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Message started by sandmanred on 02/07/18 at 19:46:24

Title: Exhaust design
Post by sandmanred on 02/07/18 at 19:46:24

I'm thinking about going with a short exhaust with an internal baffle something along the lines of this sketch. I did pipes with baffles on another bike and kept the stock jets by adjusting the baffles to get the plugs to look right. Any performance concerns or suggestions in trying the same thing with the Savage?

http://https://s6.postimg.org/8284hmu1d/exhaust.jpg (https://postimages.org/)

Title: Re: Exhaust design
Post by batman on 02/07/18 at 23:36:54

Stock header length for the bike is 32" the range for a 650 cc bike should run between 30-34" shorter pipes favor higher rpm ,longer favor lower. You should design for the rpm range you'll be riding at most of the time . You might run a short pipe under the motor with a square muffler , I think I've seen these on Ryca Sportster kits. What you've planned looks to be very short which might be OK for WOT but produce a bag sag in midrange , and dumping in front of you might not be great for your hearing.

Title: Re: Exhaust design
Post by ohiomoto on 02/08/18 at 06:31:36

I'm not seeing the sketch???

Title: Re: Exhaust design
Post by Dave on 02/08/18 at 06:52:12

You aren't really looking for an engine that makes top HP......so you can try a short exhaust and see what happens. The issue that I see is that with such a short system, there is very little room to do an pulse dampening and almost anything you try is going to be pretty loud.

Title: Re: Exhaust design
Post by sandmanred on 02/08/18 at 08:42:37

3631303634362D36590 wrote:

I'm not seeing the sketch???

There seems to a problem with postimage, all my pics on all posts are not showing right now :-?

Title: Re: Exhaust design
Post by Dave on 02/08/18 at 08:43:35

I can see the photo/sketch on my computer....here it is using tinypic.

http://i66.tinypic.com/16k71op.png

Title: Re: Exhaust design
Post by sandmanred on 02/08/18 at 08:53:43

506B6671606C77716A626F70030 wrote:

Can I compensate for short length with a different diameter or anything else?

Title: Re: Exhaust design
Post by Dave on 02/08/18 at 09:02:15

For performance, you want the exhaust to go out the header with a high velocity, and that rushing exhaust helps to scavenge the burned fuel out of the cylinder.....and it may even help to allow some movement of fresh fuel/air as the intake valve opens. If the rushing exhaust has no place to go before it is restricted, or if you make a large diameter pipe that reduces the velocity of the exhaust flow.....the efficiency of the system is reduced.

Compared to all the other work you have done....fabricating a trial exhaust system (or 3) is pretty minor. I think you just need to build something that looks good.....and see if the bike will run with it.
(The Board Track headers you are trying to copy were used on motorcycles that ran wide open and were unmuffled........not really acceptable for a modern street bike).

Title: Re: Exhaust design
Post by Fast 650 on 02/08/18 at 09:24:07

You can compensate somewhat with a smaller ID but there is a point where reducing the diameter creates enough restriction to hurt performance. With the stock pipe, 4500 rpm (about 70 mph) gives an exhaust velocity of over 320 fps. At some point the engine can't push the exhaust any faster and the pipe chokes power then.

Title: Re: Exhaust design
Post by ohiomoto on 02/08/18 at 09:55:36

2A38373D3438372B3C3D590 wrote:

[quote author=3631303634362D36590 link=1518061584/0#2 date=1518100296]I'm not seeing the sketch???

-------------------------------

There seems to a problem with postimage, all my pics on all posts are not showing right now :-?[/quote]----------------------------

We cant' have that. Fix it! lol

Title: Re: Exhaust design
Post by ohiomoto on 02/08/18 at 10:11:26

I would think something like the stock header would work fine...it already fits with the engine. If you want more length, throw some extra curves and wrap it around something.

http://https://i.pinimg.com/736x/1d/8b/76/1d8b760b0296264110022852a8acea69--racing-motorcycles-vintage-motorcycles.jpg

Title: Re: Exhaust design
Post by Dave on 02/08/18 at 10:22:29

The stock header does work fine....with a muffler on the end.

Open and/or drag pipes are know to create a flat spot or sag at certain rpm's in the mid-range.

(Something that didn't bother folks riding real board trackers that ran wide open all the time.....they didn't even have a clutch or a gearbox).

Title: Re: Exhaust design
Post by LANCER on 02/08/18 at 11:09:17

486F7D7A383B3E0E0 wrote:

What is the formula you are using to figure fps ?
Are the variables just pipe length, ID and engine rpm ?
Or are there more for this particular formula ?

Title: Re: Exhaust design
Post by batman on 02/08/18 at 18:29:01

It might pay to know that there are two components leaving the exhaust pipe ,exhaust gases which move more slowly, and a pressure wave that starts when the valve first opens that can travel at the speed of sound. It is this pressure wave that forms the basis for tuning the exhaust ,and the length of the header pipe. when the pressure wave reaches the end of the header it expands and moves backup the pipe as a negative pressure and if it arrives to find the exhaust valve open it can help pull out some of the remaining exhaust gases as well as pull in a fresh charge due to valve overlap . the return speed of the wave is slowed by the fact that as the gases move down the pipe they cool .Shorter pipes favor high rpm , which is why they were used on board trackers running at WOT. Using a stepped exhaust pipe increasing in size as it get farther from the head would allow the gases to expand and cool slowing the wave and might help in shorting the header length so it might work better at lower rpm , but the noise would ,without a muffler ,still be painful . As a test place the stock header on with no muffler and run the bike (wear ear plugs!) Running a "lollypop" baffle may help reduce mid range sag by breaking up the returning wave ,but overall performance will suffer.

