backpressure

[LITTLE FOOT]

http://www.youtube.com/watch?v=JODoi21BI9c&feature=channel_page
ff to 4:00

 

[rurouni20xx]

thats a decent explanation, however, he didnt take into consideration that most 4 stroke vehicles have what is known as valve overlap (the time that both valves are open, exhaust valve is closing while the intake valve is opening), which backpressure also helps keep the charge where it belongs as well. a no overlap engine can run w/ little to no backpressure fine, just difficult to idle since it lopes a lot. an overlap engine is easier to idle, less pulsation going on but requires some amount of backpressure to keep the charge where it belongs. much like tuning the expansion chamber on a 2 stroke engine by changing where the backpressure is made and when it affects the engine most (tuning for low end, midrange or pure hp) you can do the same on a 4 stroke by changing the pipe size, length of muffler, type of muffler and how restrictive the system overall is. even moving the muffler closer or further from the manifold changes the engines performance and pressures. motorcycles and atvs for example use fart can style mufflers w/ dif internals that change when and how much exhaust gas flows from it at dif rpms. things such as inner pipe diameter, outer pipe diameter (packing percentage) how tight the packing is the depth of strait pipe on the end (torque output length as its sometimes called) and the length of muffler are all tuned for performance. when the engine is retuned to match the muffler, the results are beautiful. cars can be done in a similar manner, just larger scale and w/ dif muffler types. even the size and length and placement of the cat can be used to help tune the vehicle, almost all newer model motorcycles have cats in the headers and their pipes are tuned around the fact they have cats in them as well. the 2007 r6 alone has 3 cats in the headers, removal of any of them will cause the engine to not have enuff pressure to run at all just for use as an example.

 

[shadetree]

The exhaust system in an engine works in conjunction with the intake system.It is not stand alone.

Backpressure is caused by a restriction in the exhaust system. You want as little backpressure as possible, but you can't get rid of it all.

Camshaft overlap is when both valves, intake and exhaust, are open at the same time. The purpose of this is twofold:
1. To clean the combustion chamber of any unburned gasses. You do not want residual exhaust gasses mixing with the intake charge. This decreases engine power and efficiency.

2. Airflow thru the engine is traveling at sub sonic speeds. During valve overlap this high speed airflow creates a "ram effect" when the exhaust valve closes. The sudden closing of the exhaust valve causes the airflow to ram additional charge into the combustion chamber, a mini-supercharging effect. This is one reason for the long intake runners on todays engines, to increase the ram effect.

After combustion, the exhaust valve opens allowing the burned gasses to exit the combustion chamber. As the gasses leave and travel down the pipe, a low pressure area developes behind the gasses pulling more of the burned gasses out of the combustion chamber. This is called "scavenging."

In the case of your truck, with the exhaust restrictions in place, the carb was tuned for the system. When you opened up the system to more airflow, the carb leaned out. The best thing to have done was to re-jetted the carb for more fuel for the additional airflow, not put a restriction back in the system. This would have allowed the engine to perform at its peak power potential.

Backpressure doesn't "block" anything, it simply dillutes the incoming intake charge making it less efficient, and increases pumping losses. It especially effects carb systems with fixed fuel jetting, but with an EFI system the O2 sensors monitor the amount of oxygen in the exhaust and adjust the fuel trim accordingly so changes in the exhaust do not effect the incoming charge in the same way.

Backpressure is not good, the less of it you have, the better. As wicked says, using your logic, put a potato in the exhaust pipe and you will have a power monster.

I'm not going to mention "reversion," pumping losses, etc., just sticking with basics.

www.veryuseful.com/mustang/tech/engine/

shady

 

[rurouni20xx]

your correct, but at the same time some is required to help you low end torque. running a strait pipe allows you to blow every bit of the exhaust out, esp at high rpms, i agree w/ this completely. the problem w/ this is your trk when taking off will require more throttle, and midrange suffers a hair, so your going to use a lot more throttle. a fine tuned balance must be found, less backpressure is great for high end performance, adding some your top end will suffer slightly, but help you get off the line better. in carb units increasing jet size is a must, and fuel injection compensates a good bit for the most part, but not completely, and retuning is def a plus. i dont have access to tuning facilities etc so i try to get the most out the tune available, even if it means i cant tune it to run the best w/ the best combination of exhaust. i do carbs and efi on bikes and atvs all the time i can do those w/ little effort, the cars i dont have a dyno nor a computer to analyze everything and tune it out, so ive learned to modify other aspects of the unit to match the desirable outcome. my previous exhaust experience w/ the 2.3 was to correct the potato in tailpipe, and i know what you mean by too much is not desirable. i minimize it, but retain some for engine balance characteristics.

 

[shadetree]

What does "engine balance characteristics" mean????????? I have never heard of using backpressure to tune a multi-cylinder automobile engine, never. You want as little backpressure as possible, but you can never get rid of all of it. But then maybe I can learn something. Also, you evidently did not read the link I posted.shady

 

[rurouni20xx]

pardon my poor terminology passed w/ a c average in english class and no i didnt get to read the link didnt have much time i will later however. by engine balance characteristics i mean balance of power thruout the powerband. no backpressure you have high horse lil torque, some backpressure you have smooth torque increased midrange w/ little sacrifice to hp (what i aim for), more pressure you have increased torque, equal to less midrange and major hp loss, excess pressure you suffer all across the board and poor driveability. im sure your aware they dont just put a muffler together and market it, there is time on the flowbench and dyno to tune each aspect out, more flow at certain rpms less flow others, this is true w/ tuned fart cans (not universal jobs) and chambered mufflers. glasspacks and universal fart cans are something you play w/ to get right but most of the time installed for noise. a long glasspack can help but you have to find the right combo of diameter and packing vs diameter of the pack plus the length.

 

[shadetree]

You are confusing backpressure with gas flow velocity.

Backpressure is where a restriction in the system causes the exhaust gasses to back up into the combustion chamber, and have to be pushed out by the piston on the exhaust stroke. This is a pumping loss, and have been measured to be quite high, as much as 8hp in some cases.

Basically, the gas flow velocity is what moves the power band up and down. High velocity thru small piping at low rpm give good low end torque, but hampers top end power. Larger piping slows the velocity lowering low end torque, but gains at the top end.

In effect, velocity simply moves the power band from a lower to higher rpm depending on how the system is tuned. shady

 

[rurouni20xx]

i stand corrected then, it was always explained to me as backpressure or pressure in the system vs flow. restriction to flow is pressure, backpressure was the term used in exhaust classes and many books ive read over time. your term makes sense and i agree, but at the same time i believe backpressure and gas flow velocity go hand in hand.

 

[Funnyman141]

The Battle Over Backpressure

Stay tuned for more action!!