Back Pressure

[cammerfe]

Here's a chance to start a debate. I've read in several places here at LVC that there is a necessity for back pressure in order for an engine to work properly. For those of you who think so---WHY? And for those with the other attitude, again WHY? My own position is that an engine is, in essence, an air pump. Any flow restriction is an efficiency 'buster'. Performance starts with opening up both the 'in' and 'out'. What say you?

KenS at Ben's Place

 

[ILLS]

Here's a chance to start a debate. I've read in several places here at LVC that there is a necessity for back pressure in order for an engine to work properly. For those of you who think so---WHY? And for those with the other attitude, again WHY? My own position is that an engine is, in essence, an air pump. Any flow restriction is an efficiency 'buster'. Performance starts with opening up both the 'in' and 'out'. What say you?

KenS at Ben's Place

Your question is stated too simply. A certain amount of backpressure does assist in torque production to an extent in a stock vehicle. An engine making more power is much more complicated than more air in and more air out. When you get into thermodynamics, flow, fueling and whatnot is when the more full picture is painted. In a highly modded vehicle there are many things you can ndo with fueling and other mods that counteracts the affects of less backpressure. There is a point of diminishing returns on both sides. If you increase backpressure in a stock vehicle there will come a point in time (usually very soon) where you will not make any more torque and your top end will be limited too. Same when going the other way. Exhaust mods are not a one stop shop. However, people just do one at a time and see losses and just equate lost backpressure to less torque when that is not totally the case. One thing that backpressure does do for stockish vehicles is allow equalization of the bank 1 and bank 2 exhaust pulses a little more too. Sorry if my answer didn't come out too straight, but that is how performance modding works. There is always much more than one angle to it.

 

[cammerfe]

Backpressure Continued

Great---Now we start a discussion. I intentionally made my original comment short to allow the greatest latitude for reply.

It's my belief that back pressure is NEVER a good thing and any time it is part of the picture, it's something to get away from and certainly not desirable.

I'll also state my belief that back pressure is virtually always present in a stock vehicle. I once had a Pinto that needed a new exhaust system. I had access to a header and made the rest of the system from readily available parts---including a new SuperTrapp 2 1/2 inch muffler. From the header to the muffler was a straight tube. With all the disks in the muffler, the increase in usable power (torque) was surprising. Unfortunately, the noise was more than I found livable and I took about half of the disk pack out. I now had a much quieter car---at the expense of most of the torque improvement.

'Stock' cars are often built so that the back pressure present is not a serious detriment; it's allowed-for in the factory tune. And, sometimes, such as when running a turbocharger, some degree of backpressure is inevitable. The turbo(s) simply do such a good job of making power when spooled-up that the slight impediment of the free-wheeling turbine in the exhaust at moderate speed isn't anything to complain about even if it costs a little mileage. The more freedom in and out the better. That's why headers and a more open intake manifold and carb have been staples of hot-rodding since the days of Offenhauser and Smitty's.
KenS from Ben's Place

 

[ILLS]

Great---Now we start a discussion. I intentionally made my original comment short to allow the greatest latitude for reply.

It's my belief that back pressure is NEVER a good thing and any time it is part of the picture, it's something to get away from and certainly not desirable.

I'll also state my belief that back pressure is virtually always present in a stock vehicle. I once had a Pinto that needed a new exhaust system. I had access to a header and made the rest of the system from readily available parts---including a new SuperTrapp 2 1/2 inch muffler. From the header to the muffler was a straight tube. With all the disks in the muffler, the increase in usable power (torque) was surprising. Unfortunately, the noise was more than I found livable and I took about half of the disk pack out. I now had a much quieter car---at the expense of most of the torque improvement.

'Stock' cars are often built so that the back pressure present is not a serious detriment; it's allowed-for in the factory tune. And, sometimes, such as when running a turbocharger, some degree of backpressure is inevitable. The turbo(s) simply do such a good job of making power when spooled-up that the slight impediment of the free-wheeling turbine in the exhaust at moderate speed isn't anything to complain about even if it costs a little mileage. The more freedom in and out the better. That's why headers and a more open intake manifold and carb have been staples of hot-rodding since the days of Offenhauser and Smitty's.
KenS from Ben's Place

When running a turbo there is actually quite a bit of pressure that is built up in the exhaust. Depending on the size of the compressor (rotating mass) and the size of the turbine A/R you will run into more or less pressure. There is always a sweet spot where you will get good spool but not so quickly that you are maxxing out the turbo and losing top end power. If you get a really small turbine housing then you will spool very quickly but lose top end. Go too big and you will spool too late but have more top end. Also with a turbo while at cruise you will usually see a slight increase in gas mileage assuming that your AFR's are stoich and you are not running such large injectors that proper fuel atomization doesn't become a problem.

