Backpressure

[Matthew]

Ok, so by friday ill have my headers, no cat converters, and my hks rear section...im not sure the effect backpressure has or even how it affects performance...but from what i understand i wont have much with those mods, so what exactly does that mean as far as launch times, 1/4 times, or just racing from the stop light

 

[twilightprotege]

the idea of back pressure is to keep "fresh" fuel and air in the cylinder and stop it from going out the exhaust before the valve closes. at low rpm back pressure is the key to keeping decent torque and emissions, at high rpm you want as little back pressure as possible because the gasses are moving so quickly, you want them to move even quicker.

so, doing what you are razorp5, you'll be loosing bottom end power and torque and fuel economy, but gaining high end power...and you want high end power for racing

hope this helps

later

 

[aftershock63]

I have no have no idea why everyone worries about back pressure. You don't want it you don't need it. Theats why you get exaust and headers so everything flow easyer and the engine doesn't have to work to force things out the motor. That is my train of thought, and I get that from a guy that does IHRA Pro Mod. Then again that is a full out drag car with ~2800bhp, could be different the street cars. We'll have to get perfect in here or jaun or spool to get their take. They'd probly know best.

 

[20ESGUY]

I have no have no idea why everyone worries about back pressure. You don't want it you don't need it.

that statement only applies to turbo cars, and even then they need a slight amount. twilights explination was good so im just gonna add a little...in a N/A car backpressure allows you to make the torque at lower rpm's as previously stated without it you loose the scavenging effect across the cylinders and you also lose some return pressure on the valves, kind of like a secondary form of compression. in a turbo car backpressure is not so good because it causes resistance on the turbine wheel which doesn't allow the turbo to spool up quickly. in full out race cars making power above 2-3K is where it's at, but on a street car if you move the power curve up that far it would almost be usless cause you'd have to floor it every time you wanted to move just about.

 

[flat_black]

I have no idea why everyone insists that 'you don't want backpressure'... Personally, I DO want backpressure so the car is more drivable, and so I don't have to wind the car up just to get it moving along. But hey, that's just me! I'd rather see better gains along the whole powerband than just at the peak.

 

[twilightprotege]

to add further back pressure for turbo's - the turbo creates all the back pressure the engine needs to survive well....it's after the turbo you want as little back pressure as you can because the turbo is already creating enough

 

[Matthew]

so youre saying im not going to be able to launch fast? i dont know about that...

 

[mypfizzle]

your launch will start off slow.. race from a rolling start -=) race from a dead stop will kill u off the line.. and say bye bye to your decent gas milage -=)

 

[Matthew]

now thats funny because someone said they had the same setup and got 5 more mpg...cant i just launch at a higher RPM?

 

[Matthew]

also, if i launch normally, the person may get the lead but as soon as i hit 3k or more rpm shouldnt i pull ahead?

 

[1FASTMP5]

you'll be fine! keep your resitnator and run you silencer if you want. or dont run the silencer. you will lose a little down low butt.. you will gain twice the amount up top. your car will pull harder to red line and not starve(sp) to breath up top.
are cars have enough tourqe down low that lossing a litte wont be that bad.

 

[Matthew]

werd im excited...

 

[1FASTMP5]

i launch @ 3-3500 rpm and my car spins alittle and doesnt bog at all.

 

[mypfizzle]

launch at a higher rpm you have to worry about traction.. you dont want to sit there and burn tires.. you will deff be faster on the high end.. say over 3 grand or 3500.. but below that.. lil slow.. some things u can do to fix this are
lightened flywheel (will make the car rev up faster)
diff ignition system (create more spark )

 

[Matthew]

Ok, so as you can see, backpressure is actually defined as the resistance to flow. So how can backpressure help power production at any RPM? IT CAN'T. I think the reason people began to think that pressure was in important thing to have at low RPM is because of the term delta pressure. Delta pressure is what you need to produce good power at any RPM, which means that you need to have a pressure DROP when measuring pressures from the cylinder to the exhaust tract (the term "pressure" is what I think continually confuses things). The larger the delta P measurement is, the higher this pressure drop becomes. And as earlier stated, you can understand that this pressure drop means the exhaust gas velocity is increasing as it travels from the cylinder to the exhaust system. Put simply, the higher the delta P value, the faster the exhaust gasses end up traveling. So what does all this mean? It means that it's important to have gas velocity reach a certain point in order to have good power production at any RPM (traditional engine techs sited 240 ft/sec as the magic number, but this is likely outdated by now).

 

[mypfizzle]

ok.. but if thats true.. why wouldnt everyone just do like a 4 inch exhaust?
i always thought u had to have some kind of back pressure.. like u had to find a happy medium

 

[Matthew]

yea he says that later in the article i believe, but i have stock size piping which is what - 2.5 or 2.25? so i should be pretty good to go i would hope...

 

[mypfizzle]

lightened flywheel.. the car will rev up faster.. eliminating the lag u may get.. smile.. all will be good -=)

 

[mrpopnfresh]

Ok, to make things simple think of a straw. Imagine your stock exhaust system as a little coffee stirer. You have to blow out really hard to get the air through, but you'll feel the air comming out real strong.

Now upgrade to a normal straw and you don't have to blow so hard, but you don't feel the same strenght comming out unless you start to blow harder.

The harder you blow, the higher your "rpms" are.

Now for a turbo, you'd want a boba straw. It takes a great deal of pressure to blow out of it, but that job is taken care of by the turbo. The size of the straw is allowing all that excess air to breeze on by w/o a problem.

If you tried to blow that same amount of pressure from the boba straw, into the coffee stir, you'd have too much pressure building up w/o any real gains.

I hope this helps a little in visualizing things for newbies.

 

[Matthew]

so am i at the happy medium? (can see this becoming a heated debate over the next few days)