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post #1 of (permalink) Old 04-12-2004, 10:30 PM Thread Starter
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Question about exaust flow..

I got a quick question for you fellas, I need to know if it is ok for my exaust to be stepped down out of the exaust ports? Meaning this- the ports on the swift head are smaller than my aftermarket swift flange that will have sami tubes welded into it. Is it ok for the exaust to immedietly drop from the smaller swift head ports, into the bigger tubed header I built?

I just got done porting my intake side to match the dual carb manifold i just finished shaping. I don't really want to open the exuast side up if i don't have to..Will it retain more Back pressure with the step down?


Thanks, Scott

This dual carbed swift swap is quite interesting and fun..Can't wait to drive it.. [img]images/graemlins/grin.gif[/img]
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post #2 of (permalink) Old 04-13-2004, 07:20 AM
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Re: Question about exaust flow..

Coming into a larger header tube a lot of turbulence would be created at that spot and would likely detract from any gain the header would provide. At this point the stock manifold may work better. Just my .02- Good luck with the project, though. Sounds like a kick-a$$ setup.
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post #3 of (permalink) Old 04-13-2004, 09:34 PM Thread Starter
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Re: Question about exaust flow..

The problem with the stock manifold is that it will not clear the motor mount, and would not face in the right direction...

Anybody else....Sarge??

Thanks, Scott
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post #4 of (permalink) Old 04-14-2004, 07:38 AM
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Re: Question about exaust flow..

build a new motor mount ????? any chance of cuttin the flange off the stock manifold?? if you used the stock flange could you weld the headder to it?? just brainstormin or brain fartin however ya wanna look at it.....
post #5 of (permalink) Old 04-14-2004, 01:52 PM
 
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Re: Question about exaust flow..

for my swift gti engine swap i used a samurai headman hedder and had a machine shop fab up a flange with the gti bolt pattern. that flange was then welded to the samurai headman hedder and it worked great. sounded mean as hell too....well as mean as a jap 4-poper can sound i guess.

the stock gti exhaust manifold flows good in stock form, but it dumps into the samurai motor mount so it will not work.

i am not sure how bad the turbulance will be with the set up you have described. i am sure RRICH or Sarge will have some good answers on that.
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post #6 of (permalink) Old 04-14-2004, 06:25 PM
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Re: Question about exaust flow..

If I understand you right - you are going from the smaller head port to a larger manifold header?
If so, that sure is far better than going from big to small!

Expanding is best done a little farther down stream, but sometimes ya gotta do what ya gotta do.

If there's room, see if you can adapt with a taper - as smooth on the inside as possible. A "step", even a step up to larger, creates lots of turbulence. Even if the taper is only 1/2 to 1" long, it lets it expand without much restriction - the longer the taper the better.
A muffler shop may be able to expand something to make the taper - I'm sure it's worth the effort.

Try something -- dig a small trench about 1" wide in your garden - sloped gently downhill. At one point, make a square cut and expand it to 3" wide. Run water slowly down the trench from the small to the big part. You'll see where it expands the water swirls around because of the step . It's the same thing as the gasses do flowing down a tube or pipe when they come to a step up.

Expanding means the velocity in the pipes becomes slower, nodes and peaks are moved up toward the engine, but not really a problem.

You did the intake but not the exhaust -- better - a little better than stock. When and if you can, open the exhaust up too. ANY restriction, intake (choking) or exhaust (constipation) limits power just like it affects a jogger.
About the same as Athsma - you can inhale, but exhaling's difficult.

By the way - the backpressure myth - if the exhaust dumped right out of the head's port - no pipes, that's 0 backpressure -- hardly will run. Adding any sort of backpressure to that - restriction - makes it even worse!
Adding an exhaust system DECREASES backpressure to LESS THAN 0 - to NEGATIVE PRESSURE (you can measure it with a very fast vacuum transducer right at the port!) The exhaust "system" uses the pulses to extract, clean, or scavenge the old exhaust from the chamber so new AF mixture can get in with a fuller charge.
More A/F = more power.
The myth that you need backpressure comes from those that opened the exhaust up too much and killed the scavenging effect.

NO? Isn't a potato stuck in the exhaust pipe a restriction? More backpressure? Try it.
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post #7 of (permalink) Old 04-14-2004, 11:14 PM Thread Starter
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Re: Question about exaust flow..

Thanks for all the help guys..

blue zuk, I did the same thing as you as well. The new flange matches exactly with the head about the header tubes are bigger than that new flange. So I machined the flange to the size of the header tubes (1.5 in.) and now the flange and tubes match but not the head..

I think iam just going to run it as it is, and I can always port it out down the road. Rich is right, at least it is not stepping up.. I am sure it will still be able to scream pretty good..


later, Scott
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