Exhausting thoughts !

[N2mesnob]

So I'm in the process of doing up my current exhaust. I'm stepping up from 2" to a 3" setup so i ordered the headers (stainless steel), the catalytic converter and the muffler. The only thing left is what to do bout the tip...? So the collector on the headders is still 2" the cat is 3" and the muffler is 3" but not straight through and with the bend leading to the tip i should still have enough backpreasure...

Still waiting on the muffler to come in

[MY1PATH]

um sorry to rain on your parade but if you put 3" anything on that exhaust system you are not gonna be happy except for maybe near redline.

stock is 1.875" 2" and 2.25" are good choices and 2.5" is getting large.

 

Even if you have 2" going into your 3" cat and muffler the sudden expansion followed by the re-compression of the freshy cooled gasses(cause they cool and SLOW DOWN when they expand) is gonna create a turbulance situation.

Sorry to hear you already bought the components before checking....

 

On the bight side, nice avatar.

[N2mesnob]

Even if you have 2" going into your 3" cat and muffler the sudden expansion followed by the re-compression of the freshy cooled gasses(cause they cool and SLOW DOWN when they expand) is gonna create a turbulance situation.

 

Interesting you should say that... the impression i was given was that i should have enough backpreassure and with the two headers dumping into one pipe should not hurt current level of performance.I saw there was plenty of room under her and was tempted to go dual exhaust. I haven't installed it yet can you give more info on why turbulence is not good? Also is this the rule of thumb on V6's, VG30's, or jus for the Pathy setup?

[Mr.510]

Given the parts you already have here's what I would do: Make a Y-pipe out of 2" that smoothly merges the two headpipes into a single 2-1/2" pipe with a smooth transition (think cone) up to 3" right before your cat. Do the rest in 3". Then I'd marvel at how good it sounds while I noted that it makes more power than stock at any RPM and gets better gas mileage too.

 

I have a VG30 with headers, 2-1/2" headpipes, and 3" exhaust. It's got a 4" auger muffler just after the Y-pipe and a Flowmaster way at the back of the car as that's the only place it'll fit. It works great, sounds amazing, and makes plenty of torque from idle on up. It's not in a Pathfinder, BTW.

 

Here's the deal: Backpressure is always bad. It never helps HP or torque (same thing) at any rpm. Ever. (Hearing the myth of backpressure makes torque repeated like the gospel by generations of ill-informed knuckleheads drives me friggin nuts.) Ok, back on track: What does help bottom end is the scavenging effect of a smaller diameter exhaust pipe. The smaller the pipe the more scavenging you get at low rpm as the exhaust impulses are farther apart. The further you get from the exhaust valves, the less effect scavenging has. As the RPM goes up the amount of time/space between impulses narrows and scavenging stops altogether. In layman's terms scavenging is when the slight vacuum following one exhaust impulse helps to pull the one behind it down the pipe... kinda like race cars drafting on a super speedway.

 

One more thing while I'm on my soap box: Properly sized single exhaust always makes more power than properly sized dual exhaust. This is true on all engines within the scope of automotive use.

 

If you run 2" headpipes that Y into 2-1/2" back to the cat your tube ID is going to be just about perfect for the first, what, ten feet of the system? What happens after that probably doesn't make any measurable difference so long as you don't have excessive backpressure which obviously you will not. Even if you run 3" pipe from the Y I don't think you could find the difference on a chassis dyno.

 

In the application of "typical Pathfinder" the ideal performance exhaust system would have: 14" long, equal length 1-1/2" primary header tubes merged into 2-1/4" collectors with 2-1/4" headpipes down to a Y into 2-1/2" pipe and components for the rest of the system. This assumes the engine is not intended to produce power above 6k rpm. I've done all the math straight from the book 'Scientific Design of Exhaust and Intake Systems'. I highly recommend it to anyone that wants to know the formulas to figure this stuff out.

[Dowser]

 

Wow. Excellent reply with an actual explanation. I think I'll book mark this thread to use for reference for any other exhaust question in the future, which will no doubt be in the next few days. Nice work 510.

