Is 3" exhaust needed?

[DomChassis]

he's running a 3" crush bent system. Crush bends make for bad sounding cars.

[Polygon]

Quote:

Originally Posted by DomChassis he's running a 3" crush bent system. Crush bends make for bad sounding cars.

Well that would explain a lot. Of course it is going to sound like crap. Why on earth would you bother replacing the exhaust and use crush bends?

I'm sorry, that is completely illogical.

[tkelly27]

My setup, 2.5" dp to 3" cut out, to 3" high flow cat, to generic 3" "turbo" muffler, to side of van. I love my 3", wish I would have gone bigger/less restrictive/louder. Well, any combination of those 3.

[turboshadow91]

3" manderal with 3"SV and 3"Dp is the best way to go and side exit, is the best sound ever there is nothing like the sound of your turbo spooling up or down.

[vcrpro3]

Engine is back in car, when out- i saw that the opening at the turbo flange was 2-1/4 inches into the downpipe, a piece was welded to the 2-1/2 pipe to match it to the mitsu turbo. I am going to put another downpipe on the garrett again 2-1/2 inch, but transition to 3 inch where the exhaust system is horizontal under the car, use a 3 inch cat for emissions only,replace with a straight tube, then to a 3 inch straight through muffler.

[vcrpro3]

For those that have used them, who makes a good, accurate 3" pre-formed exhaust system for a J-body?

Quote:

Originally Posted by vcrpro3 Engine is back in car, when out- i saw that the opening at the turbo flange was 2-1/4 inches into the downpipe, a piece was welded to the 2-1/2 pipe to match it to the mitsu turbo(have the 2-1/2 Garrett in now). I am going to put another downpipe on the garrett again 2-1/2 inch, but transition to 3 inch where the exhaust system is horizontal under the car, use a 3 inch cat for emissions only,replace with a straight tube, then to a 3 inch straight through muffler.

[RoadWarrior222]

I've been realising lately that there's one or two very limited situations where crush bends might outflow mandrel bends. These are only when you're forced to use a very tight radius, not quite sure where the critical point is, and it depends on exhaust velocity so overall flow potential in the rest of the system counts too. It starts at around 6 inch radius and by the time you get to as tight as 3 inch, you're definitely into that territory. To get that tight with a mandrel bend, the piping often gets necked down quite a bit, but that's not all, the exhaust when forced to take a tight turn at high velocity will start to flow seperate, and large unstable vortices will form, hindering flow. Now in a crush bend of the same tight radius, the ridges that normally just impede flow at lower velocities or wider radiuses, help keep the turbulence under control by stimulating smaller vortices, thus pre-empting the instability and overall flow is less chaotic than with large vortices. The ridges kind of trip the air up to get it round the corner rather than letting it slam into the outside radius and back things up. Now also, mandrel bends usually suffer no or very little reduction in cross section, until like I mentioned the radius gets real tight, then they can neck down quite a bit. However crush bends don't lose a whole lot more area on a tighter curve (to a point of course) than they do normally. Thus for a real tight curve you might have a 2.5" ID in the 3" mandrel bent, but have a 3"x2" oval (often close to a rounded off rectangle) in the crush bent, which would be under 5 square inches in the MB vs nearer 6 in the CB. In a wider radius bend the reverse would be true, MB would be 3" right through still and have 7 sqin vs the CB having a little under 6 still. The bigger the tube of course, the bigger the radius gets where there will be necking problems.

Smaller pipes will go round tighter radiuses without necking. So there may be cases where Joe and Fred have the same vehicle, both have 2.5 mandrel bent systems and both decide to go for 3" systems. Joe gets a mandrel bent 3" and Fred cheaps out and gets a crush bent. Joe goes to the dyno and sees..... nothing... it feels and seems as if he didn't change it. Fred goes to the dyno and picks up a few horsies and can feel it in the seat of the pants and is quite happy. What happened here? Well if the same routing was followed in both cases, and there was the necessity of having a tight bend or two, what could have happened is the 2.5 MB systems made that bend without necking, but the 3.0" MB system couldn't. So Joe ended up with a couple of ~5 sqin restrictions which kept things the same as his previous ~5 sqin right through pipe, whereas Fred had ~6 sqin restrictions on his crush bends, but still got an improvement over his previous ~5 sqin.

So, how do you avoid that, if you want a full potential MB system? Easiest way would be to route so you don't have to make any tight bends, plan carefully. If it's impossible to avoid a really right bend, you can either just use a CB at that one spot, or you could source a constant ID cast piece, also I think there's some spendy pressure formed MB pieces that keep ID better than conventional MB ones on tight radiuses, or you could have that bend welded up out of thin 10* slices of pipe. Another option might be to use twin pipes. If smaller pipes will make the turns without necking, then in a real tight and involved install 2 might be better than one. Remember though that if you think you "may as well just get a crush bent system" rather than just use one crush bend or get a special piece, that there is viscous drag on the crush bends too so an all MB with one CB at a critical spot should just outperform an all CB system of the same layout. There just might be some situations where you could go up a size on the CB, use a reducer before and after it, and keep the CSA, it's possible that due to the turbulation effects of the ridges that that would flow higher than any other way of doing it. (Unless you cast a custom piece with optimised turbulation ) Though situations where you'd have room to do that, but not run a wider radius with MB would be limited.

