SAFC2 untuned. Just waiting to get my hybrid in and switch to 1000cc secondaries.

 

Don't completely leave out the backpressure issue man... it makes a big difference.

And Digi7ech I feel your pain!! haha... man that CS exhaust is LOUD. I had JUST THAT with all stock for the rest... and it was still incredibly loud... so when you say people hear you a mile away... I don't doubt they hear you at maybe 2 miles away, then at 1 mile away they're sure it's you. lol


But relating to this thread... you guys with smelly cars, you're running way to rich it sounds like... I have a gutted cat now (MY CAR DID IT ON ITS OWN!!! I SWEAR... I even made a thread with proof a while ago haha... my car blew the bits of the cat out with backfire) and it runs pretty good, sounds pretty cool too (I have the stock exhaust back on now... gotta get it checked off for a modified exhaust ticket).

Dunno on gas efficiency... about the same time I swapped them I got an SAFC. I need to get a wire run to my O2 sensor so I can at least semi-tune it.

--Gary

 

What I described is backpressure. It's a terrible term, but all it means is the pressure in the pipe at the point furthest from the end, cause by the restriction of the exhaust system.

 

Sort of... backpressure are the waves of pressure that bounce back TOWARDS the engine... they are good. You want a certain amount of backpressure, opening up the exhaust too much will make it produce less power.

I thought this was only true for piston engines but from my own experience... it's just as true for rotary.

Another important thing, not sure what the term for it is but it's not backpressure... but I guess you could just call it the venturi effect... that is the exhaust already exiting also creates a pressure as it goes along which "pulls" exhaust out of the chamber on its exhaust stroke. So both of those are important when choosing the exhaust style.

--Gary

 

And I show up 300 miles away to visit the girlfriend, and she says "You smell like car again."

S4 NA, Racing Beat presilencer/cat replacement, no other emissions stuff, and it stinks a decent amount. People following me usually comment on it, and I've had to make a point of informing EVERYONE following me on cruises that I do in fact emit some interesting smells - last cruise, the '72 Stingray Corvette driver commented on the fact that he was a bit concerned about his engine for the first 20 miles, because he kept smelling oil & exhaust, then he remembered I had commented on it. He was 2 cars back, and we weren't holding a tight formation.

I have a SAFC-II wired in, and if I turn the gas up at idle (10% or so makes it *really* happy), my exhaust is really nasty. Lean it out at idle, and my exhaust is a lot less horrific, but the engine isn't as happy.

So... in summary? How is life with no emissions? Pretty much normal. I don't give rides to anyone who is picky about such things, I don't mind the smell (it's actually kind of comforting), and I seldom get tailgated.

-=Russ=-

 

so you guys are saying that maybe our cars are way outta tuning? then ****, my car idles great, but i did notice that my air/fuel mix is a bit rich. i just thought that the faze gauge was not up to par with the rotary engine.

 

Stock tune on a FC *is* pig rich. The Mazda ECU seems to believe that if a little fuel is good, a lot is better. Launching flames out of the tailpipe should be a good clue as to the stock tune.

I'm willing to bet the standalone guys without cats don't have exhaust that will kill small furry animals.

-=Russ=-

 

Gary though my experience in air flow is not with cars I think I can shed some light on both of these myths because suprisingly they are related.

First I will adress your "venturi effect." They venturi effect AKA the bernoulli principle is simply that when air travels from a small pipe or through a small hole to a large pipe for some reason it will create a ring, donut, or torus of higher pressure near the walls of the larger tube that extends well down the tube and thus have a zone of low pressure in the middle. This low pressure will not "pull" more air along becuase remember in the case of the engine the high pressure on the other side of the exhaust port is already expelling the air. Another way of looking at it is that the air in the engine is trying to equalize pressure with the outside world while it has access to the exhaust port.
The thing is that the difference in pressure between the outer high pressure ring and the inner low pressure ring in the pipe will cause turbulence. This is where backpressure comes in. That backpressure will equalize some what the difference between the layers and smooth out the turbulence. One way to eliminate this difference is to match your headers and pipe to the exhaust port. And I mean all the way to the cats/presilencer/muffler/whatever. Your exhaust should be made so that there is no major change in cross sectional area except when multiple exhaust pipes join and then the cross sectional area of the inside of the downstream pipe should only equal the sum of the cross sectional area of each joining pipe.
So if you want to get best performance(and this is for nit picky tuners) and not have to worry about/eliminate back pressure here is what you do. PORT and POLISH your block, MATCH your headers to your ports, and be sure that those headers don't all of a sudden flare out because now you will lose flow to turbulence.

oh yeah and all you stinky drivers here is an idea. Get on a dyno and tune a little bit. If your car is tuned it will burn clean anyways and not smell like a burning mountain of tires.

