LT1 exhaust stock 2.75 vs fabricated 2.5
04-04-2014, 10:27 AM
#1
Drifting
Thread Starter
LT1 exhaust stock 2.75 vs fabricated 2.5
Currently, I have a system that consists of a 3 pipes off the cats into a 2.5 x-pipe, 3 pipes out of the X pipe back to the rear diff, and then reducers into factory pipes/mufflers.
Im not entirely happy with it. The mufflers ended hanging slightly misaligned and the exhaust note is more aggressive but the x-pipe almost smooths it out too much. I do like the factory mufflers though, they keep it civil and I dont get any noticeable drone despite the lack of resonator. I intend on reusing the mufflers in the new system that I am planning to fab (hopefully this weekend).
I have a variety of pipes to choose from ranging from 2.5 to 3. Im considering using 2.5 is that too small? That is my main concern. I dont want to choke the motor considering the factory exhaust system is mostly 2.75 though it does appear to bottleneck down before the mufflers to 2.5 maybe even less. Second concern, I want to abandon the x-pipe to get closer to the sound that I want. Considering not using a crossover and then adding an h-pipe in the future if it feels down on power or I want to clean up the sound. I dont think a power difference between X, H, or none would really be noticeable on the street, could be wrong. Again, regarding the 2.5" exhaust... this is a street motor only, stock 350 LT1, stock cam, does have LE ported heads and the usual supporting boltons. Thoughts?
Im not entirely happy with it. The mufflers ended hanging slightly misaligned and the exhaust note is more aggressive but the x-pipe almost smooths it out too much. I do like the factory mufflers though, they keep it civil and I dont get any noticeable drone despite the lack of resonator. I intend on reusing the mufflers in the new system that I am planning to fab (hopefully this weekend).
I have a variety of pipes to choose from ranging from 2.5 to 3. Im considering using 2.5 is that too small? That is my main concern. I dont want to choke the motor considering the factory exhaust system is mostly 2.75 though it does appear to bottleneck down before the mufflers to 2.5 maybe even less. Second concern, I want to abandon the x-pipe to get closer to the sound that I want. Considering not using a crossover and then adding an h-pipe in the future if it feels down on power or I want to clean up the sound. I dont think a power difference between X, H, or none would really be noticeable on the street, could be wrong. Again, regarding the 2.5" exhaust... this is a street motor only, stock 350 LT1, stock cam, does have LE ported heads and the usual supporting boltons. Thoughts?
04-04-2014, 12:53 PM
#3
Drifting
Thread Starter
Thank you --- seems 2.5" diameter will be more than adequate for my motor. I know x-pipe is best for power but I'm seeking a particular sound and willing to compromise a bit on power to see if I can get it.
04-04-2014, 02:15 PM
#4
Tech Contributor
Member Since: Jun 2004
Location: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Posts: 18,346
Received 767 Likes
on
549 Posts
If you put an H pipe way downstream like an X pipe then the (poor) efficiency of both is rather close, as most X pipes are shaped poorly and create a lot of turbulence.
04-04-2014, 06:27 PM
#5
Racer
An H-pipe at the collector will kill its effect.
The reason an X- or H-pipe works is the effect it has on reflecting the pulses coming from the collector. The higher your RPM operating range the closer it needs to be to the collector, but anything less than about 15" will be too close for a street-driven car. The typical 5500-6000 RPM upper range of a SBC calls for something on the order of 20-30".
Understand the purpose of an X- or H-pipe is not to "equalize" flow, or to allow both sides to provide flow from either side. The purpose is the same as a "torque tube", as it is commonly called, on the collector vs. an open collector. The pulsing nature of exhaust flow means the pressure in the exhaust system rises and falls, and when a high pressure pulse reaches an opening or "break" in the tube, a pressure wave is reflected back up the tube. It is also reflected back when it comes to a break in the tube upstream (in this case, the 4 tubes coming together in the collector). When the pulse is reflected, it leaves a low pressure pulse behind it - if that low pressure event is timed to when the next exhaust pulse arrives, it will "pull" that exhaust pulse down the tube. The timing of the reflected pulses is determined primarily by where the "breaks" occur in the tubing.
Having the H-pipe at the collector destroys this effect (unless you have another downstream in the proper spot in the exhaust system).
The same phenomenon occurs in the header primaries, and in the intake tract (ever heard of "Tuned Port Injection"?).
