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/cgi-bin/YaBB.pl General Category >> Rubber Side Down! >> stright pipe and back pressure? /cgi-bin/YaBB.pl?num=1248843347 Message started by JohnBoy on 07/28/09 at 21:55:46 |
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Title: stright pipe and back pressure? Post by JohnBoy on 07/28/09 at 21:55:46 Any idea just how much pressure is generated by this engine? I am running a 63 inch pipe (measured from the exhaust port face). The baffle I installed is a 1 and 1/2 inch washer drilled out and supported on both sides with a bolt running through the pipe at the end. I have made three different styles using brass brazing rod to fasten the pieces together. The longest a piece has lasted is three days. A couple of 20mile runs at freeway speed and I am back to an empty pipe. I am new to brass brazing, but I have been making silver jewelry for years. I feel comfortable with a torch. The brass rod has a melt temp of about 1500 degrees so it ain't the heat! the break strength is supposed to be in excess of 50,000 lbs. Any ideas? And before you answer, I'm not the best engineer on the planet :) JB |
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Title: Re: stright pipe and back pressure? Post by justin_o_guy2 on 07/28/09 at 22:45:17 If a washer had 3 legs that pointed out & back, you could run that up in the pipe & the legs ( given they are made of stiff, hard wire) wouldnt back out the pipe. Go to a hobbyshop & ask for piano wire. Be careful cutting it & bending it in a small radius. Its so hard it breaks sometimes, in the sizes above 1/16th. It can screw up cheap diagonals,too. Not sure how it will behave after being brazed on. You may have to quench it. Or not? |
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Title: Re: stright pipe and back pressure? Post by LANCER on 07/29/09 at 05:00:29 You can buy short baffles from www.Jirehcycles.com for just a few bucks, in assorted lengths. That said, a straight pipe is the worst type of exhaust you can choose when considering performance and tuning ability. If you really want a straight pipe type of look you should browse ebay for one of the better quality mfg's drag pipe style mufflers; they at least have some baffling that has been testes and will help in making power and enabling you to tune the carb reasonably well. |
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Title: Re: stright pipe and back pressure? Post by justin_o_guy2 on 07/29/09 at 07:50:49 that has been testes and will help in making power EVERybody wants an exhaust system that really has balls.. |
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Title: Re: stright pipe and back pressure? Post by bill67 on 07/29/09 at 08:45:04 Get a couple baffles more than one should make it quieter,And have more back pressure,If you rap than with fiberglass it will take out the higher sounds and sound lower,The fiberglass will have to be repacked once in a while. |
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Title: Re: stright pipe and back pressure? Post by Charon on 07/29/09 at 18:58:52 Most likely nobody wants to hear it, but the issue is not "back pressure." If it were, the easiest way to do it would be to make the exhaust valve very small and the exhaust pipe very small diameter. Instead, the pipe acts like a resonant system, with pressure waves bouncing around in it. The reason a straight pipe is hard to tune is because it is resonant only at one frequency (speed), and at any other speed it doesn't work correctly. A smaller diameter pipe has a bit more "resistance" which helps broaden the peak. In electronic terms it has a higher resistance and thus less "Q." Those pressure waves will help or hinder engine breathing depending on engine speed, and during the valve overlap they may even influence the intake tract. Note that the intake tract can also be resonant, and its resonant frequency is almost certainly different from the exhaust. It is easy to say the engine needs back pressure, but it is nowhere near the whole truth. It is a little difficult to find concrete numbers, but as far as I can tell a four-stroke engine, operating at wide-open throttle, will have a cylinder pressure on the order of 50 psig when the exhaust valve first cracks open. That relatively high pressure is why the exhaust valve is smaller than the intake - there is more pressure differential to get the gas moving than on the intake side. |
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Title: Re: stright pipe and back pressure? Post by justin_o_guy2 on 07/29/09 at 19:28:39 I savvy PSI & Ive seen PSIG several times, but dont know what the diff is. |
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Title: Re: stright pipe and back pressure? Post by verslagen1 on 07/29/09 at 19:34:20 607F797E63645565556D7F73380A0 wrote:
No diff to the layman. psi (pounds per square inch) is the same as psig(pounds per square inch gage) reading. Why is there psig? cause there's a psia (pounds per square inch absolute) same as psig + atmospheric pressure, usually 15 psia or there abouts. |
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Title: Re: stright pipe and back pressure? Post by justin_o_guy2 on 07/29/09 at 19:45:19 So, psia is the pressure created above atmospheric pressure? I thank you for the answers. |
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Title: Re: stright pipe and back pressure? Post by Charon on 07/29/09 at 19:46:00 To be slightly more pedantic than Versy, pressure gauges actually measure differential pressure. That is, they measure the difference in pressure between the atmosphere (usually) and the inside of the container (such as a tire). PSI means Pounds per Square Inch. PSIG means PSI measured by a Gauge. Almost always PSI really means PSIG. PSIA means PSI Absolute, which is referenced to a vacuum. For most any sort of technical work you need to use PSIA, and you need to use temperatures measured from absolute zero (Kelvin instead of Celsius; Rankine instead of Fahrenheit). |
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Title: Re: stright pipe and back pressure? Post by verslagen1 on 07/29/09 at 20:03:37 yeoow, I've been out stepped. |
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Title: Re: stright pipe and back pressure? Post by verslagen1 on 07/29/09 at 20:04:40 756A6C6B7671407040786A662D1F0 wrote:
psig would be the pressure above atmospheric. psia includes it. |
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Title: Re: stright pipe and back pressure? Post by justin_o_guy2 on 07/29/09 at 20:36:36 So, I poke the gauge on the tire. Says 30 psi, we call that psig, right? But, if I want to talk about the tire pressure using PsiA, I add 15 & call it 45 psia? Ohhh Gawwd, let that be right... |
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Title: Re: stright pipe and back pressure? Post by Charon on 07/29/09 at 20:59:29 Yep, you got it. With the caveat that you are at about sea level. To a rough approximation, atmospheric pressure drops about one PSI for every two thousand feet of elevation. So in some city at 6000 feet (Cheyenne, Colorado Springs) the atmospheric pressure is only 12 PSIA instead of the nominal 15 at sea level. There, a 30 PSIG tire would be 42 PSIA. |
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Title: Re: stright pipe and back pressure? Post by Serowbot on 07/29/09 at 22:31:35 Thanks Charon,..... I always try to catch your posts... |
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Title: Re: stright pipe and back pressure? Post by justin_o_guy2 on 07/30/09 at 06:37:54 I have it on good authority that Charon ( is that Moon, Charon?) contributes to nearly half of the scientific Wiki entries. |
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Title: Re: stright pipe and back pressure? Post by prsavage on 07/30/09 at 09:16:22 4C5355524F4879497941535F14260 wrote:
I call that time to ride! LETS GO!!!!! |
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