Alright folks, here it is, I was reading the performance tuning book by Alexander Graham Bell, and I have the scoop, from his own mouth, on why a straight plug engine is almost ALWAYS better than an angle plug for performance. He writes "Quiescent chamber with offset spark plug end gases detonate." To translate the words of this Confucius of two strokes this means that when your plug is angled so it is to one side of the dome, it takes longer for the gases on the farthest side to detonate, so there is uneven detonation. What he says about the straight plug is "squish chamber with central spark plug ignition flame advances smoothly." This smooth advancing of the flame kernel means a more even burn and a more rapid and efficient use of the energy in the fuel. If you do choose to use a slant head you should NOT point the plug toward the carb side of the engine, this CAN lower performance and create hot spots. It is best to have the plug facing toward the exhaust so the flame kernel is directed toward leaving the engine. He does not recommend an angle plug, nor do I. Stick with the straight plug if it fits, if you need to use the angle remember, face it toward the exhaust. Let the comments begin!
why to not use a slant head...
- Thread starter LS614
- Start date
No matter what, yes, a slant head causes an imbalance. It is the lesser of two evils to have the head on the way you describe it, but a straight plug is better. Also, be careful torquing the headbolts. I recommend getting sick bike part's headbolt set and loctiting it into your engine block. This will be much better and more secure than the stock bolts if you still have those in.
I always thought the angled plug was a big joke with these engines . All they do is give more room for installation. The Manic billet head should have been a straight plug too. Its is just a funny coincidence that the slant head offers more compression is all.
the slant head on a GT5 does have a smaller and better shaped combustion chamber than a GT4. that should give it more compression, and more power.
in my experience, it doesn't. why? i dunno.
this is an issue that i don't see going away soon. there's a lot of people saying how much better one is over the other. but how many of these people have actually done any real testing? and how many of those engines run right in the first place? i mean, if you got one motor from here and another motor from there, and one of them runs better, you're gonna jump to the obvious conclusion that the motor from here runs better than the motor from there.
happens all the time.
but how many people try to figure out why one motor runs better than the other?
maybe one motor has less compression due to bad castings and restricted ports. it could be fitted together badly, the carb could be machined worse, the exhaust, the timing, the.........
in my personal experience, fitted to a Grubee GT4 Starfire, the slant head was worse. on the Skyhawk GT5, there was no noticeable difference between heads.
a few people i respect claim the slant is better, and a few people say it's not.
out of the multitudes i don't respect (just kidding, i mean the people i don't know
) some say it's better, some say it's not.
another problem, is the manufacturer of the motor. maybe someone's slant head has a better design than others, or maybe it just works better for their motor and their motor only.
according to "experts" like Graham Bell, it's a bad design.
according to Pablo at SBP, it warps easily, but has more power.
according to me, it didn't pass empirical testing.
according to some other guy, it's freakin' awesome!
and according to this forum, it's another subject that won't die until something better comes along to argue about.
in my experience, it doesn't. why? i dunno.
this is an issue that i don't see going away soon. there's a lot of people saying how much better one is over the other. but how many of these people have actually done any real testing? and how many of those engines run right in the first place? i mean, if you got one motor from here and another motor from there, and one of them runs better, you're gonna jump to the obvious conclusion that the motor from here runs better than the motor from there.
happens all the time.
but how many people try to figure out why one motor runs better than the other?
maybe one motor has less compression due to bad castings and restricted ports. it could be fitted together badly, the carb could be machined worse, the exhaust, the timing, the.........
in my personal experience, fitted to a Grubee GT4 Starfire, the slant head was worse. on the Skyhawk GT5, there was no noticeable difference between heads.
a few people i respect claim the slant is better, and a few people say it's not.
out of the multitudes i don't respect (just kidding, i mean the people i don't know
) some say it's better, some say it's not.another problem, is the manufacturer of the motor. maybe someone's slant head has a better design than others, or maybe it just works better for their motor and their motor only.
according to "experts" like Graham Bell, it's a bad design.
according to Pablo at SBP, it warps easily, but has more power.
according to me, it didn't pass empirical testing.
according to some other guy, it's freakin' awesome!
and according to this forum, it's another subject that won't die until something better comes along to argue about.
