Exhaust Scavenging Perfected

00:00 some people say that for an engine to make Power you need a little exhaust back pressure some people are wrong but what is exhaust back pressure in the first place well today I’m going to walk you through what causes back pressure how it negatively affects Performance

00:15 Engine life and fuel economy and where the myth that exhaust back pressure is needed actually comes from and if that’s not enough I’m going to show you how we made an exhaust tip that creates a negative back pressure I’m Eric this is entry level let’s get into it this is Banks entry level presented

00:32 by amoil the leader in synthetics exhaust back pressure is the force required to overcome the exhaust systems flow resistance or in other words the formation of pressure on the exhaust path from cylinder all the way to the tailpipe so basically any resistance to exhaust flow is considered

00:52 back pressure so the muffler the emissions equipment any bends in the tubing the tubing itself and even the air the exhaust has to p push out of the way to leave the tailpipe is a source of back pressure in many ways an engine is like an air pump it inhales air on the

01:07 intake stroke and pushes exhaust gases out on the exhaust stroke this process requires horsepower so where does that horsepower come from from this think of this crankshaft as your horsepower storage device power is put into the crankshaft on the power stroke and every

01:23 other stroke exhaust intake compression withdraw some power for an engine to work efficiently it needs to intake air and exhaust air there with as Little Resistance as possible or in other words use as little horsepower as possible from the crankshaft back pressure is

01:38 resistance the higher the back pressure the more horsepower is required to overcome it when the exhaust valve is open the Piston is pushing exhaust gases out but back pressure is resisting that Force so more horsepower gets eaten up pushing that exhaust gas out the higher

01:55 the back pressure the more exhaust will remain in the cylinder Gail calls the this poor man’s EGR or exhaust gas recirculation now when the next combustion cycle Begins part of the space in the cylinder is already taken up by hot inert exhaust gas this means there’s less space for fresh oxygen-rich

02:15 air and therefore even less power being generated on the power stroke not only that but hot exhaust gases in cylinder will increase combustion temperatures produce more harmful emissions and potentially damage your engine any component in the exhaust that would reduce or strict flow also creates back

02:36 pressure so a muffler or emissions components are obvious restrictions but changes in diameter or even bends and tubing create back pressure as well you might think air turning a corner in a pipe looks like this when actually because of exhaust gas momentum and the

02:53 compressibility of air it looks more like this you’re no longer using the full diameter of the pipe right after the bend you’ve just created back pressure but on this truck do you know what’s creating the most back pressure it’s the turbo on a turbocharge engine back

03:15 pressure is a much bigger Factor because the turbine is driven by exhaust gases the back pressure acting on the Piston is much higher than a naturally asped engine and this turbine Inlet pressure is commonly referred to as turbine Drive pressure so Drive pressure and back

03:32 pressure are not the same the pressure difference between the drive pressure at the inlet versus the back pressure at the outlet directly correlates to turbocharger speed this is called a pressure differential and in order to spin the turbo you need a pressure differential from the inlet to the

03:48 outlet and this is why you want to minimize back pressure here I have a cutway of an older turbo you can see the compressor wheel the turbine wheel and the common shaft that joins the two together now I’m going to use shop air to show you how exhaust gases make this system

04:11 work now obviously this isn’t the most efficient turbocharger since about a quarter of it is gone but you can see and hear just how fast that turbine wheel was spinning and that’s because of the pressure differential of the air acting on the turbine wheel against the ambient air now if the pressure

04:27 differential was larger the turbine wheel would spin faster and because they’re connected by that same common shaft so with the compressor wheel so back pressure after the turbine slows down turbine shaft speed and back pressure before the turbine may be good for spinning the turbine faster but it

04:43 pushes on the Piston requiring more horsepower to overcome when you think about that extra force needed you begin to realize superchargers aren’t the only forced induction system with parasitic loss back pressure changes at various points along the exhaust so you might

04:60 have 40 lb of Drive pressure at the turbine Inlet but only 4 lb of back pressure further up the exhaust now the back pressure we care about is primarily at the turbo inlet and outlet and at the cylinder but reducing back pressure anywhere in the system can translate to

05:16 back pressure reductions throughout the system this is our 5in monster exhaust for a late model Ram 67 Truck now when we develop exhausts we instrument the stock system and our system with pressure sensor at key points along the exhaust the goal here being to see what

