Enough back pressure?

[Tungsten]

So after looking around, I can't find anything being related to how much back pressure the VG30E was designed for. I need the info because I need to know what to do once the Thorley headers are in. Would there be enough back pressure if I had headers with no cat in the down pipe followed by a Magnaflow cat and a Magnaflow muffler? I can keep the muffler and cat as stock but I want to put as little force as possible on those headers. Is it possible to have the exhaust exit midway instead of at the back? What would be a good setup for this?

[KovemaN]

I don't think there is an actual back pressure spec. Too much will not allow the engine to rev and too little will lower the engine's ability to produce adequate torque at low revs. I believe that the Magnaflow converter has flow characteristics similar to stock and that the part on the downpipe is a resonator and not an actual converter. The setup you listed sounds good except for the exit. I would keep it as far back as possible to prevent CO from entering the cab.

[Tungsten]

CO entering the cab is not a problem since the wind will just blow it back and away while driving.

 

I only wanted a mid exit to avoid having to get another pipe to compensate for the length when the cat and muffler get installed. Also I thought it would be nice to avoid bends around axles.

[nunya]

Still get some fumes with side exit even while moving. Close to walls/drive thrus suck when not moving. I kinda feel bad fo rpeopel beside me at red lights...kinda.

[beastpath]

CO entering the cab is not a problem since the wind will just blow it back and away while driving.

 

I only wanted a mid exit to avoid having to get another pipe to compensate for the length when the cat and muffler get installed. Also I thought it would be nice to avoid bends around axles.

 

 

what about sitting in traffic with no wind? lol

 

how about this: a custom mid exit pipe which bends up and throws up the exhaust at the roof, could do one on each side.

 

would probably only work on a 2 door though..

[dhardison]

What about in front of the rear wheels. I don't recall seeing many dead people driving Corvettes and Vipers with sidepipes. Or old Challenger T/A's which is more what I'm thinking. Or what about Lil Red Truck style, or not.

[nunya]

'Lil Red Express style is kinda what beastpath means, kinda a form of stacks. And Vettes/Vipers with sidepipes are designed for them generally and don't have opening windows directly above where the exhaust exits. I know all about fumigating myself, especialy before I re-adjusted the angle my exhaust tip ended up being...

[shift220]

I wouldn't worry about backpressure so much. Even if there was some value, it would be silly and time consuming to design around it. Just put as best you can with the least restriction and enjoy the extra power at all RPMs. Any loss of low end torque is going to be a mental thing unless you run open headers.

 

I like going right out the back because sometimes dumping underneath resonances in the cab and can get annoying!

[Kingman]

No engine needs "back pressure" to run properly. What it needs is the right size tubing to promote the hot exhaust gas velocity as it travels down the tailpipe and out. Think of it like blowing through a straw, the perfect straw size allows you to blow the most air out with the least effort. Also think of back pressure as a paper towel or cotton ball (at the extreme) over the straw. You must blow harder to pass the air, now relate that to how the engine must work harder to pass the exhaust down the tailpipe.

 

Back pressure is also a large reason why Flowmater, or any big name baffled mufflers are, for the most part, a crock. All they do is deliver sound, at the same time they keep a relevantly close amount of back pressure to the stock muffler.

 

The best flowing and performing exhaust would be with proper headers, no cats, no muffler, and proper size tubing. There are things you can do to change the torque curve, and that involves the length of tubing on the headers, the downpipe angle, and a cat. The extreme case of no back pressure would be open headers or manifolds, then yes a lack of "back pressure" would be a problem.

[andreus009]

Ok...i'll try to make this a short story.

 

Installed Borla CatBack Muffler = 0 improvement in performance. Looks pretty though. Well at least it did until I covered it in mud multiple times.

 

Punched and Sleeved Main Cat = Wow...that really helped. Good low end to mid power improvement from stock. Top end is definitively still limited.

 

Punched the small Cat before Main Cat = Hmm.....my low end isn't as good as it was.. Dang it. Definitely not mental. But it will hit 90+ mph and hold it pretty easy. Not very useful, but a fun fact to know.

 

Practical lesson learned = the most free flow system may not be the best or it needs to be properly designed to be most effective.

 

Oh yeah...one more thing. I did all the above mods with stock manifolds. Just recently installed headers and didn't seem to notice much difference. Well at least I don't have that darn ticking noise anymore.

[180sx]

No engine needs "back pressure" to run properly. What it needs is the right size tubing to promote the hot exhaust gas velocity as it travels down the tailpipe and out. Think of it like blowing through a straw, the perfect straw size allows you to blow the most air out with the least effort. Also think of back pressure as a paper towel or cotton ball (at the extreme) over the straw. You must blow harder to pass the air, now relate that to how the engine must work harder to pass the exhaust down the tailpipe.

