YACT: why does decreased backpressure reduce low end torque?

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Compton

Platinum Member
Feb 18, 2000
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Originally posted by: Eli
Originally posted by: halik
Originally posted by: Eli
Originally posted by: Howard
Backpressure decreases torque.
What's the proper term, then?

I know for a fact that a slightly more restrictive "can" type exhaust increases low end and decreases high end torque on a 2-cycle engine, while a free flowing expansion pipe decreases low end but increases high end.


because the way the carbs are calibrated.

In fuel injected cars, nothing like that applies. The more open the exhaust the better.

Only carbureated have the double-soaking effect calculated into the fuel mixture. IE when the intake valve opens the pressure from the exhust will push the air-fuel mixture thru the carb again, which mixes even more fuel into it. That way playing with the backpressure can make your car run richer or leaner and hence change the output
eh.. Sorry, but you don't know what you're talking about.. or at least, you have a very poor grasp of it. :p

To further explain, it has nothing to do with carbs. It's also independant of fuel injection... It doesen't really have anything to do with the fuel system, other than.. thats what the reaction effects.

Read my long post farther up. It does not matter that a 2-cycle engine has ports and a 4-cycle engine has valves, the same forces are still at work.

The bolded part is absolutely NOT a universal rule. If you use a pipe that is too big for your exhaust flow, when the exhaust gasses hit the huge tube, they lose velocity, and the negative pressure behind the positive pressure front dissolves.

To explain "scavenging"..

Like I said, when the exhaust valve opens, the hot exhaust gasses rush out and down the header. Behind this positive pressure front, is a negative pressure wave. This helps scavenge the leftover exhaust gasses out of the combustion chamber by pulling them out. In cases of extreme tuning, it can even help draw the fresh air(or air/fuel mix) into the chamber, due to valve overlap.

Of course, like I said.. it will only be operating synergistically at certain RPMs.

One of the first things I thought about was carbs. So many people will put an 800cfm carb on a 300ci engine, and it will barely idle, or it will run like crap because at low rpm the intake velocity is so low. But thats like the opposite thing from what the OP was asking.

Oh yeah, and that can be a problem on fuel injection too. People often use larger throttle bodies than they need because an advertisement somewhere said it would add 13HP. :)
 

Eli

Super Moderator | Elite Member
Oct 9, 1999
50,419
8
81
Originally posted by: halik
Originally posted by: Eli
Originally posted by: halik
Originally posted by: Eli
Originally posted by: Howard
Backpressure decreases torque.
What's the proper term, then?

I know for a fact that a slightly more restrictive "can" type exhaust increases low end and decreases high end torque on a 2-cycle engine, while a free flowing expansion pipe decreases low end but increases high end.


because the way the carbs are calibrated.

In fuel injected cars, nothing like that applies. The more open the exhaust the better.

Only carbureated have the double-soaking effect calculated into the fuel mixture. IE when the intake valve opens the pressure from the exhust will push the air-fuel mixture thru the carb again, which mixes even more fuel into it. That way playing with the backpressure can make your car run richer or leaner and hence change the output
eh.. Sorry, but you don't know what you're talking about.. or at least, you have a very poor grasp of it. :p

To further explain, it has nothing to do with carbs. It's also independant of fuel injection... It doesen't really have anything to do with the fuel system, other than.. thats what the reaction effects.

Read my long post farther up. It does not matter that a 2-cycle engine has ports and a 4-cycle engine has valves, the same forces are still at work.

The bolded part is absolutely NOT a universal rule. If you use a pipe that is too big for your exhaust flow, when the exhaust gasses hit the huge tube, they lose velocity, and the negative pressure behind the positive pressure front dissolves.

To explain "scavenging"..

Like I said, when the exhaust valve opens, the hot exhaust gasses rush out and down the header. Behind this positive pressure front, is a negative pressure wave. This helps scavenge the leftover exhaust gasses out of the combustion chamber by pulling them out. In cases of extreme tuning, it can even help draw the fresh air(or air/fuel mix) into the chamber, due to valve overlap.

