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Author Topic: exhaust diameter  (Read 17639 times)
bondsman
C1
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Posts: 10


« on: January 02, 2008, 03:57:43 PM »

i have 3" exhaust on my boat and the mufflers are a magnaflow straight threw design serrated(sp) with packing around tubes.now straight headers and flex pipe i get 200 more rpm's than with my mufflers hooked up.can i go to 3 1/2" exhaust and keep those extra rpm's or would another muffler help or lenghthen my exhaust. from collector to muffler is 2 ft. from head to exit is 4 ft.have to run mufflers by law.
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fatboy0054
C2
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Posts: 29


« Reply #1 on: January 02, 2008, 06:01:33 PM »

TRY JETTING UP ONE JET ON THE BACK SIDE OR DOWN ,TRY BOTH  WAYS AND SEE IF YOU RPMS STAY UP ! I THINK YOU  PIPE SIZE IS FINE ! YOUR FUEL CHARGE IS NOT AS DIRTY AS WITH THE MUFFLERS ON ! I THINK JETTING WILL HELP !THAT SIZE  PIPE THEY SAY WILL SUPPORT 700 HORSE !
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bondsman
C1
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Posts: 10


« Reply #2 on: January 02, 2008, 08:08:44 PM »

so maybe without the mufflers its leaning me out is if were tuned to run without mufflers.maybe? i will rejet it maybe i am keeping it alittle to fat.
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caddycarlo
Guest
« Reply #3 on: January 07, 2008, 12:34:24 PM »

timing needs can change to from muffler to no ........ but yes if you go to a bigger pipe and muffler you should do better if it runs better uncorked ........ a longer pipe will make it worse .... on my car I run a duel 4 inch pipe and mufflers and now when I take the ex off the car it slows down just a little at the track ......... back pressure is never good for an engine
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bondsman
C1
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Posts: 10


« Reply #4 on: January 07, 2008, 05:17:33 PM »

thanks for the input, i jetted the carb down plugs were showing excessively fat,with the leaning out and fresh plugs i gained 125 engine rpm. i still may go to 3 1/2 inch exhaust thanks for letting me know longer is not always better on exhaust.thanks again.
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fatboy0054
C2
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Posts: 29


« Reply #5 on: January 12, 2008, 08:47:37 PM »

 Grin  COOL DEAL SAM
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sMiles
C4
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Posts: 321


« Reply #6 on: January 29, 2008, 09:23:57 PM »

I have been reading info related to exhaust systems,  pipe sizes,  flow rates of pipe sizes,  cfm / hp,  log manifolds, etc.   

cfm per horsepower is stated to be  2.2cfm / hp.

The flow of a pipe is 115cfm / sq. in of cross section area.

The flow of the muffler must be equal to or greater than the pipe cfm sufficient to serve the hp of the engine to achieve zero loss through the system.  A large oversize system offers no advantage and is louder.

2.25 dia. flows  457 cfm
2.50 .............  564
3.00 .............  813
3.50 .............  1106
4.00 .............  1445

These are some of the source info,  for your reading pleasure ..

http://www.superchevy.com/technical/engines_drivetrain/exhaust/0505phr_exh/index.html

http://speedtalk.com/forum/viewtopic.php?t=8305

http://www.dynomax.com/ecat/pdfs/ultraflowelded.pdf

http://www.geocities.com/motorcity/track/6992/vizard.html

http://speedtalk.com/forum/viewtopic.php?t=1435&postdays=0&postorder=asc&start=0





« Last Edit: January 29, 2008, 10:39:12 PM by sMiles » Logged
Stiney
C3
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Posts: 114



« Reply #7 on: January 30, 2008, 12:36:42 PM »

So a head flowing 330 cfm's per would be 1320 total, or 2640 cfm's for the complete engine.

Looks like Scott is correct to be running 4" pipes.  3.75 might be a closer fit to the needs of the engine, but where do you find that size?

Good stuff.



I have been reading info related to exhaust systems,  pipe sizes,  flow rates of pipe sizes,  cfm / hp,  log manifolds, etc.   

cfm per horsepower is stated to be  2.2cfm / hp.

The flow of a pipe is 115cfm / sq. in of cross section area.

The flow of the muffler must be equal to or greater than the pipe cfm sufficient to serve the hp of the engine to achieve zero loss through the system.  A large oversize system offers no advantage and is louder.

