kc5zom

I wonder what the Mazda ECU would think about that? Why don't you write a lengthy explanation for the entire board to read?

Or you could just say something stupid in one line like "you DON'T need back pressure".

Silver5

i dis-agree...you need back pressure unless you have any kind of force induction, if your going Normally Aspirated then you need back pressure...remember guys our cars are only 4 cyl. it's not a big block, its factory compression...unless you balance and blue print your motor and raise the compression who need's back pressure. why do you think these ALL MOTOR racers in the circuit run 10's all day long, because they bore them out and raise the compression. i ran nitros @ one point then i switched to turbo in my civic 8 years ago, i took my exhaust off from axle back with a hi flow cat...straight flow!!!!

macdaddyslomo

OK I'll revisit this again for the Man,I'm sure he is tired of explaining it....YOU DO NOT NEED BACKPRESSURE...however you also dont want a 3" exhaust on a 4 cyl non turbo engine..WHY?...well to keep it brief,you want as much flow as possible,but you also need exhaust velocity....for the dummy's you want to move a lot of air but you need to move it fast...a 3" exhaust would move large volumes of air,but not veryquickly...a 1 7/8" exhaust wont move a whole lot of air but it'll haul *** out of the pipe..what is the premium setup??? somewhere in between...best would probably be between 2 1/4 to 2 1/2" pipe...this gives a balance between flow and velocity....i.e.: you can suck air thru a coffee stirrer and the air moves very fast,but not a whole lot comes out...you could also suck air thru a toilet paper tube but you cant get any power behind it....but if you use a mcdonalds straw and you move a lot of air but still fast...not the greatest comparison but you get it....back pressure is just that..pressure.....the exhaust gas isnt moving fast enought away from the exhaust ports because the pipe cant flow enough air to keep up, so it creates pressure that "backs up"...this is in no way beneficial to an engine....hope this helps!

kc5zom

That wasn't even the issue I was bringing up. The issue is if he keeps his stock ECU. The stock Protege ECU is tuned for a certain amount of back pressure, changing that too much in either direction is likely to throw the engine out of tune and cost him output.

But, if you really want to get picky about it, and you have a standalone ECU or a vehicle you can tune it for it is best to have a good header and scavenge.

macdaddyslomo

wtf?? the ecu doesnt measure exhaust pressure!!! the only sensor in the exhaust is the 02 sensor and that is for emissions...I can see why the Man gets pissed off sometimes!!! where the hell did you come up with that...the stock ecu is tuned for some backpressure???

kc5zom

Think about this for a second (I know its hard for some of you):

On the exhaust stroke the ECU "expects" the exhaust to place pressure on the gas escaping the engine (no, of course there is no exhaust pressure sensor, but if it was it could be called EPS). Needless to say there is only a limited amount of time before the valve is closed again, the time when the gas can actually escape. What gas that does not escape remains in the cylinder, the ECU is programmed to expect this and actually adjusts air/fuel mix and timing to compensate for it:

You want a certain amount of air/fuel in the cylinder before it is fired, right? In a normal exhaust there is positive pressure when the valve closes (because of backpressure). The intake timing is set for this, it only brings in the air and fuel it thinks it needs for a good burn. If it brings in too little (there isn't enough air left in the cylinder because of too little backpressure) this effectively lowers compression (is that good or bad?). There are other effects at work here also. There is a certain amount of unburned fuel expected in the remnant exhaust. The ECU expects this and trims fuel to insure a target burn (although the ECU does not generally shoot for a proper burn, I believe they actually run lean, for fuel conservation at least that is what old carbs are tuned for). If that fuel isn't there then you have created a lean condition, more useable horsepower lost. You can add other factors and make this even more complicated (less positive pressure in the cylinder, too much air makes it in, etc). I'm just really trying to make this simple.

Therefore lowering backpressure for an engine that does not expect it can cause one of two things:

1. A power gain, but not as much as would be gained from tuning the engine for it. Power gained from the engine being able to rotate easier (the power gain from backpressure loss).

2. A small power loss, if the engine just really does not expect it. This mostly shows up on V8's because what is small loss/gain on a 4-cylinder can have a huge effect on a V8 (more cylinders, generally less tuned from the factory).

And of course there are the issues of a pipe being too big, as previously mentioned by others. And there are the issues of exhaust wave effects and properly tuning those for the ECU or the ECU for them.

macdaddyslomo

that sounds backwards too.......the ecu adjusts for back pressure inside the cylinder???...there isnt a sensor inside the cylinder wall or the valves either!!!!...pressure is sensed inside the manifold, and air flow is sensed by the maf and fuel and timing is adjusted/monitored for premium running by the ecu with these signals...along with the 02 sensor in the exhaust manifold and the TPS(throttle position sensor)..help me out if I missed one.....cylinder pressure is not measured/monitored by the ecu!!!....what the hell do you think would survive in there to send the signal!!!!If your ecu had one,you'd never need to get compression checked would you????....not to mention you talked about the ecu shooting to run lean...that is based on manufacturers programming and actually if you talk to any of the moderators on this board they will tell you Mazda's always tend to run rich .....once again back pressure does nothing for an engine...it us just a popular misconception because for exhaust to be optimal there has to be a balance between flow and velocity...so a little back pressure is there on a N/A engine because usually if the pipe size is large enough to remove all backpressure the velocity is so low you end up with low end torque loss,though you probably feel all right at WOT,and may have a very small gain at high rpms....whew!

kc5zom

Its all in the code. Like I said the ECU "expects" the pressure to be there. It does not have any way to verify it. All they have is an educated guess based on the characteristics of the exhaust design and readings from other sensors. Programmed by the engineers before it leaves the factory!

If Mazda tunes their ECU to run rich at all times then there would be a point where reduced backpressure would lead to stoichiometry and after that too much reduced backpressure would move toward lean. Also like I said it depends on the engine and ECU. Some engines will not experience a power loss. But if tuned properly they will gain even more power from the reduced backpressure.

The fact of the matter being that there really is no way of measuring backpressure on the engine from the cylinder, but the air left in the engine does create a positive pressure (I've heard that extreme scavenging can leave pressure less than 1ATM at TDC, but I've never seen anything written about it).

I just wanted to use a lot of exclamation points here!!!!!! And some question marks here?????

AFireInside

I have a cold air intake, no second cat, no resonator, 2 1/2" pipe and a straight through muffler. Right after I had the cat removed the car ran kinda weird so I had the car scanned. It had thrown 4 codes!?! How did this happen? The rear cat had no O2 sensors on it. Is it because my car is running lean? How do I richen my mixture? I also run a 75 shot dry nos. I need help. My car runs like sh*t.

LinuxRacr

Did you research your modifications thoroughly before implemntation?