Compression and Backpressure - is there a direct relationship?

[bake315]

OK so this is a classic "noob" question.... do you have to account for exhaust back pressure requirements when going for a certain compression ratio (we'll use 10:1 as an example on a 350)?  Does it even matter?

[zieg85]

Great question, I'd like to know as well...  I have heard if too big you loose torque... I bet bd can tell us...

[Stewart G Griffin]

Coming from a strictly novice background when it comes to engines, based on my limited knowledge and intuition, i'm thinking it's not so much backpressure, but exhaust velocity that matters;   If you go too big tube-wise then you might lose velocity.  We want the exhaust to be drawn out as quickly and efficiently as possible.   It's sort of the same principle if you look at certain intake manifolds that have long runners---like a tunnel ram.   It's not so much we are trying to increase volume, but more so flow velocity?

Am i on track?

So as far as the relationship between compression and "backpressure,"  i would say the more compression you have, the more the effects of decreased exhaust velocity are magnified.

[Captkaos]

Back pressure is a restriction in the exhaust system, hard turns, kinks and bends in the exhaust all add to back pressure.  Back pressure kills power.  This is the inability to move gas unrestricted out of the combustion chamber.  Reversion is the extreme of back pressure, where back pressure is so high that the exhaust cannot get out and it fills the intake chamber.

This should not be confused with Exhaust Velocity where say headers are sized accordingly to promote efficient flow of the gas out of the combustion chamber.

[bd]

Here's a low tech, empirical (qualitative) approach, although an engineer might argue differently based on quantitative data:

As stated by others, exhaust backpressure is effective resistance to rearward (normal) exhaust flow and restricts an engine's ability to breathe.  The greater the backpressure, the lesser the exhaust flow, the lesser the airflow through the engine, and the lesser the torque development. 

Compression ratio is a fixed arithmetical relationship expressed as: cylinder volume with the piston at BDC, divided by cylinder volume with the piston at TDC.  Compression ratio is directly proportional to (not the same as) cylinder pressure development and relates directly to torque.  The greater the compression pressure (short of detonation), the greater the torque development.

Pragmatically, for any engine with which we are working on this forum, typical factory backpressure parameters have negligible direct impact on choosing "streetable" compression ratios in mildly built engines.  Although, decreasing backpressure by substituting headers, less restrictive mufflers, cats, and mandrel formed tubing is generally beneficial to power production and power adders, like cams, manifolds, heads, air valves, and boosting compression, allowing the engine to breathe more efficiently, as long as exhaust energy (temperature and velocity) is maintained; hence, using a modest exhaust tubing diameter.

Exhaust backpressure is created by an engineered (intended), or consequential (unintended), restrictive "effect" that can impede airflow through an engine.  Restricting airflow at any given RPM, decreases torque, so "excessive" backpressure will diminish power output.  Yet, in some cases backpressure, properly tailored and applied to a specific powerplant and drivetrain, can benefit performance.  However, determining the correct amount of backpressure seems to beg a trial and error method of discovery.  There is no universal formula that applies to all power systems.  Nevertheless, if backpressure is properly tuned to valve timing, a reverted exhaust pulse moving up-pipe will reach the exhaust port of the cylinder head at the optimum moment to reflect back down-pipe and assist exhaust gases exiting the combustion chamber - essentially drafting spent gases from the chamber much as a bicyclist drafts behind a vehicle.  This process is referred to as scavenging and is beneficial to power production, because it purges the combustion chamber of spent gases and promotes greater filling of the chamber on the subsequent intake cycle.  Therefore, for any given engine, a unique amount of backpressure is desirable for optimal scavenging across the RPM range of maximum torque.

*** As a sidebar, the newer breed of smaller displacement engines producing heretofore unattainable power and fuel mileage on pump gas are evidence of engineering subtleties and finesse that employ concepts, such as backpressure, to real world mechanical advantage. ***

“Exhaust reversion” occurs when excessive backpressure is so poorly timed that it pressurizes the exhaust port at valve opening and prevents adequate purging of the combustion chamber, or worse, forces spent gases back into the intake port, contaminating subsequent cylinder charging.

There are numerous variables that affect backpressure - e.g., an actual physical restriction in the exhaust; reverting pressure waves reflected off of both (a) solid surfaces and edges within the exhaust conduit, as well as (b) dynamic pressure boundaries resulting from changes in conduit cross section and changes in gas density in the exhaust stream; the length of individual head pipes before joining; overall length of the exhaust conduit; volume and placement of cats and mufflers; energy content of exhaust gases; turbulence; etc.  The cognitive tuner is compelled to develop a balance between engine use, build parameters, conduit diameter, conduit length, conduit bends, and the size, shape and placement of acoustic traps, such as mufflers and cats.  See the two linked discussions for a few interesting additional details...
http://www.veryuseful.com/mustang/tech/engine/exhaustScavenging.pdf

[bake315]

Wow, great information!  All of this really makes it easier to understand the forces at work, and help when planning a build.  Thanks a lot!

[bd]

There's nothing friendly about swimming in a sea of questions....  But, it's always Great Fun planning a new build!!   

[1980c10]

I'm going to simplify the formula even more. If your engine can breathe in more air and push it through the engine, then it needs to be able to push it out at the other end.  Add proper timing/ignition, compression and sufficient fuel delivery you have an efficient and powerful engine.
free-er flowing exhaust will only imrpove how the engine runs if it is the most limiting factor in this equation.

[thirsty]

If your engine can breathe in more air and push it through the engine, then it needs to be able to push it out at the other end.  Add proper timing/ignition, compression and sufficient fuel delivery you have an efficient and powerful engine.

You just got a lesson in "performance mods 101"

Right on the money 1980c10.

[bd]

If your engine can breathe in more air and push it through the engine, then it needs to be able to push it out at the other end.  Add proper timing/ignition, compression and sufficient fuel delivery you have an efficient and powerful engine.

You just got a lesson in "performance mods 101"

Right on the money 1980c10.

I agree.  There's no better way to sum it up!