2.25 dia. flows  457 cfm
2.50 .............  564
3.00 .............  813
3.50 .............  1106
4.00 .............  1445

These are some of the source info,  for your reading pleasure ..

http://www.superchevy.com/technical/engines_drivetrain/exhaust/0505phr_exh/index.html

http://speedtalk.com/forum/viewtopic.php?t=8305

http://www.dynomax.com/ecat/pdfs/ultraflowelded.pdf

http://www.geocities.com/motorcity/track/6992/vizard.html

http://speedtalk.com/forum/viewtopic.php?t=1435&postdays=0&postorder=asc&start=0






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sMiles
C4
****
Posts: 321


« Reply #8 on: January 30, 2008, 01:41:58 PM »


Please note that the discription I gave above is not based on Head cfm numbers.

I don't know that using Head cfm #'s is totally inaccurate,  just not what the above info is based on.   If you know your hp you can easily compare.

At 2.2 cfm/hp,  an engine requiring 2640 cfm would be making 1200 hp.
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ST Dog
C4
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Posts: 499



« Reply #9 on: January 30, 2008, 01:51:57 PM »

So a head flowing 330 cfm's per would be 1320 total, or 2640 cfm's for the complete engine.

Nope.

339cfnm would be that one cylinder at a certain valve lift, and a certain pressure differential.
In actual use, the flow will be based on the flow curve, as a function of time, taking both lift and pressure diff into account.

If, the flow is 330 @ .500 and you have a great cam that opened/closed instantly, with a duration of 270o. That's still only be flowing 3/8s of the time (open for 3/4 of one revolution for intake and closed the next 1-1/4 revolution for compression, combustion and exhaust).

If all 8 cylinders match than you have 330cfm * 3/8 * 8 = 990cfm into the engine.
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caddycarlo
Guest
« Reply #10 on: January 30, 2008, 02:34:06 PM »

I am not sure it works that way ...............

but if you think of it in steady state flow then with 1 3/4 in dia hedders


you have 1/2 of 1 3/4 or .875 radius


pi r squared    is   .875 times .875 = .765625 times 3.1416 is 2.405 sq inches

times 4 = 9.62115 sq inchs of the main pipe

devided by 3.1416 and you get 3.0625 and take the sq root to get 1.75 radius times 2 and you have 3.5 in dia pipe off the header
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Nick Campagna
C3
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Posts: 180



« Reply #11 on: January 30, 2008, 03:22:17 PM »

    Remember that all pipes are not flowing in parallel, the pulses are serialized to a great degree in the collectors, like peas down a straw. Each pea is a pulse, and some will interfere with others coming down, but all eight cylinders are not flowing steady state in parallel.
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Is the defect in what I see, or in what I'm seeing with ?
MTS Marty
C4
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Posts: 282


« Reply #12 on: January 30, 2008, 05:52:20 PM »

Also how much pressure is there pushing the exhaust through the pipe. Seems this would allow for a smaller diameter
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sMiles
C4
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Posts: 321


« Reply #13 on: January 30, 2008, 08:22:18 PM »



Taking CC's example of 3.5" dia collector,  A=9.621 sq in  x  115 cfm/sq in  =

1106.43 cfm  /  2.2 cfm/hp  =  503 hp  is what a 3.5 " pipe will flow with zero

loss. 

More hp needs a larger pipe dia.
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caddycarlo
Guest
« Reply #14 on: January 30, 2008, 11:23:56 PM »

    Remember that all pipes are not flowing in parallel, the pulses are serialized to a great degree in the collectors, like peas down a straw. Each pea is a pulse, and some will interfere with others coming down, but all eight cylinders are not flowing steady state in parallel.

I am not sure I agree with this picture ..................... though I can see the pulses going down the header tube like this once it leaves the end of the tube the pressure pulse dissipates as it depressurizes into the collecter and you get a homogeneous mixture traveling down the main ex pipe ......Now this mixture is driven by pressure waves that build at the end of the tubes as the pulses depressurize but it is not of the same frequency as the pulses in the header tube it is more a build and push deal this is why the tone changes as you change the size of the main ex pipe ......... it becomes like a organ pipe.........

as a side note to your lined up peas my first question is are you sure all of the header tubes are the same length and will flow at the same rate with whatever bends they have .......... second if you look at the firing order on the pass side header you have cyl #1 fire then right after you have #5 then a pause the #3 then two pauses then #7 ........With the pauses the timing is going to be off to get them to line up ........


anyway that is the picture I have of how it works ..........

my steady state thought comes from how we test the flow of the intake and ex ....... these systems are all pulses but we look at them in steady state so if I carry that out the ex I get the math I did above ......


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