The reason an X- or H-pipe works is the effect it has on reflecting the pulses coming from the collector. The higher your RPM operating range the closer it needs to be to the collector, but anything less than about 15" will be too close for a street-driven car. The typical 5500-6000 RPM upper range of a SBC calls for something on the order of 20-30".
Understand the purpose of an X- or H-pipe is not to "equalize" flow, or to allow both sides to provide flow from either side. The purpose is the same as a "torque tube", as it is commonly called, on the collector vs. an open collector. The pulsing nature of exhaust flow means the pressure in the exhaust system rises and falls, and when a high pressure pulse reaches an opening or "break" in the tube, a pressure wave is reflected back up the tube. It is also reflected back when it comes to a break in the tube upstream (in this case, the 4 tubes coming together in the collector). When the pulse is reflected, it leaves a low pressure pulse behind it - if that low pressure event is timed to when the next exhaust pulse arrives, it will "pull" that exhaust pulse down the tube. The timing of the reflected pulses is determined primarily by where the "breaks" occur in the tubing.
Having the H-pipe at the collector destroys this effect (unless you have another downstream in the proper spot in the exhaust system).
The same phenomenon occurs in the header primaries, and in the intake tract (ever heard of "Tuned Port Injection"?).
04-04-2014, 08:43 PM
#6
Team Owner
Pro Mechanic
04-04-2014, 11:17 PM
#8
Tech Contributor
Member Since: Jun 2004
Location: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Posts: 18,346
Received 767 Likes
on
549 Posts
An H-pipe at the collector will kill its effect. Vizard recommends that the crossover be located at the collector area. I've read enough of his stuff to respect his engineering opinion on things.
The reason an X- or H-pipe works is the effect it has on reflecting the pulses coming from the collector. The higher your RPM operating range the closer it needs to be to the collector, but anything less than about 15" will be too close for a street-driven car. The typical 5500-6000 RPM upper range of a SBC calls for something on the order of 20-30".
Understand the purpose of an X- or H-pipe is not to "equalize" flow, or to allow both sides to provide flow from either side. The purpose is the same as a "torque tube", as it is commonly called, on the collector vs. an open collector. The pulsing nature of exhaust flow means the pressure in the exhaust system rises and falls, and when a high pressure pulse reaches an opening or "break" in the tube, a pressure wave is reflected back up the tube. It is also reflected back when it comes to a break in the tube upstream (in this case, the 4 tubes coming together in the collector). When the pulse is reflected, it leaves a low pressure pulse behind it - if that low pressure event is timed to when the next exhaust pulse arrives, it will "pull" that exhaust pulse down the tube. The timing of the reflected pulses is determined primarily by where the "breaks" occur in the tubing.
Having the H-pipe at the collector destroys this effect (unless you have another downstream in the proper spot in the exhaust system).
The same phenomenon occurs in the header primaries, and in the intake tract (ever heard of "Tuned Port Injection"?).
The reason an X- or H-pipe works is the effect it has on reflecting the pulses coming from the collector. The higher your RPM operating range the closer it needs to be to the collector, but anything less than about 15" will be too close for a street-driven car. The typical 5500-6000 RPM upper range of a SBC calls for something on the order of 20-30".
Understand the purpose of an X- or H-pipe is not to "equalize" flow, or to allow both sides to provide flow from either side. The purpose is the same as a "torque tube", as it is commonly called, on the collector vs. an open collector. The pulsing nature of exhaust flow means the pressure in the exhaust system rises and falls, and when a high pressure pulse reaches an opening or "break" in the tube, a pressure wave is reflected back up the tube. It is also reflected back when it comes to a break in the tube upstream (in this case, the 4 tubes coming together in the collector). When the pulse is reflected, it leaves a low pressure pulse behind it - if that low pressure event is timed to when the next exhaust pulse arrives, it will "pull" that exhaust pulse down the tube. The timing of the reflected pulses is determined primarily by where the "breaks" occur in the tubing.
Having the H-pipe at the collector destroys this effect (unless you have another downstream in the proper spot in the exhaust system).
The same phenomenon occurs in the header primaries, and in the intake tract (ever heard of "Tuned Port Injection"?).
I also like Vizard's thoughts on exhaust pulse termination boxes. (An older vintage picture.)
