Is the best exhaust system still to come.

[Dessertrunner]

This thread is going to send peoples thought pattern into tooth fairy land. Reading all the different ways of fixing the SVX system I got thinking that there must be sciencetefic way to work it out so her we go. A model airplane engine used in racing has an exhaust call a tuned pipe. The link explains, www.bmaps.net/info/pipes.html these are used to give more power to the engine by getting the exhaust away from the engine.
Next point is if you think about a turbo charger it uses the exhaust gases to power the air pump : so why can't we design a exhaust system for the SVX that acts like a pump and pumps the gase away from the engine.

Given all of the above I think it is safe to say the following which you can all feel free to challange:-

1- If the length of the pipes from one side of the engine is longer then the other you run the risk of the timing of gases resulting in two cylinders gases colliding at certain revs.
2- The performance of the engine is best if both pipes go into one then there is a more consistant airflow through the pipe.
3- The volume Dia of the pipe (total size) needs to decrease the further it gets away from the engine. Logic being that the air cools and the velicioty will slow down if the pipe size is to large. This is how I think a turbo charger powers the air pump.
4- The two pipes from the engine need to meet in a way that enables one banks gas to suck the other banks gas.
5- Next theory is that the overall diameter in the pipes from the engine affects the the power band of the engine. Example being that if the first pipes from the engine are to large in ruins the low rev power.

Okay that's my $'s worth I am happy for everyone to tell me that I am wrong or that there is a better way. If we all put heads together we might work out a better exhaust system. Depending on the interest I will later explain what they found out about VW exhaust systems back in 1940.

Tony

[thundering02]

#3 you'd have to coble several sections of barely-smaller-each-time pipe to even attempt to do this properly (and I see no one doing so)
#4 if you make the #1 cylinder side 1-2 inches longer(before the join) I think maybe that will work
My $.02

[mbtoloczko]

Some of the theory of 2-stroke exhaust system is applied to 4-stroke exhaust systems. Harvey (oab_au) has described it on the board several times. The same basic theory applies to either type of motor: use the energy of the exhaust gas to help scavenge any exhaust gas remaining in the cylinder late in the exhaust stroke where it doesn't easily come out. Its been scientifically proven that sonic reflections can help scavenge exhaust gases, but there is no solid evidence that I've seen that proves that the suction created by the exhaust gas plug has any effect on exhaust scavenging. The suction concept is very common though when speaking to people about exhaust system design. If you want to read about the sonic reflections, there is a good book out there on the topic. I can't remember the name or author, but I have a copy at work. I'll post the author/title info after I get to work if someone here doesn't post it before I do.

I know the stock headers are supposed to flow nicely, but has anyone tried a setup with longer exhaust primary pipes? From every car I've owned except the SVX I have put extractors on and achieved a power gain...I don't see why it would be any different with the SVX. After I get an engine rebuild I'm going to have a set fabricated. Of cause, you all will be informed how it turns out. I already have a dual exhaust system in, but that's going to be replaced with a better quality mandrel bent dual system after the extractors are made.

I guess I can't recommend a product without trying it first, right???

[SVXRide]

Originally Posted by MisanthropyV

I know the stock headers are supposed to flow nicely, but has anyone tried a setup with longer exhaust primary pipes? From every car I've owned except the SVX I have put extractors on and achieved a power gain...I don't see why it would be any different with the SVX. After I get an engine rebuild I'm going to have a set fabricated. Of cause, you all will be informed how it turns out. I already have a dual exhaust system in, but that's going to be replaced with a better quality mandrel bent dual system after the extractors are made.

I guess I can't recommend a product without trying it first, right???

Tom (SVXFiles) has built headers with different primary pipe lengths -- check his locker out for pics.
-Bill

[Red SVX 92]

I always thought a true-dual system with longer header lengths would be optimal for an NA application.

[drivemusicnow]

Tony,

You're grasping the overall concept of the exhaust system pretty well... You aren't exactly grasping the turbocharger aspect, however since our cars are NA we don't really need to worry about that.

The best exhaust system is one that lets the exhaust gasses out of the cylinder with as little back pressure as possible. and for all of you who just read that and said "wha?!?1 My tuner friends say you needs the back pressure" Realize that they are not smart and you probably shouldn't listen to them anymore.

