Normally Aspirated Engines need a properly sized exhaust (diameter, length) in order to create a scavenging effect. This SE helps to draw in air from the intake side for the next air/fuel charge. Back Pressure is a side effect of this. With forced induction, a turbo or SC, scavenging is not needed, since the intake is pressurized. So an exhaust is not really needed and the back pressure is not needed since it only obstructs flow.
Of course, an exhaust overall manages noise, but I am not worried about that. Just thinking of performance.
(I think I remember reading in Corky Bell's book on Turbo -- "the best exhaust is no exhaust")
Normally Aspirated Engines need a properly sized exhaust (diameter, length) in order to create a scavenging effect. This SE helps to draw in air from the intake side for the next air/fuel charge.
The thought the "scavenging effect" was helping the exhaust gasses to be sucked away from the engine.
An exhaust conduit is necessary to help draw heat away from the exhaust port on the head. Over the last three months Hot Rod Magazine has had a continuing article on proper exhaust considerations and it gave some information on proper diameter and lengths but if you are seriously interested you can contact Hooker Headers in Bowling Gree, Ky. It can be a complex subject.
Isn't the intake side draw depend if you have valve overlap..? Otherwise the valves are closed/open at separate times?
When I did my exhaust, I tryied to take in consideration the bigger diameter will slow down the volocity, ruining the SE because of gas expansion. So with that in mind, a lot of "performance" pipes I've seen (for 2.8 -3.4) have 1-3/8"ID /1-1/2"OD.. but I've also heard that some setups aren't really showing much to the owner.. they wish there was more.. maybe they thought it would be an engine swap experience.. :l
for Mine, 3.4, and to be different-- I started with 1-3/8 ID through the flange and to gasket match/my porting work to the head--follow to 1-1/2" ID pipe /1-5/8 OD - they are the primaries, but then I went the log style like stock... with much better curves, of course, and for the T sections... unfortunately, you got to look stock where I am.. or come up with a official bar number for them.. I even use all stock (87) heatshields.. I think I hit the mark plus performance though... when I slapped those on the first time-- you could feel the breathing mid to high rpm.. so geeked!... or maybe the new Y section did it.. 😏
I think any 1-3/4 OD/1-5/8 ID or larger pipe would start hurting the flow with our engines.. piston size
I know if I went equal lengths, the effect would be more evident.. 3-1 collector.. but my log design seems to help to pull through the pipe... after my primaries, I went to a 1-7/8 ID/2"OD.. I don't think I harmed the negative back wave that the video talks about... the timing seems right..I definitely don't feel a loss.. after the Y starts my 2-1/2 out.. so considering 2 size changes for my setup..
[This message has been edited by unboundmo (edited 01-07-2018).]
When I was a teenager, and our cars were new(ish) ... I was told repeatedly that headers would increase HP slightly, but decrease torque, i.e. the loss of backpressure somehow hurt torque.
I suspect that's one of those generalized statements that may or may not be right based on a number of things.
One, I am going to drag this summer. Just upgraded to a 3.5 pulley on the 3800 SC. So I was thinking of running an open exhaust not that I am seeing 10psi on the boost gauge.
Two, I have a woodstove. (Stay with me here.) When I open the door on the stove, all the way open, there is a greater air supply, but less velocity of air across the fire. The fire does not benefit. When I open the door about a quarter of the way, the air velocity increases, fans the flames, and there is greater flow up the chimney. Of course, the chimney by now is creating a vacuum pulling air into the wood burner. So the chimney is like an exhaust pipe.
When I was a teenager, and our cars were new(ish) ... I was told repeatedly that headers would increase HP slightly, but decrease torque, i.e. the loss of backpressure somehow hurt torque.
I suspect that's one of those generalized statements that may or may not be right based on a number of things.
It's a holdover from people not understanding what's going on and assuming that they do.
The essence is that pressure wave tuning is important for max power and torque and tuning for one can decrease the other. 99.9% of the time, back pressure is a detriment to both hp and torque.
[This message has been edited by mender (edited 01-08-2018).]
One, I am going to drag this summer. Just upgraded to a 3.5 pulley on the 3800 SC. So I was thinking of running an open exhaust not that I am seeing 10psi on the boost gauge.
Two, I have a woodstove. (Stay with me here.) When I open the door on the stove, all the way open, there is a greater air supply, but less velocity of air across the fire. The fire does not benefit. When I open the door about a quarter of the way, the air velocity increases, fans the flames, and there is greater flow up the chimney. Of course, the chimney by now is creating a vacuum pulling air into the wood burner. So the chimney is like an exhaust pipe.
I'm running a Camaro muffler currently.
I would caution you about assuming that chimney draughts and such are the same as an exhaust pipe, mainly because a fire is a continuous process that doesn't use any pressure wave dynamics. It's part of the process but it would be about the same as a flow bench test of a head vs running it on an engine.
I suspect you know that and are alluding to the scavenging phase of pressure wave tuning but just want to be clear.
