Well, yes, and no. The exhaust has to overcome 14.7PSI of back pressure, at sea level, to get out of the pipe.

A typical stock exhaust system runs at around 1-3 PSI above atmospheric.

If there weren't some back pressure the motor wouldn't run. You need atmospheric pressure to charge the engine with fuel and air on the intake side of the cylinders. An ICE is a positive displacement pump, after all. It just uses air to push the piston down in the bore. It doesn't make any more.

So, yes,mostly.

nickname, and some others can put up the equations and such to describe the process.

Could you expand a bit more on the comment 'if there weren't some back pressure the motor wouldn't run'? Are you considering some aspect of blowdown and scavenging when you say it wouldn't run? Also, what contributes to the 1-3 psi backpressure and is there a theoretical limit to how low you can bring this down before you see a performance degradation in the engine? Thanks.

"relatively short pipes (as seen in rail dragsters) work best"

True, and bigger diameter pipes allow for even less turbulence and back pressure in the pipes, but it is critical to control that rev range very carefully, because with very short, or very low back pressure, pipes, it is easy to reach a rev range that allows for COLD (relative to the engine internal temps) air to reach the exhaust valves. This sudden influx of relative cold, with a sudden spiky temp changes in the valves, can cause cracking, breakage, poor seating of the valves, and all kinds of other mischief.

I once took the exhaust manifold off my old 6 cylinder Chevy Nova engine (64, before the ones everyone now loves to hate) to replace a blown head gasket ( a lot more came off before I was done), and when I was reassembling it, I decided to start it up to see what it would sound like without the exhaust restrictions. Wild sound, and a friend urged me to shut it down before it warmed up, then, when I had, explained, and showed me on another engine in the shop, what COULD have happened. The engine he showed me, mistreated by another hobby mechanic, and let reach full operating temps, had cracked and spalled valve seats, more than one splintered valve "flare" (the head of the valve) and one valve stem that had it's head laying 1/2 inch or so to the side of it's normal resting place, totally severed. All done in seconds of running at normal temperatures without an exhaust system.

He explained to me that when the return low-pressure valley in the exhaust pulse curve travels back up the pipe it is heated by residual heat energy stored in the pipe walls, with the result that it is within acceptable range of the operating temperature of the valve itself. Thus, in a properly designed and installed system, no thermal shock, or at least not a damaging thermal shock, hits the valves.

Others may have a rebuttal to offer, and for any further knowledge I'd be grateful, but I don't even think about running my engines without exhaust manifolds, and suitably long pipes now. Besides, it saves my ears. And keeps my neighbors happier.

I ran my Olds 350 V-8 freshly rebuilt engine without exhaust manifolds for a few minutes and didn't have any problems at all. Ended up putting 20K+ miles on the engine before I sold the car. I think there was other factors involved in the engine your friend was talking about besides cold air returning back up the exhaust port.

I would suggest that it is POSSIBLE you ran it for a short enough time that your exhaust valves didn't achieve full heat, or that even without the manifold, the temperature you did reach was not high enough to react to the temperature of the air reaching the valves due to the passages in the ports on the head itself. If you look at the exhaust passages in some of the older V-8's, they were fairly convoluted on stock engines. No guarantees because I didn't look at the block myself to see what it looks like.

Here's some more opinions on it...

http://www.jalopyjournal.com/forum/showthread.php?t=458593

K Fry, Thanks for the explanation. I read this in a book on turbocharging by Watson. In turbocharged engines, matching is sometimes done by deliberately placing a restriction at the exhaust - i.e, a minimum backpressure, by introducing a small bore piping system. If a leak then happens at the exhaust, because the turbo was matched to specific conditions , it can create havoc. The backpressure will reduce because of the leak and turbocharger work will increase, possibly resulting in excessive boost pressure, combustion knock and engine damage.

Here's a nice little blurb on exhaust pressure....

http://www.aa1car.com/library/exhaust_backpressure.htm

I can't add to K_Fry's excellent explanation.

My point is that with no external (air) pressure there would be no way for an engine to take in a fuel/air charge. It's that 14PSI that pushes the air into the cylinders. It also causes back pressure on the exhaust outlet.

