Custom exhaust for gs850, tuneable straight pipe

[blackstag]

well, thank you for your input. i may not seem grateful, but I am.

no, its not going to be a straight pipe

and no, im not half assing anything here


these engines were built on mathematical theory and thats what I am doing. the mathematical theory to provide some answers to me and other folks.

hopefully some good can come out of this topic. I can understand that everyone gets tired of answering the same things, thats why i have not asked the same questions.

theres no saying that what I am planning on will even happen. it may all end up sucking and thrown in the scrap pile, but like i said earlier, im gonna try.

so heres what I am doing to stop speculation;
designing a set of velocity stacks that will target a mid range RPM to help with power where it is most useful

from these calculations I am going to design exhaust system that will imitate the look of a straight pipe system but have a tuneable baffle welded in place, it will consist of a baffle, packing and a 3/4 in od washer that can be turned to adjust back pressure.

heres a link;
http://www.nightrider.com/biketech/exhaust.htm

used to be snuff or nots
http://tomstoys.50webs.com/snuff.html


these guys make em too; http://www.dkcustomproducts.com/Thunder-Torque-Inserts-118-Model-Family-DK-TTI-118.htm

and as a follow up to why i have been talking about acoustic wave design for the intake


Acoustic Tuning
Acoustical tuning in the exhaust system works on the same theory as pulse tuning in the intake system. When the exhaust gases explode from the cylinder and head through the exhaust system, a high pressure sound wave is generated. A low pressure wave reflects back from the high pressure wave as it exits the pipe and the idea is to time this low pressure wave to come back to the exhaust valve at the right time.

and so we continue

 

[Agemax]

i am sure Dynojet did not spend countless thousands of hours and money developing a needle that works with pods/stacks and aftermarket exhausts for some amateur wannabee "expert" to come along with a few figures and prove all their work is a waste of time.
listen to what people are telling you, hundreds of thousands (probably) of Dynojet kit users must be some proof that they are actually required, and work!!!

 

[49er]

You're quite right about acoustic tuning considerations for both the inlet and exhaust systems. They aren't simple equations though, with lots of on going theory testing by practical trials. Things like varying gas/fluid temperatures affect your calculations too. The speed of sound varies with temperature and air fuel ratio fluctuations, causing spikes of performance at some rpms, and drops in others. These variables are often accentuated buy using pods or stacks. If you over lengthen your stacks, you'll have great mid range torque, but drop peak power.



IMO, the inlet tuned lengths of stock 850's are extremely good............good enough to pull strongly through the mid range, and noticeably so from 7000 - 9000 rpm.

Your acoustic influence on exhaust systems is well proven on 2 stroke engines. Tuners trying to save space on dirt bikes, jinked the cones around the frame and engine. The result was a widening of the power band, caused by the sound waves travelling back to the port window at different intervals due to the varying truncated cone angles.

Multi cylinder 4 stroke engines also benefits from acoustic tuning. 4-2, 4-1 and 4-2-1 systems all perform differently because of this acoustic influence, and the accuracy of their tuned lengths.

I believe that a slash cut pipe has the affect of broadening a power band, without reducing the engine's output.

Good luck with your experiments............ it's real easy to loose performance in part of your power band. You may need to change your needles too!

 

[Guest]

I don't need no stinking mathematical theory, I talk to folks who know what works and what doesn't. Why reinvent the wheel?

 

[blackstag]

I hear you guys; and i am not expert, just some fellow putting a little forethought into something before he tries it. thoery is thoery, it could all be wrong or all for nothing really.

as for the needle, yea it probably does have a place in my system. hell the dynojet kit is on my christmas list. I have a hard time paying 100 dollars for 4 needles and a couple jets but hey, if its needed, it needed

it does concern me that I have read more than a few experiences with the needles wearing out. mainly due to them being made of apparently softer brass than stock needles and partly because of their design. but, i believe the new ones are made out of titanium, sooooooo shouldnt be a problem.

I would love to understand the necessity of it before i install it also. You know, what its doing to the air, fuel; its effect on it. that sort of stuff. My belief is that you cannot properly troubleshoot a system if you do not understand what the system is doing.

so thats where I am...

calculations

 

[tkent02]

You know, what its doing to the air, fuel; its effect on it. that sort of stuff.

