Part Six: The Exhaust System

[Ketchup]

I don’t totally understand back pressure in a can muffler. It doesn’t seem like a can is able to back-stuff the cylinder like a pipe.

My crude thinking is that a saw that blasts all the exhaust out should run the same, no matter how much raw charge makes it out after the exhaust. Have you guys tried tuning a saw with no can? I’m tempted to think it will make the same power if properly tuned.

I guess too little back pressure could inhibit scavenging. Especially if the transfers weren’t aimed well.

@Dieselshawn , have you put the 661 on the dyno after a carb reset with no muffler?

 

[Dieselshawn]

I don’t totally understand back pressure in a can muffler. It doesn’t seem like a can is able to back-stuff the cylinder like a pipe.

My crude thinking is that a saw that blasts all the exhaust out should run the same, no matter how much raw charge makes it out after the exhaust. Have you guys tried tuning a saw with no can? I’m tempted to think it will make the same power if properly tuned.

I guess too little back pressure could inhibit scavenging. Especially if the transfers weren’t aimed well.

@Dieselshawn , have you put the 661 on the dyno after a carb reset with no muffler?

A 2 stroke needs some back pressure to prevent some of the air/ fuel charge from being sucked out of the cylinder by the high speed exhaust pulse.

I have actually tried a 661 without a can, the result was less power and a lot of noise.



I have not reset the 661 at all. All the runs on the dyno including changes in the setup that were done had no reset in between.

 

[Ketchup]

I was thinking 661 because the Mtronic carb will reset relatively quickly and reliably. No muffler at all may be outside of the carb’s working perameters but it’s a relatively quick experiment.

I’m not convinced the saw loses more fuel without a muffler than it replaces. If the transfers are clearing the whole chamber of spent gasses, then the chamber should be full of fresh charge. It seems more likely the scavenge loop is short circuiting and spent gasses are staying in the chamber while fresh charge gets ejected.

But I guess some weird stuff could be happening on the piston up stroke. Cold air might get pulled back in through the exhaust (seems unlikely) or the transfers may stop flowing too early (also not convinced). Maybe the upstroke will push out more fuel during the reverse of the blowdown phase, lowering compression (that one sort of makes sense). Doing compression tests on the same saw with and without a muffler might indicate something.

My general thinking is that if you can make a good scavenge loop, a saw should perform well without a muffler of any kind (or a mushroom can).

Noise and fuel economy are different.

 

[Dieselshawn]

The piston port intake and crankcase is a fixed volume. Only so much can enter through there for a given throat size. So the Mtronic has no issues making the saw run like normal.

The saws can run easily enough without a muffler.

The 661 had no trouble running the full test cycle on the machine.

Made better power than a can with a baffle and no front cover.

The can with the baffle, no front cover made a lot of noise, with only stock power.

That would be too restrictive, preventing the spent gases from escaping. Low, Booooring power.

The other way is, no muffler, just a teeny bit of air/fuel mix making it out the exhaust port before the piston closes. Not enough to make the saw run poorly but enough to make not quite the most power. And ridiculous noise....

A properly setup muffler porting is the final tuning for the best performance.

 

[ferris]

@Dieselshawn
Half a year since ur last post. Do u make more test with the 660 and 661.
And what are ur results?

 

[Dieselshawn]

@Dieselshawn
Half a year since ur last post. Do u make more test with the 660 and 661.
And what are ur results?

I haven’t been out to my shop all winter.

It’s wood stove heated and I haven’t had time to light it to warm the building up.

Been busy doing a lot of snow plowing and wrenching on big trucks.

I’m hoping to get back at it when the weather warms up.

Also the dyno itself will require warming up before putting under load which uses up saw gas.

 

[Ketchup]

