Conversation about Maxima exhaust I had w/ buddy I work with
05-29-2004, 05:38 AM
#41
Originally Posted by Stephen Max
Quicksilver,
I think you and I pretty much agree on exhaust system dynamics and the trade-offs involved between flow velocity and exhaust restriction when sizing pipe diameter.
We seem to have different definitions for "backpressure". I have stated my definition, which is consistent with what you are calling flow restriction, and I also stated that backpressure does not by itself result in exhaust gases flowing back into the cylinder (although reverse flow can happen because of a cam with large intake and exhaust overlap, but that is another discussion entirely), which seems to be your point of contention to the term.
I don't know where to go from here, so I'll just say it is nice to have a level-headed discussion without it getting into a flame war.
Peace.
I think you and I pretty much agree on exhaust system dynamics and the trade-offs involved between flow velocity and exhaust restriction when sizing pipe diameter.
We seem to have different definitions for "backpressure". I have stated my definition, which is consistent with what you are calling flow restriction, and I also stated that backpressure does not by itself result in exhaust gases flowing back into the cylinder (although reverse flow can happen because of a cam with large intake and exhaust overlap, but that is another discussion entirely), which seems to be your point of contention to the term.
I don't know where to go from here, so I'll just say it is nice to have a level-headed discussion without it getting into a flame war.
Peace.
I also believe that you and I see and agree on the physics side of it. It's so good to finally have another technical discussion (in depth) that doesn't spiral down into the deepest recesses of hell due to misinformation and blatant post whoring. I salute all who participated in this thread for that...
My point to this discussion was really to define "backpressure" and to show where it originated from...and in so showing, I had hoped to shelve the usage of the word for those of us here who are currently naturally aspirated. Backpressure does exist, but I firmly believe that using that particular term here (as an example in our engine) is a total misappropriation of the true definition and origin of the word. I know it's been fed to us over the years as something we "must have" and "all engines have"...but it's just no longer true. Backpressure to me was the early 1970's, when big V8 carbureted cars were fitted with the first (and very restrictive, I might add) emmissions control devices. At idle, these carbureted cars were so pent up and restricted with the nearly solid block catalytic converters that they actually did regurgitate exhaust, fuel, and air back through the top end of the carburetor! Talk about a severe fire hazard! That's my thoughts on the definition of backpressure. I won't even go fully into the 2 stroke side of things, as it really has no bearing on our discussions here...
Again, GOOD STUFF! Thanks for conversations
05-29-2004, 08:31 AM
#43
Originally Posted by Jeff92se
Stay outa this, the smart people are talking 
haha
You getting this stuff??? It's your thread!
haha
You getting this stuff??? It's your thread!
Well now a lot of it is just about backpressure. Mecca....from what Coop would say, duals will not make any gains at all. A "TRUE" dual system again, may have 1-2 HP difference. But, it would be your choice.
I honestly dont think a TRUE DUAL system will fit, or is worth making for a Maxima.
05-29-2004, 09:33 AM
#44
Originally Posted by bijan gxe
Well now a lot of it is just about backpressure. Mecca....from what Coop would say, duals will not make any gains at all. A "TRUE" dual system again, may have 1-2 HP difference. But, it would be your choice.
I honestly dont think a TRUE DUAL system will fit, or is worth making for a Maxima.
lol, i dont want duals, i was asking what size test pipe, and catback is recommended for best flow/hp gains for boosted and N/A cars
I read "ideal" as dual........
05-29-2004, 11:44 AM
#47
Supporting Maxima.org Member
Join Date: May 2002
Location: Atlanta
Posts: 1,815
OK here's a little (lot 
) more for you guy to chew on...
Most of you have the right thought but not the dynamics. Pressure, velocity, backpressure, flow and so on are all terms used here. But you're all forgetting the most important part. When the exhaust valve opens it's not only is the piston that's coming up that forces the gasses out, there is also more pressure in the cylinder than there was in the intake part of the cycle. This pressure along the up coming piston forces these gasses out at high velocity. At this high velocity it is a pulse, or shock wave. The shock wave has a negative pressure behind it. The negative pressure is what pulls the next exhaust pulse along. Hence scavenging. (Have any of you seen those toy air guns at the mall. It's a bucket with a handle that comes out the side with a hole in the bottom of the bucket and a sheet of... This will take to long. One of THESE ) It the same process except it's in your exhaust pipe.
