[attachment=40016:Stock_ca...tor_size.jpg]Here are some thumbnails of a recent porting clean up I performed on an older 670cc (type 669) but same principals apply to all Rotax rotary mills. I'll start at the intake tract working through the transfers and finish with the exhaust port. The Rotax RV motor suffers from a long tract from the throttle valve to the combustion chamber. A few methods of shortening that distance is to utilize shorty intake boots (coupling between carbs & rotary valve cover). The next operation (usually reserved for drag / high speed runs) is to retrofit an earlier RV cover from an non oil injected machine thus further shortening the intake tract. I prefer to leave the injection system intact. Image 1 & 2.The 2 piece injection RV cover needs to be opened up to match your selected carburetor size keeping in mind the valve side of the cover should remain close to stock size (approx 41 mm). I would suggest tapering the opening from the carb side (large) down to the valve side (approx 39 mm).
[attachment=40015:Stock_ca...tor_size.jpg][attachment=40017:Stock_ca...ide_size.jpg]
These photos are stock carb adaptor covers which taper into the case port with a large step (red arrows).

[attachment=40018:Stock_ca...per_size.jpg][attachment=40014:Carb_adaptor_stock.jpg]

Image 1 & 2 I resize the entrance (carb side) of the adaptor to 42 mm on the taper bore then I pass a rotary wire brush through the port after resizing to achieve a rough finish.

[attachment=40022:Carb_ada...close_up.jpg][attachment=40023:Adaptor_..._boring2.jpg]

Left photo illustrates before and right photo is after which I have increased the bore to a very conservative 42 mm carburetor side and 40 + mm rotary valve side.

[attachment=40027:Adaptor_...ing_size.jpg][attachment=40028:Adaptor_...r_boring.jpg]

Image 3 & 4 Moving upstream to the engine casing side of the cover, I machine 18 mm radius "eyebrows" into the 10 O'Clock (PTO side) and 2 O'Clock position of the cover.

[attachment=40024:Stock_R_...ce_plate.jpg][attachment=40025:Modified...ce_plate.jpg]

Stock cover on the left and modified on the right. These "eyebrows" will corrospond with machining done to the crankcase tunnels to provide a direct path to the rear boost port and rear cut away on cylinder sleeves.

[attachment=40026:Location..._eyebrow.jpg][attachment=40029:Close_up..._eyebrow.jpg]

As a reference to the placement of the eyebrow I use the back side of the screw hole and measure 18.5 mm to the center line of the 18 mm radius. This work will safely lengthen the intake duration and provide more fresh charge to the rear ports helping push out spent gases.

Image 4 The next step is to mount the cylinder barrels (with Y-pipe & head attached to retain working alignment) to the upper case half and transfer (scribe) dimensional differences between the case & cylinder's transfer ports. The base gasket will help in this procedure by offering you a template (after trimming to the largest dimension) to work with

Image 5 The upper crankcase & cylinders can now be matched using care not to remove too much material on the transfer port gasket rail (this can lead to base gasket blowout & meltdown). Attention will also be needed in the front (exhaust side)of the case. There is a square cut away which the con rod travels through, this is a windage baffle, opening it up will cause mid to high speed lean out. This slot also reduces high RPM pumping losses.

Thanks thumbdoctor. Exactly what I was looking for. The pictures help a lot.

Good job Thumbdoctor!!! :thumbsup:

Now what about the pics for the 800 twin ;)

Image 6 This brings us to the barrel porting (transfers & boosters). The best way to blueprint those is to create a baseline. To do this you will need to fashion a sleeve of paper to fit inside the cylinder barrels. Tape them in place to keep alignment then using a soft leaded pencil or charcoal stick, make a rubbing of the port layout in each cylinder, cut out the port openings on the paper, then compare the two by overlaying one on top of the other, Voila!, now you have a baseline of the port layout, differences in shape, misalignment and so on.

[attachment=40030:Port_Map.jpg]

Image 7 This is where the real fun starts, after you have your baseline, you need to establish: Lowest port cut, Highest port cut (Manufacturer's). By drawing straight lines horizontally & vertically over your port layout you can visualize inaccuracies in factory port sizes and location. This is where some imagination / restraint prevails. You only need to match the port sizes and locations. Note: If the cast iron sleeve is misaligned with the aluminum jug, Bombardier says you can realign it by baking the cylinder in the oven at 350 degrees F for 20 minutes then quench the sleeve with rags soaked in icy water. Using this procedure you have to work fast as the window of opportunity is very small. It's best to jig the jug into a wooden template before & fashion a piece of hardwood to twist the barrel by the transfer openings at the bottom of the cylinders. Enough said on that topic.

