Exhaust-ed

[Wildkat]

Okay... somebody walk me through this s-l-o-w-l-y, please...

On the 81 Special - there is a MAC 4 into 2 system with 1 & 2 going to the left and 3 & 4 going to the right... no connecting tube between the two sides.
On his 80 Special - John has a Jardine spaghetti set with 2 & 3 going to the left and and 1 & 4 going to the right... There is a connecting tube between the two sides.

How does the firing order affect the exhaust? and is there any reason why 1 & 3 can't go to one side or another and 2 & 4 go to the other?

[xschop]

exhaust

my 1,2 exit left and 3,4 exit right with no cross-overs...they are jardine non-spagetti...

[Pat Kelly]

Jardne did that (1-4, 2-3, with crossover) for exhaust pulse scavaging.
The engine is an air pump. Air and fuel get drawn in the intake by the downward movement of the piston (intake valve open, exhaust valve closed), compressed as the piston moves up (both valves closed), the sparkplug fires igniting the compressed air?fuel mixture pushing the piston back down. The piston goes back up as the exhaust valve opens forcing the burned air/fuel mixture (exhaust) out of the motor.
This pulse of exhaust travels down the exhaust pipe. As it moves along in front of the pulse is a high-pressure front, behind it is low-pressure. This low pressure helps to draw the following pulse (from the next cylinder in the firing order) along. This is called scavaging.
The crossover balances the two sides and somewhat scavanges one side to the other.
Remember, one coil fires cylinders 1 and 4 (left side of the Jardine system) and 2-3 are fired by the other coil (right side).
The Mac is just moving the exhaust without as complete scavaging as The Jardine.
The Mac should actually work better at high RPM as the pulses aren't scavanged at high RPM. Scavanging makes the air pump (engine) more efficient at mid-lower RPM.



I think

[79XS11F]

The idea when executed properly is that the exhaust gases that are leaving in the pipes of one cylinder will create a vacuum in the pipes and draw out the exhaust gasses of the next cylinder that is entering the exhaust stage. The gases expand as they exit the cylinder and as they go through the pipes. This expansion of the exhaust gases speeds up their movement through the exhaust system. The entire process allows intake fuel / air to enter FASTER so a fuller charge enters. This is why good exhaust to head seals at the manifolds is so important. Even on the stock bike exhaust leaks can reduce performance. This is also why all pipes have a "sweet" spot where everything is working as planned. True high performance headers will have been flow bench tested to maximize this effect and increase horse power. It is of course a lot more completed then that with everything from collector pipe length and DIA to megaphone size and shape having an effect on the system. True performance pipes for these bikes include the KerKer, The RC engineering and Vance and Hinds 4 into 1 systems.
Rob

[Brian_D]

Jardine 4-2 pipes

Originally posted by Pat Kelly
Jardne did that (1-4, 2-3, with crossover) for exhaust pulse scavaging.

Did all the Jardine 4-2 spaghetti pipes (for the XS1100) include a crossover between left and right sides?

Not that I can add any knowlege to the thread, but HERE is some more discussion on this topic.

[Wildkat]

That helps, but I still have questions...

Thanks! John had explained the scavenging to me (I have to see these things in writing though... ) and I think I understand the purpose of the crossover tube... kind of an equalization process? more efficient...

It still leaves my questions though... unless I missed the answers (which is entirely possible)

Okay... here's the deal... Since MAXIMAN brought up the 4 into 1 into 2 system... it started me thinking (yeah... look out! ). This is the set up that I have on the FJ and until the time of that post... thought it just a PITA that I had to drop the exhaust to change my oil... Then after reading his post, I realized what a performance difference this might make. I know Cody had problems because of the size of the collector on his system being too small though.

So I was wondering if routing a 4 into 1 into 2 system with 1 & 3 together, 2 & 4 together and having them come into a collector would work? Wouldn't even need to change the way they are routed I guess... but why would routing 1 & 3 together be any different than routing 1 & 2 together? This is why I wonder about the firing order and the way each cylinder responds...
In my futuristic engineering plans (ahem)... I was wondering about the advantage of having two venturi tubes in a two chamber collector 1 & 3 routed into one tube, 2 & 4 routed to the other and then have both tubes empty into a second chamber in the collector that also provided a small amount of space for back pressure.