Backpressure is bad, always, Whats worse however, is putting a pipe so large that the exhaust loses so much speed that it doesn't go through the pipe.

What would be "best" would be a very very insulated pipe that gets progressively smaller towards the end, however at a calculated rate. say 1/4 inch radius smaller per 4 feet of piping (just a random number, I haven't taken fluids or thermo yet) Thats all well and good, but who's going to make this pipe for you? therefore you can't do this. so you keep it the same diameter all the way through. but again, insulate it such that the exhaust does not radiate heat.

Then we need to look at what the "best size" is for the SVX, I haven't crunched the numbers, but i'm sure Oab_au has and could again enlighten us as to the exhaust flow properties of an eg33. you would need to look at exhaust flow from teh power band you want.

Even if we did then figure out what Radius pipe to use after all 6 cylinders have been collected, we need to look at how to collect them. If you look at a long tube header from an LS1 for instance you see the 4 pipes in a square pattern. We would need to create a similar "long tube header", equal length in a triangle pattern for each bank, and then merge those two headers into a single pipe. I'll look into seeing what size pipes should be used for the header tubes, each banks tubes, and then the single straight pipe that goes all the way back. This should increase the scavenging effect, however as much as theory says this should work, when looking into the stock exhaust system the engineers at Subaru did a pretty good job of this originally.. so gains would be minimal.

[oab_au]

As you say Tony, everyone has a different "trick" exhaust system. You can read conflecting statments from a Goolgle search of exhaust systems.

There is the old iron block school, that says "you got to have enought backpressure", then there is the modern tuned length school, that applies to the modern design of engines that the SVX is.

When considering the way the exhaust works, there are two different functions that occure. When the exhaust valve opens there is a sound wave released, traveling at around 1500/1700 ft per sec. There is also a gas pressure front of about 65/70 pis released traveling at about 300 ft per sec.

Considering the gas pressure front first. This high pressure has to drop as fast as possible, as if this pressure does not drop quickly enought and remains in the cylinder, the piston will start to rise aganst this pressure on the exhaust stroke, robbing us of power. The sooner this pressure is dropped the better. The best is a stub exhaust , that will drop the pressure instantly, but as we have to run it out the back, it has to be longer . The way to get around this is to fit an expansion chamber to the pipe, to allow the gas pressure front to expand in, that will drop the pressure faster than waiting for it to get to the back of the car. The action of this gas front travelling down the pipe will pull a lower pressure behind it, that helps to lower the pressure in the cylinder around BDC. This is the back pressure that the old school talk about, it does work, but as it will only travel at the same 300 ft/sec, regardless of engine speed, so it will become a hinderance as the engine speed increases, so it is only used for the short term around BDC.

Now to look at the sound pressure wave that was released at the same time. It is a positive pressure wave that will travel from the exhaust valve, down the pipe at around 1500/1700 ft/sec till it reaches the open end, here it expands into the atmosphere, to be replaced with a negitive pressure sound wave,( this is organ pipe theory)that will travel back up the pipe arriving at the cylinder,about 15/20 degs BTDC, to lower the cylinder pressure as the inlet valve opens, to induce a faster start to the inlet gas flow, and a higher cylinder filling.

To get this low pressure to arrive back at the cylinder at the right time, we need to set the pipe length, to a length, that the wave will travel, at its speed, at the desired engine speed.This length is worked out by multiplying the wave speed by the number of degrees, till it is to return, and dividing by the engine rpms. 1600 X 180 / 5000 = 57.6" (this depends on a heap of variables, gas temp, header volume, cats ). So if we position the expansion chamber at this distance from the exhaust valve, the wave will expand at that point, as if it was the end of the pipe.

To put it all together, we need an expansion chamber/resonator/straight through muffler, that will terminate the sound wave at the right distance, and allow the gas pressure front to expand into, to drop the early cylinder pressure. Just like Subaru has provided the SVX with.

This is the same if two pipes are run, each will have a resonator located at the right distance from the exhaust valve, after that, it does not matter what you do, as long as it is free flowing.

