is exhaust back pressure a myth?

[DeaconFrost]

keyword is: scavenging effect

I agree that's the keyword.

Here's from wiki:

For example, fast flowing heads and a tunnel ram intake combined with a poorly planned camshaft and exhaust system will cause the air to "slow down" and "speed up" throughout its journey, thus reducing its scavenging potential. So, to increase scavenging potential, the air must maintain a positive linear acceleration curve.

Air "slow down" and "speed up = reduced scavenging
Air positive linear acceleration curve = increased scavenging.

For me that's a very simple concept: you want to kick air (exhaust) out from the piping as fast as you can.

So imagine if there's no piping at all?

It's as if trying to kick out something, that is already out.

---------- Post added at 03:35 PM ---------- 6 hour anti-bump limit - Previous post was at 03:26 PM ----------

even F1 have manifold but nothing after that...

That's why I said we need to define the problem. Is it:

1. Performance problem? (performance loss)
2. Aesthetics problem? (dirty engine etc.)
3. Practicality problem? (noise, fumes, pollution, tikus & lipas masuk block etc.)
4. Law/legality problem? (JPJ, F1 rules, etc.)

Imagine one person who decides to focus only on performance and performance alone, he will do without exhaust piping at all. B-U-T as I said of course dirt & cockroach will enter the block, engine will have to be rebuilt after every run, engine will be dirty, loud unbearable noise, fumes and F1 will say no you cannot enter the race.

 

[cvkit17]

I know shit about exhausts and only know what everyone knows. My own car has a 4-2-1 header, flexible joint, a large straight resonator and a large rear true-sflow baffled muffler. All 2" piping and connections. Tested with straight flow rear mufflers and other designs like twin loop and all.

Conclusion is what I have now is the best setup for me. Wish I had a highflow catcon instead of that resonator but no one really can afford a magnaflow highflow catcon in m'sia. Stupidly high priced.

And I've tested straight flow with my ex-3-speed auto before. For some reason the gearbox needs the back pressure or whatever you call it to shift. With almost no back pressure from the straight flow setup the bloody thing shifted at much higher RPMs. I couldn't even use the foot liftoff trick to shift 'manually'. Stupid thing increases FC and moves the shift point to be much higher.

So if you ask me, I think autos need the back pressure to shift properly, not because of low end torque or whatever.

And straight flow for my manual does have some loss in low end torque. SOHC has so many downsides. Sigh.

2" pipe sounds abit big to me...my hunch. Coz my old junk sentra with SOHC was on 2" initially when I men-ah-beng-kan the car. 4-2-1, silencer, S flow muffler all in 2" pipe. Power seems ok but FC increases. And it is noticeable that the power band at 4-5k rpm is better. But all by butt-dyno la. Then I changed to 1.8" and FC reduced. Lower band power is better.

I guess you can re-tune your auto car to suit the decreased back pressure for a better performance. But since you like to play with sticks...:rolleyes:

 

[DeaconFrost]

Let's imagine this:

Engine: 4 cylinder inline
Position: Hotside (exhaust side) facing backwards

1. Exhaust manifold (2.25 inch collector) installed + full piping (2.25 inch)
2. Exhaust manifold (2.25 inch collector) installed with no piping afterwards (infinite inch=atmosphere)
3. No exhaust manifold installed (infinite inch=atmosphere). Replaced with 4 trumpets similar to throttle trumpets.

If these 3 setups are dyno-ed, which one will yield the most horsepower/response? I think it will be no. 3.

That's why I think, the older Honda B16/B18 have their hotside facing forward, while the newer Honda K20 have their hotside facing backward.

It's the ultimate testament to the 'get exhaust gas out ASAP' concept.

 

[ixeo]

Let's imagine this:

Engine: 4 cylinder inline
Position: Hotside (exhaust side) facing backwards

1. Exhaust manifold (2.25 inch collector) installed + full piping (2.25 inch)
2. Exhaust manifold (2.25 inch collector) installed with no piping afterwards (infinite inch=atmosphere)
3. No exhaust manifold installed (infinite inch=atmosphere). Replaced with 4 trumpets similar to throttle trumpets.