 

Back pressure is also a large reason why Flowmater, or any big name baffled mufflers are, for the most part, a crock. All they do is deliver sound, at the same time they keep a relevantly close amount of back pressure to the stock muffler.

 

The best flowing and performing exhaust would be with proper headers, no cats, no muffler, and proper size tubing. There are things you can do to change the torque curve, and that involves the length of tubing on the headers, the downpipe angle, and a cat. The extreme case of no back pressure would be open headers or manifolds, then yes a lack of "back pressure" would be a problem.

 

This is what i was gonna say! The old myth that you need backpressure for torque is not true. It's just a side effect. Its a properly designed exhaust the can can provide enough velocity to expell the exhuast. What you want is the venturi effect at the exhaust ports. Like when a car drives by a plastic bag, the bag wants to follow after from the 'pull' of the wind from the car. keep cars driving by, and the bags keeps going. This pulls the exhuast out, and helps suck in more fresh air for the next cycle. Its not an easy thing to accomplish. This is used to help change the powerband also.

 

Best setups to use for the VG30 (this is my opinion, by the way):

 

2-1: 1.50" piping to a 2-1 muffler, with 1.75" piping out of it. Best to include a resonator or cat.

 

2-2: 1.50" pipng to a resonator, then to a muffler, with 1.50" continuing out.

 

If these seem like small piping diameters to you, then just think of this. A Big block V8 with duals usually has a 2-2.5" exaust. they make big power, have big displacement, and have 4 cylinders to each exhaust. We have little power, little displacement, and only 3 cylinders to each exhaust.

 

Again, this is my opinion, and just a guidline to what would help.

 

Jose

[Tungsten]

No engine needs "back pressure" to run properly. What it needs is the right size tubing to promote the hot exhaust gas velocity as it travels down the tailpipe and out. Think of it like blowing through a straw, the perfect straw size allows you to blow the most air out with the least effort. Also think of back pressure as a paper towel or cotton ball (at the extreme) over the straw. You must blow harder to pass the air, now relate that to how the engine must work harder to pass the exhaust down the tailpipe.

 

Back pressure is also a large reason why Flowmater, or any big name baffled mufflers are, for the most part, a crock. All they do is deliver sound, at the same time they keep a relevantly close amount of back pressure to the stock muffler.

 

The best flowing and performing exhaust would be with proper headers, no cats, no muffler, and proper size tubing. There are things you can do to change the torque curve, and that involves the length of tubing on the headers, the downpipe angle, and a cat. The extreme case of no back pressure would be open headers or manifolds, then yes a lack of "back pressure" would be a problem.

 

Kingman, this is for you

 

http://www.youtube.com/watch?v=wAn8rgrcGRU

Edited October 9, 2009 by Tungsten

[Kingman]

That's hilarious!

[Tungsten]

yeah it would have been even better if it was an 88 pathfinder

[Kingman]

Fo sho, with no cat and a huge pipe coming out the back!

[Toby]

I never saw the appeal of side exit exhausts in front of the rear wheels. Plus the exhaust heat would probably make an already rust prone spot more likely to rust.

 

But that's just my limited knowledge.

[Tungsten]

right now im running headers and a stock cat and muffler

1000-2000 rpm is awesome but over that it doesn't make any more power

i need a magnaflow muffler badly

[MY1PATH]

Exhaust OPINOINS are very diverse.

It is my personal beleif the 2.0 is the most Ideal diameter for the vg30 (i & e) if you want to it to have the most useable tq band. A quick rise to 2.25 for a high flow cat and back down to a 2" free flow muffler is kinda the Ideal setup.

If you are looking for more power higer up I would go 2.25 max and that should do the trick.

 

seach youtube for my1path and my exhaust vid should pop up.

[BowTied]

Kingman is correct. It has to do with exhaust gas velocities. There are a couple ways to look at things for a n/a engine. Design the exhaust for bottem end or top end. "Smaller" tubing forces the exhaust to move faster through the pipe. The beneficial effect here is the scavenging effect. "Larger" pipes tend to lose this and in comparison can make acceleration from a stop feel sluggish if the pipes are too big. The trade off is the larger pipes with breathe easier at higher RPM. Case in point, when I dyno my 6.6L V8 it gained 10hp with open headers, but lost about 12 ft.lbs of torque.

 

For heavy vehicles, especially trucks and off-road rigs that are running at lower RPM, it is all about the torque to get it moving. The extra few horsepower at the top end will not be worth the loss of low end grunt. IMHO.