Exhaust design (was Exhaust cam opens when?)

[Ugluk]

Hi guys
I'm in the early stages of planning for a home brewed tuned exhaust, and got myself a copy of Graham Bells 4-stroke tuning-book. Problem is that I'm rather thick and can't even begin to try to understand the formulas for calculating as I miss a lot of facts.

Does anybody know when the exhaust valves open before BDC?

And what would be a good tuning rpm to aim at? I guess it should be somewhere between torque peak and hp peak?
Just figured I'd try to read up a bit instead of my usual approach of copying and guessing.
As I'm going to do the cutting and welding myself a complicated system would not be more expensive, and as it is a hobby time is not an issue.

Help? I did search for hours here as well as on google and found nothing..

[007brendan]

http://gstwin.com/racers_corner.htm

[Ugluk]

Thank you very much sir!

[Ugluk]

So, according to the A. G. Bell book of 4-stroke performance tuning I've ended up with these figures:

Tuning rpm 7k
Primary pipes Id= 1.2" or 30,6mm
Primary pipe length= 26.5" or 670mm
Secondary pipe Id= 1.62" or 41mm
Secondary length including collector= 29.5" or 749mm
Total system length ex muffler= 56" or 1419mm

Tuning rpm 8k
Primary pipes Id= 1.29" or 32,8mm
Primary pipe length= 22.8" or 560mm
Secondary pipe Id= 1.74" or 44,2mm
Secondary length including collector= 25.8" or 655mm
Total system length ex muffler= 48,6" or 1234mm

Tuning rpm 9k
Primary pipes Id= 1.36" or 34,5mm
Primary pipe length= 19.95" or 507mm
Secondary pipe Id= 1.84" or 46,8mm
Secondary length including collector= 22.95" or 583mm
Total system length ex muffler= 42.9" or 1090mm

Secondary pipe should end abruply and squarely so as not to mess up the tuning. I'm guessing it could instead do so in a sligthly larger muffler inlet to keep the noise down instead of the atmosphere or a reverse cone megaphone. I think tuning frequency will much depend on the physical dimensions of the bike since I don't want to use smaller than optimal pipes to get a meatier midrange. After all, the gear box is there for a reason..

My plan so far is to make a single normal sized or twin small cone muffler with a reverse tapered perforated pipe inside. Next step is to think some more and start looking for a supplier of dairy industry pipe. They have nice thin walled bends and pipe in a lot of different diameters.

Anyone tried anything like this?

[black and silver twin]

That sounds Like a cool project! One thing to keep in mind though, the Heads exhaust port is smaller in diameter than even the lowest rpm tuning numbers you have.

[Ugluk]

Damn! That means I've got to read up on the porting section as well..

Thanks for the input. Got to check that out. Perhaps measure the ports and redo the calculations.

If I end up with pipes thick as a pinkie, so be it! As long as it gives the best performance and sound character..

[bill14224]

As many have found out here you won't get more than +5 RWHP without pouring serious money into it, so if you want more than 5 more ponies it needs head work.

I gotta admit I'm fascinated reading all about those who squeeze all the power they can out of this great little bike, but at the same time I can't stop thinking about how much cheaper and easier it is to find a good 500 Ninja.

[black and silver twin]

I think with some head porting the bike would respond well with a good header system. but to really get great improvements you would need to port inlet and outlet and at least open up the stock valves or put in larger valves, then run larger carbs to match the larger inlet ports. and deck the head to get some higher compression.
The stock exhaust ports look to be approximately 1" where as all exhaust systems are at least 1.2" so puting an exhaust with some mild head porting should give good gains.
I have been thinking about this seriously for a while, at my work I have access to head porting/polishing/decking stuff, if I ever take the head off my bike (or car for that matter  ) I will port it. mostly on the exhaust side (because I dont have larger carbs for the inlet) But I will also open the valves up a little and deck it slightly.

[gsJack]

I always like to recall the magazine article about the Vance & Hines GS500 project bike they created while developing their V&H exhaust shortly after the first 89 GS500 came out.  It was in Cycle World, I have a copy of it around here somewhere.

They used a Wiseco big bore kit to increase displacement to 555 cc and did the usual intake and jetting mods including some porting work.  They only claimed about a 10% increase in HP and Torque, reported a 12.8@102 quarter, and a 121 top speed.

Could have traded it for a EX500 and got about the same performance for lot less money than they spent on the GS mods.

http://img.photobucket.com/albums/v443/jcp8832/GS500tests.jpg

[Deros514]

When I worked on cars, I always liked the mathematical part of making improvements. Some of it I understood, and some went right over my head. Sometimes the equations were simple and at times really complex. Might be because I think more like an engineer. Anyway idk where I'm going with this but good luck and I hope you see some good results.

I had the opportunity to get an EX500....took the GS instead cuz it looked cooler naked 

[Ugluk]

Thanks for the input guys. I'm not on a quest for absolute power here. If I wan't to go really fast I'll take my turbobike. The reason I've come to like the GS so much is rather that it isn't fast. It can be whipped so hard without going crazy fast that nostalgia kicks in and in my mind I'm taken back to my first bike, a ratty cafe'd Z400 twin. Me and my buddies all had small bikes at the time and did som wonderfully unresponsible nighttime cafe racing in the larger cities of Sweden..