05:34 back pressure reductions post emissions translates to up at the turbo or ultimately up at the engine frankly the results speak for themselves especially the drive pressure here you can see the back pressure readings of our 2020 Ram with the stock exhaust and with our 5-in

05:50 monster exhaust our exhaust is a DPF back system meaning that everything between the engine and the diesel particulate filter is still stock and yet yet the back pressure at the turbine Outlet drops from 9.2 PSI on the stock vehicle to 8.5 PSI with the banks monster exhaust we changed the piping

06:10 after the emissions equipment and yet it still affected the back pressure before the emissions equipment but does a reduction of 7/10 of a pound of back pressure really make a difference well in this truck it absolutely does pressure at the turbine Inlet is much higher than at the outlet over five

06:28 times higher at 4 9.5 PSI this is the pressure differential I mentioned earlier and on this stock truck it’s a little over 5 to1 we also call this an expansion ratio because pressure drops dramatically once the exhaust energy has transitioned to the turbine with the

06:46 lower turbine Outlet pressure on the banks equipped truck here’s how that affects turbine Inlet pressure 45 PSI because the expansion ratio stays nominally the same across the turbine that’s 4 .5 PSI less pressure pushing back on the Piston during the exhaust stroke meaning less horsepower wasted

07:06 pushing exhaust gas out of the cylinder with less back pressure you’re liberating horsepower so we’ve proven that exhaust back pressure is all bad all the time reducing back pressure directly reduces horsepower loss to parasitics and allows you to get more horsepower by burning

07:27 the same amount of fuel so where where does that rumor come from that you need a little bit of exhaust back pressure in order to make Power well the first place the idea popped up was in the 1950s straight through glass pack Mufflers installed by muffler shops were good for sound but

07:45 highly restrictive if anyone asked these installers about the negative impact of that restriction like true salesmen they’d say a little exhaust back pressure is important for your engine to run properly this type of Muffler is still used today today and enough people have heard that back pressure is

08:02 important that they paired it with confidence but there’s another reason people think exhaust back pressure is important and it’s probably because they’ve heard the term exhaust Scavenging exhaust Scavenging refers to an effect whereby exhaust gases flowing down one pipe affect exhaust gases

08:19 coming down another exhaust Scavenging is a result of the fact that exhaust gas comes out and pulses one cylinder worth of air enters the exhaust manifold every exhaust strope as the pulse of exhaust gas travels down the pipe it’s accompanied by a pressure wave pretty cool huh but here’s the

08:38 problem exhaust Scavenging like this doesn’t happen on most Vehicles many OEM exhaust manifolds are designed in what’s known as a log manifold they’re cheap and easy to make they’re a basic iron alloy casting that can be automated and pumped out by the thousands so in a

08:55 header Style exhaust manifold we have primary tubes that end in a single collector but in the log style design we have multiple points where the exhaust flows meet the log style have pulses that are constantly running into each other and creating fluctuations in pressure and

09:12 velocity and therefore no exhaust Scavenging exhaust Scavenging doesn’t only pertain to headers we actually have begun using it in our exhaust tips this latest patented design for the 5-in moner exhaust actually creates a partial vacuum literally sucking exhaust gas out

09:28 of the pipe let’s say you’re in the market for an aftermarket header on your 4 L Jeep if you stick with the stock style you’re going to get something like this that has no Scavenging there’s no Scavenging because the runners are connecting at different points along the

09:43 pipes now with our header you can see that each of the runners connect down to a single point and this allows Scavenging to happen because each pulse is pulling along the next one because exhaust pulses occur more frequently the higher in RPMs you get the exhaust

09:58 Scavenging effect often works best in a certain RPM range that range is determined by Runner length and Runner size and that is why people often think you need back pressure get this diameter wrong and you end up putting the optimal range outside of the Power Band of your

10:13 engine but with our headers you know that we tune them correctly and with the full Banks exhaust kit you can add over 20 horsepower to your Jeep 4 L how exhaust Scavenging ties back to exhaust back pressure is honestly a pretty complicated subject and I could do a

10:28 whole other video just on header design alone you’ve got short tube versus long tube diameter versus velocity equal length versus unequal length diving into that however might be a video best left for Gil so for now I’ll leave you with this back pressure is never your friend

10:44 parts that reduce back pressure reduce the work done by your piston on the exhaust stroke reduce parasitic loss and increase the lifespan of your engine and turbo and if you’re interested in a well-engineered exhaust system for your truck SUV or RV you can check out

10:57 everything we have to offer at Banks