I must say that although I agree w/baird's review - a "slant head" or angled spark plug does facilitate plug indexing... which is admittedly also a controversial practice (that I don't bother with TBH) and isn't mentioned by any of the slant head retailers lol
Spark plug - Wikipedia, the free encyclopediawiki said:
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i index all my plugs. old habit. have i noticed any difference?
no.
i also file my plugs to produce a better spark. another old habit. have i noticed any difference?
i think so.
i'm trying to figure out how to re-curve the distributor on these things...
no.
i also file my plugs to produce a better spark. another old habit. have i noticed any difference?
i think so.
i'm trying to figure out how to re-curve the distributor on these things...
I got a slant head China and MM billet physically in my hands right now as I type. No amount of indexing this plug to me moves it back to the center of the head. I can see the argument clearly as I look at it. While it can be deemed to make a difference and I understand that. It won't put the plug dead center in the head where I think it belongs.
I know that I argued this before as well. But if we sound it all out for a second. Which direction does the piston tend to twist? This question occurred to me the other night.
Now lets say perhaps the kernel belongs on the exhaust side? Perhaps one could argue it takes the twist from the piston. We just put more heat there..heat at the exhaust port. A motor well tuned running at its proper momentum verses load tends not to twist a piston and oval out a bore. The exhaust ports on these motors always wear out first I see no purpose for the combustion chamber or the piston here. Other than clearing the bike frame. In my humble opinion.
I think also if this motor ran in reverse the exhaust port would perhaps run cooler? Dunno? It is what it is.
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i seriously doubt the angle of the plug is going to make any differance on such a low performance engine as the HT. better squish band and more compersion would, on the HT the transfer ports are on the sides of the cyl. the intake port is never open directly to the cyl. the slant head is for frame clearance for ez mounting of the engine. in a high performance 2 stroke the chamber is hemispherical and you would want the plug in the center. many non hemi engines have a pent roof chamber and the plug is set at an angle to reach the center. it all depends on the shape of the chamber.
There is probably more than one chamber shape for the different 'straight head' manufacturers... All I can say is that I compared a RAW 'slant' to a RAW 'straight' head, & I could see that the slant had a smaller combustion chamber.
I switched the heads on two of my bikes for a test & I could feel a noticeable difference in the power of both!
I still think the slants are ugly & I have heard all the stories about them warping; (but the one's I've installed are doing just fine.)
I switched the heads on two of my bikes for a test & I could feel a noticeable difference in the power of both!
I still think the slants are ugly & I have heard all the stories about them warping; (but the one's I've installed are doing just fine.)
On my slant head the plug is at an angle, but the the electorde is still in the center of the combustion chamber. I fail to see how that will make any difference at all to flame spread. Depending on plug indexing it could even be better because there's the possibilty if it's just right the electrode won't be in the way in any direction. That would be impossible to say about a straight plug.
My old CZ 250 had 2 coils and two spark plugs that fired simultaineously. If you indexed those, you'ld have the ultimate combustion chamber of all, right?
Come on designers and machinists, get with it, give us dual angled, 2 plug high-compression heads for our HT's. Yeah, right!
Some old Yamaha's and a few others had 2 spark plug holes, but only one coil firing one of the plugs. Kinda silly. You could always install a compression release in the second hole though....handy on the 360's back then.
Whatever.....
A hemi domed head with a center located plug seems to work best, indexed or not. But iridium plugs.....that's another story.
Come on designers and machinists, get with it, give us dual angled, 2 plug high-compression heads for our HT's. Yeah, right!
Some old Yamaha's and a few others had 2 spark plug holes, but only one coil firing one of the plugs. Kinda silly. You could always install a compression release in the second hole though....handy on the 360's back then.
Whatever.....
A hemi domed head with a center located plug seems to work best, indexed or not. But iridium plugs.....that's another story.
i tried running 2 coils off of one mag, it would only fire one plug, spark bounced back and forth between the 2, not in any order either.
also tried 2 plug wires spliced out of one coil, still would fire only one plug.
also tried 2 plug wires spliced out of one coil, still would fire only one plug.
I honestly doubt there would be any advantage running dual plugs in any size bore. Fuel injection on the other hand......
