Custom exhaust

[94SubieSVX]

Originally Posted by Seraphinwolf

Has anyone tested to see if the Legacy GT cat-back meets up at all. Even just the axle-back maybe? I don't know I just want something other than a GIANT Camero looking muffler in the back. Makes it hard to get a jack under there.
-Gaddis

Actually the flowmaster 80 muffler is about half the size of the stock SVX muffler, and it's thinner so it would be farther up against the body

[shotgunslade]

Back pressure in the exhaust never helps power. It always hurts, because it increases the mass of the products of combustion that remain within the cylinder when the exhaust valve closes. Tuned exhaust systems typically help within a narrow rpm range. When the exhaust valve opens there is an increase in pressure as the exhaust gases enter the header. That pulse will flow down the header and after it passes there is a period of minimum pressure before it bounces back again. The pressure wave travels back and forth along the length of the header pipe. If the next opening of the exhaust valve coincides with a minimum of the oscillating pressure at the entrance to the header, there will be a greater pressure differential between cylinder and header improving the exhaust gas exit from the cylinder.

The tuned exhausts on kart engines don't actually cause exhaust gases to re-enter the cylinder from the exhaust header. Instead the exhaust port is uncovered by the piston on the power stroke before the intake port. The exhaust gases rush out at a high velocity followed by some of the intake charge which is being pushed into the cylinder from the crankcase by the downward movement of the piston. Some of the intake charge actually goes out the exhaust port. The reflected exhaust pressure wave actually forces much of that escaping intake charge back in the cyclinder. Since the time between the opening and closing of the intake port is entirely a function of the engine rpm, and the time it takes the pressure wave to reflect back is primarily a function of the length of the exhaust chamber, the exhaust can be tuned to maximize power only over a very narrow rpm range.

In a 4 stroke engine, most of the tuned pressure wave reflection effect will occur in the header. The idea here is to time the pressure wave returning to the entrance of the header when the exhaust valve is closed, rather than when it is open. High performance header design seeks to avoid combining pipes from cylinders such that the reflected pressure wave from one cyclinder bounces back to another cyclinder when that exhaust valve is open. Once again, the effect will be limited to a narrow rpm range, usually near the peak power point. By the time the exhaust gases mix with the pipes from the other cylinders and hit the pre-cats and, it's all over. The whole intent from the pre-cats back is to avoid back-pressure. I really don't think X-pipes that are downstream of cats do much to help engine power, except possibly to equalize the modulating flow in the two pipes to decrease pressure drop. I think they're mostly about customizing the exhaust system sound.

[benebob]

It does however even out the powerband and make flat spots somewhat less noticable. NA Subie engines do tend to run better with some backpressure. I was talking with OT the other day about longer headers being a power source (yeah this is coming from the one who likes his 5.5 foot long unequal length 6 into 1). Look at motorcycles that are tuned for performance. There is very little in the way of back pressure and they make the header as long as humanly possible so they can still get the needed sound deadening device on it so that it doesn't stick out beyond the rear end. Most performance bikes do suffer from very uneven power maps that you can feel easily due to the weight to power ratio. My guess is this has something to do with the free flow exhaust.

Originally Posted by shotgunslade

Back pressure in the exhaust never helps power. It always hurts, because it increases the mass of the products of combustion that remain within the cylinder when the exhaust valve closes. Tuned exhaust systems typically help within a narrow rpm range. When the exhaust valve opens there is an increase in pressure as the exhaust gases enter the header. That pulse will flow down the header and after it passes there is a period of minimum pressure before it bounces back again. The pressure wave travels back and forth along the length of the header pipe. If the next opening of the exhaust valve coincides with a minimum of the oscillating pressure at the entrance to the header, there will be a greater pressure differential between cylinder and header improving the exhaust gas exit from the cylinder.

The tuned exhausts on kart engines don't actually cause exhaust gases to re-enter the cylinder from the exhaust header. Instead the exhaust port is uncovered by the piston on the power stroke before the intake port. The exhaust gases rush out at a high velocity followed by some of the intake charge which is being pushed into the cylinder from the crankcase by the downward movement of the piston. Some of the intake charge actually goes out the exhaust port. The reflected exhaust pressure wave actually forces much of that escaping intake charge back in the cyclinder. Since the time between the opening and closing of the intake port is entirely a function of the engine rpm, and the time it takes the pressure wave to reflect back is primarily a function of the length of the exhaust chamber, the exhaust can be tuned to maximize power only over a very narrow rpm range.

