Another Exhaust question..help
02-19-2003, 08:20 AM
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Another Exhaust question..help
ok..well last year i bought a obx b5 muffler and just had a local import shop put it on..well i am ordering a high flow cat and a glasspack online but i wanted to know which size piping should i do
the shop that is doing it says that they don't do crush bent (no ripples) but it isn't mandrel bent...i said i wanted to do 2.5 but he suggested 2.25..keep in mind i am purchasing i have a CAI..and i am purchasing the cnk headers once the groupbuy is one..and right after that i am buying the sr bbtb and having a machine shop bore out my im...and then hopefully by summer put a small shot of nitrous..so what would you guys suggest..i neeed to know b4 i go ordering online for the cat and glasspack.
Thanks
Steve
the shop that is doing it says that they don't do crush bent (no ripples) but it isn't mandrel bent...i said i wanted to do 2.5 but he suggested 2.25..keep in mind i am purchasing i have a CAI..and i am purchasing the cnk headers once the groupbuy is one..and right after that i am buying the sr bbtb and having a machine shop bore out my im...and then hopefully by summer put a small shot of nitrous..so what would you guys suggest..i neeed to know b4 i go ordering online for the cat and glasspack.
Thanks
Steve
02-19-2003, 05:44 PM
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I think 2.25 would be the standard answer, but given your goals I think I'd go 2.5. Some people will say that 2.5 in too big for a 1.8, other
"experts" will say its fine... depending on your plans. The concern is usually about losing some low-end torque, even if the top gets bigger.
The low-end on my 2.0 seemed slightly effected around town, but not when I put the pedal down, so maybe only under 1500 rpms...if at all. Part of this may be that as the top end get stronger, the low end "seems" weaker by contrast. I haven't seen any definitive evidence
that proves or dis-proves the theory of low-end torque loss and larger diameter piping.
"experts" will say its fine... depending on your plans. The concern is usually about losing some low-end torque, even if the top gets bigger.
The low-end on my 2.0 seemed slightly effected around town, but not when I put the pedal down, so maybe only under 1500 rpms...if at all. Part of this may be that as the top end get stronger, the low end "seems" weaker by contrast. I haven't seen any definitive evidence
that proves or dis-proves the theory of low-end torque loss and larger diameter piping.
02-19-2003, 07:18 PM
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I have a perfectly good explanation. It's all about the back pressure effect. More back pressure will push low end torque (obviosly not too much back pressure) and less back pressure will free up high end power. I noticed the same thing MAXX that you're talking about when I installed my muffler. I hollowed out one of my cats (Ohio doesn't really have emission laws), removed my stock resonator and put on the muffler. The muffler I got has a removable baffle. Enforcer, I would suggest you get a muffler with a removable baffle as well. I noticed a considerable difference with the baffle in and out. In, I had a crap load more low end and out, I had a lot more top end. It's really in your driving preference which you would choose but it is FACT that back pressure plays a serious role in the type of power band you have (mainly N/A motors). Turbo motors can react very differently in some cases to back pressure and most of the time you don't want to have that much back pressure with a turbo because it won't spool fast enough so if you had a normal amount of back pressure you wouldn't see the low end torque because your turbo could lag the engine if it can't spool. Turbos and back pressure can be a vicious cycle. Hope this helps.
02-20-2003, 06:16 AM
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Yeah, I would love to see more serious testing on this subject of low-end torque loss. How much IS lost? Is it real loss,or just a perception? Its easy to imagine gains or losses if we are expecting em. I have seen some of Bill Luton's
exhaust research, years of work on
this subject alone, and heres an important point: On the various cars he tested, peak torque and hp always increased as the pipe size increased. He didn't mention if torque was lost elsewhere though. But its good to bear this in mind....you may lose a little low end torque, but still see gains in peak torque. Larger diameter pipe may not cause torque loss OR gain....it may cause both at different places. The percentage of torque loss vs. gain, and where it occurs in the powerband, is what I would like to know more about. Is it a good trade-off? I guess it depends on your driving style and goals for the car I suppose. smile.gif
exhaust research, years of work on
this subject alone, and heres an important point: On the various cars he tested, peak torque and hp always increased as the pipe size increased. He didn't mention if torque was lost elsewhere though. But its good to bear this in mind....you may lose a little low end torque, but still see gains in peak torque. Larger diameter pipe may not cause torque loss OR gain....it may cause both at different places. The percentage of torque loss vs. gain, and where it occurs in the powerband, is what I would like to know more about. Is it a good trade-off? I guess it depends on your driving style and goals for the car I suppose. smile.gif
02-20-2003, 12:29 PM
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Well yeah, that's pretty much what I was saying is the shift in the power band. My car is loud as crap without the baffle so that is ONE of the reasons I keep it in most of the time. The peak torque would probably increase but like I said, it's the trade off for the low end to top end. After the turbo goes on my car, I will probably prefer the high end but until then the low end is dandy for me plus I don't want to go deaf at 21 years old! I am proud of the low end torque on the beta2...there aren't a whole lot of 4cyls out there that can boast low end torque quite as well.
