Back Pressure vs Exhaust Velocity
03-10-2007, 10:35 AM
#1
Back Pressure vs Exhaust Velocity
Ok, over the years I have grown SICK of constantly repeating myself about the differences between Back Pressure and Exhaust Velocity. n00bs say "you're gonna loose power cus the new tubing is too big and you'll loose back pressure! thats not good!"... no, that ADVICE is no good. So let me put a big effort this time into exposing the differences in plain, non-technical English for the masses to comprehend.
Note- I'm using references from www.wikipedia.org, and paraphrasing it to Lame-man terms, also Im explaining this as it related to a Naturally Aspirated Engine
Backpressure usually refers to the pressure exerted on a moving fluid by obstructions or tight bends in the confinement vessel along which it is moving against its direction of flow. For example, an automotive exhaust muffler with a particularly high number of twists, bends, turns and right angles could be described as having particularly high backpressure. Back Pressure in the exhaust sense of the term, is usually termed as being a "bad thing" for performance
In other words, if you think back pressure is good, stick a potato into the muffler, you'll quickly find out how "good" back pressure really is (disclaimer, DONT do this)
Exhaust Velocity is simply the speed in which the exhaust can flow.
Ok now that we've separated the two concepts in your mind, lets move on... n00bs get pink, Guru gets Dark Blue, lol...
n00b- You want to make more power? Put the biggest exhaust tubing you can get! It'll flow like CRAZY!
Guru- The best way to make more power with the exhuast is to set it up in a way to promote exhaust velocity. Which is not nessesarily larger tubing. The tubing diameter selection is dependant on the amount of exhaust it will have to flow.
For a Naturally Aspirated VQ30DE, general concensus AND dyno proof has shown that 2.5" exhaust tubing is the best balance between larger tubing while still maintaining adequate exhaust velocity.
There are reasons why we dont just go with as big a tube as we can find. One is that with the amount of airflow as a constant, comparing say a 2.5" catback to a 3.5" catback, the 3.5"er will allow the hot exhaust gasses to expand, and thus cool down. When it cools down it becomes denser and sort of becomes a restriction to the outgoing hotter gasses behind it. Second, A 2.5"er catback isnt that much bigger at all than the stock tubing (2.3x") and thus there isnt much of a chance for exhaust to expand and cool. This keeps the exhuast gasses moving fast out of the engine, thus make power.
n00b- A GREAT way to make power for cheap, GUT THE CATALYTIC!
Guru- Meh, I admit, I've been there. My 1st mod was an STS, and in the process of doing that I stuck a broom through the cat and gutted it. Did not feel a difference at all from it. Something to keep in mind is that if your catalytic is relatively clean (hardly any carbon build up) you wont see an increase in power from gutting it. Granted, if your catalytic is PACKED with carbon then you might have a noticeable difference, doesn't mean that you should gut it.
A few reasons why NOT to gut your catalytic...
- Its Federal Law to have a Catalytic Converter, but its up to the state in which you live in (from my understanding) to enforce this (via visual and emissions testing). Some states, like good ol' Florida do not test emissions.
- Its bad for the environment (sorry, had to throw this in there to be fair to all parties)
- It disrupts flow!!!!! The CC when intact has a honeycomb pattern that you can see right through (linear exhaust flow). When you gut it, exhaust exits the y-pipe's end, into this "larger chamber" that is now the gutted CC, allowing the exhaust to expand (thus cool and slow down) which disturbs the flow. Any time you have these drastic differentials of tubing cross-section area it will create turbulence in the stream of flow, which as you assumed, is bad for power.
What can you do about it? Well if they test emissions in your state, I suggest a high flow catalytic. If they do not, and you want to squeeze every last bit of power from your setup (and you assume the risks involved, whatever they may be. Do not come back at me later complaining, its YOUR decision to make) I suggest a straight pipe- they are cheap ($35 shipped from WSP) so no excuses.