 

hahahahahah burn baby burn. well opefully today after i get off work i'll take my car direct to tune her cuz i was stuck in the holland tunnel and i wanted to die.

 

That's called resonant tuning, and applies to extractors and exhaust manifolds, but not complete exhaust systems. I've never heard the term backpressure used to describe this effect. It is used to describe the average pressure in the exhaust system, and is created by the resistance to airflow from the pipe's surface friction, bends, diameter changes, obstructions, etc. Put a pressure gauge in the exhaust system (the O2 sensor bung is a handy location) and go for a drive. The reading you see is backpressure. If you measure at several locations along the exhaust you'll see pressure getting lower as you get closer to the end. If you replace the exhaust with a bigger or and/or less restrictive one and you'll see lower backpressure.

 

Yes, and to expand slightly, resonant tuning is only good for a narrow rpm range, so you must decide where you want to have optimal "scavenging" by this negative pressure (ie, at ~4000 rpm)

 

I have the RB presilencer with catback too, i noticed a noticable increase in power in the high end, but it definately lost some power at low rpms. It doesnt smell that bad either, i only notice it when i am idling in traffic, when the exhaust fumes drift toward the front of the car.

 

Well i run staright pipes and apexi n1 knockooff mufflers (option brand) and its considerably louder than stock but you feel the kick more, IMO. As for the smells i never noticed anything unless i was running ti in my garage (pipe out the door), in which case my eyes burned and teared. On the street i never noticed a thing.

 

i feel good at the fact that the people driving their jaguars, hummers, lexus's etc cant enjoy their vehicles with their windows down. (the ******* inside of me) haha. One other thing is that it is really loud. Almost too loud. well it is. -alex

 

well my SAFC shorted out and I havent had the chance to buy another so....

 

Mmmm, thanks for the explainations NZ and ironpanther. =)

--Gary

 

i don't have a cat and it doesn't stink at all, but maybe i'm just used to the smell of it. with my header and no cats it IS a little louder but just the right amount IMO...i feel for u guys with emmisions laws lol

 

I have a full RB RoadRace Header/exhaust system. I NEVER smell anything. It is relatively quiet, but sounds good 2 me. Smells like U stinky guys need some tuning or something. Maybe U have the wrong "combination" of mismatched or hodgepodge headers /downpipe /mufflers. Maybe U should be using a system that is properly designed to work with your Car instead of a bunch of different components. My old exhaust was all burned up & sounded like a swarm of really angry bees. Now it is nice & sweet at "normal" revs & really comes to life with higher revs. It just seems to me like U guys all got what U paid for, just like me. My RoadRace system was expensive but will last 4ever & looks & sounds great. My buddy says he saw flames out the tailpipe once. That's my $.02 worth.

Ramses666

 

for me with an exhaust leak. Since my cat ran away, Mine Smells really bad. People will not stay behind me for long. they either Move, or pass out and pull over on the side of the road, For those that Dont smell anything, Try Driving in reverse for a few blocks and tell me you cant smell anything!

 

I tell you one negative about having an ampty cat (and leaky one at that), is you are breathing those awful fumes. I found my throat getting real raspy and it was not good for my health haha.

After dynotuning I popped in a 3" high flowing cat (3" straight through exhaust on S4 n/a) and she smells nice now, well not nice, but like a new piston engine would if you ran a hose from the exhaust to your nose :-) breathable most other road users still think it smells awful and they drop back about 3 car lengths, even at the lights.

 

So even with the high flow cat it still smells? What cat did you use....>?


Gil

 

It smells like regular processed exhaust, the fumes arent harsh but they are breathable

Before I had a rotor, I thought this sort of smell was bad and I shouldnt be breathing it.... then when I got a rotor and gutted the cat, I discovered how much worse the fumes could be. I had the car dyno-tuned so my new investment wouldnt burn up, then got a 3" 'high flow' cat. I didn't find out what brand it was.

All that is written on the underside of my cat is: 5A/CE 98009 07/04 54505

I don't think it's possible to get rid of the rotary fume smell, because a rotor produces something like 3 times the amount of exhaust than a piston engine. Put your hand behind a piston engine exhaust and there is hardly any flow, put it behind a rotor and you could end up burning your hand, it flows a lot more.

Since I got the cat my throat has been much better :-) I can talk and people don't ask what's wrong with me

Perhaps if your really worried about flow you could get like a 4" high flow cat (if they even exist) and have that welded in. Shouldnt be restrictive at all heh.

HTH

Paul.

 

i m ok with it
but my mom + sister + friend hated the smell
so i figure i should put a Metal Substrate Cat on when i got some $$

 

Like I said, you guys just must have freaky fuel...

 

I think we have freaky fuel too.....