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Posted by: moggy Jul 3 2014, 03:33 AM

Hi guys

What's the significance of the insert inside the end of this tailpipe?

IPB Image

I see it a lot on big vintage 911 engines too. Is it there as a noise control solution, breaking up the sound wave in some way? or is there something else going on?

Posted by: wndsrfr Jul 4 2014, 09:18 AM

QUOTE(moggy @ Jul 3 2014, 01:33 AM)

Hi guys

What's the significance of the insert inside the end of this tailpipe?

I see it a lot on big vintage 911 engines too. Is it there as a noise control solution, breaking up the sound wave in some way? or is there something else going on?

Keeps the competition at bay since it looks like a missle aimed at your nose!

Posted by: crresind Jul 4 2014, 09:51 AM

This is called "cookie cutter exhaust". Porsche claimed that this was helpful above 8200 rpm on their race cars. I have found that they take some of the harshness out of open exhaust. This is not an "insert", the megaphone is built this way.

Posted by: campbellcj Jul 5 2014, 10:24 AM

I thought it was simply to make the pipe stronger...

Posted by: moggy Jul 7 2014, 03:43 AM

QUOTE(crresind @ Jul 4 2014, 07:51 AM)

This is called "cookie cutter exhaust". Porsche claimed that this was helpful above 8200 rpm on their race cars. I have found that they take some of the harshness out of open exhaust. This is not an "insert", the megaphone is built this way.

Interesting. Thanks. Although my engine will not see the dizzying heights of 8200rpm, anything to help lower the db is good for me at the moment.

Posted by: Cap'n Krusty Jul 7 2014, 05:50 PM

A long time ago, these were called anti-reversionary tail pipes. Looking the term um on Google, I'm not sure they meed the definition. They were all the rage with the PCA/POC long track guys. The Chief had 'em on his 904/6, Otto on his 914/6, and most of the other fast guys used them. It could be that they were more trendy than effective, just like a lot of car fads. They DO look cool, though ..................

The Cap'n

Posted by: '73-914kid Jul 9 2014, 08:11 AM

The story I always heard was that they were to prevent the pipes from getting pinched closed if the car was tapped from behind. Given how far 911 megaphones stick outward from the bumper, it's not a bad idea to have them installed.

BTW, I ran my car with 2 sets of megaphones of the exact dimensions. 1 pair without cookie cutters, and 1 pair with inserts. No difference in sound, power, or anything of that sort.

Posted by: zymurgist Jul 9 2014, 11:32 AM

https://www.youtube.com/watch?v=PitGkp1ncXE

Posted by: Seabird Jul 9 2014, 02:28 PM

QUOTE(zymurgist @ Jul 9 2014, 09:32 AM)