon as there was enough atomized mixure at a pressure suporting combustion. That event comes long before you've reached TDC. So you are trying to compress an expanding atmosphere. This hard to do and it is hard on parts. Notice that it winds up turning backwards? This is because it fires so far before TDC that it is easier to go back the other way rather than compress it.
Hope I've got you fully confused as usual.
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THanks alot for the information, I even believe I was one of the threads that I started was to ask what kind of plug to use. On top of that I completely understood your info you put out however the question I have for you is... Would the factory plugs work fine then for the turbo setup?????? thanks alot...
That is exactly what I was saying but with a caveot. Only if you are not invoved in some event that you are on boost for extended periods of time. If you have a track day event I would for sure put a colder set of plugs in. Then remove them before you go home. That way they will still be fresh next track day.
When I was going to school I had A Paxton blown car that I drove every day and ran at the Drags, lakebed and Bonniville. I would change plugs, open the exhaust and change tires at the races. Things haven't changed all that much except you now have speed rated tires. Plus no one could stand a open exhaust rotary. Nor should you try that as you will loose power.
The same goes for wires. The plug only uses enough juice to jump the gap. More spark only helps if it can't fire under some condition. Like when you have a lot more fill in there or are compressing it or have fouled plugs. Other then that hotter spark is not needed.
Now if you can use that spark to reliably jump a larger gap there might be worthwhile improvments. Just remember if there where anything to be had the factory would have done it. It cost them nothing to get more spark. In fact they have already done it on some cars and the plugs have a larger gap for those. Normal being .035 and some GM cars have been spec'd as large as .05+.
This might have been done to fire lean mixtures for EPA purposes.
Richard, on the rotary engines, the leading plug hole into the combustion chamber is the same size as the plug but the trailing plugs fire through a hole 1/8" in diameter. From the factory the RX-7's (not sure about the RX-8) all came with 7 heat range leadings and 9 heat range trailing plugs. On my street driven RX-7, I run 9's all the way around.
On NGK the higher number is colder. On champions the higher number is hotter. I'm not tuned up on any of the others so is it two ranges hotter or cooler?
If they are one of the above then I know. Just which one?
How long? Awhile I guess, never time myself. I think it consumes a lot of my time though not as much as it used to.
Last edited by Richard Paul; 02-21-2005 at 06:50 PM.
And what is it with the fire slot? Is it there to protect the plug? Or they just want to control where the spark starts? It seems to me that you are greatfull everytime a charger fires why make it more difficult. There could be lean spots around there that they don't want to start off with.
Prey tell, why do they do that? And what happens when you drill it out? I know sombody has tried it.
Actually the trailing plug hole is smaller because they don't want any leakage past the apex seals into the next chamber. The location of the apex seal as t crosses that point is a very critical one in terms of where the most pressrue is in the chamber. We'd lose efficiency if it were larger. It actually doesn't do a whole lot for power.
that's backwards from what I thought. I figured that the heat would be somewhat less for the trailing plug so it would be hotter. What do you think the theory is behind the stock set up. Also the trailing plug would be somewhat protected from the flash by the small hole.
Rich, I think the reason that we run colder plugs on the trailing side is because that plug never gets cooled off by a fresh intake charge, unlike the leading plug. Like R.G. I plan on running 9's on all 4 corners when I get my turbo. Like you once did, I am able to use the factory plugs/heat ranges even with the nitrous because it is only used in short bursts. However, it should be noted that I once raced a nice 3000GT and forgot I had the nitrous on. We went at it for quite a while and I have never had a problem. A 55 shot is mild as far as the Renny is concerned. The turbo should be well-received also.
With the leading plug firing before the trailers, except at idle, I am assuming that the trailers see much less cool air than do the leaders. Use intake and exhaust valves on a four-cycle as an analogy. Even though they are in the same combustion chamber they are exposed to vastly different environments. I may be wrong but that's my observation. I wonder what Abrams and R.G. think. The NGK website does a great job of explaining the other lesser-known jobs that spark plugs perform. I am a pretty smug know-it-all and I learned a few things from the site. Check it out.
Last edited by Charles R. Hill; 02-24-2005 at 08:23 AM.
Do the plugs get disected when the engine is firing. for instance, the rotor/apex goes by the first hole(leading plug) and it fires while in between the leading and trailing plug then the trailing plug fires. Or do the plugs fire while the rotor/apex is past them both just at offset times to get full combustion? not quite clear to me.
I am thinking there are two issues at play. First, that the leading plug spends more time immersed in the flame front as it burns(because that's where ignition starts) and, second, the direction of the flame front as it travels through the housing. It seems to me that the trailing plug spends far more time surrounded by heat than does the leader. What may be more important than all of this might be something as simple as plug shrouding differences. Again I am only guessing. This is one of those issues that the details are far less important to me than the fact that others know far more about it than I do and they can direct me to the proper parts use. For me, this conversation is strictly academic and a challenge for my puny brain.
Charles, your logic is not far from mine. Except it must be wrong because it is backwards. Mazda uses the colder plug in thhe trailing hole. NGK plugs get colder with the higher number. Most others get hotter the higher number. The new way of listing plugds like with Autolite and now Champion have no reason to exist, there is no connection to range or ither specs.
It used to be the letters told yo the plug configuration. Like a 3/4 reach 13/16 hex plug was a AG in Auto lite and N in Champion. Then the number was the heat range ie. 42 in Autolite and 4 in Champ. The letter afterwards was the tip type like R is retracted or y is extended and so on.
Top fuel plug would be an Autolite AG 403 or a Champion N54R.
Man, now that is something I hadn't though about in 30 years and I still remember all those plugs we ran. Want to know the plug for a Studebaker R-3 at Bonniville? Glad you asked, J61Y Champion. A good alt in Autolite AZ2. (And that was 40 years ago.)
A blown gas big block Chevy at the salt? AG901 or N57Y.
Wanna know the Hilborn nozzles and jets? No! OK then you don't have to.
Last edited by Richard Paul; 02-24-2005 at 01:58 PM.