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Old 06-20-2001, 11:53 PM   #1
g6
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Default Performance gains from a true duel exhaust?

Hi All Im wondering if there is any performacne gain from a true duel exhaust on a stok engine.
Or will there need to be extensive(read Expensive)
engine mods.

Thanks

g6 g6 Racing Team
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Old 06-21-2001, 01:22 AM   #2
Brian Mayfield
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Only one exhaust pipe is needed on a 4 cylinder. Since only one cylinder is firing at a time you only need one pipe. If you had an 8 cylinder, two cylinders would be firing at the same time and taking up twice the space in the exhaust, then you would need two pipes.
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Old 06-21-2001, 02:31 AM   #3
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So you're saying the Cavalier with the dual exhausts I see driving around Tampa isn't getting twice as much HP from his exhaust?
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Old 06-21-2001, 02:55 AM   #4
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the cavalier that you seen probably doesn't have a real dual exaust. its probably split into two pipes after the midpipe. having a real dual exaust wouldn't really do any good because you will loose alot of low due to not enough back pressure.
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Old 06-21-2001, 09:35 AM   #5
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where to start...

alright first of all, only one cylinder fires at a time in any piston engine, even a V12. the crankshaft would not spin very well if it were accelerated in two separate places at the same time, to say nothing of balance.

secondly, I don't believe that a flat four needs or could benefit from dual exhaust either. besides, there's really only room for a single pipe from the midpipe back.
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Old 06-21-2001, 05:32 PM   #6
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Well thanks guys, It was worth a shot to see if it would help besides looking really cool.

g6 g6 Racing Team
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Old 06-22-2001, 10:04 PM   #7
Section 8
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Quote:
alright first of all, only one cylinder fires at a time in any piston engine, even a V12. the crankshaft would not spin very well if it were accelerated in two separate places at the same time, to say nothing of balance.
The Viper fires two cylinders at a time. Though you are right, it is the exception to the rule (there may be more....) Probably why it sounds so dissapointing when you hear one.

Greg
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Old 06-23-2001, 09:02 AM   #8
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Default firing

well it's gets down to the def. of "firing"
as all newer Subes with coil packs fire two
cyl. at a time. BUT only one is on it's compression stroke.
The "waste spark system" it is refered to
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Old 06-23-2001, 10:11 PM   #9
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Thumbs up

aRSe is right. I learned that way back in high school auto shop 13 years ago when coil packs were a strange new technology on American cars.
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Old 06-23-2001, 10:59 PM   #10
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Thats true, but it is not what Colin or I are talking about. The V10 in the Dodge Viper ignites two cylinders filled with a compressed mixture of gasoline and atmosphere at the same time, propelling the piston down the cylinder bore with the force of the expanding gasses in both cylinders. Not arcing the gap in a spark plug in an exhaust filled cylinder as it leaves the exhaust valves to lower emissions, while an oposing cylinder has its compressed mixture ignited. Though it has 10 cylinders the Vipers crank is only propelled every 144 degrees of crank rotation, like a 5 cylinder, though it is recieving power from two points unlike the 5 cylinder that only recieves power from one cylinder at a time. Where as a V8, propels the crank every 90 degrees. For the ultimate in smooth power, a V12 fires every 60 degrees. There is probably more differences than this that make the Viper V10 and a V8 sound different, I was really dissapointd that the american supercar sounded the way it did compared to a "garden varity" V8.