Once again, I do think that your "more freedom in and out the better" statement to be a little too general. While that statement will hold true with some vehicles with certain applications it is not globally applicable. If that were the case then there would be no such thing as long runner intake manifolds. Also if that were the case then head porting would be less of a work of art and more just metal hogging. There are more factors present here than just maximum airflow that create more power.

 

[cammerfe]

Back Pressure exploration

When running a turbo there is actually quite a bit of pressure that is built up in the exhaust. Depending on the size of the compressor (rotating mass) and the size of the turbine A/R you will run into more or less pressure. There is always a sweet spot where you will get good spool but not so quickly that you are maxxing out the turbo and losing top end power. If you get a really small turbine housing then you will spool very quickly but lose top end. Go too big and you will spool too late but have more top end. Also with a turbo while at cruise you will usually see a slight increase in gas mileage assuming that your AFR's are stoich and you are not running such large injectors that proper fuel atomization doesn't become a problem.

Once again, I do think that your "more freedom in and out the better" statement to be a little too general. While that statement will hold true with some vehicles with certain applications it is not globally applicable. If that were the case then there would be no such thing as long runner intake manifolds. Also if that were the case then head porting would be less of a work of art and more just metal hogging. There are more factors present here than just maximum airflow that create more power.

As seems to be becoming usual, you and I are largely in agreement---just looking at things from different angles. I made the original comment very general so as to keep from restricting the discussion. My present project---Land Speed LS---is my fourth personal turbo'd daily driver. My first, an early Fox Mustang, started as a Weber'd, sprayed draw-through and ultimately became a Holley carb'd blow-through using a NASCAR-type four barrel. Since I ran that car both with and without a turbo, and fed the turbo by redirecting the exhaust flow from the headers through the turbo, I have a little experience with 'before-and-after'. I lost about 2 MPG with the turbo in place'

Long runner intake manifolds give up a little to extra friction in the tract, but gain MORE from the resonance and 'ramcharger effect' (remember 'The High And Mighty' Ramchargers team car at the '59 Nationals here in Detroit?)

I believe back-pressure, by definition, is the 'stopper' effect of not having a free-enough-flowing exhaust system to enable the exhaust gasses to get out of the way. If the exhaust gasses back up in the system, you have back pressure. I expect to have to live with some of it with turbos in place, but I'll do everything reasonable to cut down on its effects

The 'homogenizing' effect of a turbo on the intake charge is a good thing, but not, in itself, wonderful enough to completely overcome the back pressure effects caused by the flow complications offered on the exhaust side.

Your comment about head work is right on the money---we agree completely. But I separate 'back-pressure' from flow potential. Although they are akin in some respects, I believe back-pressure is a function of components external to the engine itself.

KenS from Ben's Place

 

[ILLS]

Since I ran that car both with and without a turbo, and fed the turbo by redirecting the exhaust flow from the headers through the turbo, I have a little experience with 'before-and-after'. I lost about 2 MPG with the turbo in place'

It is a somewhat similar situation with the 05 TurboLS. Average gas mileage has dropped a little bit since I did the turbo. However, considering the minute drop and the different way I have been driving the car in testing phase I believe it to be more a result of my driving habits than the new combination. I am a fuel injection guy myself. I do not have allot of experience with turbocharged carbe'd setups so when I stated that people usually gain a little mileage at cruise speeds with a turbo it was from a EFI perspective. Carb and EFI have some large differences in efficiency and finetuned fuel ratios so that may be where the disparity is here.

 

[cammerfe]

Back Pressure Exploration

It is a somewhat similar situation with the 05 TurboLS. Average gas mileage has dropped a little bit since I did the turbo. However, considering the minute drop and the different way I have been driving the car in testing phase I believe it to be more a result of my driving habits than the new combination. I am a fuel injection guy myself. I do not have allot of experience with turbocharged carbe'd setups so when I stated that people usually gain a little mileage at cruise speeds with a turbo it was from a EFI perspective. Carb and EFI have some large differences in efficiency and finetuned fuel ratios so that may be where the disparity is here.

Howdy---
I did the turbo Mustang in 1980. At that time I'd had only one experience with an FI set-up and it was entirely mechanical. I haven't owned a carb'd car for more than ten years now. I converted the FE engine in my '63 F-100 with a Holley ProJection package.

The 'rush' of putting your foot in it may very well be a significant part of the difference in MPG.

