Exhaust crossover...

[Guest]

Hi all,

Every picture I have seen of the crossovers on the 850's shows considerable rust (mine included). My question is: is there anything inside that can fail (e.g. rust completely through, clog, etc) that isn't obvious from the outside? I have yet to see one on Ebay that looks better than mine, and mine isn't particularly good. Just trying to pinpoint all the possible causes of a poor running engine.

 

[GS1150Pilot]

The basic function of that crossover is for cold weather, cold engine ride-ability and basic exhaust scavenging. You'll note that performance exhausts tend not to have them. Now, if the pipe has rusted out and there are holes in it, that close to the exhaust ports, it seems that it could cause a lean condition and concomitant running issues.

 

[posplayr]

A lot of Yoshi Gsxr pipes as well as stock have cross overs closer to the head. Most of the 2nd gen gsxr oem pipes are a 4:2:1 "x" pipe.
i know on a large 440 dodge the cross pipe significantly smooths and quiet end the exhaust at idle and pulling away from a stop.

 

[JTGS850GL]

The basic function of that crossover is for cold weather, cold engine ride-ability and basic exhaust scavenging. You'll note that performance exhausts tend not to have them. Now, if the pipe has rusted out and there are holes in it, that close to the exhaust ports, it seems that it could cause a lean condition and concomitant running issues.

Note that most "performance exhausts" are 4 into 1 so no crossover needed.

 

[GS1150Pilot]

Note that most "performance exhausts" are 4 into 1 so no crossover needed.

That is very much a point.

 

[wymple]

I got jiggy with the welder & materials & now have a 4 into 2, without the crossover. Can't tell any difference.

 

[Steve]

If it is only the crossover that is rusted, take it out, replace it with two sections of straight pipe that are 1 1/2" diameter, 7 inches long. I used to be able to get them at the auto parts store, but they no longer carry 1 1/2" pipe. They used to carry ONE pipe at 12" long and ONE pipe at 18" long. They would not pre-order a pipe for me, so I got those two pipes and cut them to 7" long. If you stop by a muffler shop, they will be happy to cut to sections of pipe for you, but have them expand about 1 1/2" of one end so it will slide over the other piece. Cut at least four slots in the expanded section so you can clamp onto another pipe.

Not sure exactly WHY the crossover is there. If it were to smoothe out the exhaust for scavenging, it should be AFTER the Y section, not just between cylinders 2 and 3. The only plausible reason that makes sense to me is to change the sound a bit. Each bike has its own sound, having the crossover between cylinders 2 and 3 is what makes it sound like a Suzuki. It's a very subtle change, but noticeable. Note that the '79 850 did not have the crossover, so your 'new' exhaust with straight pipes replacing the crossover will sound like a '79.

.

 

[Guest]

Note that the '79 850 did not have the crossover, so your 'new' exhaust with straight pipes replacing the crossover will sound like a '79..

So does that mean that the '79s sound differently than the later ones?

 

[Guest]

Note that most "performance exhausts" are 4 into 1 so no crossover needed.

I'll bet the reason for that has a lot more to do with economics than performance. The logic that 4 into 1 somehow gets you more performance runs counter to what car people have been doing for decades; spending extra money for dual exhaust where there used to be a single exhaust. The companies that make performance exhaust systems for bikes are selling to a crowd with limited financial resources, and I'm sure that engineering and making mufflers is expensive, so if you can convince your buyers that one is better than two (or more), then you've got a sale.

 

[Steve]

So does that mean that the '79s sound differently than the later ones?

Ever so slightly, yes.

If you look critically at the pattern of sound pulses that come out each muffler, you can see the difference.

The firing order is 1 2 4 3. In the following example, “BANG” is an exhaust pulse that only goes through one muffler, “bang” is a pulse that goes through both mufflers. Here is what you would hear:

. . . Left . . Right
1 . BANG
2 . bang . . bang
4 . . . . . . . BANG
3 . bang . . bang
1 . BANG
2 . bang . . bang
4 . . . . . . . BANG
3 . bang . . bang

You can see that there is a softer pulse, a louder pulse, a softer pulse, then silence from each muffler.

In the four-cylinder GSes before 1980, there was no crossover, so this is what you would hear:

. . . Left . . Right
1 . BANG
2 . BANG
4 . . . . . . . BANG
3 . . . . . . . BANG
1 . BANG
2 . BANG
4 . . . . . . . BANG
3 . . . . . . . BANG

When you hear it, it’s a very subtle difference, but it’s there.

I have no idea WHY the crossover is there, except for the difference in sound. If it were meant for performance, it would have been placed after the Y, where 1&2 and 3&4 come together.

.

 

[posplayr]

I'll bet the reason for that has a lot more to do with economics than performance. The logic that 4 into 1 somehow gets you more performance runs counter to what car people have been doing for decades; spending extra money for dual exhaust where there used to be a single exhaust. The companies that make performance exhaust systems for bikes are selling to a crowd with limited financial resources, and I'm sure that engineering and making mufflers is expensive, so if you can convince your buyers that one is better than two (or more), then you've got a sale.

