Curious about exhaust

[81xsproject]

I am just currious, why is it that dual exaust relieves conjestion and improves performance on car, but is opposite for motorcycles where the 4-1 improves performance?

[TopCatGr58]

Hey 81XS Project,

Well, funny how this thread has been viewed 51 times, but not a single response!? So....I'll give it a try!

I'm going to assume on a car, you mean back in the old days, with the V-8's, many were made stock with very constricting exhaust manifolds, and were joined together into 1 pipe to run thru 1 muffler, cheaper! So... putting on a decent set of nice flowing headers, and running both sides into exhaust pipes for each side, greatly increased the breathing ability of the engine, less restrictive so the pistons didn't have to push as hard to push out the exhaust, and allows more air and fuel both in and eventually out, so it increases the horsepower that way!

And one side of a V-8 is a 4 cylinder, and a well designed exhaust header actually performs a scavenging action, so that the exhaust pulses don't work against each other, but flows into each other as they progress thru the pipe.

So...on our bikes, with a well designed 4-1 pipe set, a similar scavenging effect occurs, reducing restriction, increasing outflow, and eventually a bit more horsepower.

Any car/engine tech heads want to rebuttle??
T.C.

[Hobbit]

Headers

As usual, you've got it right T.C.
We even used to worry about having all of the tubes an equal length and joined together properly in the collector so that the firing pulses coming out of one pipe would create a low pressure area in the next pipe to fire.

[prometheus578]

One of the 51

Last night I was gonna post about "pressure waves" and "scavenging effects".... But not being well versed enough on the topic to bluff my way through a possable argument, I decided to shut up and crawl back into my hole.

Nice job again, TopCat

"Searching for the truth is ugly, frightening and dangerous
- and the only worthwhile choice."

[uk1.1s]

Another one of the 51...

Hmmm... well I looked at the post and thought... yeah of course it's to do with... erm... I know... just a minute.. it'll come to me... then I too crawled away in disgust!

It's a bit like when your little girl asked "why is the sky blue daddy?"..... erm ...ummm... oh let's se....... it just is OK.

Luckily I have about a year to go before my daughter can talk..... I should have worked out the answer to blue sky by then... ummm - it must be to do with...erm.... Oh I give up let's "Google" it.

[randy]

The sky is not blue. The atmosphere reflects that particular wavelentgth of light. The other wavelengths pass through.

[fbcaf]

Sky isn't Blue!

I've known that for years. I'm colorblind, so it can be whatever I want it to be, just like the "blue" ocean, or Kentucky "Blue" grass. But, back to the subject, which hasn't been exhausted yet. I don't pretend to understand pulses and wavelengths much, but would not a "well-designed" 4 into 2 work as well or better than a "well-designed" 4 into 1? I have the stock 4 into 2 on my 80G; how much improvement would I experience with a change to 4 into 1? Would it be that significant?

[Pat Kelly]

The 'pulse' is the exhaust shooting out of one cylinder. A 'tuned' exhaust system has the 'pulse' from one cylinder help 'drag' the 'pulse' from the next cylinder in the firing order.
Stock exhaust isn't 'tuned'. It's just pipes routing exhaust out the back and trying to be as asteticly pleasing. Thus the left two cylinders run out the left and the right two cylinders run out the right. There is a crossover pipe connecting the two side to keep the pressures balanced. The stock system also is made of double-tubes, one tube inside the other, to make the system quieter and keep the chrome from deteriorating as quickly.
My Jardine 4 into 2 takes the outer two cylinders (1 and 4) and routes them to the right and cylinders 2 and 3 to the left. This creats the 'scavaging' of 'pulses'. It also has a cross-over to balance the pressures from the two sides. This benefits horsepower in the mid and upper RPMs. The headpipes are single wall so the system is louder but is also lighter.
A 4 into 1 system has all four cylinders routed to a single point or 'collector' where each 'pulse' 'scavenges' the next 'pulse'. This creates the most horsepower at upper RPMs. They are single walled tubes and only one muffler to be even lighter.
Some systems run under the bike to avoid conflicts between gound an bike when leaned over in corners. Gound clearance (and oil changes) suffer. Others run the exhaust to one side or the other (or both) which can create lessened lean angles.
It's great to have choices of exhaust to suit each persons performance desires. The stock system is a compromise of performance and durability, the major plusses of the stock system. The fact that the bike is already tuned for the stock system is a plus too.
Some want more power or noise or looks or whatever. The engine will usually need other changes also to see any benefits or in some cases to even get back to stock levels of performance.
I like to tinker so I didn't mind 'tweaking' my bike (the E). My MN Special is stock as far as engine and related systems go. I'm resisting the urge to 'tinker' with it in that respect (it's not easy to resist).
Sorry for the wordy post here, I hope it makes sense. It's Tuesday (my Sunday) afternoon and I'm bored.