The box has an interior volume of about 760 cubic inches. Depending on who's figures you believe for pressure wave velocity at typical exhaust gas temperatures and doing the math with my cam specs, the system is "tuned" at the 4000-5000 RPM range. My track days are at road courses, and this RPM is used quite often at corner exits and medium length straights, along with minimizing the needless wear and tear (due to excessive RPM) on the engine during a typical 75-125 mile track day.
I also have a pressure tap on the box to monitor what the ambient pressure is inside it and to confirm that the mufflers are not restrictive. It's a three inch system, and I've had several compliments on how quiet the car is at the track.
04-05-2014, 05:22 PM
#9
Drifting
Thread Starter
It's all done. I really took my time with it to ensure that the mufflers end up sitting correctly and the exhaust tips line up perfect. The wire feed on my welder was well behaved today and the welds came out really nicely.
I ended up using the front half of a factory LT1 system cut right after the first set of spring loaded hangers. In place of the resonator I used 2.5" pipe straight back and then right into the rest of the stock LT1 pipes/mufflers. So if you read that, I didn't mention X or H pipe. Because there isn't one.
Now, as far as power goes - I drove it and I could tell no difference at all between the prior system with X pipe and the current system without crossover.
Sound - way better than the x pipe. The tone of the exhaust is noticeable more aggressive and really close to what I was hoping for. It is a stitch louder than with the x pipe but the factory mufflers keep it in check and best of all - still no drone.
The one odd thing - at warm idle, accompanied with the exhaust note I do get a "puff, puff" sound out the exhaust. Possibly, the result of no X or H? X is out of the question - I've already found I don't care for its sound and it'd be a complete hassle to weld in. I'd consider an H - but I don't know what it does to the overall sound when compared to the 'true dual' that I have now, and like. I could live with the puff sound for now which is only audible if you stand by the tailpipes. I don't feel like working on it again anytime soon, its time to drive and enjoy. But I'll keep an h pipe in mind for the future.
I ended up using the front half of a factory LT1 system cut right after the first set of spring loaded hangers. In place of the resonator I used 2.5" pipe straight back and then right into the rest of the stock LT1 pipes/mufflers. So if you read that, I didn't mention X or H pipe. Because there isn't one.
Now, as far as power goes - I drove it and I could tell no difference at all between the prior system with X pipe and the current system without crossover.
Sound - way better than the x pipe. The tone of the exhaust is noticeable more aggressive and really close to what I was hoping for. It is a stitch louder than with the x pipe but the factory mufflers keep it in check and best of all - still no drone.
The one odd thing - at warm idle, accompanied with the exhaust note I do get a "puff, puff" sound out the exhaust. Possibly, the result of no X or H? X is out of the question - I've already found I don't care for its sound and it'd be a complete hassle to weld in. I'd consider an H - but I don't know what it does to the overall sound when compared to the 'true dual' that I have now, and like. I could live with the puff sound for now which is only audible if you stand by the tailpipes. I don't feel like working on it again anytime soon, its time to drive and enjoy. But I'll keep an h pipe in mind for the future.
05-13-2014, 09:41 AM
#10
Drifting
Thread Starter
Just an update.... having driven the car about 150 miles since my last post I can say that there is some level of difference that I can feel between the prior exhaust that had an x-pipe to this one that keeps the two banks completely separate without a crossover. Thats the only difference between the two systems really (same mufflers, headers, cats).
That difference is at part throttle, low end torque. It isnt a slug or anything, but I can discern that low speed torque has taken some kind of hit. Wide open and above 2500 rpm I still can't tell a difference in power, but its the "everyday" type driving where its noticeable. So, I'll be adding an H pipe (maybe this weekend if it rains like they say it will). It will be 2.5" diameter and I'll be using a flange in the center of the H pipe, this will allow me to unbolt the flange and remove the exhaust as 2 separate halves rather than one big lump. Just thinking ahead for ease of removal for future maintenance.
That difference is at part throttle, low end torque. It isnt a slug or anything, but I can discern that low speed torque has taken some kind of hit. Wide open and above 2500 rpm I still can't tell a difference in power, but its the "everyday" type driving where its noticeable. So, I'll be adding an H pipe (maybe this weekend if it rains like they say it will). It will be 2.5" diameter and I'll be using a flange in the center of the H pipe, this will allow me to unbolt the flange and remove the exhaust as 2 separate halves rather than one big lump. Just thinking ahead for ease of removal for future maintenance.