Of course, like I said.. it will only be operating synergistically at certain RPMs.

Well the whole argument about pressure generally revolves around cutout pipes and/or open air exhaust (down pipes and thats it). I never thought about it in terms of the pipe radius.

Either way though, i dont see how the velocity would make any difference. If the exhust gas hits bigger tube it will slow down, but the total amount of the gases would still be the same...wouldnt it? If theres a wider opening the air will go slower, because it would expand and lose some of the velocity, but then it would be more of the gas coming out at once, just because of the bigger opening.


And that thing about non-EFI cars is true btw...
It has everything to do with velocity. When the flow of exhaust gasses hits a larger diameter tube, it is like hitting a brick wall as far as velocity goes. The gasses are allowed to expand, slowing them down, hindering the flow of exhaust out of the combustion chamber. Of course it all has to be tuned for the engines exhaust output, your desired tuning, the position of the moon, etc.

You do realize how many intake and exhaust strokes are happening per second, especially at high RPMs?

Imagine a positive pressure front traveling through a tube(like a large explosion in space, a'la Star Trek).. Behind that wave is negative pressure. The energy wave traveling through the exhaust header is actually sucking gasses behind it forward, hence scavenging. It's all about the synergy between your specific engine tuning, your specific exhaust setup, your specific desired output, etc.

Only carbureated have the double-soaking effect calculated into the fuel mixture. IE when the intake valve opens the pressure from the exhust will push the air-fuel mixture thru the carb again, which mixes even more fuel into it. That way playing with the backpressure can make your car run richer or leaner and hence change the output

No. That is specifically what I was referring to when I said you do not know what you are talking about. The above statement makes no sense with a even a beginner/intermediate understanding of how a carbureted internal combustion engine works.

There is no "double-soaking effect". There is no pressure in the combustion chamber when the intake valve opens, there is negative pressure. How else would the air and/or air fuel mix be drawn in? The air/fuel mixture never travels through the carb "again, mixing more fuel into it". Finally, adjusting the air fuel ratio is NOT how an engines output is modified.

:)
 

Eli

Super Moderator | Elite Member
Oct 9, 1999
50,419
8
81
Originally posted by: Amorphus
I thought the scavenging effect was only for two-cycle engines, and cars were 4-cycle? Thus, the contoured exhaust for two-stroke bikes, and the like.

:confused:
It greatly effects the operation of a 2-cycle engine. A properly designed expansion tube is sorta like a turbo charger, in that it can fill the combustion chamber to more than 100% of capacity.

The same forces are working in a 4-cycle engine, but they don't have as big of an effect because of the seperate intake and exhaust valves.

There is very little valve overlap in a 4-cycle engine, but in a 2-cycle engine, there is plenty of "port overlap".

Since the path from the intake header, through the combustion chamber and out the exhaust header is a virtually restriction free, straight shot(generally).. when that negative pressure is formed inside the combustion chamber due to the exiting exhaust gasses, it freely pulls new fuel/air in through the carburetor, and actually somewhat into the exhaust header.

Now heres where the similarities end. In a 4-cycle engine, you only care about exhaust gas velocity. That's why you have nothing more than a straight header on high performance engines.

In a 2-cycle engine, exhaust gas velocity certain matters, but have you ever wondered why an expansion pipe gets narrow towards the end? That narrowing of the tube reflects that positive pressure wave backwards through the expansion chamber, stuffing the fuel air mix that has been pulled into the header back into the combustion chamber, effectivly filling it to a greater capacity than normal, increasing power.

Expansion Chamber Animated Gif
 

halik

Lifer
Oct 10, 2000
25,696
1
0
Originally posted by: Eli
Originally posted by: halik
Originally posted by: Eli
Originally posted by: halik
Originally posted by: Eli
Originally posted by: Howard
Backpressure decreases torque.
What's the proper term, then?