4:1 exhaust have a "scavenging effect" which refers to the exhaust pressure pulse that enhances flow at between 8-12K RPM. This also leads to back pressure and the dreaded "dead spot" around 4-6K RPM (1/2 the peak). The 4:1 was popularized by Pops Yoshimura racing back in the late 1970's and it is nostalgia that keeps strong demand today.

For the street, a 4:2:1 has a much more linear power v.s. RPM response while giving up only a small amount of power at the 4:1 peak power band. Rideability is greatly enhanced with 4:2:1 but if you are going retro on a 500 lbs 40 year old bike, the 4:1 fits the theme much better. Basically the 4:2:1 provides different tuning lengths in the pipes that when summed together do not exhibit the scavenging peak or dead band.


https://en.wikipedia.org/wiki/4-2-1_engine_exhaust_systems

[h=2]Short 4-1 vs. Long 4-2-1 headers[edit][/h]Short 4-1 headers are conventionally used to obtain high torque at high rpm. However, they face such problems during scavenging as internal mixing of exhaust gases with the incoming fresh air/charge.
Consider a four-stroke engine, with a 1-3-4-2 firing order. When the exhaust manifold is short, the high pressure wave from the gas emerging immediately after cylinder No. 3’s exhaust valves open, for example, arrives at cylinder No.1 as it finishes its exhaust stroke and enters its intake stroke. As a result, exhaust gas which has just moved out of the cylinder is forced back inside the combustion chamber, increasing the amount of hot residual gas. With a short exhaust manifold, the high pressure wave arrives at the next cylinder within a short amount of time, causing this adverse effect to continue from low to high engine speed. Hence, shorter length causes contamination and produces lesser power.
While in long 4-2-1 headers, exhaust gases get cooled on their way, because of the huge heat loss due to radiation, eventually delays the catalyst activation. Exhaust temperature can be increased by delaying ignition timing, but unstable combustion will result if delayed too much.

 

[Steve]

4:1 exhaust have a "scavenging effect" which refers to the exhaust pressure pulse that enhances flow at between 8-12K RPM. This also leads to back pressure and the dreaded "dead spot" around 4-6K RPM (1/2 the peak).

The actual horsepower peak of a 4:1 pipe can be tuned a bit with pipe diameter and length, but the fact remains that there is ALWAYS a dip at about half the RPM of the peak.

The 8-12K peak that posplayr mentions is true for the size pipes we have on our bikes. If you go with a longer pipe, typical of what you might find on a car,
you will find that they make their peak power in the 6-8K range, with a corresponding dip at 3-4K.

.

 

[posplayr]

The actual horsepower peak of a 4:1 pipe can be tuned a bit with pipe diameter and length, but the fact remains that there is ALWAYS a dip at about half the RPM of the peak.

The 8-12K peak that posplayr mentions is true for the size pipes we have on our bikes. If you go with a longer pipe, typical of what you might find on a car,
you will find that they make their peak power in the 6-8K range, with a corresponding dip at 3-4K.

.

If the OP knows anything about impedance mismatch and standing wave ratios, then perhaps he can relate scavenging as being a similar effect. The exhaust leaves the exhaust port, travels down the pipe and bounces off collector because there is a big mismatch between the flow restriction of an individual downpipes and the collector which is the twice the diameter of a single down pipe. This causes a back pressure wave that travels back to the exhaust port and inhibits flow through the head at 5K RPM(flat spot).

By the time you get to 10K RPM that same wave has bounced off the exhaust port and is back at the collector when the next ignition cycle happens causing increased flow at 10K RPM. Yes physical lengths are what tune the RPM range.

 

[Guest]

So what I think I'm learning from this discussion about the crossover in my exhaust, is that if I were to replace it with straight pipes, I might hear a slight difference in the exhaust note (if I had a solid memory of what it used to sound like), and I might notice a slightly different way that the torque was delivered (if I had a well-tuned backside). I don't have either, so I doubt I'd notice any difference.

Which brings up the next question: has anyone ever actually had one of these things open to see what's inside? Is it just a hollow chamber, or is there some sort of baffling inside?

Thanks for all the info guys. It's always good to see a well-informed, respectful discussion these days.

 

[Steve]

Which brings up the next question: has anyone ever actually had one of these things open to see what's inside? Is it just a hollow chamber, or is there some sort of baffling inside?

Yes, I have cut one open There is NOTHING inside, just open space.

.

 

[Guest]

I'll bet the reason for that has a lot more to do with economics than performance. The logic that 4 into 1 somehow gets you more performance runs counter to what car people have been doing for decades

The car people are wrong. 4-1 exhausts have long been proven by theory and actual performance to be the best arrangement for an inline 4 engine such as our GS's. It has nothing to do with economy and everything to do with physics. As noted by posplayr and others, 4-2-1 gives a smoother torque curve but an equivalent 4-1 always gives a higher torque peak and more peak power.


Mark

 

[bobgroger]

http://www.cycleworld.com/2015/06/26/ask-kevin-why-do-my-exhaust-headers-have-crossover-tubes
The Bandit 1200 has such tubes, supposed to broaden the torque curve

 

[posplayr]

http://www.cycleworld.com/2015/06/26/ask-kevin-why-do-my-exhaust-headers-have-crossover-tubes
The Bandit 1200 has such tubes, supposed to broaden the torque curve

It must all be magic