Back pressure is caused by friction and curves in the pipe. Hot air is less dense, so less back pressure is produced. That's the real reason people wrap headers in hi-temp wrapping. To hold the heat in. I don't think there's any way to reduce back pressure to zero in a system of rapidly moving fluid.

We did a lot with exhaust systems when I was Dir. of Eng. at a noise cancelling company. (Electronic mufflers and headsets.) That's why I appreciate Ken's knowledge and expertise on the subject. The shortest straightest path is always the best for 4 strokes.

What are you doing?

'It's that 14PSI that pushes the air into the cylinders.'

Not if you have one of these eh?

Very good answer for Fry, once again, I learn something everything new every day. Thankyou. I never understood why I should buy a tuned FMF exhaust for my Yamaha as a kid.

And that, is one very nice looking set of extractors. Hats off for sure to the people who build them. My skills on a TIG are pretty good, but I dread to even imagine the ammount of misery I would go through in making those bad boys.

"Not if you have one of these eh?"

Not exactly. The air outside the blower intake is still only at 14PSI. So, the blower is actually sucking air out of the atmosphere faster than the atmosphere can push the air with just pressure? Right?

But, how much power does the air compressor take to pump the air into the intake?

If the atmospheric pressure was 0, it would still suck the air though?

Assumptions on my part:

Power usage depends on ammount of air being consumed by the engine to meet the power output.

Engine power output would be proportional to the power the blower uses.

Blower efficency would drop if the external (atmospheric) pressure was lower.

If atmospheric pressure was zero, there would be no air to suck and we would all be dead.

Not sure I can buy in to any of your assumptions.

Mmmm...I put my foot in that one didnt I? How about .001 Pa then, just for arguements sake? (Yeah....i guess we would be dead then to.)

Also you can't suck air, it has to be pushed and with a supercharger the air is being pushed into the intake manifold by the vanes in the supercharger. This results in a reduced pressure in the intake port of the supercharger because the atmosphere can't push air in as fast as the supercharger is pushing it into the intake manifold.

Vacuums don't suck.

And?

"Vacuums don't suck."

That's exactly correct. The vacuum doesn't suck, but instead the atmosphere around it pushes towards the reduced pressure.

Try this little experiment. Take a glass tube about a metre long, a bowl of mercury and a vacuum pump. Now what you do is fix the tube so that it is vertical with the bottom sitting in the bowl of mercury while the top is connected to the vacuum pump. Now when you turn the vacuum pump on and remove all the air from the top of the tube what's going to happen.

If vacuums sucked then the tube would soon fill with mercury all the way to the top and end up in the vacuum pump. However, that's now what will happen. When the mercury reaches around the 760 mm mark no matter how hard the vacuum pump works the mercury will not rise any further up the tube. This is because it's not the vacuum that is sucking the mercury up the tube but rather the atmospheric pressure pushing on the mercury in the bowl that is pushing the mercury up the tube. Once the weight of the mercury in the tube produces a downward force that is the same as the force from the atmosphere pushing it up then the mercury will rise no further.

The same thing happens with water and any liquid, you can only get it so far up by removing all the air from the top of the tube. With water this is about 30 feet and like the mercury no matter how hard you try the water will not rise further up the tube.

The actual height the liquid will move up the tube is dependent on the atmospheric pressure at the time which can vary so the height of the column of liquid that the atmosphere can support will vary as the atmospheric pressure varies.

Actually this is how an mercury barometer works. Basically it consists of a glass tube that is sealed at the top and evacuated of all the air while the other end sits in an open container of mercury. By measuring the height of the column of mercury in the tube you can calculate the atmospheric pressure.

So no vacuums don't suck it's things like the atmosphere that push things towards the area of reduced pressure.

Darn, I think I misplaced my bowl of mercury.

"Darn, I think I misplaced my bowl of mercury."

Fair enough. Then get yourself 12 metres of good quality hose pipe and an appropriately high window and dangle the hose out the window so that the bottom sits in a bucket of water that contains more water than the hose would when it was full. Connect the top to your vacuum pump and see what happens.

I'll bet that the water will not go much higher up the hose than around 10.3 metres (that's 34 feet for Americans) regardless of how hard the vacuum pumps suck.

