Lets talk engine!

I think there is a need to expand our knowledge (or possibly just mine) on the internal mods that can be made to a two stroke engine.
Porting: Raising the exhaust ports will give you better low end while lowering will give you better high end (I think I got that right). Many OEM cylinders had the sleeves inserted poorly causing a mis match between the sleeve and cylinder. This would cause poor performance big time I would imagine.
Exhaust manifold/cylinder matching: I hear that you should have your exhaust manifold matched to the cylinder but know very little on this.
Case polishing: I assume this is the removal of small cast imperfections from the factory. Sounds like something that a rec mechanic can do.
Crank work: Truing, welding, balancing, blue printing.... what else I am at a loss in this area. What exactly do all these things mean?
Head work: Milling the head raises the compression but you need to watch out for the squish clearance. What about reshaping the domes?
Pistons: What the heck are flat top pistons???

Well, that is a start to what I hope to be a deep discussion on the internal modifications that can be done to get a little more power.
Joe.........

I saw Dan Lamey is posting on pwctoday now, he explained flat top pistons on a thread there last week I think. I used to run a flat top setup only because I heard it made more power, he explained why it does. I can't find the thread or remember what it now though.

Flat top pistons allow better piston wash, good for big bores, but they also reduce the piston's top surface (a dome has more surface than a disc of equal diameter) and therefore the pressure applied by the explosion on the piston itself.
In ltd you cannot polish cases, in S/S you can.
Trueing/balancing a crank is basically setting your crank back to ideal factory specs, people usually weigh different rods and select 2 (or 3) that weigh exactly the same, I do not thing you can add/remove material on your crank, either in ltd or s/s.
Welding is just welding the rods, just for better reliability.
As far as limited and S/S class are concerned, you are supposed to use a STOCK crank, meaning a trued crank is ok if you haven't lightened it but just set it to OEM specs, welding is forbidden but technical inspectors let it go most of the time (if they go as far as that) because it's a matter of reliability more than performance, but if the guy is a dick, you're out.
11

[Edited on 24/11/2003 by Eleven]

11,
This is for my Yami's. I don't race them so anything goes.
Joe........

Joe, besides the trued/welded crank which I would consider mandatory on any modified engine for the sake of reliability, all those other little things you mention on that other thread are really overkill, they are a lot of time and hassle for almost unnoticeable results.
11

Oh yeah and also heard from a lot of people that umperfect surfaces for areas that are in contact with gas/oil/air mixture helps making it more homogenus (sp? is it only english?) and therefore more efficient.
Also if you haven't done so yes, you will be horrified by Yamaha cylinders finish compared to a Rotax, I personaly do not like too much porting but like matching/adjustment to stock values,
11

Now this is getting interesting :cool: First off Joe, it's the higher the port timing the higher the rpm's and you wouldn't lower the exhaust port...

The surface that 11 is talking about is the golf ball effect... Air travels better/faster on riples then a polished flat surface...

Blue printing: Cleaning the imperfections out of the cylinder transfer area of the intake and exhaust gases...

Porting: Changing the height and width of a port will change the way the motor power band width will run...

Tuned Exhaust: The is the exhaust system that has to matched to the exhaust port timing...

Intake porting: get as much fuel/air into the cases without losing manifold pressure...

Enjoy...

Wider ports give more low end, a raised port gives upper end. Milling the head is shaving a bit off the surface that the gasket adheres too. It gives you higher compression and a smaller squish. That good to a certain point, I dont think that I have to tell you that anything over 190 and you should run a higher octane and anything smaller than .04 mm squish is pushing it a little tight. Blue printing is aslo for the pump. Carve out the imperfections to reduce cavitation. Your crank should be balanced too because it is spinning a high speeds and if it is not balanced correctly, your gonna have more harmonics that break down your engine faster--the whole thing cause it is the core of your engine. Another thing that is good to have is a girdled head. The head bolts directly to the cases rather than to the jugs. This also reduces stress on the jugs cause then they arnt bolted to the cases directly, harmonics dont stress them from the bolts.

From what I understand, you do get some sleaves that dont quite match up to the transfer ports very well. I asked someone about this and if it would improve performance to shave them but they said it would ruin my timing.

Timing is an interest of mine, If you advance the flywheel a few degrees you gain more low end, or is it top end and why does this effect it so.

Some manifolds dont require port matching. Factory for instance makes their manifolds a hair wider than the cylinder ports. You wont get any restriction on performance, just maybe on resonance.

Flat top pistons would require you to reshape the head or get a head for flats. The burn is better but you dont get the same punch as you would with rounded pistons. 11 is right on the money there. So flats are great for racing but poor for lowend. The domes of the head need to match the shape of the piston so if your runnin flats you need to get a head to match. I know of one mechanic in particular that reshapes his pistons to suite his racing performance needs, if he changes the shape in anyway, well you get the picture.

The surface that 11 is talking about is the golf ball effect... Air travels better/faster on riples then a polished flat surface...


Actually that is incorrect. Like 11 said, the imperfections will create a more homogenous mixture but air doesnt travel faster over rippled surfaces. The golf ball travels faster becuase the pits create anti air vectors which decrease the pressure in front of its path of flight. Decreased pressure means less drag.

OK, many riders have the 633cc engine and are planning on switching to the 701cc engine. While a person has the engine apart,,,, what should be done?
Joe..........

Well the 6M6 cylinder can be resleeved and ported to be a match to a 61/62.
But the main thing is the top case, the reed 'hols" (sorry, hangover and bad english this am) are too small on the 6M6 so you need a 61X top case if you want to get the best of that thing.
Crank is the same, lower case is the same but they need to be matched, I'd just try to find a good 701 if I were you, there was one on Ebay for 400 last week, that's less that what a Novi cost you for your XP.
11

Oh I down with that. I have a set of 61X cases, brand new 61X cylinders, and a 61X head. I figure on getting the crank trued, thanks Robert. I'm just wondering what else. I'm thinking mill the head for starters so I get about 180 PSI. This is my thrash ski so no race rules to worry about. I should have something done with the cylinders,,, Ports widened??? whatever makes this thing a low end GOD! I could even go with a lightened crank..... so now you see where the questions are coming from. I am attempting to get my questions answered while allowing other users to get something out of this thread as well.
Joe........

Yeah some JSM porting will help low end, also about lightening the crank, if it's already trued/welded I wouldn't. Get a lightened flywheel first the bigger radius will make evey gram you grind more effective.
11

Yeah Yeah... I knew what I meant to say... This All I know is what the port should look like and how to do it...