If these 3 setups are dyno-ed, which one will yield the most horsepower? I think it will be no. 3.

That's why I think, the older Honda B16/B18 have their hotside facing forward, while the newer Honda K20 have their hotside facing backward.

It's the ultimate testament to the 'get exhaust gas out ASAP' concept.

2.
Because that's what Formula 1 uses.

 

[DeaconFrost]

2.
Because that's what Formula 1 uses.

I bet F1 does that because of F1 rules & practicality. Not performance.

F1 Rule: Must put manifold.
Practicality: Put trumpets only and exhaust gas will go out faster but wind can also blow exhaust gas back in.

 

[ixeo]

I haven't seen any engine with individual exhaust runners/trumpets. Got sample to share?

 

[DeaconFrost]

I haven't seen any engine with individual exhaust runners/trumpets. Got sample to share?

I haven't seen one. I bet it will be very impractical on the road, but it will yield extra hp on dyno.

Meanwhile:

Also shown here are the exhaust tailpipes (red arrow, centre left), which must extend at least 170 to 185mm behind the rear axle line (and have a single exit).

Formula 1® - The Official F1® Website

---------- Post added at 04:44 PM ---------- 6 hour anti-bump limit - Previous post was at 04:34 PM ----------

 

[^pomen_GTR^]

I agree that's the keyword.

Here's from wiki:


Air "slow down" and "speed up = reduced scavenging
Air positive linear acceleration curve = increased scavenging.

For me that's a very simple concept: you want to kick air (exhaust) out from the piping as fast as you can.

So imagine if there's no piping at all?

It's as if trying to kick out something, that is already out.

---------- Post added at 03:35 PM ---------- 6 hour anti-bump limit - Previous post was at 03:26 PM ----------



That's why I said we need to define the problem. Is it:

1. Performance problem? (performance loss)
2. Aesthetics problem? (dirty engine etc.)
3. Practicality problem? (noise, fumes, pollution, tikus & lipas masuk block etc.)
4. Law/legality problem? (JPJ, F1 rules, etc.)

Imagine one person who decides to focus only on performance and performance alone, he will do without exhaust piping at all. B-U-T as I said of course dirt & cockroach will enter the block, engine will have to be rebuilt after every run, engine will be dirty, loud unbearable noise, fumes and F1 will say no you cannot enter the race.

Let's imagine this:

Engine: 4 cylinder inline
Position: Hotside (exhaust side) facing backwards

1. Exhaust manifold (2.25 inch collector) installed + full piping (2.25 inch)
2. Exhaust manifold (2.25 inch collector) installed with no piping afterwards (infinite inch=atmosphere)
3. No exhaust manifold installed (infinite inch=atmosphere). Replaced with 4 trumpets similar to throttle trumpets.

If these 3 setups are dyno-ed, which one will yield the most horsepower/response? I think it will be no. 3.

That's why I think, the older Honda B16/B18 have their hotside facing forward, while the newer Honda K20 have their hotside facing backward.

It's the ultimate testament to the 'get exhaust gas out ASAP' concept.

I haven't seen one. I bet it will be very impractical on the road, but it will yield extra hp on dyno.

Meanwhile:


---------- Post added at 04:44 PM ---------- 6 hour anti-bump limit - Previous post was at 04:34 PM ----------

i would say no exhaust would be the lowest of all....

its not like kicking out something.....but more like pulling something up...u climb a hill alone using rope is slow...but what is 1person already up there help u by pulling u up....and u help person below u the same way and continuous....so more ppl can go up faster and easier than one by one climbing alon....that is what scavenging effect helps in efficiency......

