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Old 03-02-2006, 09:17 PM
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Reducers after header collectors - good or bad?

I currently have a setup of a strong 355 SBC that has 1-5/8" headers that go into a 2-1/2" Y pipe and into a single 3" exhaust, all mandrel bent. The collectors on the headers are 3", and then for some reason the guys who did my exhaust put a 3" to 2-1/8" reducer at each entrance to the Y pipe. I'm aware that the exhaust port of a cylinder head needs to have a venturi effect to be the most efficient at removing exhaust gases while helping to prevent reversion. Should there be something like this at the header collectors, or is this just an unnecessary restriction? Going all the way down to 2-1/8" seems pretty drastic to boot. I'm wondering if it would be a good idea to get a Y pipe that has a coupling that just goes from 3" to 2-1/2". I would appreciate some input on whether this is even a worthwhile consideration.

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Old 03-02-2006, 11:03 PM
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I'm not sure from your description where the restrictions are on your header. However, the ideal is to have a primary that is the same area from the exhaust valve head to the header collector. Headers work on the Organ Pipe principle or Hemholtz Resonator principle (theories differ a little but the answer comes out the same). These theories account for pressure pulses traveling up and down uniform tubes, reaching the end of the tubes and being reflected back up the tube. The collector can is there to provide an open end to each header tube so it can do its pressure wave reflection with the strongest signal. Once that is done (only takes a few pipe diameters of length in the collector for the waves to do their thing), then you can taper it down to your desired exhaust pipe diameter and not harm the effectiveness of the header.
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Old 03-03-2006, 02:46 AM
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1-5/8" headers and a 3" collector is not a good scavenging setup for a street car. I would replace the 3" collectors with 2-1/2" versions and run the 2-1/2" headpipe to the 2-1/2" x 3" Y-pipe.
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Old 03-03-2006, 07:27 AM
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Quote:
Originally Posted by onovakind67
1-5/8" headers and a 3" collector is not a good scavenging setup for a street car. I would replace the 3" collectors with 2-1/2" versions and run the 2-1/2" headpipe to the 2-1/2" x 3" Y-pipe.
Why would that be, just wondering because i'm going to have the same size headers as he is, except run a true dual.
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Old 03-03-2006, 09:51 AM
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To quote Jack Burns of Burns Stainless:
One of the largest errors street enthusiasts make is in running too large of a primary pipe, and too large of a collector. For example, NHRA and IHRA Stock teams using Burns' headers in H/SA in traditional small-block powered musclecars are running 1.5-inch diameter primary pipes- much smaller than many street machines are trying to use.

The function of the collector is to provide the secondary scavenging wave, and the wave length and strength are determined by the diameter and length of the collector. Too large of a collector will impair the secondary scavenging with a serious loss of velocity.
Most headers today are replicas of parts designed in the 1960's and don't take into account progress made in header and collector design in recent years. Ed Henniman of Headers by Ed has a formula he uses to determine the collector diameter of his headers and calls it the in/out ratio. It is the comparison of the combined area of the input tubes divided by the collector area. For his street setups he calls for about 1.6:1, or for a 1-5/8" 4-1 collector about 2-1/2" collector diameter.
Another area of development in collectors is the collector entry, starting with the cross-entry modification also developed by Ed, to the merge collector developed by Performance Welding Headers. The way the gases enter the collector will also play a part in the development of the secondary scavenging wave.
Here's a quote from PWH:

So being very general, a low horsepower 350 CID V8 engine making only 1.5 horsepower per CID ( that's 525 hp ) may only require a primary tube size of 1 5/8"stepped to 1 ". This particular engine would only require a merged collector throat size of approximately 2 " with a 3" outlet. The primary tube length is dictated by what RPM the engine is run at to obtain these power levels, with the vehicle weight, transmission, torque converter, and type of racing playing a dominating role.


