Black Smoke from Exhaust? And Tire Sizes that Fit...?

[RDKamikaze]

Ok, a few things come first. It's been a while since I've been here, and I haven't read up on anything about black smoke from the exhaust yet.

I was racing a friend of mine one day and he noticed that when I shifted to 3rd, my car caughed out a bunch of black smoke. He suggested that it could possibly be something with the o-rings in the piston, but I'm unsure. It has 37k miles on it, and I always drive it like an old man (unless I'm racing, which is hardly ever). What could be the problem? I have JP header, Injen SRI, and DNX muffler.

Situation #2. I have 8.5"x18" wheels with 225/40/18 tires on them right now, but it's getting time to get new tires. What's the best size area that would fit that wheelbase with our car's height? I'm thinking 225/45/18, but can I do 225/50/18 or 215/45/18 or 235/45/18? I don't (and probably never will) understand the first 225/40/18 part (bolded). It's mm across, right? So how do you tell what fits your wheel?

Thanks for the help, and I have to get pics soon! I've gotten mud guards, took of emblems on the back, and cleaned out the car a bit. It looks better now

[ThatVietGuy]

#2: http://www.miata.net/garage/tirecalc.html

[DatSRBoi]

Black smoke means rich. Its normal and safe. When your near redline and you release the throttle fast, The car is taking in less air but still dumping fuel. Thats where you get the black smoke. On a lower mile car you dont see it often but when a car breaks in or if you tune your car and usually it only shows up on a manual tranny, youll see a puff of black smoke on shift. Some are noticable and others arnt.

You also usually dont see it on a stock car because the flow system is restricted "to keep the car quiet" that its hard to see the black smoke because the car isnt taking in allot of air in the first place. But look at american car with the huge V8 liters shift and youll see alittle but their exhaust is more open for flow.

[Sebas007]

225/40R18 tire
18X8.5 wheel

1" = 2.54cm or 25.4mm

225mm /25.4 = 8.85"
you have a 8.5" so your tire has a 0.35" extra (no problem there).
So on each side of your wheel your tire has 0.175" extra

The first number (225) will affect your tire sidewall height just a little bit. You have to understand how the tire sidewall is calculated (example : 225/40R18) 40% of 225 or 225mm * 0.4 = 90mm or 9 cm or 3.54"

So you can imagine that the 225 passing to 235 will lightly increase tire sidewall...go with 225/45R18

[RDKamikaze]

Thanks for the info on the exhaust. That helped out a ton, so that way I know I'm not burning oil or anything now.

For the tires, thanks for the info on that too! I was educated by a kid in my Auto class today about tire sizes (he works at Discount). I was planning on sticking with 225/45/18, but I wasn't sure if something else would be better. Thanks for that help, too

[Fletch]

Black smoke: fuel, usually running fat
White smoke: water, typically condensation in exhaust @ start up
Blue smoke: oil