Custom exhaust for gs850, tuneable straight pipe

[blackstag]

Hey all!

so I am in the process of trying to design a exhaust system for a 850 shaft drive that I am working on. to properly complete the project I need a little info.

1. What is the ideal rpm that I should create the exhaust system for
(im thinking 7500)

note: there will be velocity stacks on the bike, thus increasing the
amount of airflow into the engine. maybe I should tune the exhaust for
a higher rpm? 8000, 8500?

2. anyone here know how the valve timming specs in degrees for intake and exhaust valves?

my plan is to creat a baffled pipe with a loli pop in the end to aid in tunning.

basically I want the look of a straight pipe with some control, i understand that a real muffler would be my best option, but im here to learn and pass the info on.

thanks!

[Agemax]

the further you ride, the hotter the exhaust gets, the more the lolly pop melts. hence the longer the ride, the more power increase you will get.
you could of course experiment with different types of lolly pop

[Steve]

Sorry, but the answer to #1 is ZERO rpm, if you are running a straight pipe.

The answer to #2 is found in any of the service manuals. You can grab one from BassCliff's library.

.

[tkent02]

Just curious, what is your intended purpose for this bike, remembering after all, it is an 850gL, yes the ladies model, and the heaviest naked GS ever.
Loud pipe to save your life?
Not riding it is safer.

[blackstag]

well the purpose of this bike is to change what was there into something new because replacing what was there with new costs more than the bike is worth.

seriously, a decent exhaust, and I mean decent original exhaust for these bikes are spendy. and i dont want a 4 into 1

soooo instead of spending money on something that somebody else made and gain absolutely no knowledge; I plan on designing my own system.

the intake is going to be velocity stacks, the exhaust is going to be straight piped with my muffler welded onto the original headers.

it will look like a straight pipe, but it wont be a straight pipe.

in fact someone who owns a GS has already done it, im just looking to make it better and actually tuned to the engine.

http://gstwins.com/gsboard/index.php?topic=40382.80

reply #84 is what im going to do, except it will have a loli pop on the end. basically just a 3/4 od washer welded onto a bolt so that I can adjust how much I want to block or not block the baffle.

[blackstag]

found the info on the degrees!

[blackstag]

Alright, just posting this here to see if anyone elses opinion jives with mine.

here is the cam degrees documentation supplied in the manual


so what I needed to do was calculate how long the intake valve was open and closed.

calculating open intake in degrees: 276
I got this value by adding 28, 180 and 68 together

calculating closed intake in degrees: 456
since I already had the amount of time that the valve was open I began to calculate the time closed by simply subtracting 276 from 720 degrees.

anyone have obligations to these numbers? im going off of what the diagram was telling me, but thats all the info i have so I am hoping that it is correct.

[Nessism]

How does the valve timing info affect the exhaust system you want to build? Same question for the "ideal rpm" you mention earlier, and also the velocity stacks?

[49er]

Your closed figure is wrong. Adding both should come to 720 degrees.

[Steve]

How does the valve timing info affect the exhaust system you want to build? Same question for the "ideal rpm" you mention earlier, and also the velocity stacks?

I was wondering the same thing, but since he mentioned that they were basically "straight pipes", I thought the ideal RPM would be zero.

Back to what is probably his intent, the length and diameter of the header pipes need to be calculated based on displacement and RPM, but I would not call that the "ideal" RPM. A longer pipe will resonate at a lower engine speed, providing its peak power a bit lower in the band, and vice versa. I don't see where valve timing has anything to do with it. I just don't know too many people that are going to go around screaming at a constant 7500 RPM to take advantage of a "tuned" pipe.
Besides, ... your bike is only going to see 7500 RPM at four specific road speeds.
I say 'four', because I don't think it will do 7500 RPM in fifth gear.

Velocity stacks are another ball of wax, but remarkably similar. Longer stacks will enhance lower RPM operation a bit, shorter ones will work better at high RPM. The 'fun' part is that if you use short stacks for high RPM 'enhancement', they might affect lower-speed operation enough to cause problems.

Guess in which engine speed it is where MOST of your riding is done?

.

[blowerbike]

no air box= buying a st.3 jet kit.
without it you will have a bike that runs good to 4k...falls off then runs great on the main jet(given you increase the main jet).
slot your cam sprockets and degree your cams to around 106 in./ex.
you can always move them around later to change your power characteristics.
IMO
your just beating a sore dick...but hey, who am i?

[blackstag]

I am designing the exhaust with intake in mind

the degrees have to do with Intake Length - Wave Tuning

I will be running velocity stacks not pods. stacks designed to be the proper length required for smooth operation of the 850. they will be longer allowing a better use of torque in the desired power band

i asked the desired rpm range because you guys ride these bikes consistently. if you dont let me know what is the best rpm to tune for then I am going to throw out a number. I ask questions to see what your opinions and statements are.

if you dont want to help or think im stupd, well so be it, you dont have to help or even comment. I am simply trying to provide answers for alot of people out there and learn some things about the bike in the process.

with that said.....

I had my numbers wrong earlier; they should be (i believe)

camshaft specs state:
inlet open- 276 degrees
inlet closed- 84 degrees

crank rotation
inlet open- 552 degrees
inlet closed- 168 degrees

I am using these numbers to provide the following calculations for proper intake length on the bike.

I have decided to start to "tune" my rpm range for 6000rpm, this is 2/3 the 850's redline

I take my rpm and divide by 60 to get revolutions per second; 6000/60= 100

then multiply by 360 to get degrees per second; 100*360= 36000 degrees per second

now i calculate how long the valve is closed; 168/ 36000 = .0046

now i can calculate the required intake length by multiplying by 90% the speed of sound and dividing it all by 2

.0046(306.3m)/2= .714 m (28.11 in)

now you can tune according to the air wave pulse that you would like
3rd pulse= 28.11 in/3= 9.37 in
4th pulse=28.11 in/4= 7 in
5th pulse=28.11 in/5= 5.6 inch
6th pulse=28.11 in/6= 4.65 inch

this is the "total" length of the air intake system; including the intake port, tunnel, carb and stacks

the measurement from the 850's intake valve port back to the edge of the carbs is really close to 9 inches.

so this means that i can target a lower rpm range to increase power in a useable range and allow me to lengthen my stacks.

of course all of my numbers could be wrong.....

[blackstag]

now as for the rejett and stage 3 conundrum.

i honestly dont think that I will have to rejet much or replace my existing needle.

this is because my intake length will be tuned to the engine and should be long enough to provide a smooth consistent air flow.

I am shooting for about 6" in length for my velocity stacks. I need to get a measurment on the length of the airbox plus the boots to the carbs.

I understand that that air box helps the air settle and allow for a smooth transfer to the carbs.

the reason pods suck is because the suck in from all directions.

there have been fellows with cv carbs that use the uni foam filters with beer cans inside and not having to rejet or retune.

a true looks without consequences swapout.

so using that theory along and creating tuned stacks, i dont foresee any need to change the needle in the carbs.

I may need to re-jet or adjust needle height, but i dont think i will need a different tappered needle by going this route.

honestly, i havent seen anyone here do this; i would like to see it work, and if it dont, i would like to figure out how to get it work.

my dad always had a saying; thinking and knowing are two different things...

[Nessism]

Well blackstag, you are either a genius or delusional. Hope it's the former.

[Steve]

We will be looking forward to the thread called "How do I rebuild my melted 850?"

.