I know for a fact that a slightly more restrictive "can" type exhaust increases low end and decreases high end torque on a 2-cycle engine, while a free flowing expansion pipe decreases low end but increases high end.


because the way the carbs are calibrated.

In fuel injected cars, nothing like that applies. The more open the exhaust the better.

Only carbureated have the double-soaking effect calculated into the fuel mixture. IE when the intake valve opens the pressure from the exhust will push the air-fuel mixture thru the carb again, which mixes even more fuel into it. That way playing with the backpressure can make your car run richer or leaner and hence change the output
eh.. Sorry, but you don't know what you're talking about.. or at least, you have a very poor grasp of it. :p

To further explain, it has nothing to do with carbs. It's also independant of fuel injection... It doesen't really have anything to do with the fuel system, other than.. thats what the reaction effects.

Read my long post farther up. It does not matter that a 2-cycle engine has ports and a 4-cycle engine has valves, the same forces are still at work.

The bolded part is absolutely NOT a universal rule. If you use a pipe that is too big for your exhaust flow, when the exhaust gasses hit the huge tube, they lose velocity, and the negative pressure behind the positive pressure front dissolves.

To explain "scavenging"..

Like I said, when the exhaust valve opens, the hot exhaust gasses rush out and down the header. Behind this positive pressure front, is a negative pressure wave. This helps scavenge the leftover exhaust gasses out of the combustion chamber by pulling them out. In cases of extreme tuning, it can even help draw the fresh air(or air/fuel mix) into the chamber, due to valve overlap.

Of course, like I said.. it will only be operating synergistically at certain RPMs.

Well the whole argument about pressure generally revolves around cutout pipes and/or open air exhaust (down pipes and thats it). I never thought about it in terms of the pipe radius.

Either way though, i dont see how the velocity would make any difference. If the exhust gas hits bigger tube it will slow down, but the total amount of the gases would still be the same...wouldnt it? If theres a wider opening the air will go slower, because it would expand and lose some of the velocity, but then it would be more of the gas coming out at once, just because of the bigger opening.


And that thing about non-EFI cars is true btw...
It has everything to do with velocity. When the flow of exhaust gasses hits a larger diameter tube, it is like hitting a brick wall as far as velocity goes. The gasses are allowed to expand, slowing them down, hindering the flow of exhaust out of the combustion chamber. Of course it all has to be tuned for the engines exhaust output, your desired tuning, the position of the moon, etc.

You do realize how many intake and exhaust strokes are happening per second, especially at high RPMs?

Imagine a positive pressure front traveling through a tube(like a large explosion in space, a'la Star Trek).. Behind that wave is negative pressure. The energy wave traveling through the exhaust header is actually sucking gasses behind it forward, hence scavenging. It's all about the synergy between your specific engine tuning, your specific exhaust setup, your specific desired output, etc.

Only carbureated have the double-soaking effect calculated into the fuel mixture. IE when the intake valve opens the pressure from the exhust will push the air-fuel mixture thru the carb again, which mixes even more fuel into it. That way playing with the backpressure can make your car run richer or leaner and hence change the output

No. That is specifically what I was referring to when you said you do not know what you are talking about. The above statement makes no sense with a even a beginner/intermediate understanding of how a carbureted internal combustion engine works.

There is no "double-soaking effect". There is no pressure in the combustion chamber when the intake valve opens, there is negative pressure. How else would the air and/or air fuel mix be drawn in? The air/fuel mixture never travels through the carb "again, mixing more fuel into it". Finally, adjusting the air fuel ratio is NOT how an engines output is modified.

:)


hmm couple of thigs.... first off i read that thing somewhere online and it made sense to me... (never owned non EFI car).

But i know for fact that if you take the muffles off a carburated car, the car needs to be adjusted. Also how does the a/f mixture not travel thru the carb? The whole point of the carb is to provide the a/f mixture - the vacuum from the narrowing sucks the fuel from the lil opening in the carb.