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Old 06-24-2001, 02:23 AM   #11
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Well, don't be so quick to say that a 4cyl can't benifit from a true dual exaust. Many of the old carbed cars have shown a nice boost in power with the true duals. There hasn't been alot of comparison of good single exasut vs good dual exaust, but there must be some reason that over 90% of the people who do a full exaust end up with a true dual exaust with a h-pipe. I personaly would run a X-pipe true dual configuration for a high output N/A motor. Recall that the Subarus have two heads just like every V4, V6, V8, V10, V12, etc. that is out there. So if they can see benifits from two pipes why cant the subaru motors. Now weather the power gain over a good single is worth the extra cats, and mufflers is up to you.
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Old 06-25-2001, 05:13 PM   #12
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I think you are talking about large V8's and such when you say 90%??? They move alot more air than a 4 cylinder, and need to evacuate it. We can do that by just getting a slightly larger exhaust pipe. On a 502 supercharged Chevy engine with headers (located in a 22 foot tunnel hull boat), the header tubes from each bank move into a collector and exit over the stern of the boat through a 5 inch diameter pipe (one per side). It makes alot of exhaust by itself, but with the supercharger it was not effecient to run the 4 inch (I think what was on it stock), because the exhaust wasn't flowing well. (we know this because we made more power with the larger collector and exhaust pipe, accually the whole header, but the primaries were the same diameter). We tried the larger exhaust when the engine was without the supercharger, and it made less power, so it was not the headers that were making power, it was the combination of supercharger and headers. So, to take this to your Subaru theory of a duel exhaust, you would need to use smaller pipes than you do now to keep your exhaust velocity up to aid in scavanging. 2.5 or 2.25 inch pipes being good for NA in the usual set up, you would have to use a .8" to .9" inch pipes, to keep the exhaust velocity the same since you are using half the exhaust (divide the exhaust cross section in half) Also, you need to use opposing cylinders to get the scavaging effect in our four cylinder because of the firing order. You will be running pipes all over the place, and they end up going to the center of the car anyway because it is the only place to pass the pipes unless you don't mind dragging them along the street. If you used a system that uses one pipe, you will have much smoother and effecient scavanging due to the mass of the exhaust traveling down the pipe drawing a vacume on the exhaust valves when they close, if you had a duel exhaust or half that mass (half the exhaust) you risk loosing some or all of that vacume because the exhaust slows down faster due to less enertia. If you had the single exhaust, you also have two more exhaust pulses to keep pushing that volume out the pipe keeping its velocity up and the vacume on the back valves. The V8's do not use a "quad" exhaust for more power. They are basically two straight four engines connected at the crank. Even then, the most effecient exhaust systems connect the pipes at one point to keep scavagng at its maximum and aid in power extraction. If you want a closer example of an engine that is not a Subaru, look at a VW Beetle power/tuner catolog, and see how many of the headers there end in one pipe. All of the off road buggies that I have seen in off road racing all have their exhaust end in one pipe. They want the most power, while there may be some people who build headers for the VW that end in two pipes for style. I trust the racers. Not to mention some math, logic and common sense.

If you really wanted to add some zing with your exhaust, add a tuned pipe to each cylinder. You would sacrifice some power in your rev range, but when you were in your power band, it would be a kick in the pants. Then you could have a quad exhaust. "Thats two mo', innit"

Greg

Last edited by Section 8; 06-26-2001 at 10:35 AM.
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Old 06-25-2001, 10:39 PM   #13
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Just to throw my 2 cents in.
I had a 1980 subaru Brat that I used in 87 to win the class championship with true duals. It sounded like a modified porsche. it was sweet! and very fast! for a 1600 engine. I also got more revs out of the eng becuse of this, and some carb mods. Since most auto-x tracks require you to stay in second gear all the way around. This was sometimes prohibitive with a very peaky engine, third gear would bog tremendously. So i had to do something. I contacted subaru to get some technical assistance. I worked for subaru from 83-90.
I was told to make some jet adjustments and open up the exhuast. The brat was designed to reach unbelievable levels of rev range. 7200 rpms to be exact and never an eng problem. I miss that vehicle. I rolled it in 87 as a result of a decreasing radius slalom course. The car became unstable and rolled over. But the point is I did it, true duals that is. I wanted to do the same to the RS but there is a canister on the right side of the eng that prevents that. I assume if you move that you MIGHT be able to do it. Only problem I see is the cat. The brat was a non-emissions cert vehicle. The muffler shop just cut the headpipe and added from there. On the RS, you would probably have to do quite a bit of moving stuff around to make a true duals exhaust.

