2 stroke - techniques and methods

[Quan-Time]

Does anyone have any information, or experience, on porting 2-stroke engines ?
For those playing at home who dont really know much about them, ill give a quick bit of info about them.
2 strokes are just that, they provide "power" every 2 strokes they do, as opposed to a 4 stroke, such as a car, diesel trucks, most motorbikes nower days.
2 strokes are mainly snow mobiles, water craft, some trial / dirt bikes, and road motorbikes from several years ago (no longer emission compliant for road use)


If a 1000cc 4stroke engine produced 100hp, a 500cc 2stroke would produce 100hp under the same "theoretical" conditions, because its providing power twice as much.
There is no cam or valve train to move, which lightens up mechanical load, at the expense of shorter service life.
The ports are on the side wall, and are static (they dont move).
As the piston moves up and down it exposes the holes and fuel can flow in onto the top of the piston, and complete its compression stroke and provide power. Because of these holes in the side wall, it provides excess wear on the piston and ring.

- LARGE picture where you can clearly see the holes in the cylinder




You can also see the piston is vastly different. Also note the top dome shape because of lack of valves but extremely long skirt and transfer port.

If anyone has some techniques, methods, and can provide info with possibly some before and after shots, it would be muchly appreciated !

Info im mainly after is:
Do you flow with the piston in, and move it up and down, or just flow without it ?
Do you flow each oriface individually and try to match them so there is no bias in port flow ?
how do you reach the inside transfer port, and smooth out the casting (if you do actually do that).

Thank you in advance for anyone who can help.

[106-1194218389]

You forgot to mention one two stroke I used to have to do machine work on once in a while. Detroit Diesel 2 stroke truck engines. They would hurt your ears when they dyno'd them.

[Quan-Time]

yer ive heard a 2stroke diesel V8.. quite the monster

[106-1194218389]

Flowing your two stroke will be quite the chore and in most the combustion chamber and piston can be quite the integral part of your flow. Do you have "Two Stroke Performance and Tuning" By A. Graham Bell?

John

[blaktopr]

[Tony]

And the really big power from two strokes also comes from optimum exhaust system design.

[larrycavan]

An excellent source is "The 2 Stroke Tuners Handbook" by Gordon Jennings.

Get one. You won't be without it once you start reading it.

[Quan-Time]

fantastic information ppl.

Thanks heaps for it !

I think i have some work cut out for me. This is more "research" at this stage. Basically so i know what im getting into !

[blaktopr]

The research is key. I know the Macdizzy site is a pay site now, but the guy got flooded with so many members and couldnt afford to keep the site going. A diy site, with more info than professional shops use. You can get good gains from reading those books and utilizing the info about the time/area of the ports. With the understanding of that information and utilizing the bench and the tools that go with it should give you an added edge. That website will help you learn how to map the ports to know exactly what you can work on. Then there is the expansion chamber. Oh boy. Google search hydroforming and maybe add in expansion chamber and there are sites out there to show you how you can form your own chamber to test if that is your cup of tea. Of course you can buy one if you are working with a motor/vehicle combo that is out there, but maybe not. For instance, I stuck a YZ465 in a Yamaha Blaster frame and rewelded the stock 465 pipe to fit. Once I changed the port arrangement, There was no pipe that fit the tuned length criteria that motor needed. I never got a chance to try hydroforming, but took a LT500 drag pipe and did a little hacking to try to make it work. It did, but there was probably more left with the correctly designed one. Now the reed cage area became a major restriction. At the wheels, it dynoed at a flat 46.5 hp peak at 48 from 4 to 6500rpm. The reed cage area was too small for the port timing to achieve the 7500 to 8000 rpm peak I was looking for. Im trying to give you an idea what you will be tackling. Check up on epoxies. I think I read somewhere that the oil mix plus vibration doesn't let it hold as well. You need some good stuff. Tig welder if you want to maybe fab a different intake on to your case. Reed valve angle does show a difference on particular motor. Between 30 and 45 degrees showed a change in powerband location and "hit" just like the pipe would.
I miss working on that stuff. I wouldn't mind watching what you work on and the gains you get, especially when utilizing a flowbench. Have you come up with a rig to adapt it onto the bench? Maybe you can make a plate you can bolt a jug on to. Then in order to test separately the transfers and intake, take an old piston and attach a 1/4 rod to it that will attach to a bar that goes across the deck. That bar then can adjust up or down. Then you can go through the piston movement for testing tranfers and the exit angles of the charge by blowing through it and just block off the reed cage inlet. Then bring the piston to tdc and the draw into the bench for testing that reed cage area. For the exhaust, that would be more difficult. Maybe if you turn the jug upsidedown, and use the piston, you can check the blowdown area before exposing the transfers. I feel the timing is more inportant than the actual flow, but you can record the gains you get by trying different shapes of the exhaust port during that blowdown time without having to increase the hight but only in the width, to keep the rpm range from getting to high. Just make sure you check the piston type, because that dictates how wide you can go. Sorry about all this typing, but I like these two strokes too!:D

Chris.

