Back Pressure

[JamesHigdon]

For years I heard you need back pressure to help build low-rpm torque, now I am hearing any back pressure is detrimental to an engine...I have heard back pressure helps a turbo spool up faster and inversely that no back pressure is best for a turbo application. I don't have access to a loaded dyno or I would test this myself but I wanted to get some facts and opinions from the folks on here. This seems like such a simple question but the internet lends itself to opinions dressed like facts...so whose got some?

[jwmbishop]

back pressure is really not a good term in a four stroke application. In a two stroke application back pressure is created in the exhaust chamber to help force the intake charge through the transfer port. Two strokes respond to pressure in the exhaust more then velocity.

 

If you have backflowing PRESSURE anywhere in the exhaust in a fourstroke you have problems as the parasitic losses of charge velocity diminish the cylinder pressure at burn real fast - and the exhaust from one cyl will actually back purge and smother the fire in the next cyl in valve order.

 

REVERSION velocity is what when created or negated in a four stoke application effects the torque curve. Its been known for years that larger pipes = less reversion velocity = torque curve higher in the RPM range. The whole thing you are playing with there is the boundary layer - the turbulent area against the wall where the flow "shears" and the vortex effect causes the gasses (and heat) to go the other way. Smaller pipe and higher velocity both or either will increase the thickness of the boundary layer - the thicker the boundary the higher its velocity and momentum. The thicker the boundary layer the more gasses and heat going the wrong way. The gasses coming back with the reversion are inert. They do NOTHING for the new charges burn - HOWEVER the heat they carry can be beneficial by preheating the charge before the burn - as the burn is then started on a larger volume - when the burn increases volume it is proportionate to the higher starting point. Gains are made until the inert gas overcomes the burn. Conversely less reversion helps scavenge - the exhaust flow momentum pulls the intake charge during valve overlap increasing its momentum (which allows momentum fill after BDC to increase). So its a fine line you are on when tuning exhaust! Percentage of horsepower wise - for most applications I am not real convinced its worth the trial and error. You will always give something up to make any gain. Moving the torque curve 200-300 RPM may change the PEAK - but rarely changes the area under the curve line. And THAT is what is important in most applications.

 

In a boosted application I would think the exhaust should be tuned to maximize spool up. Common sense - the more velocity (and thus momentum) the exhaust carries from the valve to the turbo - the more weight the gasses throw against the turbo vanes = faster spool up. Smaller header pipe and larger turbo pipe would do this. No real tuning experience with turbo tuning - the only dyno work I have done with them was just breakin - but I would think the tuning between the valve and turbo would be extremely sensitive and the "too small" break point (sudden drop in total torque output at target RPM) so fine that its a hard target to hit! And only for a 200-300 change in curve rpm... from the turbo out I would not expect more than a 1-2% change with each 1/32 pipe diameter.

[KISS_Racing]

Sorry ya'll... I am going to remain quiet and in the background on this one. There are theories and proven fact on this. We are in the process of making some big changes on a car we sponsor and once I have some data to back what we have proven I will share on an individual basis.

[jwmbishop]

Damn Mark - I was looking forward to your input!

[KISS_Racing]

Damn Mark - I was looking forward to your input!

 

[KISS_Racing]

I will say this... When we went from the stock exhaust on my '02 F-350 to the 4" Turbo back exhaust I gained Hp & Torque but lost back pressure. I set an EBPV Code at around 27lbs boost. I also noticed I lost about 3 MPH on the top if I gently roll into it. If I can do it and it not start to de-fuel I can still get around 128MPH out of it.

 

So, does back pressure effect Hp/Torque? Hummmmmmmm!

[jwmbishop]

That makes sense (I would expect the same result) and I will ponder for awhile why (and how to express why) against some of the theories and physics at play (you did change more than just backpressure - unlike the old potato in the pipe trick)!

 

The first one I am pondering is the simple reality that if you take two pipes of the same diameter - one straight and one with a 90 degree bend - the straight one will flow more total air one way and have less reversion (back pressure) than the curved one...

 

The second being (especially shifting into oil burn theories) is the scavenge - too little reversion can cause overscavenging - actually pulling unburnt fuel into the exhaust by over sipping the intake during overlap (flow is always from high pressure to low - so during overlap when the exiting charge's momentum is still pulling on the cyl - the low pressure BEHIND the wave or doppler depression if you will - the pressure on the outside of the exhaust port will be lower than the cyl as the piston is still dwelling at top). Not good on any engine (speaking power per lb) but BAD on a turbo (really drives the exh temp up which will lead to defuel (or worse) - which of course is contrary to primary goal of making power!) This theory points out the OBVIOUS - the exhaust flow and reversion tune MUST match the cam!

[KISS_Racing]

So, Can and should a short track car/truck have tuned exhaust? Hummmmmm!

[jwmbishop]

Can? Yes any exhaust can be tuned to be more beneficial in the overall give\take within your specific application. No different than softening your exit throttling by changing valve lash! You can give up a bit at the end of the straight but the softer application of torque gets you forward motion with less loose! You gain more there than you give up at the end.

 

Should? weeeelllllll that's one of them dollars and sense questions. I strongly believe that in the 1/2 miles and under the gains made by the $$$ in the exhaust are not NEARLY as beneficial as the gains made in the $$$ on suspension and a GOOD solid base engine (even though the whole crate concept drives me nuts). Depends on if you're chasing 10ths, 100ths or 1000ths of a second!

 

Even above 1/2 I question the value of the whole tuned exhaust thing - what speed (and thus pressures) do you tune at? On a dyno the headers are stationary - any thing you see there changes when the pipe set is moving 100+ and the pressures ahead, behind and beside have changed. A combo dyno/wind tunnel would be cool!

 

On a high end drag car exh tuning is invaluable. Many people see the top fuelers spend thousands on getting the perfect match to the cam and induction so believe its the holy grail of finding horsepower. The engine turns so few times on a top fuel that every pop counts at 100% effort! Do the math - how many times does each plug fire per 1/4 mile run (avg RPM 3000) and how many on a half mile lap? Even using averages it comes to 3000 RPM (50 RPSec) for 5 seconds = 250 rotations is 125 fires - 15 per plug so each fire is 1/15th of that cyls total contribution and each cyl is 1/125 of the total effort. 4500RPM avg on a half for 16 seconds a lap is 600+ fires per plug lap - spread that out over 30 laps and well each plug fires contribution to whole is significantly smaller. Drop the HP by a percent and you drop each fires contribution the same. When you are trying to squeeze one more atom into the fire are you really getting much reward for the effort? With higher RPM and over longer time - the value drops (rule of seventy two backwards).

 

For a qualifying lap any and all tuning is more valuable than for racing laps. But once another driver is on the track at the same time - handling and the ability to work traffic is FAR more valuable than Raw HP. the money just seems better spent elsewhere (driving lessons in some cases). And personally with the costs of fuels - if I were to play with exhaust tuning on a oval car - I would tune for economy - that would return some of the cost faster win or loose... to me Racing (and thus the spending and tuning decisions involved) is not about converting fuel to power - its about using fuel to convert dollars and personal efforts to checkered flags. That's a pretty wide field of vision - sometimes things lock us into blinders - a team that learns to avoid those the blinders (or at worse peek over or around em from time to time) are the one that succeed..