It does nothing to the air, it's a fuel thing.

My belief is that you cannot properly troubleshoot a system if you do not understand what the system is doing.

so thats where I am...

This sounds very true.

 

[Chuck78]

you are definitely going to want the dynojet needles and kit. You are de-restricting both intake and exhaust by removing the filter and replacing the original baffle with something less restrictive. Mikuni BS carbs on these bikes with intake & exhaust mods always do much better with the dynojet needles. The fuel requirements are changed and have nothing to do with the RPM that you tune the intake length for. you are concerning yourself with wavelength and all this math, but not giving attention to the fact that you are allowing a much greater amount of air (and increase in velocity slightly) to enter the engine, which will require more fuel, or else you will run very lean and melt your pistons or burn valves

 

[Steve]

... , or else you will run very lean and melt your pistons or burn valves

Which is exactly why I posted this, way back in post #15:

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

.

.

 

[blackstag]

i am giving plenty of attention to the necessary components here. I have already stated that I will be required to upgrade my jets. as for a needle upgrade, that all depends on how the bike runs and performs.

this is the procedure that I follow to tune my bikes;
http://www.factorypro.com/tech/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html


honestly I do not know what will be required until I reach that point in the build.

I could buy the dynojet kit today, throw it in and hope it works; or systematically run through the process.

I simply am not to that point yet.

theres no saying how much of a air increase my setup is going to produce.

honestly my plan is to create a system that has the looks of a very simple system but is still able to provide the airflow needs that the engine desires and was designed for. hopefully when I am done, the airflow into the engine and out is changed minimally.

my ideal design would be to have to make no changes to the carbs factory settings.

 

[850 Combat]

Take a look at the diagram of how the CV carb works. There is a good one in one of the Appendix in the factory manual on BikeCliffs site. You can get basics there. There are a couple air passages which run from the bell mouth of the carb to the emulsion tubes in the main circuit and the pilot circuit. Removal of the airbox and its restriction definitely changes the balance of air in the carb, and the amount of pressure at these locations. These bikes are actually sensitive even to air leaks in the airbox. They won't run right even if the airbox side seals need replacing.

That bike runs about 5500 RPM at 70 mph as I recall. Where a GS850 is weak, is in low and mid range power off idle to 4000 or so. They run ok in higher RPMs. lots and lots of people plan on making changes to the intake and exhaust. They receive the same good advice. Often, they throw it back. You should consider a little more research before arguing.

There are lots of old 850Ls out there. They aren't worth anything. You can't ruin anything of value here. You can annoy people with excess noise, which is a bad reflection on all of us, and many of us resent it for good reason.

 

[blackstag]

thanks for the information

if we know that the air flow is increased in the carbs and we compensate by increasing jets and changing the jet needle, to accomodate for this. theres still a issue is what your saying.

due to the way in which air is delivered, even if i do accomodate for the change, the air flowing over these points will create a larger issue.....

i dont want to waste time or money here. brainpower and thought come free. dynojet kits dont

maybe there is another route.

ive seen plenty of pod setups on gs's so they have to be somewhat effective eh?

 

[blackstag]

so i have been continuing the research on the exhaust system design and in continuing the wave length tuning read about torque reversion cones.

anybody ever use these before?

ive read about them before but have never had a reason to use them. they make the exhaust gases exiting from the engine make an abrupt change in pipe size that helps the exhaust pulse flow from the engine without interruption.

basically they help with low end torque and help stabalize the entire torque range. something that I will probably have to do.

these are the ones that I am looking to get;
http://www.ebay.com/itm/LA-Choppers...Parts_Accessories&hash=item51a240c274&vxp=mtr

if i dont use them for the 850 I will definitely use them for my xs650 project.

also, does anyone here have any idea how hot the gases exiting from these engines are? I need the temp of the exiting exhaust gases to accurately size the exhaust system.

if nobody knows, ill get a reading once I get the electronics back up and running but it would be lovely to know.

thanks all

 

[Steve]

... ive seen plenty of pod setups on gs's so they have to be somewhat effective eh?