There are also different types of construction to achieve different effects from the primaries as explained by Rodney Davis in this thread:

http://www.camaros.net/forums/showthread.php?t=16596

Some interesting reading at:
http://performanceweldingheaders.com/index.htm
http://headerdesign.com/
http://www.headersbyed.com/
http://www.popularhotrodding.com/tech/0310phr_burns/
http://www.burnsstainless.com/

I practice what I preach and use 2-1/2" collectors on all my 1-5/8" primaries and have had great performance from them. My daily driver 67 Nova ran 11.75 @ 117 with a mild 406 and small headers. Our class winning 331" road racer also runs 1-5/8" x 2-1/4" primaries and produces over 415 rwhp.

Tell me what a "true dual" exhaust is.
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Old 03-03-2006, 01:41 PM
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third gen f-bods are pretty tough to run a real dual exhaust so most people put dual tipped mufflers on them. So by me saying i'm using a true dual is exactly that, dual pipes, mufflers and exits. Of course i'll be using an h-pipe. thanks for the explanation.
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Old 03-03-2006, 02:43 PM
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Don't ask me to explain the mathematics but a single 3" pipe will work as well, or better, than two 2 1/2" pipes. It'll sound a lot better too. It mostly boils down to what design will fit. Exhaust systems are compromised from the beginning because they are in a car and have to fit.

If you have one large channel from the firewall to the rear, use a single pipe. If not use two and an x or h-shaped connection. The X design works better by design. It's smoother.


Larry
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Old 03-03-2006, 05:24 PM
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A single 3" pipe will have about 73% of the cross sectional area of a pair of 2-1/2" pipes. That's a little bit of the math.
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Old 03-03-2006, 05:48 PM
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Quote:
Originally Posted by onovakind67
A single 3" pipe will have about 73% of the cross sectional area of a pair of 2-1/2" pipes. That's a little bit of the math.
It also provides, using your math, around 13-14% more cross sectional area for individual cylinders to use. Only one firing at a time. Theoretically speaking, it also provides a better scavenging effect for both cylinder banks. But that's just a theory.

As you and I have learned with our cars, exhaust tuning is difficult within the confines of a chassis. It's never any better than a good compromise.


Larry
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Old 03-03-2006, 07:05 PM
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Thanks for the good info so far, I was actually planning on stepping up to 1-3/4" headers but from what I'm hearing 1-5/8" is more than I need. The restriction I am speaking about is actually on the Y pipe itself, not the header. At the mouth of the Y pipe where it connects to the header, there is a coupling that reduces the 3" collector down to 2-1/8", and then back to 2-1/2" where it is welded to the 2-1/2" Y-pipe. It's kind of like an hourglass shape - is this a restriction created unnecessarily by the exhaust guys, or is this supposed to be like that? From what I observe from common sense, it should just go from 3" to 2-1/2", like a funnel shape.
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Old 03-03-2006, 07:23 PM
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Sounds like the guy was trying to be tricky. I know of no theory that would call for that modification. As I said before, the organ pile principle used to tune headers requires a constant pipe diameter from the exhaust valve to the header collector where there should be an abrupt size change so the pipe thinks it is open ended. Any diameter change between those two points will result in, I would think, unwanted pressure pulse reflections and reduced effectiveness of the primary pressure pulse (actually the reflected pulse is a vacuum) that does the cylinder scavenging. The two cycle guys use a converging/converging cone on their bad boys but those pipes function differently from a straight tube, Helmholtz resonator header.
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Old 03-03-2006, 07:56 PM
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FWIW, here's Vizard's take on exhaust:

http://www.popularhotrodding.com/eng...exh/index.html
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Old 03-03-2006, 08:34 PM
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IMO, a single exhaust no matter how it's configured will sound worse, I've heard both single and dual and dual wins out with no matter what kind of muffler. A single sounds muddy and higher pitch as a dual is crisp and deep, again just my opinion.
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Old 03-03-2006, 10:25 PM
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I beg to differ on that, my single 3" sounds pretty damn mean with the tips I got on the muffler
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Old 03-03-2006, 10:40 PM
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I'm with Maggot. My 85 Z28 sounded wicked with a single 3" in/out Dynomax on it.
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