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and if u take example of dragster car using alcohol fuel....their engine already at 2000-4000whp and if u look at their valve and port size...there is no way to make it bigger to allow higher C.E.F (coefficiency of flow) hence the just dump the exhaust without any piping since if they put the piping, it would reduce C.E.F (when maximum port size already being used but still not enough CEF)

and another reason they use very short piping because their engine was forced induction (supercharged)...hence shorter dumping is better.........if u take any turbocharged engine...and remove the downpipe....i would say the boost came earlier...higher hp would be very possible.....higher boost also.....but their turbo manifold still have to follow scavenging effect design very much like n.a tuned length exhaust manifold......


try find a picture of v8 F1 engine and compare with your dragster v8 engine...see closely their design...u'll see the differences...

 

[DeaconFrost]

its not like kicking out something.....but more like pulling something up...

I humbly beg to differ taikor.

I think it's pushing (blowing through a straw) rather than pulling (sipping through a straw).

since if they put the piping, it would reduce C.E.F (when maximum port size already being used but still not enough CEF)

I think putting piping on a dragster will simply cause more restriction. More restriction = bad, more 'backpressure' = similar, also bad.

 

[cvkit17]

Let's imagine this:

Engine: 4 cylinder inline
Position: Hotside (exhaust side) facing backwards

1. Exhaust manifold (2.25 inch collector) installed + full piping (2.25 inch)
2. Exhaust manifold (2.25 inch collector) installed with no piping afterwards (infinite inch=atmosphere)
3. No exhaust manifold installed (infinite inch=atmosphere). Replaced with 4 trumpets similar to throttle trumpets.

If these 3 setups are dyno-ed, which one will yield the most horsepower/response? I think it will be no. 3.

That's why I think, the older Honda B16/B18 have their hotside facing forward, while the newer Honda K20 have their hotside facing backward.

It's the ultimate testament to the 'get exhaust gas out ASAP' concept.

My bet is on no.3 if I am a dragster. But if I am not, I will go for no.2 set up. It has more advantages than the little power it can give using no.3

I bet F1 does that because of F1 rules & practicality. Not performance.

F1 Rule: Must put manifold.
Practicality: Put trumpets only and exhaust gas will go out faster but wind can also blow exhaust gas back in.

It's not the rule. If you look closely at an F1 car, they have this 2 huge exhaust pipe coming out of the body and terminated before the end of the car. Here's why I prefer no.2 if I am not a dragster. The exhaust air amount from an F1 car is not little. This hot air, blowing out from the exhaust like a jet actually helps in giving more downforce to the car as it hits the rear wing.

I humbly beg to differ taikor.

I think it's pushing (blowing through a straw) rather than pulling (sipping through a straw).

I think putting piping on a dragster will simply cause more restriction. More restriction = bad, more 'backpressure' = similar, also bad.

Actually, pushing or pulling, it is the same if the pressure differential value is the same. When the exhaust gas escapes from the cylinder, it is not only because it is pushed but it is also pulled. The pressure inside the combustion chamber is higher than the atmosphere pressure thus the gases find its way to travel. Of course, when the cylinder moves up, the pressure inside gets higher and the exhaust gases flows with better "push".

Now scavenging is a different thing. It is the effect of using the exhaust air to pull fresh air in. Because the exhaust air moves forward, it actually creates a low pressure point, a vacuum point, that help draw fresh air in. But scavenging is great to improve efficiency at low rpm because the duration between pulse is high. If the engine is revving high, scavenging does not really help much until a point that it is negligible in terms of giving more powahhhhhhh

 

[TitanRev]

I think everyone is sharing based on experience and trial and error....well here I want to share mine...this is based on observation and my theory....

From the exhaust port to the primary runner down until the bottom of the engine. This section the exhaust gas will be at the hottest since it's the closest to the exhaust port. That's why you see most headers on turbos or NA they start to glow hot 1st around this area....so as the exhaust gas travels further away from the engine it starts to cool down. The whole exhaust acts as a cooling conductor and in physics "heat will always conduct to the a cooler area"

So to improve scavenging, flow or velocity of the exhaust out of the port is very important because you want the exhaust to exit the chamber as much as possible before the valve closes and the intake stroke starts.