Forrest
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Old 06-25-2001, 11:35 PM   #14
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You were moving much more exhaust when your engine was reving that high, and your exhaust was working effeciently. Low in the rev range was poor because you didn't have enough exhaust velocity. At the higher RPMs, your exhaust was in its "tuned" area, much like how a turbo lags but makes power all the way to red line, or it spools quickly but runs out of steam up high. You chose to have the power up top, and tuned it accordingly, though you had the ability to Rrev higher. This is the reason why there is the "argument" between 2.25 and 2.5 inch exhausts for the RS. The bigger pipe flows better at high RPMs while the smaller pipe keeps the power lower in the range with higher exhaust velocities while it becomes a little less efficient at high revs because it isn't as free flowing anymore, but on the edge of becoming a restriction. This would become more apparent if our engines were allowed to rev higher then they do. If you were to raise your rev limit I succed that a true duel exhaust could work better than the stock pipes with a performance muffler. I would have to have it proven to me with dyno figuers that it would be better than a larger single pipe that wasn't some kind of amature job. Just today I saw a BMW race car rolled on its side, and it had two pipes merging into a single muffler. I wish I knew if it was a V8, or an I6, I assume that it was an I6. 6-2-1, simular to the Honda trend of 4-2-1. If it worked better at extracting power, someone would be making alot of money making duel exhausts for Hondas.

Greg

Oh, and the peak HP number means nothing on a dyno, its the area under the graph that counts.

Last edited by Section 8; 06-25-2001 at 11:37 PM.
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Old 06-26-2001, 12:31 AM   #15
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Well it was 90% of the old brat people. If they don't do it all they tend to simply slap on a glasspack, and a new pipe from there to the bumper. The ones that do run duals I think use 1 3/4" pipe.

The honda is very easy to do a proper 4-1 design which in my opinion is a whole hell of alot better than a 4-2-1. The pipes merge and become too large too soon. I have heard that the primaries on a car should be 32" or greater in length. Thats almost 3ft. None of the 4-2-1 headers seem to have more tha 12" AT MOST for the honda's.

There might be extra power for a true dual on a honda, but I doubt it. However the honda's have a SINGLE head. Subarus have 2!. If you could do a proper 4-1 collector with equal length primaries I might agree that a single exaust is best, but you can't as ther eisn't enough room from what I've seen.
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Old 06-26-2001, 10:29 AM   #16
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Quote:
If you could do a proper 4-1 collector with equal length primaries I might agree that a single exaust is best, but you can't as ther eisn't enough room from what I've seen.
Which is why I didn't use it as an example.

Quote:
The pipes merge and become too large too soon. I have heard that the primaries on a car should be 32" or greater in length.
To big for what? You need to double the cross section of exhaust when you double the exhaust gas to keep the same velocity. Again, after a certain point of exhaust tuning, you need to decide where you are trying to put the power band. If you want your power way up in the revs, you use a larger exhaust and a cam that overlaps more. If you want more grunt, stay away from big exhausts and high overlap cams. Again, to big for what?

Quote:
There might be extra power for a true dual on a honda, but I doubt it. However the honda's have a SINGLE head. Subarus have 2!.
What does this have to do with anything? I used the example of Honda becasue there is so much money going into research and developing parts and the fact that that it is a single head makes it easy to make quite a few different variations of exhaust. Not one of which is a true duel exhaust. Two heads or one, you still have four pulses of exhaust, one every 180 degrees of crank rotation. It would be easy to do a duel on a Honda, yet they don't do it.

Quote:
I have heard that the primaries on a car should be 32" or greater in length. Thats almost 3ft. None of the 4-2-1 headers seem to have more tha 12" AT MOST for the honda's.
Quit looking at Honda headers, they don't fit your car. Here is a Subaru header that has quite long priamries.



Greg
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Old 06-26-2001, 10:42 AM   #17
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hell, quit looking at SYMS headers... you would be well on your way to paying for a low-boost turbo kit if you didn't buy those!
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Old 06-26-2001, 11:02 AM   #18
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He He He

Depending on which kit, I think you could already be there if they decided to sell them. Since they are physically larger than the turbo headers, my guess is that they go by size. Twice as long, twice as much.

Now Colin, what makes you think that turboing an EJ25 instead of using a "proven" EJ20 turbo engine is better? You can get so much more RELIABLE power out of the EJ20!!!

Greg

Disclaimer:
None of the above statements accually represent any of my thoughts and views on the EJ25 or EJ20 debate. Or buying a Syms NA header or a low boost turbo kit for that matter.
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