@Cerberus

Wanted to ask if anyone's tried / tested "Grunt Gaskets"? Amazing idea by Dominant Saws, basically a header tube to extend the very-short "exhaust system" of our saws:
View attachment 335116
Spent days thinking of how to make one for myself, given I don't have much for metal-working, then finally had the AHA moment when I noticed my pile of "portings I didn't like/did better later on" for 660's and realized I can just use the angle-grinder to literally "remove a jug's exhaust port" and then I'll have a perfect fit, identical material etc 'grunt gasket' that's a port-length long if I want it to be!!

~~~~~~~~~~~~~~~~~~~~~~~~~

We all know that the port SIZES of the (engine, but specifically)the exhaust system are important, much time is spent on exhaust-port sizing/shape/etc for instance, however for mufflers it seems the size of the exit-hole(s) is optimized at "Probably about 55%-->75% of the exhaust flange area" (is this an OEM flange or a ported flange?), have never heard a good solid approach here but came up with an approach I suspect will work for my next 660 muff attempt:
- Cut out a big square on the side/top/wherever of the muffler like 1.5"square;
- Mount tight steel plate over this square, and drill your muff's exit-hole into this plate (which, of course, does not have muffler-wall behind it, since you cut it out )
- Make 3 cuts. Drill the hole a touch bigger. 3 more cuts. Drill. Cut. Drill. Cut....
IF your 1st exit hole was OEM sized then the 2nd size larger should in almost any case give you faster cut-times.... and the 3rd size-up... At some point, though, you'll LOSE power -- and now you know your optima!! Then simply put a new metal plate on or, if you're lucky enough to have a saw with <$20 mufflers, just get a new one & do it nice&pretty

This ^ is the only way I can think of to practically find your optimal exit-size, and that size matters a lot (being a lil beneath it is OK, you lose power real quick going beyond the optima, so IMO knowing this would be quite important, but almost no attention seems to be given to hard #'s in this regard)

If you read what @Dieselshawn is up to above, you’ll see his findings support some need for exhaust back pressure, but considerably less than stock. I’m not clear why any is needed, but the dyno doesn’t lie.

Instead of making an outlet plate and continuously enlarging it until you go too far, just make an adjustable outlet hole. A simple slide that covers a large rectangular hole would be pretty easy and you could probably tune it mid cut.

I also wanted to touch on the “grunt gasket”. There have been exhaust spacers available for quite a while. Some saws actually came stock with one (034Super comes to mind). The manufacturers claim they increase horsepower, but I have not met anyone that has noticed a measurable difference. Certainly a spacer that wasn’t well matched to the port would create bad flow dynamics.

@Red97 did a very nice YouTube vid of several alternative muffler designs. It’s somewhere in the last 20 pages of the “Another chainsaw dyno…” thread. The muffler with the strongest performance had no divergence and no surfaces to bounce off of on the way out. It was just a long bent tube. It was up against a “megaphone” style divergence cone that performed unremarkably. To me it indicates that complex muffler designs are largely for emission control. A simple tube that pulls all the burnt gasses out and replaces them with fresh fuel is all you need unless you go full expansion chamber.

 

[Ketchup]

Here’s the vid:

Joe is a chainsaw hero.

 

[Bjorn]

Hello. How much extra compression Will a well made expension chamber make? It helps the engine suck fuel from the crankcase, and then puch the unburned fuel back in the cylinder. How many psi?

 

[Adirondackstihl]

0

 

[Bjorn]

Dyno Joe have made some tests

 

[Ronie]

Dyno Joe have made some tests

I've been thinking about getting one off ebay to play around with.

https://www.ebay.com/itm/144520617729?_trkparms=amclksrc=ITM&aid=1110006&algo=HOMESPLICE.SIM&ao=1&asc=238698&meid=53ad8732f38446ad9dcff4d4e2402d9b&pid=101195&rk=1&rkt=12&sd=175216706180&itm=144520617729&pmt=1&noa=0&pg=2047675&algv=SimplAMLv9PairwiseWebMskuAspectsV202110NoVariantSeedPLXV3Ranker&brand=STIHL&_trksid=p2047675.c101195.m1851&amdata=cksum:14452061772953ad8732f38446ad9dcff4d4e2402d9b|enc:AQAGAAABIF5w9SIYA7LCsjjN3P6Yl26VztH0VKUH9PSWDJnasZbD1yCZJUhd3q2YXl6I7nNELSwP159v9id1K2rKKUMaxSk32jl0wL0pnpFQ5%2BPZxTmYGt4lqUrTk26fKJd35FEGID%2BDyVAGC2P8JRCgXD3sI3SMAJT74V1X2JxdCNk7g5XOmmn0lRpzDbEFTZMHMw0GXE4Z3wkuPo7lHZ8kOHO0VL1zIeeX74ylxi3Uz5qxzIsM4D4%2FoiSlbGQejUo%2BVfbo2rBJQRBV%2ByxkAtW5JLrsL8IfAael2IBqPeY3l0KFHWGRZGs9fTZqY6jiBuraH2SF3UwtKgjtFblA4YDlMXkfjVXLSY4UdhrUNGNLkuF6o2dEMIl3xOSTn3WGckTjIM3ohg%3D%3D|ampidL_CLK|clp:2047675

 

