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PostPosted: Thu Apr 12, 2018 8:43 am 
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Board Sponsor & SL6 Racer

Joined: Wed Oct 23, 2002 7:57 pm
Posts: 8283
Location: Waynesboro, Pa.
Car Model: 65 Valiant 2Dr Post
How do I determine the correct length? I have new extensions that are @ 14" long. I remember that some say to use a paint method. What is that exactly? Do I just give them a coat of spray paint? Then some say that is a myth? I can use trial and error but if there is scientific way to decide I am all ears! :lol: :lol: Thanks for any help!

Rick

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PostPosted: Thu Apr 12, 2018 10:27 am 
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Board Sponsor & SL6 Racer
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Joined: Fri Nov 08, 2002 4:48 pm
Posts: 5835
Location: Burton BC canada
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I don't know a scientific method other than starting long and cutting shorter...
( non - slant application....35" tire mudbogger rail ....carbed 6500 rpm V8) but the process is the same.

I used 3" exhaust tube flared to fit over the collectors...I started with a 20ft length of pipe and got the shop to make me pieces starting at 3ft (x 2 of course)....then 2 1/2 ft...2 ft....18"....

Trying out the 3 ft and 2 1/2 ft pieces quickly showed me they were too long. The sound alone told the story. The 18" extensions sounded pretty good. (better than longer or just the collectors)

At the track I tried 12" ....15" (I cut the 3ft and 2 1/2 ft extensions down)

In the end I settled on 16"

The cost of 20ft of pipe ...some cuts and flaring was cheap

I know tuning by sound seems weird but its an easy way to get close. Track times tell the real tale.

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PostPosted: Thu Apr 12, 2018 12:13 pm 
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Turbo EFI
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Joined: Mon Aug 02, 2010 7:36 am
Posts: 1200
Location: Rome, GA
Car Model: 1963 Dart 270, 1980 D150
Back when I was road racing a Honda CB350 there was a calculation for determining the length of a megaphone added to the header pipe. It was primary length (measured from the head of the exhaust valve to the end of the primary pipe) X .85. So 85% of the primary length. Surely someone somewhere has come up with a similar rule of thumb for header collector length. I will bet it would be less that 85% of primary length though.
With collector and primary length data from racers here we could come up with a percentage of header primary length for a starting point calculation. Just sayin'.

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PostPosted: Thu Apr 12, 2018 1:13 pm 
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Board Sponsor & SL6 Racer

Joined: Wed Oct 23, 2002 7:57 pm
Posts: 8283
Location: Waynesboro, Pa.
Car Model: 65 Valiant 2Dr Post
Right now I have a 6" header collector and 14" of extension giving me 20" total. I'll start there and trim if needed. Pipe is cheap.

Of course I would love some racer input on what your using.

Rick

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!
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PostPosted: Thu Apr 12, 2018 1:14 pm 
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EFI Slant 6
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Joined: Tue Mar 15, 2011 7:56 pm
Posts: 374
Location: Pauls Valley, OK
Car Model: 1975 Dodge D100
I have heard of the paint method for determining the collector extension length. Racers would paint a line the length of the collector extension, make a pass then trim the collector extension where the paint begins to burn. I'd heard of guys using crayon or shoe polish too.

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PostPosted: Thu Apr 12, 2018 1:25 pm 
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Turbo EFI
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Joined: Mon Aug 02, 2010 7:36 am
Posts: 1200
Location: Rome, GA
Car Model: 1963 Dart 270, 1980 D150
I have a full exhaust system on my street driven car so I have no real world input. I just find this kind of thing very interesting. Reyer and Morrison said, time slips were the best way to tune header extension length and that on high horsepower applications, somewhere between 8" and 14" seemed to work best. They weren't tuning slant sixes either.

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PostPosted: Thu Apr 12, 2018 1:44 pm 
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Joined: Mon Oct 21, 2002 11:08 am
Posts: 16449
Location: Blacksburg, VA
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It will change depending on primary length and a bunch of other factors. I would start with maybe 30-36" and go down from there. I used timeslips and it made a big difference from an open collector (more than 0.1 sec w/a 14 sec car). On Project V with Clifford shorties, and a smallish (2.5" output) Clifford Y-pipe, I ended up around 18" from the collector exit. Long tubes w/no Y-pipe will be different I'm sure. I then put a single exh and muffler on exiting in front of the rear dr side tire and the times were similar to the tuned length. I think you never want an open collector with no extension.

Lou

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