Simple exhaust question

[hat_man]

I was reading some of the exhaust threads to get some insight but can't find quite what I'm looking for. The exhaust pipe on my '97 Ranger rotted off some time ago. All of it....all the way back. It didn't sound all that bad so I just added a short turn down piece and left it. Now that the muffler is going I am looking at options for a new exhaust. I have a couple of questions though.

First...does anyone think the lack of back pressure is hurting my FE? I have read mixed reviews on here but the general idea is a resounding "maybe". I am planning on replacing it but was considering a custom bent pipe with as few bends as possible. Something as straight as I can realistically and physically get.

Second...this little truck still has the original catalytic converter. 220K miles of exhaust have been through it. Time to replace it? If I do (and probably will) does it help FE? I mean in the sense that a new one would be more efficient than the old one on there now. Definitely will help the environment. BTW no emissions testing in Illinois for my truck.

Third...Does it make a difference where in the exhaust system the muffler is located? Right now I have to use an 8" round muffler. All the catalogs call for an oval muffler with offset intake/outlet. I tell the counter guy it won't fit...he says "It should...that's what's in the book". I take him out and show him and he scratches his head. Happens every time. Oval rubs the driveshaft. Can I move it farther back to a more open location? Does it need to be close to the cat? I know some cars have the muffler at the very rear but I don't know if they were designed that way or not.

As for pipe diameter I wasn't planning on changing that from stock. Ford seems to have done a pretty fair job with the Ranger in terms of FE but there is always room for improvement. Unless there is a huge consensus that says Ford made a mistake with exhaust size, I think stock is the way to go.

Well that's about it. Any advice is greatly appreciated.

Thanks

[Daox]

1) Lack of back pressure will hurt FE. I've seen it multiple times with my own vehicles as well as with others. Fix it and it goes back up. Leaks and breaks kill low rpm torque.

2) If you want to think about reusing the CAT you can try looking through it. See if there is any physical damage or clogging. Replacing it at that mileage is probably a good thing, but chopping one out and clamping a new one in isn't a big deal either, and if you get say 15k more miles thats another year or so with the truck.

3) Moving the muffler will change how things sound, thats about it. Emissions wise it doesn't matter, functionality wise either. It'll just change the tone.


Smaller piping will increase lower rpm torque at the expense of high rpm torque (less peak horsepower) and also has the potential to increase FE. I wouldn't even think about going larger.

[some_other_dave]

Backpressure itself will not help fuel efficiency at all. Adding an exhaust that makes some backpressure under some operating conditions (mostly talking about how much is flowing through and how fast) can easily help or hurt FE, depending on what range of flow is helped or hurt by the exhaust.

Clear as mud, right?

You're not actually going to know until you try it out, and the results from fitting one exhaust may be fairly different from those for fitting a different exhaust.


A catalyst cannot really help your FE. One that's broken up and partly clogged can certainly hurt flow and FE.


The location of the muffler can affect efficiency, as it affects the other components in the exhaust. For instance, it could give you less room for long primary pipes (or long secondaries) if you put it too close to the engine. Or you could put it somewhere that needs lots of tight bends in the exhaust pipes to reach. I doubt that putting it in the middle of the vehicle (which seems to be what you are implying) or the tail end will affect things noticeably, though.

Exhaust design is a mix between art and science. You can build things that are optimized for a given RPM, or if you're lucky a decent range of RPM. In general, long narrow pipes are better for slower flow (i.e., low RPMs) and short fat pipes are better for faster flow (i.e., high revs). You can also tune the lengths of the primary and secondary pipes to reflect the exhaust pulses back as mini-"vacuums" which will help flow. And of course, all of the bends affect how the gases flow.

And there are exceptions to pretty much every rule, because what fun would fluid dynamics be if they were actually easy to calculate?

-soD

[RobertISaar]

think about the term of "backpressure" itself.... it's pressure(which itself is a measure of restriction to flow) between the tip of the tailpipe and the beginning of the exhaust port in the head.

resistance there is simply more work that the piston will have to push against to get the exhaust out of the combustion chamber. if anything, "backvacuum" would be beneficial to efficiency.... it would literally suck the piston up during the exhaust stroke. this exists in a relative sense, go from a sea-level altitude to a mile up... the volumetric efficiency of an engine actually rises due to less force from the atmosphere pushing exhaust gasses back into the combustion chamber. of course, since the air is significantly less dense, the engine doesn't create as much output either.

a lot of the times, when people install exhaust systems that "lower backpressure" and see/feel a hit to power/economy at lower RPMs, it's because they lowered backpressure at higher RPMs..... and raised it at lower speeds. an engine is a series of compromises, if you desire to tweak them out of factory specs, you'll get out of factory results.

[gone-ot]

ANALOGY: car exhaust system = tuned pipe

...so, unlike a trombone where pipe "length" is adjusted to coincide with the desired "frequency" (musical note), the exhaust system stays the same length as the engine rpm varies between idle and maximum rpm...and, thus goes through a series of "in/out phases" in the process...where sometimes engine breathing is "helped" and sometimes the engine breathing is "restricted."

[oil pan 4]

If you are not required to have it replace it with a pipe.

[chumly]

The cat can't clog up if it isn't there, I did it years ago on my old 81 Datsun pickup and never had any trouble. Chumly