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| collector for homemade duals https://www.slantsix.org/forum/viewtopic.php?t=9366 |
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| Author: | JohnnyDees [ Thu May 20, 2004 5:06 pm ] |
| Post subject: | collector for homemade duals |
Im just finishing up a set of homemade duals. (exhaust manifolds). I am starting to make up the 2 head pipes that will go into some sort of collector. My question is does it matter how long the pipes are before they go into the collector? Also what size pipe for the two pipes? I am thinking 2" into a collector (2.25 or 2.5") Clutch linkage is a consideration also. (ported, gasket matched,. 060 head. stock valves,backcut .030, .460 cam, 2- 1 barrel offy. )Thanks in advance......... |
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| Author: | '65 Dutch Dart [ Fri May 21, 2004 4:29 am ] |
| Post subject: | |
The /6 firing order is 1-5-3-6-2-4. So the front pipe gets 1-5-3 and the back pipe gets 6-2-4 So if both pipes are equal lenght, the fumes per cylinder (f.p.c.  ) will never take part in any collision,....2.25" for the exhaust to the rear is minimum, so I would say split that in half for the minimum of the pipes to collector (p.t.c. ) --> 1.125" editted!.Hope I'm making sense    
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| Author: | guest [ Fri May 21, 2004 7:46 am ] |
| Post subject: | pipe sizes |
Just a heads up. Two 1.25" pipes do not flow = to a 2.5" pipe. Draw a quick sketch for yourself and you'll see that you can fit the 2 smaller pipes within the larger and with much room to spare. This is a common engineering miscalculation. Sorry that I can't help with actual numbers for you, but 1.25" p.t.c. pipes would be restrictive compared to the 2.25" out the back. |
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| Author: | '65 Dutch Dart [ Fri May 21, 2004 8:13 am ] |
| Post subject: | |
Then I'll take that back...   I can see your point, thanks for sharing. Sometimes your brain can fool you, right 
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| Author: | DusterIdiot [ Fri May 21, 2004 8:57 am ] |
| Post subject: | Net Free Area.... |
The best way to look at 'flow area' is to calculate the area of the opening and compare it to what you want to do... Remember that 'round' pipe has less 'friction' on what flows through it than square/rectangular pipe (so if you really want to look at the combined area of your ports vs. the exhaust pipe, you'll have to 'fudge' the numbers about 20% to make up for the rectangular ports....) So in this case you have a pipe 2.5" round.... so area = Pi * radius*radius or A=3.1415927 *1.25*1.25====4.9087 sq. inches of 'possible flow'(it's never perfect in the real world... So if you suddenly dropped in two 1 7/8" pipes you scavenged from junkyard slant six cars.... A=3.1415927*.9375*.9375======2.7612 sq.in. each or 5.522.... You might actually say the two smaller pipes will be very close to the one big pipe due to the smaller size and restriction of the pipe vs. the ports, also due to smaller size the velocity of the exiting gasses should be greater (if you've ever driven a vehicle with big pipes in cold weather condenstion tends to gather in the 'lee' sections of the pipe, when you start the car up in the morning you can literally pour yourself a cup of water out of your pipes.....) -D.Idiot |
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| Author: | Super6 [ Fri May 21, 2004 10:20 am ] |
| Post subject: | |
Pressure has a profound effect on the flow of a fluid as well (air/exhaust is considered a fluid). To high a pressure in a small pipe will decrease flow capacity because the exhaust molecules are being physically pressed closer together (pressure) creating friction (there are other issues here as well, but I doubt anyone cares too much for hydrodynamics). Too large a pipe solves the pressure problem, but now there is excessive turbulence in the pipe, which also decreases flow capability. As always, you need to find your happy medium, where the exhaust pressure is great enough to maintain smoother flow, but not so great the gasses must physically compress to get out the tailpipe. What all this boils down to is, basically, keep the flow area throughout the system the same (as has been stated already). That is, you want your two head pipes combined area to roughly equal the are of your single exhaust. Since you can't get pipe sizes to make these two areas exactly equal, error a little on the small side for the single exhaust pipe, a little more pressure should flow better than a slightly larger pipe in this case.(The most efficient exhaust pipe would have a taper to it, IE a pipe with a continuously changing diameter. For an example look at a snowmachine or jetski exhaust pipe, but these pipes must be tailored to the specific engine to work properly, and must also be a tuned length. The taper is essentially an assymtotic (sp) curve) BTW, you don't need to mess with PI in your calcs to compare pipe flow areas. Just square the diameter's of the pipes. EG: Two 1.75" head pipes have a flow area slightly less than one 2.5" pipe (1.75^2 + 1.75^2) = 6.125, 2.5^2 = 6.25 -----these numbers are not pipe areas, but a means of comparing flow areas quickly. To get area, multiply the above by (PI/4) -S/6 |
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| Author: | DusterIdiot [ Fri May 21, 2004 11:33 am ] |
| Post subject: | That's a quick cheat... |
That's cool. We don't use that one in HVAC that much but it is available, I only wanted to make it simple in case he decided to 'apples and oranges' the thing by adding the net free area of his exhaust ports after gasket matching, then compare that to his exhaust pipe size (less some fudge factor since every bend in the exhaust pipe is equivalent to a few more feet of straight pipe, along with the 'kinks' in the pipes if they aren't mandrel bent...by the time he gets done he'll wish he'd gone to college for another 10 years  -D.Idiot |
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| Author: | sixinthehead [ Fri May 21, 2004 12:26 pm ] |
| Post subject: | |
He asked about pipe length as well, but is there a formula for that? I was taught that long, small dia primaries = low rpm power; short, large dia primaries = high rpm power. Collector length is also a factor, but it's effect may be damped by a full exhaust system; n'est ce pas? |
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| Author: | steponmebbbboom [ Fri May 21, 2004 12:52 pm ] |
| Post subject: | |
don't forget the expansion of the gases as they cool once theyve come out of the head, that cuts down velocity as well... wrapping the headers with fiberglass wrap will help keep that gas warm and keep velocity up, and will cut down underhood temperatures as well. |
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| Author: | Super6 [ Fri May 21, 2004 2:02 pm ] |
| Post subject: | |
Quote: don't forget the expansion of the gases as they cool once theyve come out of the head, that cuts down velocity as well... wrapping the headers with fiberglass wrap will help keep that gas warm and keep velocity up, and will cut down underhood temperatures as well.