So we go back to the temperature 1st, hotter gas have higher pressure and higher flow in a confined space. So at the start of the primary runner you want the runner diameter to be bigger because the gas is super hot at that stage and it will need as much space it have to flow out of the chamber as much as possible...but after it gets under the engine it starts to cool down and in order to maintain the flow you reduce the piping diameter...this forces the hot exhaust that trying to cold down won't have a too steep temp drop...you force hot air into a smaller area it's velocity increase and it's temp drop will reduce....(just like heater use in winter, the pipe where the heat is given out is small instead of big) By doing this, you maintain the flow of the exhaust in a control area. But this does not mean you reduce the size to 1 inch is good because it will cause pressure to build up also....the reduction is size needs to be calculate and done in tandem with the intake.

That's why, when you use a bigger pipe at the rear, the exhaust gas have much more area to expand and cool down (cool air is heavier) and when you use a smaller exhaust the temp drop won't be too steep....a very good actual test is, you put your leg at a small medium size muffler, you can feel the exhaust gas pushing and blowing against your leg at a distance...compare that to a big muffler, the exhaust that exit is slow and sometimes you can feel it blowing on your leg.....

For those drag car that is posted by member here especially the 3rd picture, you can see that the exit diameter is smaller. Some what tapered...this maintain the exit velocity of the gas....

 

[DeaconFrost]

I think everyone is sharing based on experience and trial and error....well here I want to share mine...this is based on observation and my theory....

From the exhaust port to the primary runner down until the bottom of the engine. This section the exhaust gas will be at the hottest since it's the closest to the exhaust port. That's why you see most headers on turbos or NA they start to glow hot 1st around this area....so as the exhaust gas travels further away from the engine it starts to cool down. The whole exhaust acts as a cooling conductor and in physics "heat will always conduct to the a cooler area"

So to improve scavenging, flow or velocity of the exhaust out of the port is very important because you want the exhaust to exit the chamber as much as possible before the valve closes and the intake stroke starts.

So we go back to the temperature 1st, hotter gas have higher pressure and higher flow in a confined space. So at the start of the primary runner you want the runner diameter to be bigger because the gas is super hot at that stage and it will need as much space it have to flow out of the chamber as much as possible...but after it gets under the engine it starts to cool down and in order to maintain the flow you reduce the piping diameter...this forces the hot exhaust that trying to cold down won't have a too steep temp drop...you force hot air into a smaller area it's velocity increase and it's temp drop will reduce....(just like heater use in winter, the pipe where the heat is given out is small instead of big) By doing this, you maintain the flow of the exhaust in a control area. But this does not mean you reduce the size to 1 inch is good because it will cause pressure to build up also....the reduction is size needs to be calculate and done in tandem with the intake.

That's why, when you use a bigger pipe at the rear, the exhaust gas have much more area to expand and cool down (cool air is heavier) and when you use a smaller exhaust the temp drop won't be too steep....a very good actual test is, you put your leg at a small medium size muffler, you can feel the exhaust gas pushing and blowing against your leg at a distance...compare that to a big muffler, the exhaust that exit is slow and sometimes you can feel it blowing on your leg.....

For those drag car that is posted by member here especially the 3rd picture, you can see that the exit diameter is smaller. Some what tapered...this maintain the exit velocity of the gas....

Very interesting I can see the point here.

---------- Post added at 08:16 PM ---------- 6 hour anti-bump limit - Previous post was at 08:08 PM ----------

It's not the rule. If you look closely at an F1 car, they have this 2 huge exhaust pipe coming out of the body and terminated before the end of the car. Here's why I prefer no.2 if I am not a dragster. The exhaust air amount from an F1 car is not little. This hot air, blowing out from the exhaust like a jet actually helps in giving more downforce to the car as it hits the rear wing.