But anyhoo, bigger is better for turbo's, for N/A be conservative or you'll shoot yourself in the foot. Here's an N/A sizing table, has useful diameter/area comparisons too, just ignore the size/HP if you're boosted...
Performance Unlimited 4-Wheel & Off Road Center - Hartford, Wisconsin - USA Technical Documents

RW222

[tkelly27]

http://us.st11.yimg.com/us.st.yimg.c...2_1946_2918114
http://us.st11.yimg.com/us.st.yimg.c...2_1946_2409699

Lotsa necking on those 4" radius pipes. Sure wouldn't want to be Joe in that situation... : /

Can you even make a radius smaller than the diameter of the tubing? I think it becomes bent at that point, doesn't it?

http://www.gibsonperformance.com/ima..._stockpipe.jpg

"but that's not all, the exhaust when forced to take a tight turn at high velocity will start to flow seperate, and large unstable vortices will form, hindering flow. Now in a crush bend of the same tight radius, the ridges that normally just impede flow at lower velocities or wider radiuses, help keep the turbulence under control by stimulating smaller vortices, thus pre-empting the instability and overall flow is less chaotic than with large vortices."

So you're a fluid dynamacist and have researched and found that having the boundry layer tripped makes up for the 15-20% reduction in area? Neat. If you're that worried about flow separation, add flow channels, don't reduce the area of your pipe and add steps. I'm also under the impression that exhaust gas is traveling fast enough to be fully turbulent at all times that matter in the exhaust system, so tripping the boundary layer will do nearly nothing. That's my own opinion though. If you can find an example of what you are stating, I'd love to read about something with evidence to back it up.

I've got a 3" cast piece that is a VERY tight bend. It is smooth on the inside, I couldn't imagine a crushed piece of the same dimensions.

At the end of the day, it's not a life or death discussion. Race cars run itty bitty pipes off the end of their turbos, and street cars are unlikely to notice the difference 2 or 3 CFM makes out their exhaust (whichever way flows better) if they use a mandrel or a crush bent for an ultra tight turn if they are using mandrel for the rest of their bends.

I agree with the last little bit though. 3" crush, or mandrel bent will be an improvement over anything stock TD. I don't think you'd see the benefits of mandrels until you started making big power.

[RoadWarrior222]

Quote:

Originally Posted by tkelly27 http://us.st11.yimg.com/us.st.yimg.c...2_1946_2918114 http://us.st11.yimg.com/us.st.yimg.c...2_1946_2409699 Lotsa necking on those 4" radius pipes. Sure wouldn't want to be Joe in that situation... : /

Hmmm did you notice I said a very limited number of situations. The Joe and Fred story was just intended as an extreme example. 99% of the time mandrel bent has near as dammit full CSA all the way through.

Quote:

Originally Posted by tkelly27 I'm also under the impression that exhaust gas is traveling fast enough to be fully turbulent at all times that matter in the exhaust system, so tripping the boundary layer will do nearly nothing. That's my own opinion though. If you can find an example of what you are stating, I'd love to read about something with evidence to back it up.

Well it won't be turbulent all the time, there will be a transition at some point in the RPM range, if you've got smaller pipes it will be lower down, bigger pipes it will be higher up. If and only if there is a tight enough bend in the system, there may be a kind of hiccup in the flow as you rev the engine, might occasionally be bad enough to feel, if you get a severe case of it in the power band, then you'll probably find it annoying enough to want to fix. That would be a large vortex "valving" in a tight bend.

But you're right, usually bends that are approaching the size of the pipe wouldn't even be attempted. However, if you're ever using a stock routing where the bend was okay in a 2 1/4 pipe, you might find it's verging on the ridiculous with a 3" pipe.

[tkelly27]

Quote:

Originally Posted by RoadWarrior222 But you're right, usually bends that are approaching the size of the pipe wouldn't even be attempted. However, if you're ever using a stock routing where the bend was okay in a 2 1/4 pipe, you might find it's verging on the ridiculous with a 3" pipe.

Progress on header

and it's insane with 4"! That guy does badass work. Yeah, you might need to crush it a bit just to get it to even fit, flow be damned.

Ohh, was there an argument that a crushed bent could possibly ever outflow a mandrel bent pipe? Yeah, you're right then, those conditions could exist. I could also win the lottery and have someone else build my cars

[RoadWarrior222]

I just thunk other ppl like me might like collecting up the "one in a hundred" tips and oddities, by the time you've seen a thousand of them, you'll have a handful that you can apply to your particular car, and then it will be quicker than anyone else with the "same" mods. There's no fun for me in mindless bolt-ons, I like to think about everything. Of course, what you'd like to do and what actually ends up on the car are usually two different things when you have time and budget constraints. Then you end up sighing, shrugging and thinking the mindless bolt-on will be good enough.