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Old 07-14-2008, 05:19 PM   #1 (permalink)
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Exhaust system question

How, if at all, will replacing the exhaust system or just muffler increase fe? A friend told me that a new, better muffler will increase fe but he couldn't explain why.

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Old 07-14-2008, 05:29 PM   #2 (permalink)
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Some exhaust systems have two cats to meet california emissions standards. Deleting one of those cats can have positive fe implications. A freind of mine just did just that on a mazda speed 6. I guess regulation of emissions went out the window when he moved to KY.
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Old 07-14-2008, 07:36 PM   #3 (permalink)
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A new performance muffer and exhaust system might improve your torque and hp but it is unlikely to have a positive affect on fe. It might negatively affect it though. Most performance exhaust systems are designed to boost torque and hp at higher RPMs. If you are not driving in those ranges, you will not see an improvement. Improving higher RPM performance sometimes adversely affects low RPM performance. If you are seriously considering a new muffler or exhaust system, I would look at Dyno charts if they are available for the RPMs that you normally drive in.

Performance exhausts are less restrictive and allow the exhaust gasses to flow more easily out of the engine. They can also be substantially lighter (especially on a motorcycle).

Here is a good link:
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Old 07-14-2008, 08:02 PM   #4 (permalink)
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so, does this mean that changing exhaust system is not a good consideration for increasing fe?
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Old 07-14-2008, 10:52 PM   #5 (permalink)
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what about the weight difference between stock cat back and aftermarket cat back?
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Old 07-15-2008, 12:55 AM   #6 (permalink)
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Before we go replacing the stock system, realize the goal of the new piping and flow.

"Performance" exhaust systems generally allow a higher flow, at higher RPMs. Eco-Drivers tend to drive in the lower-RPM range.

Here's a LINK that has some good info, including the Basics.

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Old 07-15-2008, 09:34 AM   #7 (permalink)
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Depends on what vehicle we're talking about and the specifics of the mods. A number of years back Hot Rod did a flow bench test of various OEM and aftermarket "performance" cats to determine which had the best flow for a given pressure drop and IIRC a number of OEMs came out on top (Dodge Viper was one?).

Mufflers are a different can of worms because there's so many different designs its hard to say what's better in which situation. It's true that backpressure is bad and restricting flow at all is inefficient, but any exhaust system at all will have some backpressure associated with it. The acoustical tuning of an exhaust system can have a significant impact on your torque curve and can even flatten the peaks and broaden the curve without changing the cam profile.

A long-tube header 4-1 or a tri-y header 4-2-1 are different ways of trying to do this that will allow some acoustical tuning to shift torque lower in the RPM range. "Shorty" or block-hugger headers offer nothing but a prettier and possibly lower restriction option to cast iron manifolds on 30+ year old engines.

I would be wary about saying aftermarket is lighter. Many aftermarket systems have dual outlets where the stock system would have a single outlet. Often it's added weight for no benefit.
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Old 07-17-2008, 10:45 AM   #8 (permalink)
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I have a 88 crx hf new to me and it has a huge heavy stainless after market getto blaster
and I think that it hurts my fe, I think these older lean burn vehicles especially pre vvtimeing engines relied on the small tight exaust back pressure to complete a efecient burn considering the valve overlap that was re quired to reduce the hi nox that they produce, so anyway I am going to put a oem exhaust and sell the cat back to a racer the problem I am not looking foward to is finding a oem that has oem back press, I had a cheap shop put a new muffler and tail pipe on my 3/4 ton chev and it sounded loser thjsn the holy one, chinise junk, is there a way to tune the stainless one it looks well made?,thanks Paul
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Old 07-17-2008, 03:36 PM   #9 (permalink)
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Quote:
Originally Posted by RP-CLMBR View Post
I have a 88 crx hf new to me and it has a huge heavy stainless after market getto blaster
and I think that it hurts my fe, I think these older lean burn vehicles especially pre vvtimeing engines relied on the small tight exaust back pressure to complete a efecient burn considering the valve overlap that was re quired to reduce the hi nox that they produce, so anyway I am going to put a oem exhaust and sell the cat back to a racer the problem I am not looking foward to is finding a oem that has oem back press, I had a cheap shop put a new muffler and tail pipe on my 3/4 ton chev and it sounded loser thjsn the holy one, chinise junk, is there a way to tune the stainless one it looks well made?,thanks Paul
I think the best bet is probably an OEM system. Does it have the original exhaust header? If the engine is running well, the original diamater of the piping from the header-back will likely give you the best FE. Muffler choice probably effects the situation the least, but I wouldn't choose a "high-flow" style. Look for OEM-style aftermarket versions.

Many of the aftermarket exhausts are either poorly designed for the application or are designed for running at WOT/Redline.

Honestly, I haven't figured out Backpressure vs. Pulse Width vs. Diameter of Piping vs. Length vs. Muffler Design etc. It's a complicated array of variables that I've left Honda to figure out in the beginning. Check out my links a couple posts back -- it sheds some light on the issue.

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Old 07-17-2008, 04:23 PM   #10 (permalink)
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Backpressure is bad unless you like using engine power to forcefully expel exhaust out of the engine against its will.

Not sure what you mean by Pulse Width.

Diameter of pipe affects the backpressure of the system but also the speed of exhaust flow through the pipe. Larger diameter decreases backpressure by reducing resistance to gas flow but the flow slows down.

Length of the pipe in any area, combined with the diameter, determines how you tune the acoustics of the system so that a low pressure wave intersects the exhaust port during valve overlap to scavenge the exhaust from the cylinder and prevent reversion flow. Length cannot be set in a vacuum as the speed of flow as defined by diameter and engine size (mass flow rate) informs the length choice.

Length/diameter have to be paired. Longer primary tubes tune for better exhaust scavenging at slower engine speeds. Larger primary tube diameter makes exhaust flow more slowly and will tune a given length for an even lower engine speed. Excessively large pipe diameter makes the flow slow down too much, lose too much heat (changing the speed of sound in the gas), and puts you well past the point of diminishing returns on increasing pipe diameter. Length and diameter are obviously constrained by packaging concerns in most cases so compromises are made such as tuning for a harmonic of the desired tune frequency (at each harmonic of the tuned speed you see varying percent of the full benefit of the actual tuned frequency).

Muffler design is another ball of wax from tuning for maximum flow (or acoustical supercharging). It's a more standard noise reduction technique and each method of reducing noise will do different things to the upstream pulse tuning depending on the physical design. Mufflers are typically resonant chambers often broken down into smaller or multiple differently-sized resonant chambers inside, often with Helmholtz or quarter-wave resonators inside and many take advantage of insertion losses associated with passing the flow pipe some distance past the flat plate entry into the chamber. Other mufflers use perforated pipe and deadening material within the chamber to dampen pulses (glass packs) and still others use pulse reflection and resonant volume to reduce overall sound transmission. Too many options...much more than any OEM can handle to optimize for any one enthusiast's desired outcome under the restrictions of cost and time.

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