blinx's OFFICIAL EXHUAST THREAD velocity backpressure all explained here.


blinx9900

blinx9900

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So far I’ve seen A LOT of exhaust questions and A LOT of misconceptions, so im typing this up to clear those up and hopefully some people will learn the truth about exhaust systems back pressure and velocity, I will try to keep this as brief as possible,

But first let me say I don’t care what your friend said or what the exhaust shop says or what the v8 drag racer said to you, trust me on this one:

First I will address back pressure:
its simple, back pressure is BAD always BAD! Forget the idea of some engines need back pressure it’s a LIE!! Back pressure means the engine has the waste its HP to push the exhaust out of the system (also known as pumping loss) so you will ALWAYS lose power with back pressure!

Now I will address velocity:
exhaust velocity is what’s important, increase in exhaust velocity is what will free up extra power. now if your wondering how then read on:

Why does velocity free up more power: exhaust velocity is when the exhaust flow is so smooth that each exhaust pulse leaves a vacuum behind to suck out the next pulse so instead of the engine pushing the exhaust out the exhaust itself is sucking out exhaust hence taking the load off the engine and reducing pumping losses. This is where exhaust piping is important, different size piping will give you ideal exhaust velocity at different RPM points. You can’t have perfect velocity throughout your entire rpm range. a smaller piping will give you better velocity down low but too small and your sacrificing top end power because your restricting flow, a large exhaust will give you that ideal velocity higher up in the power band but you may loose some power down low.

Why do you loose power down low with large exhaust: when I say large I mean like having a 3" piping on a stock low revving na 1.6 sohc, what this does is it takes longer for the exhaust to exit the tail pipe because it takes longer to fill up the system, so the engine has to constantly push the exhaust through the system until it gets to the tail pipe. Basically there are more pumping losses. Basically the exhaust has more space to fill before it exits the tailpipe and the engine has to work harder to do that.

So now you know BACK PRESSURE IS BAD! And VELOCITY IS GOOD. but your wondering why so many people think back pressure is good, ill explain that too:

Why people think back pressure is good: in the 1960's racers would put larger pipes on there cars but there cars felt slower sometimes because they where shifting there ideal velocity to a higher rpm, V8's most redlined at 4500-5000rpm in those days so they shifted the point past there engine operating range, then they put much smaller pipes on which gave them a great deal of velocity down low so they had a jump in hp and tq say from 1500-3000 rpm and they thought it was because of back pressure, but it was actually velocity that had increased. Basically it was all a misunderstanding.

How do I know what size to choose to have ideal velocity:
well it’s really complicated it depends on a lot of stuff like your driving style and mods you have etc... So I don’t feel like getting into it, plus everybody has there own preference so I can’t decide what is good for YOU, only you can. And I don’t want to start an argument here.

But blinx I really wanna know your opinion on piping size, I promise not to argue I know it’s just your opinion:
ok fine this is my opinion and preference only. first your exhaust piping size should match the size of your collector on your header (or larger) in most cases, the header collector can always be modded to be larger to match the exhaust size you need, don’t limit yourself to a 2" just cause your collector is 2" that just means you need to cut it off and have a larger collector welded on.

-I think most vtec engines 1.6-1.8 stock or mildly moded (stock internals) are good with 2.5" with a high flow cat or no cat at all, you can get away with a 2.25 if you have NO cat but i just don't see why you would choose such a small exhaust.

-for 1.6-1.8 vtecs with internal work like cams, raised compression, higher redline, port work etc.. I prefer a minimum of 2.5 to 2.75". You can try to step up to 3" if your heavily modded, you might see some gains.

-for 1.9-2.1+ vtecs 3" all the way! B-series 1.9+, H22, K20, or K24 setups should all use 3" for max power, even if the internals are OEM, you will in most cases see gains across the whole power band.

-for turbo Honda’s making 400whp and less 3" if your making under 300whp you could do a 2.5" with no cat but you will make more power with a 3". You can make a lot more than 400whp on a 3" but some people chose to step up to 3.5 or 4" at this point its really preference as ground clearance is heavily sacrificed at this point...

There now next time someone tries to tell you back pressure is good you can educate them
:nice:
 
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Excellent write up! sticky! thanks for sharing this info! :nice:
 
Excellent post blinx :nice:
Good idea to have this issue nailed down once and for all.



