Single exaust pipe size

CigarJohnny · #1 08-04-2003, 05:01 PM

Just read in another thread that when going to dual exaust it probably better to go with 2" pipe. What do those who have modified their exaust but kept it a single pipe recommend for pipe diameter? 2.5"? 3"? I don't recall anyone ever saying.

CigarJohnny · #2 08-04-2003, 05:09 PM

Heh...I think I just answered myself. The square surface of two 2" pipes openings is 6.28". A single 2.75" pipe would yield 5.93" and a 3" pipe would yield 7.06".

Anyone used either or both of these in the past and if so, what would you recommend? I like the idea of a single exaust for weight savings.

CigarJohnny · #3 08-04-2003, 05:15 PM

As an aside, a 2 7/8" pipe would yield the closest match with roughly 6.45 inches.

Hey, Earl! Can you relocate this to the mods forum?

oab_au · #4 08-04-2003, 05:58 PM

Johnny,
The pipe size should stay the same size as the orignal. It is not a case of total gas flow, it is still one cylinder flowing at a time, no matter how many pipes there are.

Harvey.

Subafreak · #5 08-04-2003, 06:53 PM

But 2in pipe looks so wussy.

CigarJohnny · #6 08-05-2003, 06:54 AM

Quote:

Originally posted by oab_au
Johnny,
The pipe size should stay the same size as the orignal. It is not a case of total gas flow, it is still one cylinder flowing at a time, no matter how many pipes there are.

Harvey.

Harvey, if that's the case, then why go to dual exaust at all? Isn't going dual with 2" pipes going to hurt performance by reducing needed back pressure in the exaust? What if one were to want to use NOS? Increasing the single pipe diameter would increase gas flow to the same rate as duals but if the flow of duals hurts performance why does anyone switch to it aside from asthetics?

Aside from that, I had 2 1/4" pipe on my old Horizon with a 1.7 liter engine. It ran way stronger than stock with only a change in air filter to go with it. Gas mileage was pretty poor but it spun the tires hitting 3rd gear. Like Freak said, 2" pipe is just wussy!

elninoalex · #7 08-05-2003, 08:50 AM

2" pipe is too big for true duals. 1 3/4" would be better. Stock single is approx. 2 3/8", not 2". 2 1/2" is fine for a single. If you go duals, use a X pipe or it will sound goofy and lose some low end. Believe me, I have tried everything under the sun. I am currently running 2" from o2 sensors joining into center cat with a 2 1/2" 'Y' pipe back to dual 2" and out through dual mufflers. It is awesome.

Stephen

elninoalex · #8 08-05-2003, 08:52 AM

http://www.subaru-svx.net/forum/show...hlight=Dynomax

oab_au · #9 08-05-2003, 06:12 PM

Quote:

Originally posted by CigarJohnny
Harvey, if that's the case, then why go to dual exaust at all? Isn't going dual with 2" pipes going to hurt performance by reducing needed back pressure in the exaust? What if one were to want to use NOS? Increasing the single pipe diameter would increase gas flow to the same rate as duals but if the flow of duals hurts performance why does anyone switch to it aside from asthetics?

Aside from that, I had 2 1/4" pipe on my old Horizon with a 1.7 liter engine. It ran way stronger than stock with only a change in air filter to go with it. Gas mileage was pretty poor but it spun the tires hitting 3rd gear. Like Freak said, 2" pipe is just wussy!

" REDUCING NEEDED BACK PRESSURE"

No, there is no need for back pressure. This is a myth Johnny.
The main function of the exhaust system is to reduce the pressure that exits the exhaust valve from about 70psi to zero as quickly as possible. This is to reduce the pumping losses that would occur if the piston has to push the gas out. This is why we use an expansion chamber or resonator at the end of the header pipe. The chamber acts like the end of the pipe, as the high pressure gas plug will expand, the same as if it had an open pipe there. After the chamber, the gas pressure has dropped and we just want to be able to flow the gas out without restriction.

Header pipe diameter has an effect on cylinder scavenging, by using the inertia effect of the gas plug, to pull a low pressure at the cylinder. The diameter is mainly controlled by the rpm the engine is to run at. The higher the rpm the larger the diameter, time is the reason.

Harvey.

CigarJohnny · #10 08-05-2003, 07:17 PM

Thanks for the explaination, Harvey. From anyone else I would definitely question your information. I was always under the impression that you needed back pressure to scavenge the exaust from the cylinders. What you are saying is that it is the headers that do all the limiting and the size of the pipe after that does not matter as long as it is less restrictive than the headers? That is very interesting and definitely improves one's choice of parts when redoing the exaust.

Lets face it though, larger pipes will decrease the velocity of the exaust gases, thereby limiting the potential volume of my WooWooooo tips.

oab_au · #11 08-06-2003, 06:18 PM

Quote:

Originally posted by CigarJohnny
Thanks for the explaination, Harvey. From anyone else I would definitely question your information. I was always under the impression that you needed back pressure to scavenge the exaust from the cylinders. What you are saying is that it is the headers that do all the limiting and the size of the pipe after that does not matter as long as it is less restrictive than the headers? That is very interesting and definitely improves one's choice of parts when redoing the exaust.

Lets face it though, larger pipes will decrease the velocity of the exaust gases, thereby limiting the potential volume of my WooWooooo tips.

You are not on your own in this back pressure theory. I have read it many times, mainly a US thing.

I think it is a confusion of the inertia effect that I mentioned and the effect of the tuned negative pressure wave that returns to the cylinder, as a " BACK PRESSURE ".

Two strokes use a build up of pressure in the expansion chamber to limit the fresh charge spill, out the exhaust port, as both inlet and exhaust ports are open together.

The inertia thing is why you hear that "a large diameter pipe will hurt the bottom end torque". They are referring to the amount of inertia that the gas plug will develop as it rushes down the pipe. Smaller pipe, higher velocity, higher inertia but more restriction. Larger pipe, less velocity, less inertia but less restriction.

The main objective is to have the lowest pressure from the exhaust valve on. It is all a compromise.

Harvey.

#12 08-09-2003, 03:46 PM

2.5" from the headers back has always worked very well on the EJ25 motors, but their pulse pattern is significantly different than that of the EG33. The 2.5 doesn't lose a significant amount of low end torque from increasing the pipe diameter (within reason), if anyone has good info on how the 3.3 responds, I'd be interested in hearing it.

Maybe I can get Harry up here to the dealer and we can dyno his car with the stock system and then the Stebro to see what changes, plus maybe mock up a set of 2.5" catless straight pipes and dyno the change again. Sounds like a fun way to spend a Sunday.

I love having a 4 wheel Dynapack chassis dyno 30 feet from my office.

CigarJohnny · #13 08-10-2003, 10:31 PM

That does sound like a fun way to spend a Sunday!