Exhaust Backpressure and engine knock

Hello Guys,

What do you guys think about excessive exhaust back pressure (caused by restrictive exhaust) leading to engine knock?

I have only experienced power and VE loss from exhaust backpressure, but not to the point of knock. As I believe Knock can only be induced due to excessive heat and the auto-ignition of part of the combustion gases ahead of the flame front.

Now, the only logical way (to my knowledge at least) engine knock happens due to excessive back pressure is if lets say, part of those combustion gases stay in the cylinder and dilute the next batch of intake mixture (which would in a way resemble a hot EGR system). And these gases overheat the mixture to the point of auto-ignition after the spark event.

i would like to know if excessive exhaust back pressures actually have the damaging potential on an engine.

As always i tend and it seems that i like to overcomplicate matters, and would love for you experts to share your thoughts :)

Hi. First of all you need to understand what knock and pre-ignition are different about- link to the educational video is attached.

The answer to your question - exhaust back pressure overheats combustion chamber and helps to create little hot spots - both factors in their turn help knock occurrence (if octane rating of fuel in use isn't high enough) which in its turn can overheat combustion chamber even more,creating big hot spots(as glowing spark plug ) which can lead to pre-ignition.

Video of hot exhaust gases creating little hot spots provoking knock is here

https://youtube.com/shorts/urH80xyDXUg?si=A7uSu5Jjtdv65fyh

Knock and pre-ignition video is here.

https://youtu.be/HGWKdfSF6zg?si=K9CA_ZzunbOyyj7f

Thanks for the great videos of hot exhaust gases and knock/pre-ignition. However the first video does not take into consideration the boundary layer during normal combustion, which actually protects against this sort of direct heat transfer.

Exhaust back pressure will surely lead to a loss of the engine's charge-moving efficiency, but how does it overheat the combustion chamber?

1) excessive back pressure is usually caused by restrictions in exhaust system starting from exhaust valve seat. That results in slightly more time exhaust gases evacuating from combustion chamber allowing more heat to be transferred to combustion chamber surface. The same principle works with hot iron- if you'll touch it very fast you won't get skin burn but if you will not be quick enough your skin will get more heat from iron and you'll get skin burn.

2) Ideal gas law tells us the more pressure we get in the same volume the higher temperature will be. So higher back pressure results in higher temperature inside of combustion chamber by default.

3) first video was provided just to get an idea of how hot spots would be created. Even little depositions of burnt hydrocarbones under combustion chamber pressure would be becoming little hot spots helping knock to start.

Cool links, Shota!

I'd rephrase 2/ above - the higher back pressure will reduce the temperature drop from expnsion into a lower pressure state, but same affect.

Gord, thanks a lot...

I forgot to mention one other thing about little hot spots in combustion chamber. When I do DIY head porting I polish combustion chamber metal surface. That along helps to reduce knock threshold and increase ignition timing by up to 3-4 degrees... The reason being is that flat surface has almost no potential hot spot point and on top of that polished surface reflects heat at higher rate.

Thats very insightful, thanks Shota and Gord!

The hot iron analogy and the reduced temperature drop are amazing.

So basically, as engine backpressure increases, some of the hot combustion gases will stay inside the cylinder and transfer heat there. And if soot deposits, hot spots will form and knock will happen.

As I understand, the time for exhaust gas to evacuate the chamber is fixed (for non-VVT and at a set engine speed). Now it's up to the engine to actually push all these gases out in the available window. And if the exhaust back pressure is high, it won't be able to do so.

Appreciate you experts chiming in on this matter.