I realize that the size and length of the tubes and the length of the megaphones (if any) would affect the performance as well... but how skewed are the physics of this idea?

Thanks again for the help and the link!

[Jerry]

There is a connecting tube between the two sides.

The idea of the balance tube is to improve torque at low rpms. It generally will be after the collectors, where two tubes blend into 1, and befor the muffler proper. On both the XJ OEM exhaust and the Jardine "spagetti" 4 into 2 systems the crossover is in front of and below the swing arm bushings.

I could be wrong, but every 4-cylinder 4-stroke that I know of uses a firing order of 1 - 2 - 4 - 3. Since the Jardine ties 1 & 4 to one pipe and 2 & 3 to the other, you get one pulse every 180 degrees from each pipe. Compare this to the stock pipes 1 & 2 (left), then 3 & 4 (right). You see that the pipes get a 90 degree pulse followed by a 180 degree "gap". I.E. on the left pipe 1 fires, then 90 degrees later 2 fires, but then you wait 180 degrees for 4 and 3 to fire out the right pipe before there is another pulse in the left pipe when 1 fires again. This can't be all that effective for scavenging.

I don't know enough about the math to know if 180 degree pulse is better or worse than 90 degree pulse you get from a 4 into 1, suspect over-all length, muffler design, cam profile, compression, and all the rest figure into it, but do think the Jardine 'spagetti' style scavenges better than the OEM design. The Jardine collector output is larger in diameter than OEM. Larger pipes tend to scavenge better in mid to upper RPMs and smaller pipes do a better job low down the RPM range. The balance tube is a compromise to get better low-end grunt (simulates smaller-diameter tubing) without sacrificing mid and upper range performance.

Back pressure in the mufflers and rest of the system also affect scavenging; many engines will run worse with loud exhausts than they do with an OEM system. One reason some systems have replaceble disks is not just for sound, you can actually tune the system for best performance over the RPM range you generally use by changing the back pressure characteristics of the muffler(s).

A bit off the subject...2-stroke engines are far more suseptible to scavenging changes. Try running a 2-stroke snowmobile or 2-stroke dirt bike with no exhaust and see how far you get.

[TADracer]

I think it all has alot to do with firing order and actually goes all the way back up into the combustion chamber. The scavenging effect of one cylinder on another is not just to help suck the exhaust from the pipe but is to help start the new charge of fuel into the next cylinder in the cycle.

Consider that on the exhaust stroke the piston is pushing the gasses from the cylinder and is chasing the exhaust valve closed as it reaches the top of the stroke. This is why checking valve clearance on the exhaust side is so, so critical to prevent a collision when milling the head or decking the cylinders or chaning to higher lift or longer duration cams. Anyway, the low pressure part of the exhaust pulse, that Pat Kelly mentioned, serves to make that last bit of exhaust gas rushing from the spent cylinder draw in new fuel during that critical period of valve overlap when both the exhaust and intake valves off of their seats. So, what a tuned set of pipes are doing is using the exhaust pulse from one cylinder to hasten the pulse of the next cylinder in the firing line to make the most of the valve overlap period.

In a 4 into 1 header this is all happening at the collector and is accomplished by having the correct length and diameter pipes from the head down to the collector. In the collector, any one pipe can act on the other three so the firing order and pipe placement is not so critical. That is why a well tuned 4 into 1 header will outperform about any 4 into 2 system, crossover pipe or not, in the higher RPM ranges.

The Jardine spaghetti 4 into 2 system attempts to partially accomplish this by placing the 2 and 3 pipes together in that two-pipe collector and the 1 and 4 pipes together in the other two-pipe collector.

I think

Hey, it sounds good.