[TopCatGr58]

Hey there again,

Okay, I'll take another stab at it

The firing order goes like this:
1-3-4-2-1-3-4-2-1-3-4-2-1-3-4-2

With a 4-2 system, with cylinders 1-2 and 3-4 linked like in the OEM design, there are pulse delays on either side during the 2 crankshaft rotations for a complete series of cycles for all cylinders, so you get 2 pulses on a side, but then there is a pause in pulses as both cylinders then finish the exhaust cycles and go thru the intake/compression cycles. SO..the exhaust pressure fluctuates more with OUR 4-2, and there are interruptions on the pulses or waves of exhaust being pushed out thru each side.

After closer review of the Jardine pipes, it looks like they are linked, 1-4 and 2-3, and so the exhaust pulses are more evenly destributed on each side, but there is still a bit of a pause between each cylinder's exhaust cycle as the other pair is cycling. The Jardine 4-2 should be a better designed system vs. the OEM, but they(NEW)aren't available for the XS11 anymore! The other brand (Mac) links 1-2 and 3-4 in their sets!

With a 4-1, every cylinder is pushing an exhaust supply with every half turn of the crankshaft, so there are no pauses in the pulses, the pressure is fairly constant, and the "wave pulses" are steady going into the collector and thru the muffler.

The 4-1's provide a reduced backpressure, and are often tuned to give more mid and top end power, but can drop the lowend power due to the same reduced backpressure....something about helping to close the valves at lower rpms!? A fair amount of the horsepower boost from aftermarket pipes comes from the reduced weight compared to OEM! So...don't expect a big jump in HP with 4-1 vs. the OEMs.
T.C.

[SG-Olli]

Stock exhaust

Hi guys over the pond!
I am not sure if the stock exhaust in the US is or was the same as here in Europe. But over here you will not get any replacement exhaust be it 4in2 or 4in1, that matches the stock one performance wise.
All of them are a lot louder, some of them even sound great, but they all decrease power. The sound is one of the reasons why many people think it improves performance. Did anybody ever compare a stock system with a replacement system on a dyno (except for the replacement suppliers!)?
The stock exhaust is fairly well tuned and is a very good compromise between performance and noise (sound).

[Pat Kelly]

I felt that my Jardine 4 into 2 is quieter that the stock exhaust. The stock system was still in good condition (XSecpt for severe surface rust). I replaced it in 2003 around 15,000 miles on the bike.
It is lighter which does help in the "horsepower to weight ratio". I'm sure going on a diet would help even more .

[fbcaf]

Thanks Pat, TC and S.G. Olli,
My understanding of exhaust has been enhanced a bit. What a bunch of engineers! I guess my ride has more than enough power already, but one day I will have to replace the exhaust because it is getting real old. This discussion will provide valuable information which will no doubt color my decisions when the time comes.

[planedick]

Jardine slip on's...

Top Cat sait that the Jardine 4 into 2 wasn't available anymore. I just purchased a new set from a motorcycle accessories shop here in Anaheim, CA just recently. They ordered them and it took about 3 weeks. I did have to retune the carbs some though. I found it necessary to change the float height and set the needle as lean as possible for that perfect plug color. I'm only getting 33 mi per gal though.

[John]

What type of mufflers are they? I mean are they straight cut, slash or angle cut, turn outs? If you go to the webshots address in my signature, you can see what type of mufflers that I have. Are they like mine? How much did it cost?

[desert1]

HDs

A lot of the HD Guys must have great performing engines, a lot of them have strait pipes, unbaffled, maybe burnt valves as well................

A moments silence for James Doohan, "Scotty"has passed on to the big engineering dept. beyond
Bruce