I would like to add a bit of a related subject to it if you do not mind, not for power increment but for the enemy of power: Fuel economy.

At partial throttle for maintaining speed, a small amount of actual backpressure helps improve overall efficiency.

At wide open throttle, combustion has to be at maximum potential to create maximum power. But partial throttle at 3000rpms require less combustion than full throttle @ 3000rpms, and this is where backpressure helps improve economy.

In simple terms, not all of the exhaust waste is useless, some may be retained in the cyllinder along with fresh mixture to "fill up" the displacement and give the required compression for the next combustion, with less fresh air-fuel mixture required. This translates in feeling to " more power with less throttle applied " during *Lower-partial* or *constant* throttle, and better fuel economy.

In contrast, the engine where exhaust gas is quickly extracted out of the cycle will require more pumping to intake the required volume of air for its displacement, and due to the high oxygen content, more fuel to avoid detonation. This basically means more parasitic losses and less fuel economy; both cars will be able to travel at the same constant speed but one car will require slightly more throttle applied than the other.

Some engines also employ exhaust gas recirculation valves to harness this benefit. The design of exhaust-gas recirculation however is highly relative to the other components of the engine, and only the manufacturer may really calibrate its degree of performance so take this issue lightly with a pinch of salt.

Note that this is for everyday-driving's performance.
Blinx' argument is absolutely correct when it comes to generating power. This is just to say that for an everyday car, you might want to look into maintaining some back-pressure for fuel economy. But if its all-out power youre looking for, a high-flow exhaust is the way to go. :)
 
JayH said:
Excellent post blinx :nice:
Good idea to have this issue nailed down once and for all.



I would like to add a bit of a related subject to it if you do not mind, not for power increment but for the enemy of power: Fuel economy.

At partial throttle for maintaining speed, a small amount of actual backpressure helps improve overall efficiency.

At wide open throttle, combustion has to be at maximum potential to create maximum power. But partial throttle at 3000rpms require less combustion than full throttle @ 3000rpms, and this is where backpressure helps improve economy.

In simple terms, not all of the exhaust waste is useless, some may be retained in the cyllinder along with fresh mixture to "fill up" the displacement and give the required compression for the next combustion, with less fresh air-fuel mixture required. This translates in feeling to " more power with less throttle applied " during *Lower-partial* or *constant* throttle, and better fuel economy.

In contrast, the engine where exhaust gas is quickly extracted out of the cycle will require more pumping to intake the required volume of air for its displacement, and due to the high oxygen content, more fuel to avoid detonation. This basically means more parasitic losses and less fuel economy; both cars will be able to travel at the same constant speed but one car will require slightly more throttle applied than the other.

Some engines also employ exhaust gas recirculation valves to harness this benefit. The design of exhaust-gas recirculation however is highly relative to the other components of the engine, and only the manufacturer may really calibrate its degree of performance so take this issue lightly with a pinch of salt.

Note that this is for everyday-driving's performance.
Blinx' argument is absolutely correct when it comes to generating power. This is just to say that for an everyday car, you might want to look into maintaining some back-pressure for fuel economy. But if its all-out power youre looking for, a high-flow exhaust is the way to go. :)
Click to expand...

thank you :) in regards to emissions and efficiency you are correct for the most part, and as you stated my post was for the sole purpose of performance and at that i was correct.

however adding an after market exhaust will not lower your fuel mileage, it usually increases it. i understand your point but consider this:

with the reduced pumping loses of a properly matched performance exhaust you generate more power, so you need less throttle to maintain speed. you can even look to diesels for an extreme example, adding intake/exhaust/chip to a ford diesel generally increases fuel mileage 4+mpg

i still have to maintain my point that back pressure is ALWAYS bad except for emissions it could help burn unburnt fuel in the exhaust but :blabla::blabla::blabla: when it comes to that.
 
Actually from experience I disagree with the increasing fuel mileage part... the point I was making is that a free flow exhaust actually increases parasitic losses at partial/constant throttle, but reduces pumping losses during WOT.