The way the pipes join is important. They should not join, till they have entered the resonator chamber. If the two pipes are joined before the chamber, the high gas pressure front from one side will enter the other side, to flow into the open exhaust valve on that side. They should both expand into the chamber to drop pressure, before joining. Useing a Y pipe to join the header pipes before the resonator is a NO NO, it may look good for flow, but the pressure front will expand in all directions equally, regardless of the shape.

Extractors sound like they would work. But the name means nothing. The idea that the gas flow from one cylinder will extract the gas from another, is a myth. The exhaust valve opens to enought pressure to ensure its flow, without useing a negitive pressure to help it out. The main function of most 'Extractors' is to divide the cylinders so that they don't intrefer with each other. A 4 cylinder divides cyl 1/4 from 2/3. a 6 cylinder divides 123/ from 456. Our engine has these divided already by the two header pipes.

So if I was building a exhaust system for it, it would be the same as it is now. If you were going to remove cats, you will change the effective length of the header pipes, to change the rpms that the exhaust will resonate at, and you will have to try different lengths till you find the right length that works at the right rpms. If it was for a supercharged car, I would increase the volume of the resonator to allow for the higher pressure that the engine will have, and increase the pipe dia from the resonator back to the muffler.

Harvey.

[NikFu S.]

So removing the resonator was a bad thing.
Or is the primary cat considered the first res chamber for the terminating sound wave?

[Beav]

Harvey - Not sure if they still market them here or ever did there but some header manufacutrers used to sell adaptor plates that had cones at the flange, pointing downstream. They called these cones extractors. The marketing ploy was that the reversion pulse would strike the cone's back side instead of the exhaust valve, enhancing the scavenging effect. Polyrazzmatazz! The cones are a flow restriction, not to mention the reluctance of surface-bound molecules to flow which would negate the majority of any possible reversion enhancement. Seems to me the cones would effectively be bringing the chamber effect almost to the valve head.

[Dessertrunner]

This has become a interesting debate, if back pressure reduces power then will you get more power with no exhaust as opposed to a well designed tuned one?

[thundering02]

No because you need some slight back pressure....Hot Rod mag did an issue on that a couple years back and thier camaro big block made close to 15 extra horses when more than headers were used most naotably striaght pipe all the way back

[Red SVX 92]

Originally Posted by drivemusicnow

Even if we did then figure out what Radius pipe to use after all 6 cylinders have been collected, we need to look at how to collect them. If you look at a long tube header from an LS1 for instance you see the 4 pipes in a square pattern. We would need to create a similar "long tube header", equal length in a triangle pattern for each bank, and then merge those two headers into a single pipe. I'll look into seeing what size pipes should be used for the header tubes, each banks tubes, and then the single straight pipe that goes all the way back. This should increase the scavenging effect, however as much as theory says this should work, when looking into the stock exhaust system the engineers at Subaru did a pretty good job of this originally.. so gains would be minimal.

What would be the optimal collector shape for an H6 be then? triangles on each side? Would the lengths needed to ensure that the exhaust pulses from each of the three cylinders on each side don't interfere with each other be feasible, i.e., is there enough room in the engine bay for tuned length headers?

[Red SVX 92]

Originally Posted by oab_au

This is the same if two pipes are run, each will have a resonator located at the right distance from the exhaust valve, after that, it does not matter what you do, as long as it is free flowing.

The way the pipes join is important. They should not join, till they have entered the resonator chamber. If the two pipes are joined before the chamber, the high gas pressure front from one side will enter the other side, to flow into the open exhaust valve on that side. They should both expand into the chamber to drop pressure, before joining. Useing a Y pipe to join the header pipes before the resonator is a NO NO, it may look good for flow, but the pressure front will expand in all directions equally, regardless of the shape.

Would true duals be the best option, or would there be no significant difference between that and a regular joined Y-pipe exhaust?

[oab_au]

Originally Posted by NikFu S.

So removing the resonator was a bad thing.
Or is the primary cat considered the first res chamber for the terminating sound wave?

Yes Nik, the resonator is needed for the higher rpms, to drop the pressure. The second cat that joines the two headers pipes, has a small chamber in the front of it, but it is not big enought to drop all the pressure. Though it could terminate the sound wave at higher rpms, not sure of this.

Harvey.