[Ketchup]

I've been thinking about getting one off ebay to play around with.

https://www.ebay.com/itm/144520617729?_trkparms=amclksrc=ITM&aid=1110006&algo=HOMESPLICE.SIM&ao=1&asc=238698&meid=53ad8732f38446ad9dcff4d4e2402d9b&pid=101195&rk=1&rkt=12&sd=175216706180&itm=144520617729&pmt=1&noa=0&pg=2047675&algv=SimplAMLv9PairwiseWebMskuAspectsV202110NoVariantSeedPLXV3Ranker&brand=STIHL&_trksid=p2047675.c101195.m1851&amdata=cksum:14452061772953ad8732f38446ad9dcff4d4e2402d9b|enc:AQAGAAABIF5w9SIYA7LCsjjN3P6Yl26VztH0VKUH9PSWDJnasZbD1yCZJUhd3q2YXl6I7nNELSwP159v9id1K2rKKUMaxSk32jl0wL0pnpFQ5%2BPZxTmYGt4lqUrTk26fKJd35FEGID%2BDyVAGC2P8JRCgXD3sI3SMAJT74V1X2JxdCNk7g5XOmmn0lRpzDbEFTZMHMw0GXE4Z3wkuPo7lHZ8kOHO0VL1zIeeX74ylxi3Uz5qxzIsM4D4%2FoiSlbGQejUo%2BVfbo2rBJQRBV%2ByxkAtW5JLrsL8IfAael2IBqPeY3l0KFHWGRZGs9fTZqY6jiBuraH2SF3UwtKgjtFblA4YDlMXkfjVXLSY4UdhrUNGNLkuF6o2dEMIl3xOSTn3WGckTjIM3ohg%3D%3D|ampidL_CLK|clp:2047675

Put that in the Ebay thread. I wonder what other saws that would be close on.

 

[Cerberus]

@Ketchup "The reason so many people have gutted the internals of saw mufflers and made a big hole is because it works. Subtle changes to the exhaust are relevant, but the big gain is in more flow volume."

I've got a handful of other points I have to cover in another post but Re muffler exit, of course "optimal exit area" will differ based on its location (and location of course matters, IMO no saw benefits from any exits "in line" with the exhaust pulse, and there seems to be real benefit to having the exit "near & 'interfering with'" the exhaust flange -- I don't want to over-state ability of this to prevent charge spill-out but if placing the exit somewhere why wouldn't you want it this way?)

But for final exit area there's certainly an optimal, I know Blair & Jennings both make explicit points to mentioning that being beneath this point, versus going past it, you take greater losses once past it than gains when approaching it (so you'd want 97% of optimal before 101%, or something like that as obviously I don't have hard #'s) This is also kinda self-evident of course... to what degree it matters is the only point of contention, right?

Wish I had a dyno but would bet you're talking at least a 5% difference between two different "well ported" mufflers, if&when this kind of non-measured approach is taken....I used to use flange% but found measurement to be almost pointless because the porting of the flange influences it too much, the layout of the muffler influences it too much, etc, so I now try working-with the OEM layout (and would bet that in most instances the OEM is best if and when you're able to "upsize the holes" throughout it, not all mufflers allow this of course but 660's, 880's certainly do, 2511's do if you get into the left side of the muff and enlarge those (3) baffle-plate holes and, of course, the muffler intake&exit ports)

Somewhere between a 2511t's OEM and an AF muffler there is 15% gained in opening-up to that size, and no-doubt power is lost once you start making a 1" hole at the OEM location, hard thinking that size should be an afterthought (even if we're unable to properly quantify&measure,we should at least care about employing / aiming for optimization, unlike pipes there's no hard math for the #'s so you can only test...thankfully some things, like no front-plate exits, are pretty well-established --- yet even there you'll find people who bought $100+ stainless steel faceplates for their muffler swearing that it's great...I guess I mean to say anecdotal reports are worth crap soooo often and the reality is a moderate opening of OEM sized holes typically does equal improvement, that doesn't mean there isn't a good 5%+ power on-the-table here in optimizing things!

 

[Ketchup]

@Ketchup "The reason so many people have gutted the internals of saw mufflers and made a big hole is because it works. Subtle changes to the exhaust are relevant, but the big gain is in more flow volume."

I've got a handful of other points I have to cover in another post but Re muffler exit, of course "optimal exit area" will differ based on its location (and location of course matters, IMO no saw benefits from any exits "in line" with the exhaust pulse, and there seems to be real benefit to having the exit "near & 'interfering with'" the exhaust flange -- I don't want to over-state ability of this to prevent charge spill-out but if placing the exit somewhere why wouldn't you want it this way?)