I have experienced a climb in fuel consumption and reduced torque at 30% throttle switching from a S-flow exhaust system to a straight flow, both of the same size. The straight flow performed better at WOT (loud as heck too) but the S-flow which has a lot more resistance performed considerably better at slow daily driving at constant speed. I can only credit this to "backpressure", which according to a technical article I read sometime ago does aid engine efficiency at partial/low throttle. I couldnt find the article again though... but it was clear at pointing out that an engine's pumping operations at WOT and at maintaining speed is very different. Something has to fill up the cyllinders for compression when the throttle doesnt allow enough air in, and that leads to exhaust gas recycling.

Backpressure is only a layman's term of addressing it anyway, in actual fact its just a reduced degree of exhaust velocity siphoning. Of course, if what people mean by "backpressure" is A LOT of resistance working against the exhaust flow then that would be utter bollocks.

The concepts are very engine/exhaust design dependant though, and frankly there are too many variables to take account for so I guess its pointless trying to relate it in real life applications. I would however think that "backpressure (reduced exhaust velocity)" is not completely useless when it comes to optimal engine efficiency, or too much of exhaust gas siphoning can hurt efficiency when it comes to maintaining rpms.

But blah, engine designs vary so much these days I think its pointless to argue about how efficiency is obtained, since it really depends on the engine itself. One engine could probably benefit from a free-flow exhaust while the other suffers. :)



edit: I can believe I typed this much for a really small matter :p
And looks like the argument really circles around the definition of "back pressure"


blinx9900 said:
thank you :) in regards to emissions and efficiency you are correct for the most part, and as you stated my post was for the sole purpose of performance and at that i was correct.

however adding an after market exhaust will not lower your fuel mileage, it usually increases it. i understand your point but consider this:

with the reduced pumping loses of a properly matched performance exhaust you generate more power, so you need less throttle to maintain speed. you can even look to diesels for an extreme example, adding intake/exhaust/chip to a ford diesel generally increases fuel mileage 4+mpg

i still have to maintain my point that back pressure is ALWAYS bad except for emissions it could help burn unburnt fuel in the exhaust but :blabla::blabla::blabla: when it comes to that.
Click to expand...
 
JayH said:
Actually from experience I disagree with the increasing fuel mileage part... the point I was making is that a free flow exhaust actually increases parasitic losses at partial/constant throttle, but reduces pumping losses during WOT.

I have experienced a climb in fuel consumption and reduced torque at 30% throttle switching from a S-flow exhaust system to a straight flow, both of the same size. The straight flow performed better at WOT (loud as heck too) but the S-flow which has a lot more resistance performed considerably better at slow daily driving at constant speed. I can only credit this to "backpressure", which according to a technical article I read sometime ago does aid engine efficiency at partial/low throttle. I couldnt find the article again though... but it was clear at pointing out that an engine's pumping operations at WOT and at maintaining speed is very different. Something has to fill up the cyllinders for compression when the throttle doesnt allow enough air in, and that leads to exhaust gas recycling.

Backpressure is only a layman's term of addressing it anyway, in actual fact its just a reduced degree of exhaust velocity siphoning. Of course, if what people mean by "backpressure" is A LOT of resistance working against the exhaust flow then that would be utter bollocks.

The concepts are very engine/exhaust design dependant though, and frankly there are too many variables to take account for so I guess its pointless trying to relate it in real life applications. I would however think that "backpressure (reduced exhaust velocity)" is not completely useless when it comes to optimal engine efficiency, or too much of exhaust gas siphoning can hurt efficiency when it comes to maintaining rpms.

But blah, engine designs vary so much these days I think its pointless to argue about how efficiency is obtained, since it really depends on the engine itself. One engine could probably benefit from a free-flow exhaust while the other suffers. :)



edit: I can believe I typed this much for a really small matter :p
And looks like the argument really circles around the definition of "back pressure"
Click to expand...

haha yup, your right, i was thinking the same thing (in regards to all the variables) but didnt want to type that much :D
 
nice...

so would 2.25 be good for street?

Looking to get cams, pistons etc.
 
hmmm, I really like the sound of my setup right now and I just can't stand that high pitch barking sound of most honda's out there...:angry:

I guess I just gotta see how it goes... maybe I'll just put a huge high flow cat. :D
 
sound pitch is all controlled by the resonator :nice: 24" resonator will make the car sound deep :)
 
come to think of it, I do have a large back box and 3 very long resonators(hotdogs). hehehe
 
also velocity inside the exhaust creates a suction effect that will also help pull out gases more efficiently then having more mass in your pipes.
 
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