Now here is where it gets interesting. If each cylinder had it's own pipe that went to the back of the car the pulses would be too far apart to scavenge. This is where the exhaust manifold comes in, they not only join the pulses together but their length will set the timing of the pulses at a time when they will be of benefit. Changing length runners in the manifold will change the point the engine produces power in the curve. A stock manifold is designed to balance the power for daily driving. But we all want more power, i.e. HEADERS. Headers change the length of the runners to join the pulses together at the right point to help/force join the pulses together to change the power producing ability of the motor. They will even help create a negative pressure in the cylinder when the exhaust valve closes which will pull more of the intake charge it when those valves open. Bigger charge bigger explosion, bigger explosion bigger exhaust pulse. This works best in a V8 not a V6. In a V8 there are 2 cylinders firing in each bank during each cycle. Those 2 pulses will exit the header at a time when they will help pull the next two out, the Collector. In a V6 it alternates between 2 and 1 per cycle. So there is less of a negative pressure behind the single pulse...
Whew
Ok now as we get further down the exhaust pipe we hit the Y. You still want these pulses to join together at the same time. (Going in the same direction, more on that later) When they exit the Y joined together they create a bigger pulse, which in turn keeps everything going. When it comes to pipe size either here or anywhere in the exhaust track you want it big enough to maximize flow, but to big and the pulses loose energy (also remember heat=energy too) and you loose power. It's all about pulse size. NA 2.5" is as big as you want to go. FA bigger pulse, bigger piping works.
Now why does a Y pipe make power? Well, Nissan had a sh!ty design. Here is where we get to that "same direction" comment above. In the pre2k2 Y they forced the 2 banks together at a BAD angle. Look at this -->Y Ideally you want the flow to come from each end of the top of the Y heading towards the bottom so the flow goes in the same direction. But it was made so one came from the bottom and the other came from one side of the top (lets say the left) exiting out the other (the right). Also as you look at the Y the bottom is the left/front bank and the top left is the right/rear bank exiting out the upper right to the main cat.
As you can see this slams the pulses against each other, not good. With an after market Y you correct this problem and make more power! This flaw, or intentional design, was changed in the 2K2+, which is why <---
aftermarket y-pipes do very little to nothing for them!
You want to keep the flow of the pulses together all the way out the tail pipe, only if done to make the most power it would be to loud to drive around town. And we have the EPA (cats), which isn't such a bad thing BTW. So we must balance high flow cats, resonators and mufflers to get the power we want at a noise level we can stand, or not get a ticket for some of us.
In a nutshell there is not any backpressure at the cylinder head. But there are many bad flow characteristics from there on in a stock design.
Now on to 2-strokes...
LOL, Not
) more for you guy to chew on...Most of you have the right thought but not the dynamics. Pressure, velocity, backpressure, flow and so on are all terms used here. But you're all forgetting the most important part. When the exhaust valve opens it's not only is the piston that's coming up that forces the gasses out, there is also more pressure in the cylinder than there was in the intake part of the cycle. This pressure along the up coming piston forces these gasses out at high velocity. At this high velocity it is a pulse, or shock wave. The shock wave has a negative pressure behind it. The negative pressure is what pulls the next exhaust pulse along. Hence scavenging. (Have any of you seen those toy air guns at the mall. It's a bucket with a handle that comes out the side with a hole in the bottom of the bucket and a sheet of... This will take to long. One of THESE ) It the same process except it's in your exhaust pipe.
Now here is where it gets interesting. If each cylinder had it's own pipe that went to the back of the car the pulses would be too far apart to scavenge. This is where the exhaust manifold comes in, they not only join the pulses together but their length will set the timing of the pulses at a time when they will be of benefit. Changing length runners in the manifold will change the point the engine produces power in the curve. A stock manifold is designed to balance the power for daily driving. But we all want more power, i.e. HEADERS. Headers change the length of the runners to join the pulses together at the right point to help/force join the pulses together to change the power producing ability of the motor. They will even help create a negative pressure in the cylinder when the exhaust valve closes which will pull more of the intake charge it when those valves open. Bigger charge bigger explosion, bigger explosion bigger exhaust pulse. This works best in a V8 not a V6. In a V8 there are 2 cylinders firing in each bank during each cycle. Those 2 pulses will exit the header at a time when they will help pull the next two out, the Collector. In a V6 it alternates between 2 and 1 per cycle. So there is less of a negative pressure behind the single pulse...