I don't expect to match systems like the yoshimura and such properly researched and developed ones, but I think a lot can be compensated for by mere effort put in fabrication if the calculations are reasonably correct. Time spent fabricating it is no issue, nor is cost of material. I'll still expect to end up spending only a quarter of the price of a full system.

I do know a guy who do a lot of dragracing on gsxr's and briefly considered sending the head to him for epoxy porting and valve job (he's got a MIRA machine for 5-angle jobs), but I expect I won't afford it. And I'm not gonna race the bike after all.

This one has 200rwhp, but below 70mph, the GS is a lot more fun..

[Ugluk]

I just had.. something..

Either it is a moment of depressing clarity, or it is just another brain fart..

It's a twin, right?
A 180* twin?

The way I see it pulse tuning and cross scavenging will only half work, ie the first cylinder will help filling the second. The first however is on it's own..

I can't get my head around it being any other way. The 4-stroke cycle is 720*, right?

So, when one is down the other is half a turn behind. This messes up my calculations, perhaps explain the balance pipe on the stock exhaust?

Any experts out there who know about this stuff?
Mr Bell makes no mention of 180* inline twins and every google search for "180* twin exhaust tuning" only turns up v8 with dual headers..

[007brendan]

Is the gs a 180 twin?  I thought it was a 360 offset.

[black and silver twin]

Is the gs a 180 twin?  I thought it was a 360 offset.

If your asking if it fires one cylinder every revolution, then yes. if your asking somthing else then I dont understand the question.

[kman]

I believe it is 180 since one piston is up while the other is down.  The 2nd piston must fire either 180 before or 180 after the first and I am not sure which is #1 and #2.  That part is all relative.  With a dual exhaust they could work independently.

[Deros514]

I believe that's why the full exhaust options we have for the GS use individual runners and avoid the cross pipe of the stock setup. Easier to get the scavenging effect when you only deal with one pulse.

[Ugluk]

It would seem that the Ninja250 has this issue, solved by independently jetted carbs for the two cylinders..
http://forums.ninja250.org/posting.php?mode=topicreview&t=77816&tro=1

Perhaps some kind of 2-3-2-3-2 system? Sounds kinda difficult..


It would seem that pulses don't matter that much once the revs are high enough for a good flow.

Anybody with a good 2-1 and an Ir-thermometer who can take a reading on the primary pipes? Should read a difference if one is leaner..

This stuff is complicated.

Found a calculation site as well, and it agrees with my own calculations.
http://www.mez.co.uk/mezporting/exhaust_length.html

[Ugluk]

Still not much result, therse's precious little to find on the web. I'm starting to suspect that the common flat spot that is often mentioned could be caused by the odd fire sequence of the little engine and the scavenge of the original 2-1. I guess suzuki used the crosspipe to either put it there to lessen a dip in torque or shift it.

No theory on crosspipes has crossed my path yet though..

Someone suggested 2-1 or 2-2 with stepped headers for a wider power curve, so I'm gonna try and look that up too.

[Ugluk]

I'll just keep on ranting to myself here, sort of thinking aloud.. If there is a wizened engine guru lurking in the shadows here, perhaps he will eventually take pity on me..

The 2-1 seems to be something that just won't fit right with the odd fire sequence. I've been doing a lot of random reading on the web, and it has swayed me into thinking that a classic looking 2-2 is what will work best and give the right looks for my intended future bike.

I've found a site that calculates stepped header tuning lenghts in three segments, and I'll use these ratios to calculate the steps in the headers. I don't really know how to calculate the diameter increments, so I'm going to calculate the tuned pipe volume and keep the stepped header total volume the same. This would mean a smaller pipe diameter at the port and a larger at the muffler.

As soon as I get around to taking a measure at the exhaust ports I intend to calculate a starting diameter of that +10% and see if I can find a tuning that will have the pipes end where I want the mufflers and still give me some top end power. I hate when bikes just goes limp 2/3 to redline..

I'll probably make a messy prototype in regular steel before having a go at the shiny stainless that will hopefully be the final construction material.

[TheRealSpinner]

When I was building my exhaust, I talked to a local race exhaust builder.  He's got tons of hours of Dyno time.  I was looking for as much low end as possible, he gave me these dimensions:

Individual Pipes (no 2 into 1, no "H" pipe)
1 1/4" for 12"
1 3/8" for 20"
Megaphone/muffler (for more low end) that goes from 1 3/8" to 3.5" or 4" over 18"



I did a lot to my bike all at once- 33mm Keihin Carbs, Pipes (to these specs, minus the megaphone), Smaller sprocket in front, Bigger Sprocket in Back, and much taller tires.  It's hard to say what actually affected the performance, but it sure seems faster.  It's a little louder... OK, I'm one of those a$$holes that believes that loud pipes save lives.

-SPiNNeR-