After straightening the port layout on paper transfer the revised configuration onto thicker paper to create a template to scribe the ports for correction. Using machinist's blue dye in barrels before scribing outlines of new ports. Now some experts transfer the dimensions onto magnetic sheets which hold on the sleeve better but I find good preparation always prevails.

finally, the exhaust port clean up. I use the exhaust gaskets & Y-pipe as a rough guide for sizing. This is an area which you can go to town polishing. The reflective finish helps carry heat into the expansion chamber (pipe(s). Shining the port up also prevents carbon build up in the port and Y-pipe. Lastly, do a light hone in barrels to aid new ring seating, then radius all upper & lower edges of freshly ground ports to prevent rings from biting the port edges (or worse!). Tune in next week for Y-Pipe mods, then later, perhaps Idooski will enlighten us on his progress building pipes. :hallo1:

Man that was educational. You'd be a good teacher :) Now lets do some cat cylinders :devil:

very interesting ,i see you have study this quite a bit ,so whats the performance gain from matching and blueprinting on an other wise stock motor? :thumbsup:

Originally posted by z24bb@Jun 25 2003, 10:14 PM
very interesting ,i see you have study this quite a bit ,so whats the performance gain from matching and blueprinting on an other wise stock motor?* :thumbsup:
This motor (built to '86 type 537 spec) was also chamber modified, piped and canned using stock parts from a 1992 617cc PRS chassis. It has been dynoed at 111 reliable hp @ 8700 RPM & 78 pound feet of torque @ 7600 RPM calculating a 0.561 BSFC (Brake Specific Fuel Consumption) but still very good for a mill that started out with a feable 65 hp @ 7700 RPM ('89 type 536 trim). The machine ('89 F Plus LT) cleans up in vintage speed runs @ 97MPH on Radar and will match / beat type 583 equipped PRS machines most days. The performance gains depend on getting the most available V/E through port matching / blue printing then clutching to optimize the changes. An conservative guess would be 6-15 hp in the mid to top end in optimal conditions keeping in mind that this is using a stock pipe and an after-market can breathing dyno air @ 85 degrees F. Combustion chamber modifications and pipe selection will net more. A lot of manufacturers use hp figures gained from hand built "mule" or "works" engines to advertise. The motors mass produced for the common folk rarely come close in "real world testing". Another area commonly overlooked is drivetrain parasitic friction, in other words, for every Lb Ft of driveline friction there is one less Lb Ft of torque delivered to the ground from the mill. Driveline alignment is critical as speed increases. There is some "free HP" to find in your chassis.

thanks for your reply,ok now that brings up another topic,how do you reduce the frictional losses in the chassis,i take it that there is more to it than just track alignment ,tension ?

Originally posted by z24bb@Jun 28 2003, 08:16 AM
how do you reduce the frictional losses in the chassis,i take it that there is more to it than just track alignment ,tension ?
Z24bb, You hit the nail right on the head. There are a lot more sources of friction on the average sled which are commonly overlooked. Lets start with the motor. Have you checked your belt alignment and deflection giving a good look at which way the engine moves under high torque conditions. Check the engine mount cushions, chances are, if you are like most of us, you get out of the hole like a bullet out of a gun. Check the front left rubber mount, see if it has turned soft allowing the clutches to pull the alignment away during high torque episodes. Once that's accomplished you might consider a torque brace (strut). Next, remove your secondary clutch from the jackshaft then loosen both the chaincase and the left countershaft bearing assembly observing which way the shaft tends to rest. It should be in the center of the chassis flange (bearing mount). If it isn't you will need to shim the upper or lower portion of the chaincase to center the jackshaft. The same exercise can be applied to the drive shaft making sure the bearings aren't binding. Check the drive sprockets to make sure they are exactly where the manufacturer intended them to be. Offset the drivers to one side or the other will cause the track to touch the tunnel at high speed. Lastly, check the tunnel for parallel by measuring the distance from the front left axle to the rear right and vise versa. This will assure the track is not being overstressed on one side or the other. Adjustment can be accomplished by slotting the front chaincase and bearing mounting holes. Little tolerance stack up leads to large frictional losses.

i was re-reading your last post this morning and it got me to thinking ,on my drag racing cars that i have built,set up and tuned i have always scribed a permanet chassis center mark one in the front and one in the back that i use to locate 4-link settings ,fr a-arm brackets or strut mounts,body location on chassis, motor centerline etc, i have found these to be invaluable over the life of these racecars, not just for setup but also for checking for worn or bent componets or frame members,it has always amazed me what we find when we do our regular maintance. do you or have you ever done this and do you think it would help on a sled ? if so where would you think these scribes should be located on the chassis of a sled ,thanks

z24bb,

I'll answer your reply on chassis index marking on http://www.snowmobileworld.com/forum/index...hp?showforum=67 (http://www.snowmobileworld.com/forum/index.php?showforum=67)

were getting off topic here.

S. R. C.

As promised, Here are some inexpensive mods that can be performed on Y-Pipes.
To start with make sure the manifold you start with is sound, no cracks, poor welds or excessive corrosion. The best way to clean the existing carbon build up is to soak the pipe in tap water for a few days. The coke will just brush off with a good wire brush. To remove rust you have only two avenues: chemical or mechanical. I prefer to use both. First start with metal cleaner containing sulfuric acid (follow directions and use appropriate caution) to clean of any surface rust and the remainder of carbon. An optional method is to have the Y-Pipe "Glass Beaded" this is preferred to sand media blasting as it removes less material and hardeneds the surface. Once thoroughly cleaned, inspect the pipe to flange weld at the cylinder plates. I usually mig weld another bead around the factory welds (be sure to clamp the flanges to a straight surface before welding) to give me some room to blend the inside port shape. Using the flange plates as a guide, I work around the inside, opening the port size starting at the inside weld moving downward towards the junction of the two primary pipes. Once this is done the balance of the work should be done with abrasive flap wheels. The object here is to promote a good unrestricted heat path to the pipe (expansion chamber).

