I don't fully understand this but have heard servaeral people speak about it. I have heard that the mxz 800 has little crank case volume and thats why twins do not work as well as they should. I must confess, I am also fishing for info as we have a 01 800 with a crankshop head and twins on it now and it works ok but not well. This is a trail sled. I have noticed on other motors that the reed area on the case is squared off to the bottom end and that some people grind that area so that there is a smoother flow from the reeds to the case. Almost 45 degree it and make the air flow easier. Is this what the 800 needs to make the twins respond better?

The 800 Rotax has cylinder reed induction so examples from case induction engines probably don't apply very well. Check out the porting post at the top of this tech section. Thumbdoctor has done a great job of documenting a case cleanup project on this engine.

Some people swear they have had great luck with twins on this engine. There are a number of opinions on why twins are problematic on this engine (check port/pisont overlap thread here for a start). Most say that the engine needs porting to work well with twins. You might search for threads on this and check some of the recent posts for Canucklehead who runs these pipes. Send him a PM here or on SnoWest. I think he recommends Clayton at Multi-tech Motorsports (http://www.multi-techmotorsports.com/) in Alberta -> Calyton is a big Crankshop dealer and builds up engines using their 900 kits too.

Also, DNE Performance and a couple other shops offer a mod that will add two additional intake ports in the cylinder wall and open up the existing one to increase flow to the case.

Hope this helps,

SummitAK

thanks and it does help. I have only really looked at triples from the inside as that is what I run for an asphalt sled. Haven't really looked at the twins..just ride em.

I will be covering a different approach to getting more flow into the lower end on an upcoming thread. I do agree that others have great inlet mods performed by adding supplemental intake ports. I on the other hand have found good results from removing the lower bridge between the bottom rear port & cylinder base flange. Working with TSR port software, I have found this as a very economical way to wake up a few lost ponies. A few other port designers use the same strategy. I guess the final say is who crosses the finish line first.

T.D.

purple, good question on the case volume. The wind has blown both ways on that topic through the years I followed race bikes. Some recent examples in sledding, the 600 polaris liked the smaller 400/500 cases than the big block case it was on, would this support smaller case vol? The 600HO was put on the bigger 7/800 case and seems to perform well, does this support bigger case volume? Some of the modern mx motors are absolutly shrinkwrapped around the crank, cranks are lightened and plastic plugged as to not increase volume?. Hopefully thumbdoc or someone will give us more ideas.

I can't wait to see what thumbdoctor does. Like SummitAK said I run Crankshop twins on an 02 800 with porting by Clayton @ Multi-tech and an RKT head, it runs very well. More crankcase volume may not necessarily be a good thing if you ride at altitude.

You know, I think it is great that Keith and Brian took the initiative to get a forum that is purely technical in its nature. Great job guys!


Having said that.. people need to keep in mind that there will always be AT LEAST 2 sides and/or differing explainations to all subjects.... especially in 2 stroke theory and sled set-up.

I think that this forum has done great in having technical discussions about various subjects that concern engines and such.

Now, It is very apparent that the Thumbdoctor is well versed in 2 stroke engines and also does a great job in explanations. Nice job man!!
The reason I call you "man" is because I have no idea who you are. Nor does 99% of the others that read your posts.

If it is not too much trouble.. would you mind telling us who you are??? And your experience in the 2 stroke engine world? Ya see, at least for me, opinions ALWAYS seem to carry more weight if you know who is giving them. For example, if Bill Gates was on a software forum giving advice about operating systems under an alias like "BOB" many would just think that he is a guy with a good understanding of operating systems. BUT if he was giving this SAME advice under his real name "Bill Gates" His opinions would carry much more weight.. wouldn't ya agree???

OK, Crankcase volume... This is indeed a subject with varying opinions on which way to go (more or less). Which just goes to show you that there will always be the theoretical side to things and then there will always be the practical , or reality, side of things. Sometimes theory and reality do not agree.

Primary compression: Let's look at the reasoning behind why more or less may be beneficial to an engine's power output. It is not my intent to touch on every aspect of this subject so please feel free to add all other inputs on the subject.
1) More volume = less primary compression (PC). One theory behind this is like the analogy of scooping a bucket through a puddle of water or a lake of water. If the crankcase is capable of holding more F/A mix then it may stand to reason that when the blowdown phase has completed.. that there could be more F/A mix pulled into the cylinder. On that same note.... what if the crankcase was already suffiecient in its capacity to store F/A?.. Would you be able to pull anymore into the cylinder???

2) How does the PC effect the signal on the carb during the intake phase?

3) Less Volume = More primary compression. Having more PC... How does that effect cylinder filling? Having more PC will allow a greater velocity in the transfer ducts.. which COULD be interpreted as allowing for more F/A to be pulled into the cylinder simply due to its greater speed. So, what happens if the speed of the new charge is too great? Will you have more short circuiting? Will you over scavenge the cylinder? Will you deplet the crankcase of all the new charge?

4) How does the higher PC effect the signal on the carb?

Kelsey

I don't think there is a definitive answer to crankcase volume, unfortunately. As with most 2 stroke technology's you'll find this to be the case. I would say though unless a cylinder pipe combination can flow a good amount of air it would be useless to enlarge the case. And then if you did enlarge the case because you thought you had good air flow up top, unless you increase the DR (delivery ratio) to the case or cylinder you probably didn't gain a whole lot for your work. The truth of the matter, I believe, is it has to be a complete package. Everything from the carb to the stinger on the pipe and everything in between, which is allot. It would also take allot of dyno time to factually support change's that were made, and again because thing's have to be in concert within the engine package, one change made to a componet may necessitate another change that you may not be aware of. Sometime's you have to be intuitive when you make change's to verify your work on the dyno. But maybe I'am wrong, and there is a definitive answer on crankcase volume, and port timing, and port width, and port direction, and God forbid, pipe design.

Orthopedic surgeon with a degree in avionics.. Thats like an oxymoron isnt it? ..lol .. Its funny because Im a vintage mustang enthusiast and just a couple days ago a member of a mustang forum I frequent was ranting about doctors.. and I was on the defense... but his final statement was " we can all be glad theyre not fixing airplanes" LOL Boy is he in for a surprise... One question Doc, how do you get all the grease from under your fingernails before you consult your patients? Im visiting the Orthopedic surgeon tomorrow for a torn lateral miniscus.. this guys a great guy, great surgeon and a motorhead too.