Gas contracts as it cools. PV=nRT, the ideal gas law. Neither exhaust nor air is an ideal gas, but is close enough for our purposes. P= pressure V=volume n=mass of the gas R=universal gas constant T=temperature As the mass of the gas in the "system" remains (essentially) unchanged at a given rpm, it can be considered a constant. Thus, "R" and "n" have no real bearing on the equation, which leaves: PV=T, thus, as the pressure of a gas goes up the temperature goes up. (in a constant volume) Exhaust gasses expand while leaving the head because the gas is being forced out of a very small volume at high pressure into a relatively large volume, where the pressure can lower. Same equation as above applies. If temp is constant, as volume increases pressure decreases. An internal combustion engine has all three things going on at the same time, and they are all inter-related. The differential equation to describe what the gasses are doing is about a page long, and there is way to much thermodynamics & heat transfer involved for my tastes. There is really a lot more going on here, the above are just some of the basic principles exhaust design is based on. Since the exhaust comes in pulses rather than a constant pressure, things change slightly (read, become way more complicated) We all know a bigger pipe has the capability to flow more volume of exhaust, my point was there is a point where pipe size will begin to hurt flow rather than help it. Honestly, on a humble slant, it just isn't worth it to go through any hassle other than just basic matching of pipe area, because I don't see it providing much more power, especially at the rpm ranges most of our rides operate in. You'll note the tuned pipes I mentioned above are usually on 2-cycle engines, mainly because they rev so high (8000 rpm +). Street motorcycles are an example of a tuned pipe for a 4-cycle, but they also tend to have pretty high redlines. Oh, BTW, tuned pipes are only good for one rpm, possibly three or four rpms at different stages, which gets us into reversion waves and a whole host of other 'fun' DI--I mentions forgetting PI above for simplicity in comparing flow areas. the number you get is meaningless except as a comparison to another pipe or pipes. The PI can be eliminated as follows: We want the area of two pipes to equal the area of one pipe, assuming the two pipes are of the same diameter, which in this case they are, (d^2)(PI/4) + (d^2)(PI/4) = (D^2)(PI/4) where d = diameter of smaller two pipes, and D = diameter of single larger pipe. Divide both sides of the above eqn by (PI/4) and you get (d^2 + d^2) = (D^2) Just an easier calculation. Again, the actual numbers are meaningless. -S/6 |
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| Author: | JohnnyDees [ Sun May 23, 2004 4:30 pm ] |
| Post subject: | |
Thanks guys. If I'm translating you guys right. Assuming I use the bigger pipe 2.5" = 4.90 sq" - 2 - 1.75" pipe = 2.40 sq" x 2 = 4.80 sq" Now if I could only divide the clutch linkage by 2 I could fit all this in there
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| Author: | JohnnyDees [ Sun May 23, 2004 4:55 pm ] |
| Post subject: | |
I mocked up some different combos of pipe and the 2.25" fits a lot better. So 2 - 1.75" pipes at 4.8 sq" would flow into 1 - 2.25" at 3.97 sq". This setup works very well especially if the collector is before the clutch linkage. Does this seem to short to you guys? How would you compare the flow of of a single 2.25 exhaust with the original manifold. And the 2 - 1.75 pipes from split manifolds into a 2.25 pipe? |
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| Author: | Super6 [ Mon May 24, 2004 10:22 am ] |
| Post subject: | |
IMO those pipe sizes will work very well. Exhaust velocity will stay up, even as the gasses cool and enter the single pipe, which has slightly less area than the two header pipes. The longer/smoother you can make the transition from two to one pipe, the better the exhaust will flow. -S/6 |
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| Author: | JohnnyDees [ Mon May 24, 2004 2:14 pm ] |
| Post subject: | |
Can anyone explain to me a way to quantify the theoretical difference in flow between the standard exhaust system with 2.25" pipe and the split manfold with 2 - 1.75" pipes going into a 2.25" pipe? |
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| Author: | Doc [ Mon May 24, 2004 6:37 pm ] |
| Post subject: | |
153624, with that firing order you get twice as much "separation time" between exhaust pulses if you run the front 3 and the rear 3 cylinders in separate headers / manifolds. (132 & 564) If you keep the pipes the same length and exit into the collector side-by-side, ending on the same plane, the exit pulse from the one pipe creates suction or "scavaging" in the neighboring pipe while it is waiting for the next pulse. DD 
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