Title: Re: Exhaust design
Post by Fast 650 on 02/09/18 at 07:56:54

Lancer, I found the calculation online some time ago. It is in an xls spreadsheet. Here it is if you want to play around with it. Just input the bore and stroke in inches, rpm, and the pipe diameter and it gives you gas velocity and piston speed.

Title: Re: Exhaust design
Post by Fast 650 on 02/09/18 at 08:05:42

152E2334252932342F272A35460 wrote:

The stock header does work fine....with a muffler on the end.

Open and/or drag pipes are know to create a flat spot or sag at certain rpm's in the mid-range.

And CV carbs exaggerate that effect more than a roundslide carb does in my experience. I am guessing that it messes with the vacuum signal at the carb at certain rpms.

Title: Re: Exhaust design
Post by sandmanred on 02/09/18 at 08:45:35

Was that excel file loaded with the 650 stroke and bore (it came in with 3.7 x 3.7 inches)? Do you have any guidelines for the velocity range to optimize around?

Title: Re: Exhaust design
Post by verslagen1 on 02/09/18 at 09:06:03

longer pipes = deeper tone

Title: Re: Exhaust design
Post by Fast 650 on 02/09/18 at 09:41:39

5E4C4349404C435F48492D0 wrote:

Was that excel file loaded with the 650 stroke and bore (it came in with 3.7 x 3.7 inches)? Do you have any guidelines for the velocity range to optimize around?

Those numbers were from the last calculations I had done. As far as velocity, the number that I always heard was 280 to 320 fps at peak power rpm is ideal and that is what I am shooting for with my build.

Title: Re: Exhaust design
Post by sandmanred on 02/09/18 at 11:25:23

Thanks for the input so far. It's making me re-think. Now I'm thinking some thing more like the yellow line. It lengthens it out and leaves me plenty of straight to stick a baffle into near the outlet. I still have a lot to go before I get there so I could change my mind again but I wanted to think it through to make sure I have the connection points I need for the potential designs.

http://https://s6.postimg.org/41e08npsh/Capture3.jpg (https://postimages.org/)

Title: Re: Exhaust design
Post by Gary_in_NJ on 02/09/18 at 11:39:45

Where are your foot controls going? I think that location will drive the routing of the exhaust pipe.

Title: Re: Exhaust design
Post by SoC on 02/09/18 at 18:00:51

Since on the subject of exhaust , and this question may seem a little simplistic, but I am working on my exhaust. Is the basic length and where the wave encounters it's first resistance basically what's important in the design assuming you use the stock header. This is not to down play the aspects of wave dynamics, and all that's been touched on, but there is basically free flow through the header (all things equal) and the return wave is influenced by among other things the first resistance it encounters via a baffle or obstruction. I realize there is more complexity to it, but does this seem to be a realistic point to start from.

I just ask as I'm not making any change to the stock header on either of my bikes, I have a Voodoo muffler (and that's a pretty loose interpretation of muffler, if you view the internals of it) to play with on one, and my thoughts are to build a straight pipe approx. the same diameter as the header, with internal insert steel baffles and approx. the same length as the stock muffler with a more down turn out than straight out the rear.

Title: Re: Exhaust design
Post by batman on 02/09/18 at 21:42:03

SoC, The wave isn't influenced by resistance, but by expanding , the velocity drops do to entering a muffler (an enlargement) or atmosphere and is turned into pressure , it then reverses as a negative pressure wave and moves back up the header. the muffler kills noise ,if it has a baffle this will slow the gases and provide some back pressure on the motor ,this helps flatten the torque curve ,making it less peaky. A straight pipe , with or without baffle, will not allow the pressure wave to expand until it reaches the end of the pipe,which makes it bad for wave tuning, and you'll make less Hp .I'm not up on the voodoo muffler but if it's larger than the header pipe I'd go with it or any muffler before a straight pipe.

Title: Re: Exhaust design
Post by SoC on 02/09/18 at 23:23:33

Batman, thanks for the simplified explanation. The Voodoo is a larger diameter. It has an internal pipe and an external, and the baffle is basically 2 opposing cut outs in the internal, bent inward. If you look into it you can see through it to the opposite end with the 2 opposing tabs in the middle sort of blocking the path of flow. I'm sure it will be pretty loud, I am looking into ways to add a baffle insert to it, probably in front end.

Title: Re: Exhaust design
Post by batman on 02/10/18 at 00:25:57

SoC The ideal place for the baffle is right in the middle, half the openings for the gases to go round the baffle and half to return an exit.

Title: Re: Exhaust design
Post by sandmanred on 02/10/18 at 04:39:18

72787C767D747079272125150 wrote:

Where are your foot controls going? I think that location will drive the routing of the exhaust pipe.

Foot controls will be forward anchored to a pair of the bolts through the engine plates.

Title: Re: Exhaust design
Post by IslandRoad on 02/10/18 at 20:46:55

For what it's worth, IMHO, the yellow line looks much better from a purely aesthetic point of view - the curves match the frame nicely! And, from what I understand of the concept of pressure waves, that will give you a more predictable outcome when tuning.