I couldnt find the article where i saw the thing with the double soaking or what not... ill look for it some more
 

Eli

Super Moderator | Elite Member
Oct 9, 1999
50,419
8
81
Originally posted by: halik


hmm couple of thigs.... first off i read that thing somewhere online and it made sense to me... (never owned non EFI car).

But i know for fact that if you take the muffles off a carburated car, the car needs to be adjusted. Also how does the a/f mixture not travel thru the carb? The whole point of the carb is to provide the a/f mixture - the vacuum from the narrowing sucks the fuel from the lil opening in the carb.

I couldnt find the article where i saw the thing with the double soaking or what not... ill look for it some more
Yes, you are correct. Modifying either the intake or exhaust will result in the carburetor needing to be adjusted for its new operating parameters.

No, I was saying that the air/fuel mixture never travels through the carb twice. There is never pressure inside the combustion chamber when the intake valve opens. Remember that the exhaust gasses are some 1300ºF.. if the intake valve were to open while there were still hot gasses inside the combustion chamber.. well, you don't have to be a rocket scientist to infer that the results could be pretty catastrophic. ;)

And if you want to get nitpicky, it is not the engine vacuum that draws the fuel into the air stream. This actually kinda has relevance to our discussion.... velocity, tubes, etc. ;)

Carburetors work on the venturi effect principle - as air velocity increases, pressure at that site decreases. What's the best way to increase air velocity? Reduce the tube diameter.

A carburetor, in its simplest form, is nothing more than a tube with a slight restriction somewhere inside of it.pic

The jet("little opening") is positioned in the middle of the venturi, at the place of lowest pressure. Due to the lower than normal pressure at the jet site, atmospheric pressure pushes fuel out and into the stream of air, vaporizing it as it is drawn into the engine.

:)
 

GalvanizedYankee

Diamond Member
Oct 27, 2003
6,986
0
0
What halic is referring to in double soaking is the air moving back and forth through the carb picking up more fuel
and becoming very rich. Full race type headers on a street engine can cause this to happen and be almost undrivable.
As I stated above a positive pressure wave at the exhaust port while the intake & exhaust valves are still open, if
strong enough, can reverse the flow of air in the intake port. This will result in air backing out of the carb picking up more fuel & picking up even more as it re-enters the carb. This would normally happen in the lower rpm range.

Fuel injected engines do not suffer with this problem because electronics not air flow determine fuel delivery.


On a dyno with the air filter off a smoke like cloud can be seen to hover over the carb.
This is called "fuel stand off",it happens at wfo throttle and only at certain rpms. It is normal but is caused by
a reversal of air flow, caused by that positive pressure wave in the intake manifold.

EDIT: If you doubt the above, read a book, ISBN 0-8376-0140-1 or 0-8376-0309-9 :)


 

arcenite

Lifer
Dec 9, 2001
10,660
7
81
VTEC stands for Very Tall Engine Coolant, and if you etch out your intake manifold you lose 5 lbs.

Bill
 

halik

Lifer
Oct 10, 2000
25,696
1
0
Originally posted by: galvanizedyankee
What halic is referring to in double soaking is the air moving back and forth through the carb picking up more fuel
and becoming very rich. Full race type headers on a street engine can cause this to happen and be almost undrivable.
As I stated above a positive pressure wave at the exhaust port while the intake & exhaust valves are still open, if
strong enough, can reverse the flow of air in the intake port. This will result in air backing out of the carb picking up more fuel & picking up even more as it re-enters the carb. This would normally happen in the lower rpm range.

Fuel injected engines do not suffer with this problem because electronics not air flow determine fuel delivery.


On a dyno with the air filter off a smoke like cloud can be seen to hover over the carb.
This is called "fuel stand off",it happens at wfo throttle and only at certain rpms. It is normal but is caused by
a reversal of air flow, caused by that positive pressure wave in the intake manifold.