[79XS11F]

To work well the cylinders that fire one after the other should be connected to the same collector pipe. With 4 into 1 setups they all meet so there is no problem so long as the header pipes are of equal length. This is why true high performance headers are all set up with all 4 headers pipes meeting right in the middle under the bike. Any set of pipes for an in line 4 like ours that maintains easy access to the oil filter will not increase performance a lot and often do not even need to be jetted. A pipe that works will needs jetting because it does in fact speed up total air movement from the intake side through the engine and out the tail pipe. If you start running header pipes of different lengths to set up a firing order that is one after the other it just does not work well. That has more to do with exhaust harmonics then gas flow. Harmonics is the pulse that has been mentioned. If I can find it again I will post a link to a site I found about exhaust engineering.
Rob

[pggg]

Inline 4's like the XS, common firing order is 1342, for a performance 4-2-1 pipe, then equal length 1 & 3 connected and 4 & 2 connected to the 2 collectors. Good performance 4-1's can still leave room for easy filter access, just need to be spaced/designed for the particular bike.

[skids]

One thing about scavanging that I believe: If you have exhaust scavenging at certain rpms, you will have the opposite effect at other rpms. You might call that bad effect "pressure piling." In my opinion, if you aren't racing, you would want a flatter torque vs rpm characteristic curve.

Originally posted by 79XS11F
That has more to do with exhaust harmonics then gas flow. Harmonics is the pulse that has been mentioned. If I can find it again I will post a link to a site I found about exhaust engineering.
Rob

[Pat Kelly]

The main thing you want is exhaust velocity. You need to keep the exhaust temperature high. When the exhaust temp cools it becomes denser and slows creating back-pressure.
From what I've been reading back-pressure isn't needed, it just kind of exists. Going to too large an exhaust pipe after a collector reduces the velocity and flow of the exhaust creating more back-pressure. This back-pressure inhibits the exhaust from exiting the combustion chamber thus not allowing the incoming air/fuel to optimally fill the cumbustion chamber. On the flip side, too much flow will draw the air/fuel mixture out the exhaust port during the overlap phase of the cam timing (the breif time when both valves are open).
It's a case of deciding what you want from your engine and tuning it to that. If you're racing and running at redline most of the time you want a free-flowing, non-restrictive exhaust system. It will be loud and won't optimize the low and mid range of the powerband. If you cruise in the middle of the RPM range then the stock exhaust is designed for just that.
Some of us have stock exhausts that are rotting away and after market is about our only resource for replacement. With the variety of types available we should be able to get what works best for us.
For my case, I use my 78 E as my commuter, workhorse, long-distance, foul weather ride. It spends most of the time in the 3,000 to 5,000 RPM range. Yes, I do pedal it faster at times. I replaced the stock exhaust with the Jardine 'spagetti pipes' because:
a- the stock was cosmetically rusty
b- I just wanted some headers (never had a vehicle with headers)
c- They work better in the mid-range than a 4 into 1
d- got them at a great price.
My 80 LG is also my commuter but since I (mostly) only ride it when the weather is nice, I ride it a bit harder and at a higher RPM (smaller special back wheel), usually in the 4,000 to 6,000 RPM range and I go to redline more often. This one has a 4 into 1 because:
a- I want to preserve the stock exhaust which is in great shape.
b- I ride this bike more aggressively
c- It's tuned toward the upper RPM range
d- I got it at a better price that the Jardine's

Google "motorcycle exhaust scavanging" and "motorcycle exhaust back pressure" and you can find a wealth of information.

[Wildkat]

Firing order

I've seen two different sets of numbers for the firing order... 1243 and 1342 (this was posted in another thread too)... Having the pipes paired so they are in harmony with one that fires before/after it makes sense... That's why I wonder about the current set up...
If it fires 1432, then it makes sense for the 1/4 & 2/3 crossover as well as the 1/2 & 3/4...
Sooo... is the 1342 actually an option for the firing order of these bikes?

Does anyone have a 1/3 & 2/4 set up on their exhaust?

I like the idea of the venturi tubes to increase the rate of flow without adding back pressure... seems you could increase performance on one end... without sacrificing it as much on the other...

Really good information you all have given here!
Thanks SO much!
Anything more you have to add is certainly welcome...

[Wildkat]

Duh

No coffee yet this morning...
Even if the firing order IS 1342... I guess 1/3 & 2/4 would STILL be paired with one in line next to them... as it would with 1/2 & 3/4...

So... does anyone know exactly what the firing order is?

[TADracer]

Re: Duh

Originally posted by Wildkat
So... does anyone know exactly what the firing order is?

Hmmm, I dont really know. But pull the valve cover and watch the cams as you turn the engine over and that will tell you.