YACT: why does decreased backpressure reduce low end torque?

[OrganizedChaos]

?

 

[Quixfire]

It's completely dependant on the rpm range you are working in.

 

[Brutuskend]

I know when I first built my engine I had a glass pack on it (waiting until I could put together a better system) and it had crap for off the line acceleration. Then I installed dual turbo muff's on it and it worked MUCH better.

 

[DivideBYZero]

'Turbo muff's' -

 

[GalvanizedYankee]

Very good question ! Let me do a little reading and come up with an understandable answer. If no one else does.

 

[Eli]

Originally posted by: galvanizedyankee
Very good question ! Let me do a little reading and come up with an understandable answer. If no one else does.

From my knowledge of 2-cycle engines, and how expansion chambers work, it has to do with the density of the gasses inside the exhaust tract giving a better medium for the pressure fronts to travel through..

Could be wrong though. It's a pretty grey area as far as I know(although I am sure there is definitive info out there), because it's so complex. On one hand, you don't want too much backpressure.. because any real restriction in exhaust is bad, but I've also heard no backpressure hurts low end torque.

This certainly seems true with 2-cycle engines. A free flowing expansion chamber with very little if any backpressure is great for the upper RPM range, but a can-header creates more low end torque.. and not as good a high end.

 

[Compton]

My understanding was that its not the backpressure, but its the exhaust velocity. If you use a pipe diameter that is too large, you lose the scavenging effect.

 

[Eli]

Originally posted by: Compton
My understanding was that its not the backpressure, but its the exhaust velocity. If you use a pipe diameter that is too large, you lose the scavenging effect.

Hmmm.. Yes, this makes sense. But OTOH, if you use a pipe diameter that is too small... it will be restrictive in the upper RPM range.

So, like with anything, it's a tradeoff.

And yes, I don't really think "backpressure" is the correct term. Maybe though.

You have to think small...

When that exhaust valve opens, the expanded gasses FLY out and into the exhaust tract. Imagine the front[of exhaust gasses] traveling through... Behind the front is negative pressure. That is what you want. That helps pull the remaining exhaust gasses out of the cylinder, and depending on how the engine is designed, can actually help pull the fresh charge in(valve overlap). I think that when there are gasses still packed into the exhaust tract("backpressure"), it gives a better medium for the energy to travel through, creating [better, more?] negative pressure.

But since there are so many variables involved(valve timing, exhaust gas temperature, exhaust gas velocity, etc), you have to tune your exhaust system to match your engine and what you want out of your engine. This results in a realitivly narrow RPM range where everything is operating synergistically. You pick where it is.

 

[GalvanizedYankee]

I checked in several books over at the other house. The best I can explain how on a stock engine with just a muffler
swap can cause a 2500rpm flap spot, would be, a pressure wave frequency change in the exhaust flow.
The +pressure wave inside the header at the port is reflected back into the chamber, killing intake flow & polluting the
fresh charge. At worst the exhaust will flow back up the intake runner.

At best a neg.wave is at the exhaust port as that valve is closing, in the above case it would be a positive pressure wave.
This is a major problem when ppl put slip-on mufflers on motorcycles.

Generally the better street headers are a TRI-Y on 4cyl engines. 1-4 & 2-3 join to a short collector, then those two join
at the main collector. This enhances mid-range at a slight decrease in top end.

At one time I thought that the reduced back pressure caused the fresh charge to be lost as it flows out the exhaust,
while the piston is at TDC. Some maybe lost at certain RPMs but what the OP is referring to is most likely back-flow.




Velocity is king ! Thats why killer 1200cc Sportsters run well ported 883cc heads w/stock valve diameters. They loose
a little on top but have monster mid-range. Thats why Yoshimura Racing would run a 750cc head on a 1000cc engine
on shorter tracks, velocity=mid-range torque.

Interesting note: Intake velocity dies at 600fps because the pressure wave reaches the speed of sound, a sonic barrier. Good street intake velocity is around 350fps.

The above is for NA engines, turbos & supercharges are another story

 

[Howard]

Backpressure decreases torque.

 

[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.

 

[Desslok]

Originally posted by: Howard
Backpressure decreases torque.

ONLY if it excessive. Engines are designed to run with some back pressure. It improves the scavaging effect which helps the engine be more efficent and create more power.

 

[Flyermax2k3]

Originally posted by: Compton
My understanding was that its not the backpressure, but its the exhaust velocity. If you use a pipe diameter that is too large, you lose the scavenging effect.

We have a winner.

Backpressure is a myth.

It's all about exhaust gas velocity.

 

[WinkOsmosis]

Scavenging effect? Wtf are you people talking about?

 

[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

 

[Jejunum]

commonn misconnception. its not actually decreased back pressure, instead its decreased exhaust velocity.
http://www.cobbtuning.com/tech/exhaustdesign/index.html

 

[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.

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.

 

[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.

however, in upper rpm ranges when the cyldiner is moving much quicker and combusting much faster, you'd want the exhaust gas to flow out as fast as possible.

This is why some cars have variable muffler that open a second flap to let out exhaust gas faster at higher rpms (the 2000+ Maximas employ this).

 

[Zenmervolt]

Originally posted by: Compton
My understanding was that its not the backpressure, but its the exhaust velocity. If you use a pipe diameter that is too large, you lose the scavenging effect.

Ding Ding Ding! We have a winner. Exactly right, a larger pipe that can flow more volume (for high RPM operation) reduces the exhaust gas velocity and makes the engine work harder at low RPM.

In an ideal world, you would have some sort of dual-stage setup that opened up a secondary exhaust tract for high RPM operation.

ZV

 

[Viperoni]

Originally posted by: Zenmervolt

Originally posted by: Compton
My understanding was that its not the backpressure, but its the exhaust velocity. If you use a pipe diameter that is too large, you lose the scavenging effect.

Ding Ding Ding! We have a winner. Exactly right, a larger pipe that can flow more volume (for high RPM operation) reduces the exhaust gas velocity and makes the engine work harder at low RPM.

In an ideal world, you would have some sort of dual-stage setup that opened up a secondary exhaust tract for high RPM operation.

ZV

We're getting somewhere

 

[ElFenix]

Originally posted by: Zenmervolt

Originally posted by: Compton
My understanding was that its not the backpressure, but its the exhaust velocity. If you use a pipe diameter that is too large, you lose the scavenging effect.

Ding Ding Ding! We have a winner. Exactly right, a larger pipe that can flow more volume (for high RPM operation) reduces the exhaust gas velocity and makes the engine work harder at low RPM.

In an ideal world, you would have some sort of dual-stage setup that opened up a secondary exhaust tract for high RPM operation.

ZV

if anyone would have such a thing, F1 would

 

[LAUST]

Read up on X-Pipes

 

[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.

 

[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.

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...

 

[mAdD INDIAN]

Originally posted by: ElFenix

Originally posted by: Zenmervolt

Originally posted by: Compton
My understanding was that its not the backpressure, but its the exhaust velocity. If you use a pipe diameter that is too large, you lose the scavenging effect.

Ding Ding Ding! We have a winner. Exactly right, a larger pipe that can flow more volume (for high RPM operation) reduces the exhaust gas velocity and makes the engine work harder at low RPM.

In an ideal world, you would have some sort of dual-stage setup that opened up a secondary exhaust tract for high RPM operation.

ZV

if anyone would have such a thing, F1 would

well they have dual stage mufflers as stated in my previous post.