KenS from Ben's Place

 

[ILLS]

The 'rush' of putting your foot in it may very well be a significant part of the difference in MPG.

There is nothing like the 50 dollar bill on the dash test... The Turbo LS, while quick, would not be up to snuff for that test; but my other vehicle would be.

 

[Jayce 1971]

Backpressure

I installed a CAI on my 91 Daytona Iroc Turbo. Then had a backpipe fabricated from the exhaust manifold to the rear. Put a cherry bomb in just for $hits and giggles. 3" exhaust from the turbo to the tailpipe (no cat). The 30hp gain was more than noticable, (250 hp makes awful torquesteer), it was a different car. Of course we had to weld a nut on the pipe and tap it for an O2 sensor, but pretty simple all around. VERRRRY LOUD, but ran like a scalded dog. Some hold that there needs to be some backpressure for engines to run properly, who knows. I'm older now, and don't particularly care for sound of an unmuffled engine, but alot of people still hold that gutting your cats, and taking off the muffler increases power considerably.

 

[ILLS]

I installed a CAI on my 91 Daytona Iroc Turbo. Then had a backpipe fabricated from the exhaust manifold to the rear. Put a cherry bomb in just for $hits and giggles. 3" exhaust from the turbo to the tailpipe (no cat). The 30hp gain was more than noticable, (250 hp makes awful torquesteer), it was a different car. Of course we had to weld a nut on the pipe and tap it for an O2 sensor, but pretty simple all around. VERRRRY LOUD, but ran like a scalded dog. Some hold that there needs to be some backpressure for engines to run properly, who knows. I'm older now, and don't particularly care for sound of an unmuffled engine, but alot of people still hold that gutting your cats, and taking off the muffler increases power considerably.

You are in a whole different ballgame when running a turbocharger. You will want as little hindrance to outflow as possible from the turbine discharge to the tip of the exhaust. The reason for this is that flow is much more affected in low pressure area's versus high pressure area's. That is why if you are going to have to have an exhaust restriction when running a turbo it is best to place it on the pressurized exhaust side and not the downpipe side. The power gains you seen were less from backpressure for the engine being reduced and more from backpressure for the turbo being reduced.

 

[ground_zero298]

Why have a header or manifold anyway. If back pressure is the enemy skip all the bull:q:q:q:q and just run open exhaust ports, with super huge exhaust valves? And the air pump idea is well, 1 unit of air goes in 1 unit of air comes out. If a engine was like a air pump it would be called a air pump. Why not come up with some mechanically driven exhaust vacuum unit? It would run off a pulley at supersonic speeds creating a vacuum on the back end of the engine resulting in no back pressure?

 

[cammerfe]

Exhaust 'sucker' ??

There are many places where engine technology says "Enough is enough!" When an ignition has done a complete job of igniting a fuel charge, more 'spark' won't magically create more power. So it is also with exhaust flow---many subtlties help increase it, but your proposed 'sucker' set-up is most likely going to quickly reach the point of 'no further advance.' What you are proposing is overkill and would heavily complicate things with no benefit. Try it if you choose! I'd be interested to hear about the result.
KenS from Ben's Place

 

[Snowseeker]

Why have a header or manifold anyway. If back pressure is the enemy skip all the bull:q:q:q:q and just run open exhaust ports, with super huge exhaust valves? And the air pump idea is well, 1 unit of air goes in 1 unit of air comes out. If a engine was like a air pump it would be called a air pump. Why not come up with some mechanically driven exhaust vacuum unit? It would run off a pulley at supersonic speeds creating a vacuum on the back end of the engine resulting in no back pressure?

Headers and exhaust alow for scavaging of the exhaust pulse's. Kinda like a vacuum at a certain rpm where the "puffs" of air line up right (along with the intake pulse's) to help pull eachother along and out of the exhaust or down the intake runners into the cylinder to make up your powerband arch. Without a header there would be no scavaging of the exhaust and you would lose part of your powerband, the intake runners making up another part of the power band.

What you are talking about with the exhaust "pump" already exists and is called a supercharger. Forcing air in is way more efficiant than trying to suck the exhaust out. Sucking the exhaust out will cause a vacuum in the cylinder. Forcing the air in makes boost, more air and fuel in = more power.

Back pressure is old school lead in the gas day myth. Without back pressure "back in the day" you would burn up and chip exhaust valves. These modern technology days we have much stronger and harder valves that don't over heat and become brittle as easy.


Sorry I am not very good at explaining technical things, but basicly you don't want such big exhaust that there is no scavaging, but you don't want such small exhaust that you create back pressure.