Gutted Muffler Question

[1982911sc]

I was just reading the new SSI/Dansk post and did not want to hijack that thread with this question.

I have come across a few posts that talk about "Gutted" stock mufflers. What does this exactly mean? And how does one do this and what types of gains would you see as far as sound/HP gain?

I currently have the stock heat exchangers, Cat by pass pipe and stock muffler. Maybe this would be a cool spring project!

[djmcmath]

At the risk of being the silly newbie who didn't know any better, I'll take a SWAG on answering this one. For optimal performance, there should be no restriction in the exhaust -- no muffler, no catalytic converter, etc. Just straight pipes from each cylinder to the outside. Check pictures of Shelby Cobras tricked out for racing for an example.

Unfortunately, the cars we actually drive for daily use generally have to comply with both noise and emissions regulations. If you're not so concerned about either requirement, and don't have the money to upgrade to a professionally designed system, and would like to attempt to fool the people doing visual smog inspections (CA, I think?), you can take a broomstick to the inside of your muffler and cat, thus removing all the awful restricting cruddies from inside them. This makes them both totally ineffective, and generally is not street legal. It is, however, super-cheap, and seems like it ought to render some gains.

Does that sound about right? Somebody more knowledgeable, please correct me if I've hosed this up.

Dan

[masraum]

The open race exhaust is actually best for optimal performance in high strung/racing motors and is only really better for high rpm power where you are trying to flow lots of air really fast.

For a street motor or a motor that is not highly tuned you want/need some backpressure as it helps with lower rpm power.

Our motors are somewhere in the middle. They are actually pretty high strung/highly tuned (especially the older 'S' motors), but they also spend a fair amount of time in the lower part of the tach rather than all of the way at the top like a racecar, so you want a low restriction exhaust, but not a no restriction exhaust.

A gutted muffler will change the sound, but I don't know that it will actually do much for you in terms of power. Like a car with a cat, gutting the cat is not really a good thing unless the cat is bad, a Euro style premuffler is much better than a gutted cat. I have to think that a muffler benefits from internal flow tuning, but I'm definitely no expert, I just read a bunch.

[Early_S_Man]

The following article explains the procedure, and was posted last month. You can find considerable discussion by doing a search on "up-fixen+sport+muffler" ... which nets the following link ...

http://forums.pelicanparts.com/search.php?action=showresults&searchid=557447&sortby=lastpost&sortorder=descending

[Randy Webb]

Rumor has it that the amt. of backpressure is related to the MFI system.

Muffler design is very complicated because you need to account for alternating rarefaction and compression of the pressure waves -- that's how you get scavenging -- think "pulsation." Zero backpressure is not optimal. You can read an early account in Phil Smiths Design of Induction and Exhaust systems. More recent design criteria use numerical modelling and a lot better data collection methods than the old guys had. I don't know of any new books at a reasonable technical level. Be wary of books "How to hotrod your .. [whatever]" -- a good design requires a passel of PhDs, computer power, and dataloggers.

But, anyone with an engine dyno can test the _outcome_ of those designs -- yet there are few such studies beyond B. Anderson's.

[Zeke]

You can have backpressure and scavenging at the same time. It's a balance thing. You'll probably have to reset your part load fuel adjustment on an MFI car when you change the exhaust. But the post was about a CIS motor and that I don't know. The air flow gizmo may do it for you.

FWIW, I had a pretty much blown out, used up muffler on a '77 2.7 and it sounded very good. I don't think you can buy that sound, but some gutting of the muffler might come close. Not that sound has to do with performance or anything.

[williel]

sounds awesome! I used to run track events with him while living in AZ and his 'stock' muffler intimidated even the Corvettes in our run group!

[anh911]

Originally posted by Randy Webb
You can read an early account in Phil Smiths Design of Induction and Exhaust systems. More recent design criteria use numerical modelling and a lot better data collection methods than the old guys had. I don't know of any new books at a reasonable technical level. Be wary of books "How to hotrod your .. [whatever]" -- a good design requires a passel of PhDs, computer power, and dataloggers.

Thanks Randy I have been trying to get Smith's book (my dad had it but it disappeared) but couldn't remember his name.

[Randy Webb]

Sure thing -- I usually just go to the library and request it thru ILL (Interlibrary loan) -- for free (my tax $$ at work).

He also has a more genral work out on engines.

Just a caveat -- it's an old-timey book.

[anh911]

Just a caveat -- it's an old-timey book.

Yep, I remember. Some of my favs are the very old ones.
ILL is a great idea too.