This is why the following rule was enforced:
Also shown here are the exhaust tailpipes (red arrow, centre left), which must extend at least 170 to 185mm behind the rear axle line (and have a single exit).

Formula 1® - The Official F1® Website

It can be seen that the difference between F1 cars and NHRA dragsters is the distance - lots of laps vs. one run down the straight. In F1 cars, outright performance need to make some compromise for practicality.

 

[TitanRev]

Just my input for race cars, the tail pipe is bigger at the tip example like F1 because their engines are what V8 or V10 so you have a lot of primary runners joining up together at each side and in a very narrow space...the bends from the exhaust ports is much more angled that our road car....in velocity term...bend creates pressure and hence the bigger exhaust tips at the end so that all the pressure can be let off as much as possible but this does not apply to our road going car with puny little 4 bangers....Also race cars have different gear ratios than our daily car which can be made to make use of the engine powerband....

 

[Waiora_ProTuner]

nice discussion here.

regarding the heater/radiator thing, where it have smaller piping..it actually have lots of small piping rather than 1 big pipe..purpose? more area for heat exchange...and the total area should be more than inlet so the fluid inside would be slower thus better heat exchange..its not a good comparison/example here btw...

the rules for road car is = exhaust must exit at the back.
and reduce noise..

regarding the temperature drop, yes...
cooler air are heavier, yes..
but why they didn't coat it with ceramic coating...or thermal wrap? why the complexity?
and even it drops so much, its still higher temperature than atmosphere...

Ferrari 458 have 3 exit..GTR have 4...no problem..
even though all comes through 1 pipe in the center...for scavenging..

regarding the 2rd dragster picture...i bet the bigger piping is only 'cover'..no welding there..
its all same diameter..see the braided tubing goes inside?

 

[TitanRev]

nice discussion here.

regarding the heater/radiator thing, where it have smaller piping..it actually have lots of small piping rather than 1 big pipe..purpose? more area for heat exchange...and the total area should be more than inlet so the fluid inside would be slower thus better heat exchange..its not a good comparison/example here btw...

the rules for road car is = exhaust must exit at the back.
and reduce noise..

regarding the temperature drop, yes...
cooler air are heavier, yes..
but why they didn't coat it with ceramic coating...or thermal wrap? why the complexity?
and even it drops so much, its still higher temperature than atmosphere...

Ferrari 458 have 3 exit..GTR have 4...no problem..
even though all comes through 1 pipe in the center...for scavenging..

regarding the 2rd dragster picture...i bet the bigger piping is only 'cover'..no welding there..
its all same diameter..see the braided tubing goes inside?

The heater for winter use have smaller diameter piping because they want to reduce the temp drop across the heater surface...because the heated steam comes from a boiler and is fed throughout the entire house. Imagine you use the same diameter as the delivery pipe at the heater the temp would have drop more already...that's why heater pipe is smaller than the delivery pipe. This increase the steam velocity and pressure which in turn heat up the steam.

Bro, engine capacity different between R35 and daily sub 2 liter engine with 4 bangers.....the exhaust temp at the muffler tip is not so much higher than atmo....try testing with a temp meter you will see....regarding coating....it's also an option but if we do not consider coating then you need to find another way. Actually tapered or stepped system Supersprint is using you can check it out....It's not something entirely new....

About the dragstar tubing, notice the burn marks at the braided line....it's not just a cover....

Also R35 exhaust is not totally single pipe from the downpipe all the way to the back....the joint into 1 pipie section is very short only then split into 2 again. Some have longer 1 pipe design like Meistercraft. Also you can notice the smaller diameter after the split pipe compare to the single diameter. The most important is still the primary runners out from the exhaust port section. For the Ferrari 458 is you notice the center exhaust pipe is smaller in diameter than the 2 at the sides. There's a valve system in the 458 exhaust. When idling and sub 3K RPM only the center exhaust is functioning. The outer 2 is only activated at RPM above 3K. The center exit is the joining from 2 muffler while the 2 at the side is directly from the 2 muffler...