In a 4 stroke engine, most of the tuned pressure wave reflection effect will occur in the header. The idea here is to time the pressure wave returning to the entrance of the header when the exhaust valve is closed, rather than when it is open. High performance header design seeks to avoid combining pipes from cylinders such that the reflected pressure wave from one cyclinder bounces back to another cyclinder when that exhaust valve is open. Once again, the effect will be limited to a narrow rpm range, usually near the peak power point. By the time the exhaust gases mix with the pipes from the other cylinders and hit the pre-cats and, it's all over. The whole intent from the pre-cats back is to avoid back-pressure. I really don't think X-pipes that are downstream of cats do much to help engine power, except possibly to equalize the modulating flow in the two pipes to decrease pressure drop. I think they're mostly about customizing the exhaust system sound.

[Speedklix]

ohh wait.. the lack of driveshaft might not have helped either
forgot all about that... still, with stock heads it should still need at least a little right?

[94SubieSVX]

I can't wait to hear the various custom exhausts at Cumberland this month, I'm **hopefully** getting mine done Monday.

[UPnorth362]

I think a lot of people are getting confused with the term back pressure. Here is my take on the subject. A good short primary equal length header will make more difference in power,than any other part of the exhaust. Why, because as the exhaust pulse is not flat. It leads with a higher pressure than ambient(positive pressure), and tails off with a pressure lower than ambient(vacuum). Now when a header is installed, it tunes the exhaust pulses so that the tail end of one cylinder pulse(vacume) actually pulls the high pressure pulse(positive pressure) from the next cylinder with it, like the - and + poles on a magnet. This is effective because the header keeps the pulses in sync, and keeps a pulse from one cylinder from backing into the next cylinder like a manifold does. Why do auto manufactures use a manifold then over a header? Because its cheep, and easy.

Now, about the rest of the exhaust. Back pressure will not make more power under any circumstance on a 4stroke motor. This does not mean that the bigger the pipe the better. Think about this. Cold air is alot heaver and denser than hot air, and therefore harder to move. The key to a good exhaust system is to keep the exhaust gas as hot as possible the whole way down the pipe with the least amount of turbulence from bends and diameter changes. How do we do this? By selecting a pipe diameter that is large enough to not cause restrictions, but small enough to keep the gases moving quickly, and with the least amount of turbulence from bends.

As for resonators and mufflers. They dont do much for power, unless you are replacing a restrictive one with less restriction. There main function is simply to quiet the exhaust note. I see a lot of people focus on magnaflow mufflers here, but I think more people should take a look at Flowmaster and Walker mufflers. They are dyno proven many times over to be better flowing per db than most of there competitors, plus the noise canceling technique they use makes for a non raspy, deep sound.

[UPnorth362]

On a side note, two stroke engines require proper back pressure for both engine longevity, and peak performance. But that is a topic saved for another day

[94SubieSVX]

I'm not sure how many people are using Magnaflow mufflers, but I do know that a decent amount are using some sort of Flowmaster muffler.

[jeffast]

my $.02, i have magnaflow's on both my cars they flow better and sound better.
flowmasters are just annoying in my oppinion, but to each their own

[benebob]

Originally Posted by UPnorth362

A good short primary equal length header will make more difference in power,than any other part of the exhaust.

Then why would the most powerfully tuned per liter engines (motorcycles) have disproportiontely long mostly unequal headers.

[processengr]

Hi all,
Which resonator sounds better, the Aero like in the E3 on SVXCESS's car or the magnaflow #? Anyone heard both? I am going into the bullet Camaro muffler, like Shotgunslades. I only care about the sound. If appearance was a concern, the Aero wins, peroid. The two resonators appear to be different internally, based on the cut away on magnaflows site price is close... Is the Flow master resonator any better or different yet? Being near Nino's Performance in Bethany, and JP motors in Woodbridge, I get to hear a few Italian cars now and then

[94SubieSVX]

Just got back from my exhaust guy and he quoted me $450 to weld everything up, and I have all of the parts except the tubing. That price is because my boss is good friends with the owner.

I called up Mufflex which is another shop by me and they quoted me $900-1200.

Hopefully Monday I can take it to get everything welded together...can't waittt

[Hondasucks]

That third 02 sensor on the OBD2 cars is after the cat because it is there to check the operation of the cat, if you put it before the cat, it'll see the same reading as the pre-cat sensors, and will throw a catalyst failure code. There are "cheater" harnesses that piggyback one of the front sensors and fakes what the rear one should see if the catalyst is working, I see them on ebay all the time...

[SVXRide]

Originally Posted by benebob

Then why would the most powerfully tuned per liter engines (motorcycles) have disproportiontely long mostly unequal headers.

because someone actually did the calculations and they're trying to spread the hp/tq across a specific rpm band versus a single peak with equal length?
-Bill

[svxfiles]

Originally Posted by SVXRide

because someone actually did the calculations and they're trying to spread the hp/tq across a specific rpm band versus a single peak with equal length?
-Bill

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