I believe backpressure is important in a street driven engine only because that is what people-in-the-know are saying. However I dont know exactly why it is so important. What exactly is the science and mechanics behind exhaust backpressure? How would an engine behave(and what would the HP and torque curves look like) with an engine with increased backpressure? the right amount of backpressure? no backpressure(no exhaust manifold)? and a theoretical negative backpressure(vacum sucking exhaust out of engine)? I can't wait to hear everyone's opinions. This should be interesting.........

 

i believe that every engine depending on CI, cam, intake, comp., and etc has a certain exhaust size in which it creates the most efficeint flow. if you have to big of an exhaust (stock 4 cylinders with 3") the exhaust can tumble through the pipe rather than flow quickly like it should. on the street where torque is an important factor, your exhaust size needs to be at the size where its more efficeint at lower rpms (or the rpms you would mostly be turning) on the strip you are aways at W.O.T so bigger exhaust is more practical. one mistake people make is when they go to bigger exhaust they dont richen the fuel mixture. when exhaust can exit the combustion chamber quicker then air can enter it quicker also, remember intake and exhaust works togther, this causes a lean mixture which is why cars always pop and spuder when they have big exhaust with no other fuel mods. i run a mustang (302) with a comp cam, edelbrock intake, headers, stock ported heads, 3.73s, 8.5-1 comp.,and a holley 600. i have 2.25" true duals with flowmasters. i could run 2.5 and it would help if i richened the mixture unless i was already running rich. my 302 is dynoed at 312 hp @ 5500 and 335 ftlbs @ 4500. not bad for a couple hundred bucks of simple mods.

 

My knowledge on the subject is limited, but I'll attempt to explain it anyway.

First off, backpreasure is always bad. As smoke said, it's all about air flow.

Backpreasure is a situation where the exhaust cannot exit the cylinder due to obstructions in flow (small pipe, etc.).

But, exhaust flow is aided by the dynamics of the engine. As the exhaust exits, the valve closes, and as the exhaust continues down the pipe it creates a vacuum near the exhaust valve. The next time the valve opens, the vacuum helps to pull the gasses out of the cylinder.

If the pipe is too large for a given volume of exhaust, then there will be no, or very little, vacuum present.

If the pipe is too small then backpreasure will prevent the gasses from exiting the cylinder.

At low rpm, a smaller pipe will produce optimal flow. Which will help to maximize low rpm power. But will reduce top end performance due to backpreasure.

Which is ultimately why people say you need backpreasure. You don't, but optimizing for low to mid rpm performance creates the situation anyway.

 

The way I understand it is that it's all about the velocity of the exhaust gasses. Smaller piping will increase velocity, but can produce backpressure at higher RPM's due to being able to flow less. Even with a relatively small exhaust, you won't be creating any backpressure at the low RPM's simply because the engine isn't creating enough exhaust to produce backpressure, but wind it up and the back pressure may build making the engine have to work to push the exhaust out. A large exhaust will be able to flow enough so that at high RPM's there is no backpressure, but the exhaust gas velocity at lower RPM's may be too slow to help pull the exhaust gasses out. To me it seems that with a built motor, there is much more to be gained at higher RPM's than there is to be lost at the lower RPM's.

 

hmm, i see. I was taking "backpressure" in the literal sense of the word, when actually is a side effect(?) of proper exhaust size choice. Given this info, what do you think would happen if you hooked a large vacum to your tailpipe to suck out exhaust gasses and draw a slight vacum in the combustion chamber before the exhaust valve closes? Yeah, it's a crude idea, so please try not to focus so much on the shop vac hanging off your exhaust pipe as on the theory involved and the effects of doing so. My thought is that in doing so, you would lessen power loss during the exhaust cycle of the combustion process by sucking gasses out of the cylinder instead of having the piston push them out. Also, right before the exhaust valve closes, you would be drawing a slight vacum, which will help to draw in the air/fuel mixture on the next cycle. This would also lean out the air/fuel ratio because all the combusted gasses would be out of the cylinder, making room for more air to be drawn in. What do you guys think? This setup can't be very emissions friendly though, as it will probably reduce the efficiency of the catalytic converters and the gasses coming out of the tailpipe will probably not be burned to California's standards. I have never seen this tried before, but if it has, what were the results?

 

Ok, a few things forgotten here. With a propolly design exhaust sywtem a vacuum will be present in the combustion chamber. Has the piston starts to move up on the exhaust stroke and the valve opens the gas will start to accelerate. Add to this the scavenging effect and it accelerates even more. This tends to draw the exhaust gas out of the combustion chamber because the leading front of the exhaust is moving faster than the trailing end thus creating a vacuum.

Before the piston reaches the top the exhaust valve starts to close and the intake valve starts to open, overlap. This allows the intake to get a head start on the intake stroke added by the vacuum created by the exhaust flow. To much overlap and you get a very loppy idle. This is because the exhaust velocity is not great enough at ideal to maintain the vacuum. You end up sucking exhaust back into the cylinder. Not enough overlap and you don't get a good intake charge.

Now hooking a vacuum source to the exhaust may be a good idea but the implementation would be a bear or we would already see people doing it. My guess is the cost and the power required to do it does not justify the gains. It would need to be variable depending on rpm. Its easier to just build the correct exhaust system based on engine profile.

 

Great thread!

I particularly liked the “out of the box” shop vacuum exhaust. The vacuum would work like a supercharger, using vacuum deta-p to increase combustion charge, but it would be inherently less efficient. From a mechanical point of view, it’s much easier to build pressure than to maintain a vacuum. There are too many logistics problems with exhaust gas temperatures, valve timing, etc to make the vacuum idea practical.

As to the original backpressure question here is a link to another good thread on the same topic.

https://www.ford-trucks.com/forums/s...d.php?t=189316