Given that the water will only go 10.3 metres or so up the pipe, water has a density of around 1,000 kgm^-3 and the acceleration due to gravity is about 9.81 ms^-2 see if you can work out what the atmospheric pressure is that is pushing the water up the pipe.

This limit caused major problems in mining as it meant that a pump located on the surface could not extract water from mines that were more than 10 metres or so deep. To extract water from mines that were deeper you either had to locate the pump at the bottom of the mine or used a huge rod that connected the pumping device at the bottom of the mine to a beam engine located on the surface. This technique was used right up until the 1950s in some older mines.

However, now with electricity driving everything it's simpler to just run an electrical cable down to the lowest point of the mine and locate the pump there. This works because the pump is pushing the water up the pipe rather than removing the air from the top of the pipe and getting the atmosphere to do the pushing with its limited pressure. Since you are pushing the water up the pipe the limiting factors then become the force the motor/pump can generate and the pressure the pipe can withstand at the bottom of the mine.

With very deep mines draining the water from the bottom is often done in stages as it reduces the head pressure required at each of the pumping levels. You have to expend slightly more energy doing it in stages but the motor/pump assemblies are much smaller and cheaper plus the head pressure is dramatically reduced so you don't have to use pipes that are capable of holding back the full head pressure you would get with a single stage system.

GA from me, Masu. I probably should have known why the same pump can have such a limited suction head, while being able to develop a huge pressure head. But I didn't. Now, thanks to you, I can even explain it to non- technical help. This, plus all the info about mining and pumping history and application was fascinating. Thank you!!!

Your welcome.

I used to be pretty good with tig, but I don't think I am now -- at least I am pretty poor with my wire feed welder (and don't have a tig machine). When I built two stroke pipes I would gas weld them, and was pretty good at it. I once did a restoration job on a 1957 Ferrari, part of which involved fixing the headers, which had several cracks. I made the welds a very good match for the originals, which were also done with a gas torch.

But the Ferrari headers were very simple and had good access all around. The ones in the picture would require a huge amount of planning in addition to great welding skills. It's art.

(I thought I could find a pic of the old Ferrari headers themselves, but could not quickly do so. But I found this album of a sister car. The one I worked on was gold, but otherwise identical.)

There has to be a pressure difference to cause the spent (combusted) gases to push through the exhaust pipe system including the silencers and such restrictions. This pressure is achieved by the setting of the exhaust valve cam. There has to be a small sacrifice of power. As Lyn pointed out an IC engine is a positive displacement device and all actions have to be circumscribed by that.

For Turbo engines this exhaust pressure difference is expoited extensivly at high altitude. Basically the higher you go the less air pressure there is, for and aircraft using a turbo piston engine you can get the same engine power beacuse the turbo will spin faster to compenstate for the lower air pressure due the presurre difference between the exhuast gas and the atmosphere less drag on the compressor wheel. It is also why most cars use turbos at pikes peak to keep the power consistant as they climb.

Thanks. Not all IC engines operate by positive delta P across the engine. Turbocharged diesel engines with EGR deliberately operate with negative delta P to reintroduce exhaust back into the cylinders.

The combustion of air and fuel vapor causes expansion of the mixture, increasing the pressure in the confined spaces, mechanical displacement of the piston and conrod and finally, when the exhaust valve opens, following the laws of physics and thermodynamics flowing from a high to low pressure state. So, I would say unequivically, that too much back pressure (providing inadequate volume (hence the backpressure) in the path from high combustion to static atmospheric pressure will reduce performance. It serves to retard the flow of gases, both combusted and un combusted. In this case, backpressure is the differential between exhaust entrance and the valve seat. All combustion engines have this differential. Even an open bonfire has a miniscule boundary layer of backpressure.

The disadvantage to reducing backpressure? Increased size and weight of the exhaust system. A larger exhaust volume slows exhaust gas velocity, though , and reduces heat transfer from the engine to the atmosphere. In four strokes, I have not seen any downside to reducing backpressure that relates to the backpressure itself, although increasing local high temperatures can cause other performance impacts, by increasing intake air, block and fuel temps. This is why many high performance headers have heat insulating tape.

Normally up to p≈1.25 atm.For more cycles in same time (more power) you need fast and efficient scavenging..., this exhaust pressure and temperature are needed and determined with care.-