But for final exit area there's certainly an optimal, I know Blair & Jennings both make explicit points to mentioning that being beneath this point, versus going past it, you take greater losses once past it than gains when approaching it (so you'd want 97% of optimal before 101%, or something like that as obviously I don't have hard #'s) This is also kinda self-evident of course... to what degree it matters is the only point of contention, right?

Wish I had a dyno but would bet you're talking at least a 5% difference between two different "well ported" mufflers, if&when this kind of non-measured approach is taken....I used to use flange% but found measurement to be almost pointless because the porting of the flange influences it too much, the layout of the muffler influences it too much, etc, so I now try working-with the OEM layout (and would bet that in most instances the OEM is best if and when you're able to "upsize the holes" throughout it, not all mufflers allow this of course but 660's, 880's certainly do, 2511's do if you get into the left side of the muff and enlarge those (3) baffle-plate holes and, of course, the muffler intake&exit ports)

Somewhere between a 2511t's OEM and an AF muffler there is 15% gained in opening-up to that size, and no-doubt power is lost once you start making a 1" hole at the OEM location, hard thinking that size should be an afterthought (even if we're unable to properly quantify&measure,we should at least care about employing / aiming for optimization, unlike pipes there's no hard math for the #'s so you can only test...thankfully some things, like no front-plate exits, are pretty well-established --- yet even there you'll find people who bought $100+ stainless steel faceplates for their muffler swearing that it's great...I guess I mean to say anecdotal reports are worth crap soooo often and the reality is a moderate opening of OEM sized holes typically does equal improvement, that doesn't mean there isn't a good 5%+ power on-the-table here in optimizing things!

So, I don’t really like front exits on a muffler. They’re super loud and they set stuff on fire. Bark Boxes kind of irritate me. But do they hurt performance? I haven’t seen strong evidence that they do. I’m open to it though.

What do you think the advantage of all the baffles and plates in modern mufflers is? I’m not arguing, no doubt they’re supposed to do something. What?

I like muffler exits to be 80-120% of exhaust port area. Usually a touch under 100%. Just going by what feels stronger and isn’t annoyingly loud. I do feel that the more turbulence the exhaust has the larger the outlets need to be.

Is it possible that the amount of back pressure in the muffler is more important depending on the exhaust port roof height? Lower roofs need less back pressure?

 

[Ketchup]

Hello. How much extra compression Will a well made expension chamber make? It helps the engine suck fuel from the crankcase, and then puch the unburned fuel back in the cylinder. How many psi?

I have no idea. How would you measure chamber compression while the engine is running?

 

[Ronie]

I have no idea. How would you measure chamber compression while the engine is running?

I got curious and found this.

 

[huskihl]

I just whack a hole in it.

Ohhhhh……about that size……

I ain’t got time to be perfect on every saw. Larger openings are good for maximum horsepower. Smaller openings are better for bottom end power. There is no best case scenario because everyone likes something a little bit different. I try and find something in the middle of it and let it fly

 

[Bjorn]

Hello, if you are running a 2-stroke engine with a regular muffler, and then change to a calulated expensionchamber. It Will be a huge differens in performance, around 30% more power with the pipe. I think it is because, the pipe help the engine bread and it also race the compression. When it is puching the unburned fuel back in the chamber. But how much I Can not say. Maybee put a compression tester in the decomp hole on the cylindern for testing.

 

[Ketchup]

Hello, if you are running a 2-stroke engine with a regular muffler, and then change to a calulated expensionchamber. It Will be a huge differens in performance, around 30% more power with the pipe. I think it is because, the pipe help the engine bread and it also race the compression. When it is puching the unburned fuel back in the chamber. But how much I Can not say. Maybee put a compression tester in the decomp hole on the cylindern for testing.

I think you would want to build something like in the lawnmower vid. Rubber parts might not handle the heat. Please do it!