Bottom line, Do NOT gut your catalytic. Go high flow CC or straight pipe. No excuses.
n00b- "After I put in my 2.5" mandrel bent catback, my car doesnt feel as responsive at low rpms, but its got more power on the top that it did before"
Guru- Alot of modifications have the potential to "shift the powerband". Some do more at certain RPM ranges than others, some even take away power from one range and make it on anther. Exhaust tubing can have this effect. Larger and smoother-flowing (such as mandrel bent vs crush or corrugated) tubing will usually take some power away from the bottom end but make up for it on the mid and upper ranges.
n00b- Why do I want to keep my exhaust gasses to move as fast as possible?
Guru- Its called Scavenging Effect. This can be attained by alot of ways, especially with cam specs and cam timing, but exhuast can play a big role in this also.
Scagenging Effect takes place when the piston is coming to TDC, exhuast valves are open still as the exhuast moves out of the chamber. Just before the piston gets to the TDC, the intake valves open (this is called overlap BTW), yet exhaust doesnt get pushed up into the intake, its still moving through the exhuast valves, and the intake charge is coming into the chamber as the piston is still coming up. THAT is scaveging effect.
Exhaust tubing can relate to this because the faster you have the exhuast moving OUT of the chamber, it creates a vaccum-sucking effect behind it. Thus, when the intake valves open, the outgoing exhaust literally pulls the intake charge into the chamber for a much denser mixture. This is why headers such as hotshots/ Cattman make more power than the stockers. Not only are the runners slightly larger, they are much smoother flowing. This allows the gasses to move out of the chamber alot faster, thus promote more of a scavenging effect, which makes more power
Conclusion- Back Pressure=
Exhaust Velocity=
Note- For turbo applications, general concensus dictates that less to no overlap is best (because if the valves overlap, you'd have boost going out your exhuast until the valves close, not good).
I am not perfect, If someone spots a potential mistake, please point it out to me and I will fix this post accordingly. Hopefully this can become a sticky so we can just link the n00bs to this, rather than explaining the differences over and over.... and over again.
Note- I'm using references from www.wikipedia.org, and paraphrasing it to Lame-man terms, also Im explaining this as it related to a Naturally Aspirated Engine
Backpressure usually refers to the pressure exerted on a moving fluid by obstructions or tight bends in the confinement vessel along which it is moving against its direction of flow. For example, an automotive exhaust muffler with a particularly high number of twists, bends, turns and right angles could be described as having particularly high backpressure. Back Pressure in the exhaust sense of the term, is usually termed as being a "bad thing" for performance
In other words, if you think back pressure is good, stick a potato into the muffler, you'll quickly find out how "good" back pressure really is (disclaimer, DONT do this)
Exhaust Velocity is simply the speed in which the exhaust can flow.
-------------------------------------------------------
Ok now that we've separated the two concepts in your mind, lets move on... n00bs get pink, Guru gets Dark Blue, lol...
n00b- You want to make more power? Put the biggest exhaust tubing you can get! It'll flow like CRAZY!
Guru- The best way to make more power with the exhuast is to set it up in a way to promote exhaust velocity. Which is not nessesarily larger tubing. The tubing diameter selection is dependant on the amount of exhaust it will have to flow.
For a Naturally Aspirated VQ30DE, general concensus AND dyno proof has shown that 2.5" exhaust tubing is the best balance between larger tubing while still maintaining adequate exhaust velocity.
There are reasons why we dont just go with as big a tube as we can find. One is that with the amount of airflow as a constant, comparing say a 2.5" catback to a 3.5" catback, the 3.5"er will allow the hot exhaust gasses to expand, and thus cool down. When it cools down it becomes denser and sort of becomes a restriction to the outgoing hotter gasses behind it. Second, A 2.5"er catback isnt that much bigger at all than the stock tubing (2.3x") and thus there isnt much of a chance for exhaust to expand and cool. This keeps the exhuast gasses moving fast out of the engine, thus make power.
n00b- A GREAT way to make power for cheap, GUT THE CATALYTIC!
Guru- Meh, I admit, I've been there. My 1st mod was an STS, and in the process of doing that I stuck a broom through the cat and gutted it. Did not feel a difference at all from it. Something to keep in mind is that if your catalytic is relatively clean (hardly any carbon build up) you wont see an increase in power from gutting it. Granted, if your catalytic is PACKED with carbon then you might have a noticeable difference, doesn't mean that you should gut it.
A few reasons why NOT to gut your catalytic...
