Back Pressure, Good or Bad???

I know my car has a lot of back pressure cuz I can hear it in the burble every time my foot's off the gas but I can't say whether or not it's good. Sorry.

There are a lot of factors but it depends on your motor setup. A little bit of back pressue is needed though.

So would it be a bad idea to put an O/R x on a pretty stock 98' GT? I'm debating on the Bassani o/r X or the Magnaflow catted X.

no that would be fine. No back pressure would be like running Open headers.

I'm glad this topic was brought up. I have a nac with people who say back pressure is a good thing. Back pressure is NOT a good thing. Back pressure does not help produce power. I believe people have back pressure confused with something called Delta Pressure. I'll explain:

When the engine is working to draw in air (intake), it must overcome the forces that resist air movement. These forces include things like gravity, air density, internal motor friction, resistance caused by the length and diameter of the tubing used, and the resistance caused by any medium that the air is drawn through such as filters or chemical sorbents. The sum total of all these forces is called backpressure, and it is a measure of how hard the engine has to work. Any time the engine is working, it is always working against some level of backpressure. So, technically, backpressure is the resistance of air flow.

Now, as car enthusiasts, we see that backpressure is resistance and resistance is BAD for performance right? Well, this is where Delta Pressure comes in. Delta pressure describes a pressure drop through a component or a difference in pressure. Delata Pressure is what is needed to basically get exhaust gases out of the cylinder and moving through the exhaust tract for optimum performance. Now, for all the engineers out there, you understand that as a volume of air travels, it's pressure is directly related to it's traveling speed. Therefor, the faster the gases are moving, the more velocity or pressure it has. So, the higher the delta pressure (difference in pressure within the engine, the faster the gases will move through the exhaust tract.

Exhaust gases have to travel a certain speed and contain a certain velocity for optimum power production at any given RPM point. A certain delta pressure must be achieved to get the burnt air/fuel (exhaust gases) out of the cylinder on the exhaust stroke. By not having this certain delta P affects valve overlap (burnt air/fuel is sucked back into the cylinder on the intake stroke). Now, it is virtually impossible to achieve this certain speed for each and every exact RPM point, so we have to do some sacrificing in order to achieve the best power production. So, lets discuss this regarding exhaust mods. A straight through exhaust (no cats, no muffs) enables exhaust gases to move much quicker than if there were a restrictive exhaust. Now, as I mentioned earlier, each RPM point requires a certain amount of gas velocity for optimum power production.

So by altering the flow in an exhaust tract, you therefor basically affect your powerband, not overall levels of power produced. A free-flowing exhaust will inhibit better upper RPM power than lower-end. But being too restrictive (stock setup) will just increase backpressure (resistance to air flow) and hurt performance.

Now, this is a pretty complex topic and would take quite a while explaining it all, but I hope i covered the "jist" of things. To summarize: BACKPRESSURE IS BAD. DELTA PRESSURE IS GOOD. PEOPLE CONFUSE BACKPRESSURE WITH DELTA PRESSURE.

Good explanation. I now have a better understanding. So what causes the burble sound when your foot's off the gas pedal & your exhaust gases are flowing from rear to front of the car? Or is that not what happens when you hear the burble?

Thanks for explaining that! It never made sense to me why people said that you need back pressure. It all makes sense now...

downsouthman1 said:

Good explanation. I now have a better understanding. So what causes the burble sound when your foot's off the gas pedal & your exhaust gases are flowing from rear to front of the car? Or is that not what happens when you hear the burble?

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that is because of the design of your mufflers. Mine doesnt do that.

Volldamort......

You da man!! Thank you for that very comprehensive explanation. It has especially caught my attention due to the fact I felt that I lost performance when I added my Bassani Aft-cat system. I lost that “pull” that I normally felt. So perhaps what I was feeling was actually a more smooth power band?!?! With your explanation of “Back Pressure” I’m no longer hesitant to add a mid pipe.

Thanks

I have some great links to exhaust if anybody wants them. It discusses the finer techinical points of header selection and exhaust theory in general. I think most people would be surprised at the idea that maximum hp comes from the least restrictive exhaust.


Jon

Well whatever its called, you need a lil something to restrict exhaust flow on a basically stock 4.6 2V. I ran FRPP Headers, O/R X, and Magnaflows (a straight through style muffler) and it actually slowed me down.

I put some 1-chamber turndowns on it, and picked up some noticeable power. Not sure if it came from Back Pressure, or Delta Pressure, or whatever..........but it helped

-Will

TARZAN said:

Well whatever its called, you need a lil something to restrict exhaust flow on a basically stock 4.6 2V. I ran FRPP Headers, O/R X, and Magnaflows (a straight through style muffler) and it actually slowed me down.

I put some 1-chamber turndowns on it, and picked up some noticeable power. Not sure if it came from Back Pressure, or Delta Pressure, or whatever..........but it helped

-Will

Click to expand...

Well, I just covored the concept of back pressure and delta pressure. Like i said in my earlier post, this is a very complex system. You also have to include things like exhaust pulses, scavenging, reversion, pressure waves, and cam overlap. There are many things associated with the exhaust and how it affects performance.