[slracer]

Before I started playing with the Honda 4 stroke, I ran a Kawi KM-100 rotary disc valve 2 stoke. The disc valve allows the intake timing to be separate from the piston location and adds more variables to testing. The bike ran 93.465 mph with NO aerodynamics so it was making some pretty good power (but current records at Bonneville are faster). It may be the record holder at Muroc Dry Lake forever as the "Government" doesn't let us race there anymore (the track is the space shuttle landing strip at Edwards Air Force Base). I built a second engine and made the pipe for more power but a narrower band. The first time I rode it, I almost lost it as when it "hit the pipe" it almost jumped out from under me! There are a lot of things to consider with those toys and when you read the A.G.Bell book and the Jennings book you will probably find some "issues" where they don't agree. Work through them both and understand them. I wrote an Excel spreadsheet to run the numbers on the pipe sizing, I'll try to find it and post it here if anyone is interested, but I am 2 computers downstream from those days and am not sure where it is so no promises. Enjoy! -- Doug

[106-1194218389]

[slracer]

I think there is just a number of years between them and a different way of looking at things. They use the same data (as I remember since I went to 4 strokes in 04). I used the Bell book more for the pipe numbers and used the factory race bike specs for my timing. I changed to a short stroke KX125 with a sleeve just before I quit running that bike. With the stock stroke, I got 0.5mm bigger bore and an extra gear in the trans so I could stay on the pipe. I never made a run without electrical trouble on that version and was planning on going to CDI when I went to a 100cc 2 cyl Yamaha (LS-2 with a GYT kit) instead. That one would turn 16,000 RPM but it had trouble with the left piston developing holes in the crown. The frustration led me to 4 strokes! I now know what happened to both and am considering getting both out of mothballs (they run in different classes at Bonneville). I do remember one thing about the pipe numbers: the dimensions are for the INSIDE diameter. It took a couple of cuts before I figured out that for a 23mm hole, you had to cut the metal for a 26mm can (with 1.5 mm thick, about .060, metal). If that doesn't make sense, draw a picture and think about it. ???

Losing some weight is a good idea as I ran the numbers for my little bikes and it was about 5 lbs lost weight for the same increased speed as adding 1 HP. A lot cheaper to lose a little weight! -- Good luck! Doug

[Quan-Time]

I worked with a 125GP bike for a season. RS125 '04 honda. Put out ~42hp (wheel, measured) depending on a few things.

We had multiple jets, dyno runs, and a full spread sheet of data for best jet VS baro / temp / humidity. Just rejet ~3mins before a race and it worked quite well.
Obviously had a detonation counter. We planned on 3dets per kilometer (1.6km per mile). One team with good sponsorship was running 5 dets per km and constantly beating us. So we tried it, and beat them. At the cost of having to replace the piston at the end of the meet.
Pistons only last 200km at best, we usually replaced 150km intervals to be safe. But could still be used as a mule piston.

Upon rebuilding a honda factory bike, it was surprising how much detail they missed. Casting flashings, burrs on radius edges, rough casting marks.
The 125GP bikes you see on the TV (motoGP ones) they put out ~52 - 54hp depending on who you ask. They cost 50k (australian dollars) for the engine and come with ALOT of electronics. Its basically a "bolt in" package.
Every time we rebuild the bike that season, id remove some burrs, and slightly clean that area. Next race monitoring what it changed. We became more consistant with weather aswell. And noticed we got LESS dets, sometimes by 1 per km, (we had 50 - 120 per race, track dependant).

Altho im not working on that bike anymore, im interested in getting one, or possibly a aprilla RS250 / RGV250 and having a play. yes they are a V-Twin engine, but it will help me to be consistant. Replicating is a key factor in my eyes.
No point having one port which can flow great numbers if every other cylinder is letting you down and not doing the same.

Im not trying to be a world record setter, but i would like to be able to tune a bike that will make a few people get up and notice. Might help me get into another team.

For the record we ran 42:1 oil, and 105RON racing fuel for those interested.

Ill just quickly edit, these honda factory bikes are the ones you buy direct from honda. We use them on our local circuits, im not part of the 125GP world tour !! (altho we did race @ phillip island in the support race the other year and did quite well, pity our rider doesnt qualify for 125GP tour, age limit is 15 - 21, our rider was 34, and female ! )

[larrycavan]

I have the 2 stroke Tuners Handbook in PDF format if you want it. It's 7 megs. Holler and I'll make it available as a download.

[49-1183904562]

It can also be found here with other good docs.



Rick