EDIT: If you doubt the above, read a book, ISBN 0-8376-0140-1 or 0-8376-0309-9 :)




Yeah from what i read, people calibrate the carbs to take in account the multiple passes.

Come to think of it, the cloud of smoke over the carb is common in top fuel and pro street drag racing. I just never realized what it is :)


How stuff works has a decent article on carbs, including the venturi effect and everything. .. :)

Oh yeah as far as a/f ratio and performance goes, leaner mixture tends to make more power. But the engine runs quite hot and its bad for poston rings (i've seeon a ton of turbo cars where people just cranked up the boost a lot and that's al they did...). Its also what the shady "gain 10 horsepower blah blah" kits on ebay do. Its a resistor you put on the mass that weaken the signal and make the ECU acount for less air thats actually comming in
 

djNickb

Senior member
Oct 16, 2003
529
0
0
Originally posted by: aRCeNiTe
VTEC stands for Very Tall Engine Coolant, and if you etch out your intake manifold you lose 5 lbs.

Bill


Actually VTEC=Variable timing and lift electronic control


VTEC
 

LAUST

Diamond Member
Sep 13, 2000
8,957
1
81
Originally posted by: djNickb
Originally posted by: aRCeNiTe
VTEC stands for Very Tall Engine Coolant, and if you etch out your intake manifold you lose 5 lbs.

Bill


Actually VTEC=Variable timing and lift electronic control


VTEC
I detected sarcasm in his post from 8 threads away, how did you miss it? :p

 

StageLeft

No Lifer
Sep 29, 2000
70,150
5
0
Originally posted by: LAUST
Originally posted by: djNickb
Originally posted by: aRCeNiTe
VTEC stands for Very Tall Engine Coolant, and if you etch out your intake manifold you lose 5 lbs.

Bill


Actually VTEC=Variable timing and lift electronic control


VTEC
I detected sarcasm in his post from 8 threads away, how did you miss it? :p
8 threads? I had to come within 3 :p
 

scorpmatt

Diamond Member
Feb 8, 2001
7,040
97
91
Originally posted by: Eli
Originally posted by: halik


hmm couple of thigs.... first off i read that thing somewhere online and it made sense to me... (never owned non EFI car).

But i know for fact that if you take the muffles off a carburated car, the car needs to be adjusted. Also how does the a/f mixture not travel thru the carb? The whole point of the carb is to provide the a/f mixture - the vacuum from the narrowing sucks the fuel from the lil opening in the carb.

I couldnt find the article where i saw the thing with the double soaking or what not... ill look for it some more
Yes, you are correct. Modifying either the intake or exhaust will result in the carburetor needing to be adjusted for its new operating parameters.

No, I was saying that the air/fuel mixture never travels through the carb twice. There is never pressure inside the combustion chamber when the intake valve opens. Remember that the exhaust gasses are some 1300ºF.. if the intake valve were to open while there were still hot gasses inside the combustion chamber.. well, you don't have to be a rocket scientist to infer that the results could be pretty catastrophic. ;)

And if you want to get nitpicky, it is not the engine vacuum that draws the fuel into the air stream. This actually kinda has relevance to our discussion.... velocity, tubes, etc. ;)

Carburetors work on the venturi effect principle - as air velocity increases, pressure at that site decreases. What's the best way to increase air velocity? Reduce the tube diameter.

A carburetor, in its simplest form, is nothing more than a tube with a slight restriction somewhere inside of it.pic

The jet("little opening") is positioned in the middle of the venturi, at the place of lowest pressure. Due to the lower than normal pressure at the jet site, atmospheric pressure pushes fuel out and into the stream of air, vaporizing it as it is drawn into the engine.