Magnaflow guys says good post-

Dyno Testing has shown us, with straight through mufflers that you do not even need to look at 3" piping until you achieve around 550rwhp in case any one was wondering...becuase of lack of delta pressure - or lack of exhaust velocity.....

Seat of the pants feeling is good, but it doesnt always mean that much.....

The only way to really see what is going on, is to put your car on a dyno.

I dont know how many times I see- chambered mufflers low end, straight through high end

X pipe high end- H pipe low end- but in reality on a dyno this is what is going on-

Both will make very equal amounts of torque, but a straight through muffler or X pipe will often make much more power in the top end, or move the power band towards the top of the RPM band and will make the power band much smoother as well- changing the feeling of the car-

You are not making less power at 2000RPM, you are just making that much more at 6000RPM giving it the feeling that it lost low RPM punch.

Noise also plays a factor as many times a noiser car will make you feel like you are going faster.....its true.

I can give people rides in my 90 GT which is quiet, which is a mid 13 second car, and people will think it runs high 14s, then I give people rides in my high 14 second GT350, but it makes a ton of noise, and people always guess it runs 13s.....lol

Straight though mufflers provide higher velocity, as long as you don't go too large with the piping which will allow gases to overeaxpand slowing down the flow of the exhaust......

Its all about achieving the highest possible speed with out overrestricing with backpressure......basically achieving the best balance possible between delta pressure and back pressure.....in order to achieve the most hp and tq under the RPM curve.....


I think I just repeated a lot of the stuff all ready said, but I put it into words that I understand- and added some personal experiences......


Cliff Notes- You need to dyno your car to really know whats going on.

Maybe Volldmort can correct me if I have any of the concepts wrong, but Im definitely not an engineer, I just try to remember everything the engineers teach me....lol

GNN60GT500 said:

Magnaflow guys says good post-

Dyno Testing has shown us, with straight through mufflers that you do not even need to look at 3" piping until you achieve around 550rwhp in case any one was wondering...becuase of lack of delta pressure - or lack of exhaust velocity.....

Seat of the pants feeling is good, but it doesnt always mean that much.....

The only way to really see what is going on, is to put your car on a dyno.

I dont know how many times I see- chambered mufflers low end, straight through high end

X pipe high end- H pipe low end- but in reality on a dyno this is what is going on-

Both will make very equal amounts of torque, but a straight through muffler or X pipe will often make much more power in the top end, or move the power band towards the top of the RPM band and will make the power band much smoother as well- changing the feeling of the car-

You are not making less power at 2000RPM, you are just making that much more at 6000RPM giving it the feeling that it lost low RPM punch.

Noise also plays a factor as many times a noiser car will make you feel like you are going faster.....its true.

I can give people rides in my 90 GT which is quiet, which is a mid 13 second car, and people will think it runs high 14s, then I give people rides in my high 14 second GT350, but it makes a ton of noise, and people always guess it runs 13s.....lol

Straight though mufflers provide higher velocity, as long as you don't go too large with the piping which will allow gases to overeaxpand slowing down the flow of the exhaust......

Its all about achieving the highest possible speed with out overrestricing with backpressure......basically achieving the best balance possible between delta pressure and back pressure.....in order to achieve the most hp and tq under the RPM curve.....


I think I just repeated a lot of the stuff all ready said, but I put it into words that I understand- and added some personal experiences......


Cliff Notes- You need to dyno your car to really know whats going on.

Maybe Volldmort can correct me if I have any of the concepts wrong, but Im definitely not an engineer, I just try to remember everything the engineers teach me....lol

Click to expand...

Well put.....

Now hook a guy up with a good place/person on a catted X!!

GNN60GT500 said:

Magnaflow guys says good post-

Dyno Testing has shown us, with straight through mufflers that you do not even need to look at 3" piping until you achieve around 550rwhp in case any one was wondering...becuase of lack of delta pressure - or lack of exhaust velocity.....

Seat of the pants feeling is good, but it doesnt always mean that much.....

The only way to really see what is going on, is to put your car on a dyno.

I dont know how many times I see- chambered mufflers low end, straight through high end

X pipe high end- H pipe low end- but in reality on a dyno this is what is going on-

Both will make very equal amounts of torque, but a straight through muffler or X pipe will often make much more power in the top end, or move the power band towards the top of the RPM band and will make the power band much smoother as well- changing the feeling of the car-

You are not making less power at 2000RPM, you are just making that much more at 6000RPM giving it the feeling that it lost low RPM punch.

Noise also plays a factor as many times a noiser car will make you feel like you are going faster.....its true.

I can give people rides in my 90 GT which is quiet, which is a mid 13 second car, and people will think it runs high 14s, then I give people rides in my high 14 second GT350, but it makes a ton of noise, and people always guess it runs 13s.....lol

Straight though mufflers provide higher velocity, as long as you don't go too large with the piping which will allow gases to overeaxpand slowing down the flow of the exhaust......

Its all about achieving the highest possible speed with out overrestricing with backpressure......basically achieving the best balance possible between delta pressure and back pressure.....in order to achieve the most hp and tq under the RPM curve.....


I think I just repeated a lot of the stuff all ready said, but I put it into words that I understand- and added some personal experiences......

Click to expand...

Yes, you're on the right track