02-25-2003, 12:03 AM
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bump...so what does everyone think i should do..it will be awhile before i get the nx kit..but does anyone really think i'll loose alot of backpressure with 2.5..keepingin mind they said it isn't mandrel bent but it is crushed either..and i'll have a glasspack?
02-25-2003, 03:14 PM
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Enforcer, I say if you haven't all ready gotten a muffler then get one with a silencer/baffle. That way you can put the silencer in to create backpressure when desired. (silencer is a metal insert that makes the opening smaller) Or if you all ready have a muffler with a silencer GREAT! If this is the case, DEFINITLY go with the 2.5. If this is NOT the case then it depends on how you drive and what you like. Do you spend a lot of time in high rpm's or low rpm's? High rpm's then get bigger pipe. Low rpm's then get smaller pipe. Do ya see what I'm saying?
02-25-2003, 10:20 PM
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Shane (back in the Fxtreme website days) had a supercharged Tib and removed the cat. He said they immediately wanted to put it back on. Too much torque loss.
As for the back pressure effect: most people view the flow through an engine as steady. Wider the pipe, the less resistance, the more flow. This works in larger displacement engines, because they are pushing out so much exhaust that it is almost steady. However, our four-bangers do not have this luxury. Instead, the exhaust comes out in pulses. At first glance, the pulse is just a high pressure "pop" as the valve opens, but it is actually much more dynamic. There is the high pressure "front" as the valve opens, but then there's a low-pressure (actually a vacuum) "wake" behind the pulse. You want to get all the hot exhaust gasses out of the cylinder after each firing, the vacuum-wake behind the exhaust pulse helps suck out the hot gasses. When you have back pressure, the pulses are slowed down and this helps maximize the scavenging effect. Too much backpressure and the exhaust can't flow out (which is the ultimate goal of the exhaust), too little and it expands too fast and very little scavenging is done. As a result, some hot gasses stay in the cylinder and efficiency is reduced.
So like everything in automotive design, it is a tradeoff. From what I've read over the years at the various forums, keeping the cat seems to keep about an optimum level of back pressure. As usual, a tunable system (like the muffler baffles discussed earlier) would be ideal to get the right balance.
okay, I've typed enough. smile.gif
ac
As for the back pressure effect: most people view the flow through an engine as steady. Wider the pipe, the less resistance, the more flow. This works in larger displacement engines, because they are pushing out so much exhaust that it is almost steady. However, our four-bangers do not have this luxury. Instead, the exhaust comes out in pulses. At first glance, the pulse is just a high pressure "pop" as the valve opens, but it is actually much more dynamic. There is the high pressure "front" as the valve opens, but then there's a low-pressure (actually a vacuum) "wake" behind the pulse. You want to get all the hot exhaust gasses out of the cylinder after each firing, the vacuum-wake behind the exhaust pulse helps suck out the hot gasses. When you have back pressure, the pulses are slowed down and this helps maximize the scavenging effect. Too much backpressure and the exhaust can't flow out (which is the ultimate goal of the exhaust), too little and it expands too fast and very little scavenging is done. As a result, some hot gasses stay in the cylinder and efficiency is reduced.
So like everything in automotive design, it is a tradeoff. From what I've read over the years at the various forums, keeping the cat seems to keep about an optimum level of back pressure. As usual, a tunable system (like the muffler baffles discussed earlier) would be ideal to get the right balance.
okay, I've typed enough. smile.gif
ac