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Old 12-31-2008, 09:41 AM
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exhaust size ?

I got a '85 318 , 'leanburn' motor stripped of computor controls, with '70 or '71 Ignition ECU, will have about same vintage carb, Exhaust manifold out let is about 2'', and I won ( a door prize at a carshow) 2 stainless steel Magnaflow mufflers that are 2 1/2 ''.........Question ; where is the best place to go from 2'' to 21/2 ''...soon as possible at the heads, or just before the mufflers ? Do I need to keep some kind of back pressure ? Or bigger as soon as possible ?.........I forgot , has a 2bbl carb

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Old 12-31-2008, 05:35 PM
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Quote:
Originally Posted by knighthawk
I got a '85 318 , 'leanburn' motor stripped of computor controls, with '70 or '71 Ignition ECU, will have about same vintage carb, Exhaust manifold out let is about 2'', and I won ( a door prize at a carshow) 2 stainless steel Magnaflow mufflers that are 2 1/2 ''.........Question ; where is the best place to go from 2'' to 21/2 ''...soon as possible at the heads, or just before the mufflers ? Do I need to keep some kind of back pressure ? Or bigger as soon as possible ?.........I forgot , has a 2bbl carb
Interesting, my son just did this 2 weeks ago on an 86 Fifth Ave. His deceased MIL low mileage car.

No cats and true duals with 2" crimped pipe is all the size you need to move the volume of only 4 cyls per pipe at the rpm you will be driving. Flow velocity is more important than volume for you. You are not racing.

Definitely use a crossover pipe, H is fine.
However you do the up size into the muffler is not critical as long as it is smooth. The muffler outlet to tailpipe must be fairly smooth also, not with exhaust gasses hitting an abrupt step down in size into the tailpipe.
By the way, the 2.25 and 2.5 Magnaflows are identical inside, only the pipe size differs.
If you want a little more mellow sound, use 2.5 tailpipes.

To be honest, you will be just as happy with single exhaust of 2 1/2 after the Y pipe, if you choose to save one muffler. I will be quieter too, but the duals will not be loud.

Also what you need to watch out for is that horrible constant flow EGR system in the 85 manifold. Weld the holes or get an earlier intake and block it out on it also.
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Old 01-01-2009, 09:14 AM
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so it's 2'' back to the muffler then, the 'H' pipe may be a problem, I assume the 'H' is BEFORE the mufflers ? Not sure what you are referrring to about the EGR, will look at it again today.......thanks
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Old 01-01-2009, 09:31 AM
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Quote:
Originally Posted by knighthawk
so it's 2'' back to the muffler then, the 'H' pipe may be a problem, I assume the 'H' is BEFORE the mufflers ? Not sure what you are referrring to about the EGR, will look at it again today.......thanks
If you take the carb off, lean burn engines have a hole/port/valve directly below the carb that sucks in exhaust gas ALL THE TIME. Pull the intake manifold and weld the holes.

If you can get an older intake manifold, the EGR is external next to the carb and can be easily capped or plugged.

H pipe goes before the mufflers, location is not important. It balances the staggered exhaust pulses from side to side to smooth flow and smooth the sound= takes some of the pop out of the sound. Many say it improves the low speed torque noticeably.
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Old 01-01-2009, 11:36 AM
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mine ("85) has a vacumn (EGR) thing just to the left of the carb, bolted to intake with 2 bolts. Is this it ? So maybe I can just take it off, put a small plate over it or maybe just leave it with the vacumn line ?
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Old 01-01-2009, 01:35 PM
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Quote:
Originally Posted by knighthawk
mine ("85) has a vacumn (EGR) thing just to the left of the carb, bolted to intake with 2 bolts. Is this it ? So maybe I can just take it off, put a small plate over it or maybe just leave it with the vacumn line ?
a plate will work
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Old 01-01-2009, 02:06 PM
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i was told the old school way to figure out where the H-Pipe goes is to paint the exhaust pipes and spot where the paint stops burning off is where it goes
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Old 01-01-2009, 02:14 PM
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Originally Posted by matts37chev
i was told the old school way to figure out where the H-Pipe goes is to paint the exhaust pipes and spot where the paint stops burning off is where it goes
Yep and if you try that at different rpms,
the paint will burn off at different spots.

So IMO it makes little difference where you put the H.
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