- Its Federal Law to have a Catalytic Converter, but its up to the state in which you live in (from my understanding) to enforce this (via visual and emissions testing). Some states, like good ol' Florida do not test emissions.
- Its bad for the environment (sorry, had to throw this in there to be fair to all parties)
- It disrupts flow!!!!! The CC when intact has a honeycomb pattern that you can see right through (linear exhaust flow). When you gut it, exhaust exits the y-pipe's end, into this "larger chamber" that is now the gutted CC, allowing the exhaust to expand (thus cool and slow down) which disturbs the flow. Any time you have these drastic differentials of tubing cross-section area it will create turbulence in the stream of flow, which as you assumed, is bad for power.
What can you do about it? Well if they test emissions in your state, I suggest a high flow catalytic. If they do not, and you want to squeeze every last bit of power from your setup (and you assume the risks involved, whatever they may be. Do not come back at me later complaining, its YOUR decision to make) I suggest a straight pipe- they are cheap ($35 shipped from WSP) so no excuses.
Bottom line, Do NOT gut your catalytic. Go high flow CC or straight pipe. No excuses.
n00b- "After I put in my 2.5" mandrel bent catback, my car doesnt feel as responsive at low rpms, but its got more power on the top that it did before"
Guru- Alot of modifications have the potential to "shift the powerband". Some do more at certain RPM ranges than others, some even take away power from one range and make it on anther. Exhaust tubing can have this effect. Larger and smoother-flowing (such as mandrel bent vs crush or corrugated) tubing will usually take some power away from the bottom end but make up for it on the mid and upper ranges.
n00b- Why do I want to keep my exhaust gasses to move as fast as possible?
Guru- Its called Scavenging Effect. This can be attained by alot of ways, especially with cam specs and cam timing, but exhuast can play a big role in this also.
Scagenging Effect takes place when the piston is coming to TDC, exhuast valves are open still as the exhuast moves out of the chamber. Just before the piston gets to the TDC, the intake valves open (this is called overlap BTW), yet exhaust doesnt get pushed up into the intake, its still moving through the exhuast valves, and the intake charge is coming into the chamber as the piston is still coming up. THAT is scaveging effect.
Exhaust tubing can relate to this because the faster you have the exhuast moving OUT of the chamber, it creates a vaccum-sucking effect behind it. Thus, when the intake valves open, the outgoing exhaust literally pulls the intake charge into the chamber for a much denser mixture. This is why headers such as hotshots/ Cattman make more power than the stockers. Not only are the runners slightly larger, they are much smoother flowing. This allows the gasses to move out of the chamber alot faster, thus promote more of a scavenging effect, which makes more power
Conclusion- Back Pressure=
Exhaust Velocity=
Note- For turbo applications, general concensus dictates that less to no overlap is best (because if the valves overlap, you'd have boost going out your exhuast until the valves close, not good).
I am not perfect, If someone spots a potential mistake, please point it out to me and I will fix this post accordingly. Hopefully this can become a sticky so we can just link the n00bs to this, rather than explaining the differences over and over.... and over again.
03-10-2007, 05:40 PM
#4
Originally Posted by NmexMAX
It's in the stickys from mzmtg. But it's always good to have this here now since no one ever reads the damn things (stickys)
03-10-2007, 07:53 PM
#6
Senior Member
Join Date: Nov 2006
Posts: 2,391
excellent information man, much appreciated, i am a noob to exhaust science. but it makes sense what you are saying. If i ever have to replace my stock exhaust i will read this info, which hopefully will be STICKIED by then. so our stock diameter is approximately 2.35" ? i did not know that thats not a bad size.
i have read on motorvade that a proper exhaust makes a BIG HP and torque difference, but im not after that.
i have read on motorvade that a proper exhaust makes a BIG HP and torque difference, but im not after that.
03-12-2007, 06:40 AM
#7
Originally Posted by 95BLKMAX
Oh yea? oh man, lol. I must have missed it because I've never ran into it myself for the last almost 4 yrs I've been on the org. Thanks for pointing it out to me
03-12-2007, 07:10 AM
#8
Originally Posted by phenryiv1
Its in there, but your version is better. I'll stick it later today.
Just edited the post to include the subject of the gutted cat in the n00b vs Guru section
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