Exhaust vs gas mileage

[VRoman]

Several months ago after some hardcore four-wheeling one of the cats began to rattle, so now I am thinking about replacing it sometime soon. But since I will be replacing cats, I might replace the entire exhaust as well. When I got a high performance muffler three years ago, it actually hurt my gas mileage by about 0.5 mpg. If I put only one cat instead of two, it will drop back pressure even more, and I suspect I would get lower mpg again. So, should I keep two cats or just one? I think stock pipe size is 2 inch diameter (or is it 1.75"?). Should I use 2.25" pipes instead? I don't have enough money to buy equal length headers, and stock seems to be pretty much equal length to begin with, so I can simply replace the pipes between the engine and the cats with bigger size ones. Will it worth it? Overall I am looking for better gas mileage and improved torque. Of course extra horse power would be nice have too. Any suggestions? Thank you, Roman.

[BAC5.2]

First of all, you are misinformed. Backpressure is the enemy of an exhaust system. The less there is, the better off you are... until you start to reduce exhaust velocity.

Chances are your "high performance" muffler, was nothing special, and probably flowed less than stock.

You probably just have a heat shield rattling, and you didn't actually toast the cat. I'd suggest removing the shield and seeing what that does to the rattle.

[J-MoNeY]

First of all, you are misinformed. Backpressure is the enemy of an exhaust system. The less there is, the better off you are... until you start to reduce exhaust velocity.

Chances are your "high performance" muffler, was nothing special, and probably flowed less than stock.

You probably just have a heat shield rattling, and you didn't actually toast the cat. I'd suggest removing the shield and seeing what that does to the rattle.

From what I hear, that's not always true. On motorcycles, there needs to be a muffler creating a little pressure or else the bike looses massive amounts of power and it actually hurts the engine.

[rallysam]

Well, I don't believe that you could actually detect a 0.5 mpg change if it did happen. So many other things are affecting your mpg simultaneously. Maybe it actually improved your mileage by 2mpg... but you had a lead foot that week, or maybe you hit a traffic jam once, and that hurt hurt your mileage by 2.5. (2 - 2.5 = -0.5)

But, it's possible. I could believe that having a little exhaust back pressure might make it burn a bit more efficiently "around the town" even though it reduces peak power. There's a couple systems on the car designed to get some exhaust into the cylinders along with the intake charge and that supposedly makes it burns more efficiently. I dunno.

[VRoman]

BAC5.2 I wish it was just a cover, but tried everything including taking all of the covers off, it is still rattling if I punch the cats.
The less back pressure than better performance, I agree with that. But 1.3 lb is pretty low already. Stock was 1.6. I think what happens is that with lower back pressure more air/fuel mixture is being sucked through the cylinder into exhaust during the valve overlap, that's what is hurting gas mileage. Am I right?

rallysam I check my gas mileage every single time I fill up the tank. It was not just one time when my MPG went down. Maybe ECU learned how to deal with the new setup months later, but for a long period of time mpg was lower than before. Also everyone I know who has a performance muffler say that they have more power with worse gas mileage. Several years ago I argued with my friend that I can make this car get 30 mpg in our city or more, now I am only 1 mpg away from this goal!

[BAC5.2]

It might be different with the cam grinds on the N/A, but since there is virtually no valve overlap in the turbo cars (what I'm used to), backpressure is the enemy. I'm not a fluids engineer, but I would imagine that backpressure is still the enemy.

I get better gas mileage when I drive off boost, than when the car was stock.

[evolutionmovement]

There is little overlap on either cam.

Backpressure is bad. I think the benefits you guys are thinking of have to do with pulse tuning effects. This backpressure would change the rpm at which pulse scavenging effects benefit most. A completely open exhaust, for example, would have 0 backpressure, but would hurt preformance as there would be no low pressure pulses to help pull gasses from the cylinder during the exhaust stroke.

Steve

[VRoman]

So... Should I replace the headers with bigger pipes? What effect can I expect? Lower the backpressure and redusce the pulse tuning effect?

[evolutionmovement]

When you have multple cylinders meeting in a common pipe you will have some scavenging effect. Say you have two adjacent cylinders that fire consequtively (as we do). When #1 exhausts it creates a pulse of sound and energey that travels down the pipe. What follows this pulse is a low pressure zone. At certain rpms, the high pressure pulse will pass the collector when the adjacent cylinder's (#3) exhaust pulse is entering. Optimally you want this pulse to pull right in behind the initial pulse almost like merging cars in traffic, though the low pressure zone will help pull the later gasses through if this low pressure zone passes the adjacebt cylinder as the exhaust valve opens. This ncreases pumping efficiency. This optimized scavenging only happpens at a certain fairly small rpm range.

There are also wave reflectoins of energy that go back to the cylinder whenever a difference in pipe diameter is reached or a new chamber/pipe meets. These reflections can be used in the same manner to help pull gases out of the cylinders (even it's own cylinder) and the rpm range this works best at depends on the length of pipe, diameter, etc. Generally, the longer the distance between the cylinder and the change in pipe, the lower the rpm range this effect works best at. Intake manifolds are designed in a similar manner. These reflected energy waves happen regardless of backpressure, but backpressure will cause the exhaust pulses to back up like a traffic jam instead of a smooth flow and this will hurt the pumping efficiencey of the engine.

In our case, since the cam timing is designed to work best at lower rpm, the best plan without changing the cams would be to tune the exhaust to work best at the lower revs in compliment to the cams. Otherwise the two would just be fighting each other.

IMO the stock system is probably pretty well designed to match the stock cams.

Jesus, I've had a few amnd I hope this thing made sinse. I get the feeling I wrote in circles.

Steve

[Tleg93]

First of all, you are misinformed. Backpressure is the enemy of an exhaust system. The less there is, the better off you are... until you start to reduce exhaust velocity.

Chances are your "high performance" muffler, was nothing special, and probably flowed less than stock.

You probably just have a heat shield rattling, and you didn't actually toast the cat. I'd suggest removing the shield and seeing what that does to the rattle.

I was getting annoyed by my exhaust rattling so I decided to tear the heat shields off the other day (finally got a jack and jack stands ) so I got under there and, naturally, had to break off all of the bolts to get the mid-pipe shield off. Now, I don't know if it's this way on all stock exhaust systems but the cat shield is WELDED onto the cat so it didn't come off. I'll have to get a pry bar in there. But anyway, it's now louder than it was before and I've noticed how desperately I need to get my exhaust system installed. I'd do it but I'm ignorant about changing hangers and all that.