I think any 1-3/4 OD/1-5/8 ID or larger pipe would start hurting the flow with our engines.. piston size
I know if I went equal lengths, the effect would be more evident.. 3-1 collector.. but my log design seems to help to pull through the pipe... after my primaries, I went to a 1-7/8 ID/2"OD.. I don't think I harmed the negative back wave that the video talks about... the timing seems right..I definitely don't feel a loss.. after the Y starts my 2-1/2 out.. so considering 2 size changes for my setup..
Your airflow at certain rpms on a particular engine, and how much hp and torque it makes, and where in the curve, will matter. Also computer run engine, or not, etc.
Since we know this is a 3800 SC, there is a good reference point. When you force the air things do change that's for sure. The magic of tuning and people who know how to do it become important. I am not that person
I would think tube dia depends on cylinder volume that needs to exhaust also?.... is this the right formula? --- 5.0L / 8 cylinders = .625 volume per cylinder... a 3.8L / 6 = .63333 ....... So for a conclusion.. a 1-3/4 OD would be great for a 3.8L... as does the V8..
For mine, i think -- 3.4L / 6 = .56667.... the 1-5/8 OD are right where they need to be.
For kicks... 2.8L / 6 = .46667 cylinder volume ---- a 1-1/2 OD would be best...?
I would think tube dia depends on cylinder volume that needs to exhaust also?.... is this the right formula? --- 5.0L / 8 cylinders = .625 volume per cylinder... a 3.8L / 6 = .63333 ....... So for a conclusion.. a 1-3/4 OD would be great for a 3.8L... as does the V8..
For mine, i think -- 3.4L / 6 = .56667.... the 1-5/8 OD are right where they need to be.
For kicks... 2.8L / 6 = .46667 cylinder volume ---- a 1-1/2 OD would be best...?
Much more to do with HP/Cylinder for a specific exhaust size. The goal is to provide a diameter with enough velocity to allow for good pulse signal to aid scavenging. HP is proportional to total engine airflow, regardless of engine size.
The reason that a non-oversized NA exhaust is good is that it reduces backpressure; it helps to suck the exhaust out of the engine.
People make the error in automatically assuming that larger pipe = less backpressure.
When a moderately-sized pipe performs best, people then mistakenly believe that the performance is due to the "proper amount of backpressure", when in reality it's the lowest amount of backpressure.
If a large exhaust performs poorly, that's because it provides a greater amount of backpressure than a smaller exhaust.
Basically, you always want to reduce backpressure. The only mystery are the pipe diameters / lengths to achieve the lowest backpressure.
The presence of a (turbo)supercharger negates none of this logic; the better you can suck out the exhaust, the more you can fill the cylinder with fresh air/fuel mixture.
I wish I could find the article but no luck.I think it was Popular Hotrodding. Anyway it was a twin turbo v8 in a full size truck, I do not recall the make. They did dyno readings with cat back exhausts by increasing the size by 1/2" at a time to see when they would stop seeing increases in HP and Torque. At every increase in size, they continued to see more gains. I think they stopped at 4.5" because they could not fit anything bigger to the truck.
My own recent experience is similar. On my turbo ecotec I had a 2.5" diameter exhauast with flow master camaro style muffler with dual outlets. I decided to do away with the muffler and reduce the cat back pipe to a single 2" diameter outlet. I thought I would get a faster spool up time and without the muffler no loss at higher revs. I got the faster spool up but the engine fell flat at 6200 RPM. What followed was 6 weeks of wasted time and money changing to a bigger fuel pump and changing my tune almost daily. Finally I figured that the 2" pipe was the problem. Welded up a 2.5" pipe and all the lost power came back and now it pulls strong right up to 7000 rpm. And the muffler is not needed, the cat and the turbo keep it fairly quiet until you stomp on the gas. Wether going to an even bigger pipe would see more gains I do not know. There really is not enough room to make up a larger diameter exhaust.
[This message has been edited by wftb (edited 02-10-2018).]
I have a 2.8 too but not in a fiero so i have somewhat different exhaust configuration pass the y-pipe. But recently part of my exhaust/muffler fell off while im driving on the freeway. It broke off just before the cat. i instantly loss power and the coolant temp rise instantly. Pull off the freeway and couldn't get the car to start and got a code 33(low vacuum MAP error). I spend couple weeks trying to figure why im getting code 33 w/o re-install the muffler since most of the consensus online said it doesn't really matter if your muffler are attach to it. Finally re-attach the muffler, and the engine start fine and code 33 is gone. I assuming Im not getting enough backpressure which cause the code 33 or low vacuum in the intake manifold??
Sounds to me like when the pipe broke off the cat it must have gotten squeezed and pinched closed to shut. A lack of back pressure will not cause a car to overheat or make it not start. But a plugged exhaust pipe will.
Call me paranoid but I suspect when the muffler fell off, the computer went bonkers & shut the engine down to "limp home mode" because it thought it was was putting out bad emissions.
[This message has been edited by Monstertone (edited 02-09-2018).]
Call me paranoid but I suspect when the muffler fell off, the computer went bonkers & shut the engine down to "limp home mode" because it thought it was was putting out bad emissions.
Ummm... what fantastically advanced artificial intelligence do you believe is implemented by the ECU in a Fiero? Damn.
Damn.