Sure, they are "effective", but effective at what???

The thing at which they are MOST effective is making carb removal easier for all the jetting adjustments you need to make when you install them.

so i have been continuing the research on the exhaust system design and in continuing the wave length tuning read about torque reversion cones.

anybody ever use these before?

Nobody that really matters. Just people like Yoshimura, Vance and Hines, Kerker, Jardine, MAC and a few others.

ive read about them before but have never had a reason to use them. they make the exhaust gases exiting from the engine make an abrupt change in pipe size that helps the exhaust pulse flow from the engine without interruption.

basically they help with low end torque and help stabalize the entire torque range. something that I will probably have to do.

You want to do some reading on that topic? Look up information on designing 2-stroke expansion chambers.

... also, does anyone here have any idea how hot the gases exiting from these engines are? I need the temp of the exiting exhaust gases to accurately size the exhaust system.

Not sure what the temps are on a GS engine, but they might be similar to other gasoline-fueled engines, when under full load. I have seen reports of 1700+ degrees, and have personally observed temps over 1700 after the turbo, where temps are several hundred degrees cooler.

If you don't mind taking shortcuts in your design research, just look to see what the "nobodys" mentioned above are using. :-\\\

.

 

[blackstag]

Nobody that really matters. Just people like Yoshimura, Vance and Hines, Kerker, Jardine, MAC and a few others.

lol.... is that sarcasm there steve?

and i dont think anyone here is a nobody. i wouldn't be here if you guys were. this is the single greatest community for the gs series of bikes on the net, period.

pods have their place, they also create alot of mayhem. I am attempting to help reduce the negativity towards them by providing some sort of systematic approach to handling them, their affect on the bike and how to overcome the negative affects.

alot of people spend alot of money for "looks" which realistically, is the only reason they are on many bikes. definitely the reason they may end up on mine.

i however dont plan on spending alot of money, well see though

 

[Agemax]

pods have their place, they also create alot of mayhem. I am attempting to help reduce the negativity towards them by providing some sort of systematic approach to handling them, their affect on the bike and how to overcome the negative affects.

there already is a systematic approach to handling them..........FIT A DYNOJET KIT.

THEY WORK!

 

[Guest]

Some people just have to reinvent the wheel.

 

[blackstag]

im just trying to understand what is going on is all, dont want to reinvent the wheel. I usually find that I save money when i understand what is happening.

in this case I have realized that sticking with cv carbs are going to cost me at least 100 dollars.

thats before I buy intake boots.

so, is there any reason why I should not run the vm series carbs found on 79 and earlier models?

 

[Agemax]

im just trying to understand what is going on is all, dont want to reinvent the wheel. I usually find that I save money when i understand what is happening.

in this case I have realized that sticking with cv carbs are going to cost me at least 100 dollars.

thats before I buy intake boots.

so, is there any reason why I should not run the vm series carbs found on 79 and earlier models?

you will STILL need a dynojet kit to get them running correctly. countless people have fitted pods or belmouths and a race pipe believing they are clever enough to set the carbs up to run perfectly............but they all soon give up,admit defeat and go buy a DJ kit.(mainly just to use the needles DJ supply)

 

[Steve]

lol.... is that sarcasm there steve?

Me?? Sarcasm??

Never.

You asked if anyone had used wave length tuning and torque reversion cones.
I answered, saying that we basically trust those who have already done the work, and gave a few examples of who has already done it.

so, is there any reason why I should not run the vm series carbs found on 79 and earlier models?

The intake ports on the head are smaller on the '79 engines, as well as the boots. You could make an adapter, since you are adapting everything else, too, but that will be up to you.


Step back just a bit and look at the big picture. You are going to be putting a LOT of time and effort into this. Is that all going to be offset by saving the $100 for the Dynojet kit? :-k

.

 

[blackstag]

realistically, no

in the end 100 bucks isnt going to end my life. ill probably stick with the cv's and get the dynojet kit.

seems like it will save me some time and provide at least a starting point for me.

thanks for the info fellows