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General Category >> Rubber Side Down! >> Designing Exhaust Timing & Related Concerns
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Message started by Mavigogun on 01/29/20 at 10:34:38

Title: Designing Exhaust Timing & Related Concerns
Post by Mavigogun on 01/29/20 at 10:34:38

A conceptual question follows- but requires recounting my [imperfect] understanding. Or misunderstanding:

Our wonderfully crude, single-cylinder 2 stroke engine couples the ejection of exhaust with the introduction of fuel into the cylinder- the front and back door open at the same time, as it were. Surprise-surprise, some of that fresh fuel leaves the cylinder with the exhaust gas, assisted by a related negative pressure wave- and here's where my exhaust design query comes into play. With a properly contrived exhaust, that wave of negative pressure doesn't just pass out the exhaust exit, but is reflected back by obstructing geometry as a returning positive wave; this positive wave engages the escaping unspent fuel, driving it back into the cylinder where we want it, and simultaneously contributing to pressure in the cylinder. Timing this negative-positive wave to the stroke cycle is why mufflers are typically at or toward the terminus of the exhaust and not affixed directly after the engine exhaust port.

Perhaps some questions should be reflexively answered by the above understanding- but, you know what they say, "when you assume, you make an ass out of yourself, a yard ornament out of your bike, and Batman will justly dope slap you for not asking", so-

Those cool kids who dispense with their muffler, side venting the pipe somewhere between the rear wheel and transmission case, are throwing away fuel and power, aren't they? Kinda the opposite of a supercharger?

What options do we have for ascertaining if positive exhaust pressure is both adequate and well timed?

What options, good or better, do we have for tuning an exhaust to deliver the needed pressure at the ideal moment? Exhaust length trial-and-error is consumptive of time an treasure- is there some sort of moveable baffle employed at the muffler front to adjust the reflection point and flow/pressure?

I suspect the path to seemingly simplistic design solutions is typically anything but simple.

I'll attach a picture to trap you in the post long enough to reflect.

Title: Re: Designing Exhaust Timing & Related Concerns
Post by Gary_in_NJ on 01/29/20 at 11:18:48

Your understanding is way off. The Savage is a single cylinder FOUR stroke engine.

Exhaust timing of a 2 stroke (pressure waves and reflection) is very different than exhaust timing of 4 stoke engines.

Title: Re: Designing Exhaust Timing & Related Concerns
Post by Ruttly on 01/29/20 at 13:28:38

MATH !

Title: Re: Designing Exhaust Timing & Related Concern
Post by batman on 01/29/20 at 16:42:42

Mavigogun You are not totally wrong ,as this same thing can and does happen in 4 stroke motors as well ,the exhaust pressure wave can return as a negative pressure wave and if it arrives just before the exhaust valve closes and the intake has started to open ,it will help clear the cylinder of gases and help draw more of the fresh charge in increasing hp and torque. This can only occur if the cam has overlap . A stock Savage has none .a lot of newer bikes don't , the carry over of fuel causes emissions to run to high to pass the standards set for factory bikes.
so with no overlap in the cam, tuning the Savage exhaust isn't something you can do, reducing back pressure by using a less restrictive muffler (Dyna for Ex.) can help.

Title: Re: Designing Exhaust Timing & Related Concerns
Post by ohiomoto on 01/29/20 at 17:29:23

I have to say, I'm amazed at how "overanalyzed" this seemingly cheap, POS, under powered LS650 is on here. And I don't mean that in a bad way!!!

There are a lot of really smart people on here and we all love the LS. I'm just amazed how the bike inspires so many people in so many different ways.

Title: Re: Designing Exhaust Timing & Related Concerns
Post by IslandRoad on 01/29/20 at 19:22:40

Ohiomoto, I was just talking about this site with a friend today. And the point you make about the simplicity of the bike.

Personally, I really enjoy revisiting the basics again and again. There's something really satisfying about milking the most out of a really simple engine (air in, air out, and that bit inbetween where tve action happens).

In learning about the Savage on this site, I picked up that the exhaust port is the limiting factor. The fact that there's not really anything that can be done to improve it (apart from perhaps a little trimming of some casting artifacts). That has saved me from embarking on too high expectations of the intake. And encouraged me to look to where I can make some sort of practical changes.

Title: Re: Designing Exhaust Timing & Related Concern
Post by norm92de on 01/29/20 at 21:12:04

I don't know very much about two stroke exhausts. But I do remember many years ago, in England, I owned a Zundapp 201s 200 cc two stroke bike. It was very quiet relatively speaking and for its time high performance.
I felt that if I took off the HUGE silencer it would surely go faster. WRONG!
The bike dropped from 70 MPH to 60 MPH instantly, on top of which, the noise was incredible.

Those German engineers certainly knew what they were doing when it came to designing a two stroke exhaust system. :)

Title: Re: Designing Exhaust Timing & Related Concerns
Post by IslandRoad on 01/29/20 at 21:38:13

Norm, that's areally good example to illustrate the diagram posted above. The large chamber on the exhaust plays an important role on 2 strokes.

My ex-boss used to compete in chainsaw comps. He had a chainsaw with a custom exhaust. That thing looked and sounded mad.

Title: Re: Designing Exhaust Timing & Related Concerns
Post by kamelryttarn on 01/29/20 at 23:22:53

I have had EXACTLY the same idea for some time but I am thankful to batman for explaining that this is primarily a two stroke thing. Now I can keep my Silvertail exhaust and focus on other parts of the bike instead.

Title: Re: Designing Exhaust Timing & Related Concern
Post by DragBikeMike on 01/29/20 at 23:38:59

Islandroad, regarding your comment:

"In learning about the Savage on this site, I picked up that the exhaust port is the limiting factor. The fact that there's not really anything that can be done to improve it (apart from perhaps a little trimming of some casting artifacts). That has saved me from embarking on too high expectations of the intake. And encouraged me to look to where I can make some sort of practical changes."

Actually, that exhaust port responds well to some basic porting work. It's not difficult to do and yields measurable performance gains, both on a flow bench and timed performance tests. The intake also responds well.

http://suzukisavage.com/cgi-bin/YaBB.pl?num=1565076891

Try it, you wont be disappointed. 8-)

Title: Re: Designing Exhaust Timing & Related Concerns
Post by batman on 01/31/20 at 16:16:28

IslandRoad Your expectations for the intake may also be too low. Gains there can be higher than that of the exhaust . After market carbs and work like DBM did to the intake passage ( reshaping with filler etc. ) I've done neither of those, but have gained 3 to 4 hp by adding 6"of pvc pipe between my rubber carb tunnel and air filter (intake tuning).

Title: Re: Designing Exhaust Timing & Related Concerns
Post by zipidachimp on 01/31/20 at 20:35:19

I got a big laugh reading this post and seeing the words/phrases: "suzuki, 2 stroke, and 'german engineers' in almost the same sentence. To see the reason I'm chuckling, google the name 'ERNST DEGNER'. Suzuki has been at this a long time!!!!
Cheers! 8-)