tech question on Leading spark plug hole location

[730RWHP12A]

does anybody know why Mazda moved the leading spark plug down on the series 5 and 6 motors? There must be a reason mazda did this..next question, if you ran the series 5 or 6 housings will it hurt power if your a N/A application? or would you make more power using series 4 or earlier housings? N/A application.. thanks in advance guys!

[Rotary Evolution]

i would chalk it up mainly to improve fuel economy. the older rotaries had the angled compression pockets and the revision came about shortly after making the pockets in the rotors equilateral/fuel injected(with the exception of the original 13B RX4/REPU/GSL-SE engines).

the change actually came on series 4 jspec engines so it came about in the '87 era.

i've used the various combos in just about every compression ratio engine(always matched housings though), turbo and non turbo and never really noticed any drastic side effects. but i never did any back to back comparisons, just too much work for sake of testing. i would tend to think the later revision should improve performance but is less forgiving with aggressively advanced timing.

if you're trying to get every ounce of power out of it i would probably use the later series housings, the chrome is superior anyways for the added benefit of longevity. but i wouldn't expect much noticable gains in performance.

if the housing is going to be narrowed for use in a 12A engine, i doubt you will get much feedback. the position isn't drastically different so i doubt you will have much trouble with it, especially if using alcohol.

[730RWHP12A]

if the spark plug hole does not make a difference in power then why are the MFR p-port housings the old leading spark plug location ?

[Rotary Evolution]

because they were made before the revision and fuel injection?

[730RWHP12A]

Originally Posted by Rotary Evolution

because they were made before the revision and fuel injection?

possibly.. but, wouldnt mazda put the spark plug holes where they would make best power/ torque? it is a MFR part...

[Rotary Evolution]

i would say it likely has to do with carburetion versus fuel injection, giving the less efficient fuel setup less room to expand between the 2 flame fronts to increase efficiency of the carb versus the later model fuel injection.

but the differences are going to be very minor and not likely worth worrying about.

i would be willing to bet that any direct injection engine in the future will show an even further gap between the plugs and more a retarded leading plug position(moved down in the housing). i doubt you will find much research outside of the mazda papers on this, it's simply too much work doing back to back comparisons. even ambient condition changes and minor setup differences would skew results.

it would be interesting to know the exact benefit and drawback of each, but i'll be damned if i'm going to go through it all. heh. i'd also be interested on the difference of running leading only ignition versus the trailing assist in performance, i do plan on doing some testing of those combinations but it will not be on a carbed engine(well maybe, maybe not. i do have a customer with a turbo REPU which is carbed and currently running leading only). moving the lead plug position is just a royal pain to get actual figures, comparisons and results from.

[730RWHP12A]

then how do you explain this?
these motors are injected and they use 3 spark plugs.. the two bottom plugs are in the early position, not like the S5 and S6 engines

http://www.fd3s.net/787B/26b_2.jpg


http://www.google.com/url?sa=i&sourc...65447741943524

[Rotary Evolution]

looks like the R26B shifts all the plugs down even further to accomodate for the third plug. but i can't say for sure if it does use the early or late plug timing, it looks wider than the early plug gap to me.

[730RWHP12A]

Originally Posted by Rotary Evolution

the R26B looks like the late plug timing.

i measured one at sevenstock a few years back, same as leading spark location as the old motors..

the two bottom plugs go straight in, the very top plug is at a 45 degree angle

[Rotary Evolution]

Originally Posted by 730RWHP12A

i measured one at sevenstock a few years back, same as leading spark location as the old motors..

the two bottom plugs go straight in, the very top plug is at a 45 degree angle

even so, the third plug would widen the firing range. the better question is if the leading plug is in the same location as in the early engines. they did have to squeeze 3 plugs in there somehow so it may have just been a compromise they had to make. looking at a rotor housing it looks to me as if they had to move both the bottom and middle plug downward to even fit the top plug casting otherwise it would be humping the 13B trailing plug location.

i highly doubt the coolant passages were cast in the same location in order to fit everything. the housings weren't based off of any previous design specs.

[RICE RACING]

I would look at it this way:

It makes much more sense in a knock limited boosted engine to have the leading spark plug lower as when it fires it is more central to the combustion pocket = better power and more efficient.

For a N/A motor with more leading spark lead it makes more sense to have the spark plug higher as it will be more central.... and will deliver more power and be more efficient.

It would be related to the spark timing that is tolerable as to where to best position the leading spark plug. Would not loose too much sleep over it, Mazda are smart enough cuuuuuunnnnnttts and power and being efficient go hand in hand for the application.

[730RWHP12A]

Originally Posted by RICE RACING

I would look at it this way:

It makes much more sense in a knock limited boosted engine to have the leading spark plug lower as when it fires it is more central to the combustion pocket = better power and more efficient.

For a N/A motor with more leading spark lead it makes more sense to have the spark plug higher as it will be more central.... and will deliver more power and be more efficient.

It would be related to the spark timing that is tolerable as to where to best position the leading spark plug. Would not loose too much sleep over it, Mazda are smart enough cuuuuuunnnnnttts and power and being efficient go hand in hand for the application.

thank you kind sir.. im happy you replied mate.. cheers

[Rotary Evolution]

but in n/a applications the timing is more advanced, so the plug firing is closer to the leading apex tip at 35* of advance, the trailing in the 3 plug configuration is probably just before the combustion event closes right before the trailing apex tip passes over the plug port(because of its location it doesn't have much alternative unless they somehow worked out a leading>trailing>center firing configuration for optimal performance).

in 13B engines the trailing fires the closest to the center of the rotor.

[RICE RACING]

Think about it you will work it out the leading combustion pocket, not the central part of the rotor recess