:)

I have a car that has a carb, the intake hose was bad so I removed it, still havnt gotten a new one. The muffler fell off not to long ago, I didnt have to change anything on the carb. I put a new muffler on and the car still runs great
 

Eli

Super Moderator | Elite Member
Oct 9, 1999
50,419
8
81
Originally posted by: galvanizedyankee
What halic is referring to in double soaking is the air moving back and forth through the carb picking up more fuel
and becoming very rich. Full race type headers on a street engine can cause this to happen and be almost undrivable.
As I stated above a positive pressure wave at the exhaust port while the intake & exhaust valves are still open, if
strong enough, can reverse the flow of air in the intake port. This will result in air backing out of the carb picking up more fuel & picking up even more as it re-enters the carb. This would normally happen in the lower rpm range.

Fuel injected engines do not suffer with this problem because electronics not air flow determine fuel delivery.


On a dyno with the air filter off a smoke like cloud can be seen to hover over the carb.
This is called "fuel stand off",it happens at wfo throttle and only at certain rpms. It is normal but is caused by
a reversal of air flow, caused by that positive pressure wave in the intake manifold.

EDIT: If you doubt the above, read a book, ISBN 0-8376-0140-1 or 0-8376-0309-9 :)
Ahh.. Interesting..

No, that makes sense... I just didn't understand it the way he put it. And this just shows my lack of knowledge regarding hardcore race stuff. :)

Thanks for the enlightenment.
 

Eli

Super Moderator | Elite Member
Oct 9, 1999
50,419
8
81
Originally posted by: scorpmatt
Originally posted by: Eli
Originally posted by: halik


hmm couple of thigs.... first off i read that thing somewhere online and it made sense to me... (never owned non EFI car).

But i know for fact that if you take the muffles off a carburated car, the car needs to be adjusted. Also how does the a/f mixture not travel thru the carb? The whole point of the carb is to provide the a/f mixture - the vacuum from the narrowing sucks the fuel from the lil opening in the carb.

I couldnt find the article where i saw the thing with the double soaking or what not... ill look for it some more
Yes, you are correct. Modifying either the intake or exhaust will result in the carburetor needing to be adjusted for its new operating parameters.

No, I was saying that the air/fuel mixture never travels through the carb twice. There is never pressure inside the combustion chamber when the intake valve opens. Remember that the exhaust gasses are some 1300ºF.. if the intake valve were to open while there were still hot gasses inside the combustion chamber.. well, you don't have to be a rocket scientist to infer that the results could be pretty catastrophic. ;)

And if you want to get nitpicky, it is not the engine vacuum that draws the fuel into the air stream. This actually kinda has relevance to our discussion.... velocity, tubes, etc. ;)

Carburetors work on the venturi effect principle - as air velocity increases, pressure at that site decreases. What's the best way to increase air velocity? Reduce the tube diameter.

A carburetor, in its simplest form, is nothing more than a tube with a slight restriction somewhere inside of it.pic

The jet("little opening") is positioned in the middle of the venturi, at the place of lowest pressure. Due to the lower than normal pressure at the jet site, atmospheric pressure pushes fuel out and into the stream of air, vaporizing it as it is drawn into the engine.

:)

I have a car that has a carb, the intake hose was bad so I removed it, still havnt gotten a new one. The muffler fell off not to long ago, I didnt have to change anything on the carb. I put a new muffler on and the car still runs great
Of course you didn't have to. The engine will still run fine, etc.. but if you want it to run optimally throughout its whole RPM range, yes.. you would need some carburetor adjustment.

If you're going to be nitpicky about headers, performance, etc, then the car in question probably isn't an A-B econobox.
 

Howard

Lifer
Oct 14, 1999
47,982
11
81
Originally posted by: mAdD INDIAN
as i understand it, you want some backpressure in lower rpm because you want to have a relatively slower exhaust gas velocity. This is because the cylinder cannot pump out exhaust gas quickly and you'd want the "combustion" to last longer so you'd have more power.
False. How do you get any torque when the exhaust valve is open (when the scavenging effect works)? Furthermore, even if you DID keep the combustion going (won't happen), you'd only reduce the total amount of torque because the force of the gas would be acting against the motion of the piston.

EDIT: You always want the least amount of backpressure throughout the entire RPM range.