Downpipe>Exhaust

[selmerguy]

backpressure increases low-end torque to an extent (as in don't over do it) ...tell me I'm wrong

...therefore running a cutout, open downpipe, straight off j-pipe etc will subsequently sacrifice some of that low-end torque. @ got used to sitting in passenger seat too bad I've never run an open exhaust and can speak from experience welcome to the 2gnt side of the tracks


I love you

I have seen tests done in super street I think. They two cars a civic and a 2g gst. Both had 2.5 inch cat backs and a cutout. The ran 3 pulls closed and 3 open. The open was a avreage of 10 hp and like 12 torque better. I wish I could find that mag now. My wonderful wife probably tossed it out.

 

[XMasta19]

meh...you're a helpful contributing member, and I'm just more helpful than not
Goes to show what I know, I'll stop spreading misinformation now

 

[selmerguy]

meh...you're a helpful contributing member, and I'm just more helpful than not
Goes to show what I know, I'll stop spreading misinformation now

Man it's all good. This will still be a debate anyway. There are so many v8 motor heads right now they still teach the "a little back pressure is a good thing" thoery. Which NA is true. But turbo cars have a different standard. Going back to the mag I was reading, I also remembered the torque curve looking crazy with the cut out open. It had a little rise in the lower rpm and then it went straight up as soon as boost kicked in.
Even with 8.8:1 compression and a cutout open I still can spin my tires for days at stop light. Thats enough lowend torque for me. Now if I could just get ride of all that wheel hop.

 

[VelocitàPaola]

There are so many v8 motor heads right now they still teach the "a little back pressure is a good thing" thoery. Which NA is true. But turbo cars have a different standard. Going back to the mag I was reading, I also remembered the torque curve looking crazy with the cut out open. It had a little rise in the lower rpm and then it went straight up as soon as boost kicked in.

...sounds exactly like the guys at one of the shops I used to work at. They were all about the V8s, until one of them got a turbo Shelby. He started talking about exhausts and how he was going to get rid of the cat, so I said "yeah, back pressure and turbos don't agree with each other too well." Now he's a smart guy - he taught me much of what I know, but the next thing he said made me doubt that: "well, you still want back pressure... ya know, otherwise the turbo will just free-spin and burn up. But that muffler should give it enough pressure."

D'oh.

I was the only one at that shop with any sort of import - the rest all had Mustangs or classic Dodges, etc.. They should've just stuck to their V8s and let me answer the questions about turbos.

 

[Blitzeclips]

That makes sense considering how turbocharged cars are morely exhaust dependant versus intake dependant for N/T. My buddy has a cutout on his Z24 Cavi and it puts out more top end then low whenever he switches it over from full exhaust. However, I' m looking more at the 3" vs. 3.5" at this point (scavenging paragraph got me ). Ill be taking the cat out for a test pipe, so hopefully the flow can be as smooth as possible. hows 3.5" sound to the 3" pipe? think it's worth the difference(theres that damn 1/2 inch again)?

 

[dsm-onster]

I edited my last post when you asked about 3.5" vs. 3" w/ a 20G.

The best exhaust for a turbocharged application is no exhaust. You will increase spoolup times. Therefore, increase torque (work) down low. So 3.5" is better than 3"

pi * r^2 = area of a circle

where pi = 3.14 ; r = 1.5 or 1.75 (for 3" and 3.5", respectively)

3" => 7.1 square inches

3.5" => 9.6 square inches

3.5" exhaust is 35% more open. 35% closer to no exhaust which is best for a turbo application. Backpressure (pressure blowing against the turbine blade in the opposite direction) is never good for a turbo. ESPECIALLY during spoolup down low in the rpm range.

 

[BigRand]

Now if I could just get ride of all that wheel hop.

Motor Mount time, throw some polyurathene inserts in there.


Also (back to the thread) I think a lot of people over look how exahust are set up, Civic's seem to have a lot of bends to cancel out exhaust noise when you first get em. More bends create more backpressure which is bad right?

I looked at my friends GSX piping after his downpipe and HkS exahaust install, and that thing is basically straight back from the turbo, with minimul bends. I would assume this has to have some kind of efffect on exhaust effeciency.

 

[selmerguy]

Motor Mount time, throw some polyurathene inserts in there.


Also (back to the thread) I think a lot of people over look how exahust are set up, Civic's seem to have a lot of bends to cancel out exhaust noise when you first get em. More bends create more backpressure which is bad right?

I looked at my friends GSX piping after his downpipe and HkS exahaust install, and that thing is basically straight back from the turbo, with minimul bends. I would assume this has to have some kind of efffect on exhaust effeciency.

The more bends at high degrees would cause more back pressure. Which on a turbo car would be bad but on a NA not so bad or even help down low a bit. I know I need to put some pollys in just waiting to pull the motor out so I can fix a few little things like my oil leak and drop in my new clutch.

 

[Blitzeclips]

OK. ive taken all this advice into consideration and made up my mind. Im going to have a local performance shop fab an exhuast 3" back for me, but with the straightest exit as possible. I want to eliminate as many extra bends as possible, and increase the flow even more with a test pipe. This should provide enough exit for the setup I've got. Hopefully I will see max gain of HP with the 3" as well, every little bit should help ....

EDIT: 350 HP to be honest w/ ya!

 

[dsm-onster]

OK. ive taken all this advice into consideration and made up my mind. Im going to have a local performance shop fab an exhuast 3" back for me, but with the straightest exit as possible. I want to eliminate as many extra bends as possible, and increase the flow even more with a test pipe. This should provide enough exit for the setup I've got. Hopefully max gain of HP with the 3" as well....

I hope this shop has a mandrel pipe bender. Go to Autozoo in the exhaust section and take a look at their 90* bends. They are crush bent. At the bend, your 3" pipe will get crushed down to 2 1/4" or less. Most muffler shops just use a crush bending unit. Ask them before they get started .