[Tungsten]

I always wondered which diameter after the 2.25" Y-pipe will make more power and where. I know I have a stock 2.00" pipe with high flow components after and it's nice and quiet on the highway and has good power for accelerating.

[Mr.510]

 

Wow. Excellent reply with an actual explanation. I think I'll book mark this thread to use for reference for any other exhaust question in the future, which will no doubt be in the next few days. Nice work 510.

 

Thanks Dowser.

 

I always wondered which diameter after the 2.25" Y-pipe will make more power and where. I know I have a stock 2.00" pipe with high flow components after and it's nice and quiet on the highway and has good power for accelerating.

 

 

Well, in *theory* the total area of the inside of an exhaust pipe should taper and get smaller gradually as it gets farther from the engine, at least once it's past the header collectors. That said, I always say error on the side of bigger pipe since you're not going to get much, if any scavenging effect once you're six feet from the exhaust valves. There is very little chance you will hurt power output with too big of a pipe, except close to the engine in applications where bottom-end torque is of paramount importance. If you're Y pipe is 2-1/4" I would either continue the system with that or go up to 2-1/2". The biggest reason to go up a pipe size here is that a 2-1/2" cat and muffler are likely to produce far less backpressure than 2-1/4" components. For what it's worth, 2-1/2" pipe has 23% more flow capacity than 2-1/4" pipe. 2-1/4" pipe flows 27% more than 2" at the same pressure.

 

Just for grins: 3" pipe has 44% more flow capacity than 2-1/2", or 2.25 times the capacity of 2"!

 

 

A little bit on sound waves, reversion, and mufflers:

 

As far as performance mufflers go in my opinion you cannot beat Flowmaster. A Flowmaster muffler is a one-way check valve for sound waves.

 

I talked in my earlier post about exhaust impulses, these are the 'puffs' of exhaust gas that exit the cylinders during the exhaust stroke. There is another force at work emanating from the engine: sound waves. Think of a sound wave in a pipe as a shallow cupped disk, like a frisbie. The domed center is the leading portion of a sound wave. These waves can travel through the exhaust gases at a higher rate of speed than the gases themselves. The gases they travel through slow them down, and therefore part of their energy is actually pushing the exhaust gases down the pipe. Here's the catch: a sound wave traveling down a pipe will reverse direction if it hits a hard edge or sharp step-down in that pipe. A very small step of 1/32" is plenty big enough to reverse sound waves and make a jumbled mess of them trying to push the exhaust gases one way or the other. This is called reversion and it has a significant impact on exhaust flow, power output, and efficiency. The trick to a really good racing exhaust system is to have either no transitional steps in the system or to have them all go from smaller pipe to larger (such as with slip-fits) so the sound waves can travel out of the smaller pipe and into the larger but not backwards up the 'step'. If you use slip-on connections in an exhaust system the rearward pipe should always slip over the forward one! The same goes for mufflers: the inlet should slip over the pipe while the tailpipe should slip over the muffler's outlet.

 

Back to Flowmaster mufflers. What these mufflers do, among other things, is to effectively 'trap' sound waves and prevent them from rebounding and heading back toward the engine. For optimum effect the muffler has to be placed close to the ideal distance from the engine but this is not always possible due to vehicle layout. Flowmaster has charts as to what is ideal for their various mufflers. My VG30 powered 510 sedan mentioned above with 3" exhaust has the muffler at the very back with only an 8" long piece of SS tube for a tailpipe. This is not ideal but it does still work very well. Some people don't like the sound of Flowmaster mufflers and I don't think I'll be running an open chamber version on my Pathy as it will be too loud inside. My VG powered 510 wagon has a Flowmaster Deltaflow muffler and it has far less 'rattle' sound to the exhaust note but still functions as a sound wave check valve, albeit with slightly increased backpressure over an original Flowmaster.

 

This is a short video of my VG wagon with headers, an auger pre-muffler, and a 2-1/2" Flowmaster Deltaflow with about a four foot tailpipe.

 

http://www.youtube.com/watch?v=s-9PowgmokE

[JamesRich]

Thanks Dowser.

