jutny

I was looking at pictures of the rotor housings the other day, and realized that the trailing spark plug hole is very small compared to the leading. Now on our cars, we're always told that the trailing igniton does not do VERY much.

would there be ANY benefit to drilling out the trailing spark plug hole to match the leading hole?

it would allow for better flame propagation from the spark plug and seems to me, a more complete burn of the AF mixture.

i guess the real question is why did mazda make the trailing hole smaller than the leading to begin with?

oh well, just something that passed through my mind.

-Kurt

rxseven

Do a search, this has been covered before. There is no gain in doing this, sorry.

vipernicus42

iTrader: (4)

Sorry dude,

Mazda has exparimented with plug depth and plug number quite a bit. The trailing plugs are only there to help finish combustion and lower emissions. They have to be more recessed with smaller exposure because of their placement, the leakage they allow between working chambers and their volume. IE: every time an apex seal goes over a spark plug hole it opens a gap between working chambers allowing air to leak from one to the other. If the holes are too big or placed so that they allow a leak when one working chamber is under high pressure, that pressure will just be pushed into the other working chamber. This decreased power. Also if the holes are big, you can't compress the air sitting in the recesses, which lowers compression also compromising power.

Mazda even had 3-plug engine designs trying to improve spark exposure, but these engines were race-only engines which idled very high because they had too much leakage/overlap to allow them to run at low idle. This is a similar idea as to why bridgeport engines idle high, but in that case it's exhaust and intake air exhanging, not really air between working chambers.

For the amount of gain you'd get for opening up the plug more, you lose it through the other factors. The better idea is to put in a system like DLIDFIS which will just increase the power of the spark on your Leading Coils (which is where the power is made anyway) and give you a bigger/better explosion. Even DLIDFIS has little effect on HP, mostly the effects are seen on smoothness and emissions. The more powerful spark makes sure that you get a more complete combustion, and making combustion happen where sometimes it wouldn't necessarily.. which eliminates some 'misfiring' in the engine (which often isn't necessarily the plugs mis-firing, but really just an air/fuel mix that isn't ideal not igniting.. but sometimes is from plugs being slightly fouled an dnot able to fire for one or two chambers worth).

Well I hope my ramblings have helped. My usual disclaimer applies, meaning that I'm only human and thus am quite capable of being WRONG sometimes. So anyone who has corrections/additions/or suggestions of where I should 'go stick it' just feel free to post after me

Jon

jutny

vipernicus, thats exactly what i wanted to know. I just wanted to know why they made the hole so small.

i'm satisfied now.

karism

I allso thought about opening up the trailing hole.So i decided to try it.I opened up the hole to 8mm (0.31").Closed the motor up again.

I had to replace the rotorshousings,as the enlarged holes caused to much blowby.The engine wouldnt start with the starter.It only started when it was pushed.

2 Rotorhousings ruined ,but at least now i know.

Karis

Rx7carl

Also the reason the hole is so small is that when the chamber is in the power stroke, the trailing apex seal crosses the trailing spark plug hole. If the hole was bigger, the flame would travel thru the hole into the next chamber and that would ignite the next chamber way pre mature causing bad stuff to happen.

karism

Thats a very good answer Charl!I havent thought of that!
Thanks!~