Whew
Ok now as we get further down the exhaust pipe we hit the Y. You still want these pulses to join together at the same time. (Going in the same direction, more on that later) When they exit the Y joined together they create a bigger pulse, which in turn keeps everything going. When it comes to pipe size either here or anywhere in the exhaust track you want it big enough to maximize flow, but to big and the pulses loose energy (also remember heat=energy too) and you loose power. It's all about pulse size. NA 2.5" is as big as you want to go. FA bigger pulse, bigger piping works.
Now why does a Y pipe make power? Well, Nissan had a sh!ty design. Here is where we get to that "same direction" comment above. In the pre2k2 Y they forced the 2 banks together at a BAD angle. Look at this -->Y Ideally you want the flow to come from each end of the top of the Y heading towards the bottom so the flow goes in the same direction. But it was made so one came from the bottom and the other came from one side of the top (lets say the left) exiting out the other (the right). Also as you look at the Y the bottom is the left/front bank and the top left is the right/rear bank exiting out the upper right to the main cat.
As you can see this slams the pulses against each other, not good. With an after market Y you correct this problem and make more power! This flaw, or intentional design, was changed in the 2K2+, which is why <---
aftermarket y-pipes do very little to nothing for them!You want to keep the flow of the pulses together all the way out the tail pipe, only if done to make the most power it would be to loud to drive around town. And we have the EPA (cats), which isn't such a bad thing BTW. So we must balance high flow cats, resonators and mufflers to get the power we want at a noise level we can stand, or not get a ticket for some of us.
In a nutshell there is not any backpressure at the cylinder head. But there are many bad flow characteristics from there on in a stock design.
Now on to 2-strokes...
LOL, Not
.....
05-30-2004, 06:13 AM
#50
Supporting Maxima.org Member
Join Date: May 2002
Location: Atlanta
Posts: 1,815
Originally Posted by SR20DEN
That Airzooka certainly is an interesting item to have at work. 
The look on peoples faces when they've been shot with it the first time is priceless.
The look on peoples faces when they've been shot with it the first time is priceless.
Another good one is blowing smoke into it. Not only will it make a nice big smoke ring but a nice big ball of smoke will go right past them. Then they look aound like WTF was that
05-31-2004, 01:29 AM
#52
test pipe versus stock cat: the conditions were better with the test pipe runs and the condition of the max was better also.
Poor track prep and dying clutch with the stock cat runs and with a higher 60 foot i was still able to beat the best run with lower 60 foot using the test pipe.
FINDINGS: test pipe is worthless !
Poor track prep and dying clutch with the stock cat runs and with a higher 60 foot i was still able to beat the best run with lower 60 foot using the test pipe.
FINDINGS: test pipe is worthless !
06-01-2004, 07:24 PM
#54
"Residual pressure opposing the free flow of a gas or liquid, as in a pipe or an exhaust system." thats the definition of BP, so doesnt almost any exhaust system have that at some point in the power band?
06-01-2004, 08:56 PM
#55
Banned
Join Date: Sep 2003
Posts: 491
I look at it this way, every automotive magazine from Hot Rod to Ricer Digest refers to flow restrictions in the exhaust as backpressure, so do manufacturers, exhaust companies, muffler shops, etc. Always have, alway's will. So what's the point of arguing over semantics? It's an excepted and used term from one end of the automotive world to the other!
06-02-2004, 06:09 AM
#56
Originally Posted by nismos14
"Residual pressure opposing the free flow of a gas or liquid, as in a pipe or an exhaust system." thats the definition of BP, so doesnt almost any exhaust system have that at some point in the power band?
Yeah, that's what an online dictionary calls it, but that's such a poor description. It's really so much more than that (when used in an automotive sense...or any engine conversation)...
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