 

 

 

 

Well, in *theory* the total area of the inside of an exhaust pipe should taper and get smaller gradually as it gets farther from the engine, at least once it's past the header collectors. That said, I always say error on the side of bigger pipe since you're not going to get much, if any scavenging effect once you're six feet from the exhaust valves. There is very little chance you will hurt power output with too big of a pipe, except close to the engine in applications where bottom-end torque is of paramount importance. If you're Y pipe is 2-1/4" I would either continue the system with that or go up to 2-1/2". The biggest reason to go up a pipe size here is that a 2-1/2" cat and muffler are likely to produce far less backpressure than 2-1/4" components. For what it's worth, 2-1/2" pipe has 23% more flow capacity than 2-1/4" pipe. 2-1/4" pipe flows 27% more than 2" at the same pressure.

 

Just for grins: 3" pipe has 44% more flow capacity than 2-1/2", or 2.25 times the capacity of 2"!

 

 

A little bit on sound waves, reversion, and mufflers:

 

As far as performance mufflers go in my opinion you cannot beat Flowmaster. A Flowmaster muffler is a one-way check valve for sound waves.

 

I talked in my earlier post about exhaust impulses, these are the 'puffs' of exhaust gas that exit the cylinders during the exhaust stroke. There is another force at work emanating from the engine: sound waves. Think of a sound wave in a pipe as a shallow cupped disk, like a frisbie. The domed center is the leading portion of a sound wave. These waves can travel through the exhaust gases at a higher rate of speed than the gases themselves. The gases they travel through slow them down, and therefore part of their energy is actually pushing the exhaust gases down the pipe. Here's the catch: a sound wave traveling down a pipe will reverse direction if it hits a hard edge or sharp step-down in that pipe. A very small step of 1/32" is plenty big enough to reverse sound waves and make a jumbled mess of them trying to push the exhaust gases one way or the other. This is called reversion and it has a significant impact on exhaust flow, power output, and efficiency. The trick to a really good racing exhaust system is to have either no transitional steps in the system or to have them all go from smaller pipe to larger (such as with slip-fits) so the sound waves can travel out of the smaller pipe and into the larger but not backwards up the 'step'. If you use slip-on connections in an exhaust system the rearward pipe should always slip over the forward one! The same goes for mufflers: the inlet should slip over the pipe while the tailpipe should slip over the muffler's outlet.

 

Back to Flowmaster mufflers. What these mufflers do, among other things, is to effectively 'trap' sound waves and prevent them from rebounding and heading back toward the engine. For optimum effect the muffler has to be placed close to the ideal distance from the engine but this is not always possible due to vehicle layout. Flowmaster has charts as to what is ideal for their various mufflers. My VG30 powered 510 sedan mentioned above with 3" exhaust has the muffler at the very back with only an 8" long piece of SS tube for a tailpipe. This is not ideal but it does still work very well. Some people don't like the sound of Flowmaster mufflers and I don't think I'll be running an open chamber version on my Pathy as it will be too loud inside. My VG powered 510 wagon has a Flowmaster Deltaflow muffler and it has far less 'rattle' sound to the exhaust note but still functions as a sound wave check valve, albeit with slightly increased backpressure over an original Flowmaster.

 

This is a short video of my VG wagon with headers, an auger pre-muffler, and a 2-1/2" Flowmaster Deltaflow with about a four foot tailpipe.

 

http://www.youtube.com/watch?v=s-9PowgmokE

 

I read the whole thing and now my head hurts!

That is one sweet sounding 6!

James

[Kingman]

Thank you for actually taking the time to explain what you did, David. As was said it should be pinned for all members to see whenever they have a question. I've been trying to explain to many people that back pressure is never good on ANY motor, ever, and that it won't make any more or less torque. Also how after the primaries and further along the system larger piping isn't going to hurt anything.

 

Your ideal set up is basically exactly what I've been planning to do with mine for quite a while now, just haven't had the money to do it. Another muffler to mention is the older Edelbrock mufflers that are no longer in production. They use a perforated metal design that produces zero back pressure at any RPM and have a really nice sound to them. I search constantly on Ebay for them, but usually don't have the money to snatch one up when it comes along. But of course, when I do they never show up...

 

Oh, and what brand headers are those you have pictured?

Edited August 28, 2010 by Kingman