Y-Pipe2: One area of concern is the inside blend of the two primary pipes to the main pipe (outlet), this is the area of the greatest heat buildup during hard cycling. (I'll try to get a photo of this on the dyno). This is one area you should not remove any metal, polishing with a 320 grit flap wheel is enough. At the cost of a new manifold, pay attention here!. Again the object of this exercise is to pull heat away from the exhaust port and deliver it to the pipe. Another area of detail is the transition from the Y-Pipe's ball joint to the socket on the expansion chamber. Make sure the inside diameter of the Y-Pipe's outlet matches the pipe's inlet. Some folks trim some (up to 1/2 inch) material here to promote peak HP. I choose to leave that up to the driving style (drag racing / speed runs or trail use). Leaving this area close to stock length retains factory torque readings. For corner to corner or mountain climbing where a lot of engine / pipe movement is apparent, grinding the Y-Pipe outlet oval vertically will assure good exhaust flow regardless of Y-Pipe to chamber alignment. (On a track dyno I have seen 10 degrees total exhaust deflection here during severe loading and unloading).

Y-Pipe3, For those of you who have lost exhaust springs or had a new after-market pipe wear right through, here's why. The V-brackets on most exhaust couplings tend to lie down after a few good high speed runs thus reducing the clamping force provided by the springs (use good quality S/steel springs here). Carefully bending these wire clips upward will recover that lost tension and surprisingly they tend to stay put (I guess after a lot of heat cycles the clips anneal). Be sure to check for worn through or cracked clips as repairing (tig welding) them now is cheaper than pipe or belly pan replacement. Lastly, polish the ball and socket interface and check clearance of the two components. Sloppy fit here leads to noise, power loss and oil residue on everything. I have the finished exhaust parts bead blasted then High Performance Coated. To save costs, you can use Brake component paint or header paint (good for 1400 deg F) stove paint won't cut it !. Don't forget to seal all exhaust parts with good quality high temp silicone sealer when you install them then run a good bead on the retainer springs. As Dynamo^Joe said in another column "adding mass lowers the amplitude (frequency)" of the spring's natural resonance further reducing spring fatigue.

Here are some thumbnails of a recent porting clean up I performed on an older 521cc Rotax mill. I'll start at the intake tract working through the transfers and finish with the exhaust port. The Rotax RV motor suffers (a little) from a long tract from the throttle valve to the combustion chamber. A few methods of shortening that distance is to utilize shorty intake boots (coupling between carbs & rotary valve cover. The next operation (usually reserved for drag / high speed runs) is to retrofit an earlier RV cover from an non oil injected machine thus further shortening the intake tract. I prefer to leave the injection system intact. Image 1.The 2 piece injection RV cover needs to be opened up to match your selected carburetor size keeping in mind the valve side of the cover should remain close to stock size (approx 43mm). I would suggest tapering the opening from the carb side (large) down to the valve side (blueprint).[/b] hello, where can i find these pics? thank you

Paul, I don't know what happened to the photos that go along with the article. I'll look in my achives to see if I kept them.

where did the photos go?

Someone hacked in to my controls last summer and deleted them all and I don't have the originals anymore.

good info pics would help is there a chance you might be dooing the same project again and could take the pics and post again? just a thought

Someone hacked in to my controls last summer and deleted them all and I don't have the originals anymore.[/b]

oh i swear i saw them early this summer

but maybe that was in the other thread..

Thats too bad the photos are missing from these articles, its not often someone takes the time to provide such comprehensive information. Maybe if you find yourself self porting a Doo 809 you could start another thread......I know, I'm a funny guy, its just winter is approaching and wouldn't mind waking the old triple triple up a bit....

Thats too bad the photos are missing from these articles, its not often someone takes the time to provide such comprehensive information. Maybe if you find yourself self porting a Doo 809 you could start another thread......I know, I'm a funny guy, its just winter is approaching and wouldn't mind waking the old triple triple up a bit....[/b]
you did that all wrong. should be: I know where an 809 is you can doo for another thread. :p

you did that all wrong. should be: I know where an 809 is you can doo for another thread. :p[/b]
LOL! Your damn right! Why didn't I think of that...?!?

I am slowly repopulating this post with new photos I am taking using some spare parts and a 670 that I will build up. The 521 photos are unfortunately lost forever.

Do you have any updated pics of the 670 you are supposed to do?

This would help me greatly as I have my 670 apart due to a bad bearing that wore my crank.


I found a good crank, I'm punching the cylinders .020" and I want to follow your instruction on porting the twin rotax engines.