So far I'm with Jimmy Dean on the idea that the engine needs backpressure. Too much is a bad thing and too little is a bad thing. Engines need to balance out ya know?

 

Ok here is my original position on the subject. Headed home for the day now, but I'm looking forward to reading the responces in the morning......

Back pressure is NEVER desirable. For high-performance purposes, backpressure in an exhaust system increases engine pumping-losses and decreases available engine power. Maximum mid-to-high RPM power will be always be achieved with the lowest possible backpressure
IMO the whole “engines require backpressure" thing is a myth.

An engine needs the highest flow velocity possible (intake and exhaust) at any given RPM for quick throttle response and torque. The problem stems from the fact that the flows are dramatically different across the RPM range. A small diameter exhaust pipe might give you maximum unimpeded gas velocity at 2000 RPM. This would give you great off-idle acceleration, but after 2000 RPM the exhaust gas volume begins to exceed the amount that can flow unimpeded through the pipe and you have BACK PRESSURE. To overcome the limitations of the undersized exhaust pipe your engine needs to literally pump the exhaust out the end of the pipe. This seriously inhibits peak performance for any number of reasons.

If we install a larger diameter exhaust pipe, we now have enough pipe volume to flow the unimpeded gas velocity at mid-to-high RPM without BACK PRESSURE. However, at lower RPM’s the gas volume is not large enough to maintain the flow velocity in the larger pipe and low-end throttle response suffers.

exhaust systems are a compromise, you can maximize for low-range and mid-range performance or mid-range and high-end performance but you cannot simultaneously maximize for all three because of the differences in flow volume and velocity.

 

check out the response in the TB SPacer thread please.

Garrett

 

I did not know an engine would run without back pressure...but i am not a mechanical genius either...I thought the engine had to have something to push against to keep compression

 

Sigma is correct (or at least I believe he is): an internal combustion engine does not need backpressure. Backpressure, by defenition, is pressure pushing back (the wrong way) against the engine trying to expel the spent gases.
A year or so back, I got into a LONG thread in the exhaust forum. I'll see if I can find it and post a link.
Gas flow dynamics are complicated, but the key to it is exhaust gas velosity and the timing of very high velocity energy pulses that travel up and down (i.e. with the flow and against the flow) of exhaust gases.
These pulses can assist with the scavenging of spent gases.
Anyways, I'll see if I can find the old thread.

edit to add:
here's the thread.........
https://www.ford-trucks.com/forums/s...2Bbackpressure

 

Man that's an old thread!

 

So whatever happened to this Falcon guy?

 

Jimmy mentioned in the other thread that running an engine without backpressure could lead to burnt valves. This was true on older carbureted motors, but I do not think this applies to newer computer controlled FI motors. Here is why: Burnt valves are typically the result of a very lean-burn. When correctly tuned, your motor runs somewhere around 14.7 parts of oxygen to 1 part of gasoline (by mass not volume). If an engine burns with less oxygen present it is said to run rich. Conversely, if the engine runs with more oxygen present it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with slightly richer mixtures on acceleration or warm-up, and slightly leaner mixtures while decelerating. Your engine will tolerate lean burning for a little bit, but not for sustained periods of time.

Reducing the backpressure lets your engine flow higher volumes of air during each intake and exhaust stroke. Earlier cars, truck, and motorcycles with carburetion could not automatically adjust to the increased airflow and ran lean. Without carburetor adjustments to deliver more fuel, the result was possible burnt valves. Today’s engine computers use sensors to actually measure the airflow and automatically increase/decrease the fuel injection to maintain the correct air to fuel mixture.

 

ok...maybe I am dumb, or confused, or as usual both. I am not comprehending this concept of no backpressure. I get reducing backpressure...but I dont get how the engine can have compression without some backpressure (which I suggest we call BP because its a long word to type). If the engine does not have any thing to push against then how does it all work? If I am too lost to recover just tell me never to return to this thread again...and I will comply

 

Backpressure occurs in the exhaust system and is unrelated to engine compression. Combustion chamber design, valves, and cam duration, lift, and timing control engine compression. The basic steps are as follows:

1. The intake valve opens and the piston travels downward creating a vacuum to pull in fresh air/fuel.

2. Intake valve closes and the piston begins it’s upward compression stroke squeezing the full volume of fuel/air mixture into the combustion chamber.

3. Just before top-dead-center the sparkplug ignites the mixture and you have an explosion which forces the piston back down.

4. The exhaust valve opens and the piston begins it’s upward travel as the spent gasses dump into the exhaust system

 

Sigma provided a good brief summary of the four strokes of a four stroke internal combustion engine: Intake, Compression, Power, Exhaust.
The whole issue of BP comes into play (mainly) when speaking about the exhaust stroke. For it is here that the supersonic pulses, traveling in the exhaust gases, can (and will) affect the expulsion of spent gases. These pulses of high and low energy can either help suck the gases out of the cylinder or help to keep them in.
There is also consideration to be given to that brief period of time where both intake and exhaust valves are open (valve overlap) The more valve overlap, the more impact exhaust tuning and intake tuning can affect efficiency.
It is somewhat easier to think about the impact exhaust pulses have when thinking about a two stroke engine. Lacking valves (not all do) these engines rely heavily on the tuning of the exhaust for their operation. Two stroke exhaust "expansion chambers" exist to create energy pulses at various fixed rpm ranges (fixed, but variable based on size and location of expansion chamber). They function by allowing the exhaust gases to expand. This send a pulse back upstream. (nozzle effect).
This is what accounts for the phenomenon known as getting "on the pipe". you hit that rpm range where the engine works very efficiently and WOW all of a sudden a nice boost in power. It is also noticeable on 4 stroke (in particular in-line 4) bikes. They too have an rpm range where breathing becomes "more efficient". When breathing becomes more efficient, power increases.
Make any sense ????