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questions about porting

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questions about porting

Postby sno*jetfreak2 » April 25th, 2006, 4:16 pm

Polishing a port is fine on the exhaust port as it does help prevent build up. On a port as long as the transfer ports its true polished smooth can create a Laminar Layer... the air actually basically sticks and builds drag the further it goes.

Intakes should be smooth like tissue paper(light texture but no real drag makers)...exhausts can be smooth like regular paper... My engines exhaust is a mirror though.
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questions about porting

Postby AHood » April 25th, 2006, 4:21 pm

[quote=ZR800XC]Just to kinda add in here some tips for exhaust port shaping. You'll notice the exhaust port is angled down. When shaping the top of the exhaust port you'r going to want to remove only enough material to keep that downward angle. You don't want to cut into the roof a whole bunch if you're keeping a stock height. On the bottom corners try and get these to flow right into the exhaust tunnel at the factory angle. As far as on the sides...widening out the port requires a change all the way down the tunnel or the effect will be little. If you go to AHood's suggested 45.5mm you're not gettin' too crazy...1.25mm a side over stock??? but it's something to pay attention to.

When you're dressing up the transfers, pay attention to the roof angles. You'll want a fairly flat roof that goes back with a short radius where it changes from vertical to horizontal. Your back transfer, well I'd make that about as wide as the front one. Or at least as wide as the piston ring gap would allow. Aiming it towards the back of the cylinder will help clear the exhaust gases out of the back of the cylinder. Obviously can't tell it's angle from the port map, but if it's aimed straight out towards the piston crown, you have no charge filling the back of the bore...as for the front port, it's probably fine, as long as the rear wall of the front port will intersect the path of the front wall of the rear port before they meet the center of the bore. Generally you want the paths of the transfer charge to intersect with each other. Again, you're filling up more of the bore with fresh charge.

I'm sure there's plenty to do in the transfer deck of the upper crankcase and in the transfer port passages...but this gets into major material removal and even some welding in some engines. Just cleaning it up should be fine for reasonable performance. I like to get mine as smooth as I can. I don't buy the mini-vortex theory of the rough finish...again, obviously, personal preferance. A good rule is to pretend you're the air fuel charge trying to get up into the cylinder, and then out the exhaust port...make your path as easy as you can![/quote]

unless he has a small right angle grinder he wont be able to work the transfer port windows roofs or entry angles.

what is the mini vortex theory. lots of great two stroke tuners would disagree with a smooth finish. it almsot makes no difference in flow but it can have effects on jetting and fuel attomization. a slightly textured surface keeps fuel suspended and atomized best.
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questions about porting

Postby ZR800XC » April 26th, 2006, 1:10 pm

Well some builders believe that a fine rough texture provides a thin layer of turbulance on the edges of the port. Each scratch makes it's own vortex, and thus lets the rest of the fuel/air mix pass right by. Kinda like in the back of a pick-up truck...when your tail gate is up is makes a vortex of air at high pressure and the air flowing over the cab slides over this high pressure area right over the tailgate resulting in a more aerodynamic vehicle. Yes, you get better gas mileage with your tailgate up, boys, it's proven. It may be possible for a transfer port to be more efficient if this is the case.

Some guys think the rough texture "stirs" the mixture in the transfers for better atomization. When it comes out of the carb it should be completely atomized, if the fuel clumps along the way, stirring it back up just makes a more turbulant, clumpy mixture. It won't re-atomize once the molecules are stuck together. (remember molecular tension in school?!?!)

I just don't buy it...an engine built either way will produce identical numbers on the dyno, I'm told. So it's not really worth arguing over...I do believe that a smooth transfer passage will result in a better atomized fuel when it enters the cylinder. If this is the case it should make for a faster burn, especially when running race fuel in high compression engines. Possibly making jetting more forgiving to different temps and barometric pressure. I may not be right and the theories above may be, but I don't have evidence to really go either way, so for now, I make 'em smooth. LOL!

Anyway, good call on the transfers. You'll need the right equipment to make these changes.


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questions about porting

Postby toocheaptosmoke » April 26th, 2006, 4:04 pm

well, i have a dremel, die grinder, and hand files. i was going to get either the flex or right angle attachment for my dremel, but i may get both if they're worth it. the right angle adapter should be able to get into the transfer ports? i'll make sure to get a cratex bit too.

i made this drawing for the exhaust. does it look about right?
[IMG]http://i20.photobucket.com/albums/b211/toocheaptosmoke/export.jpg[/IMG]



i've been looking at the transfer ports more closely. they seem to be set up kind of odd.(not like i really know what "odd" is, lol.) according to the T.S.P.T.M. i have one old and one new style transfer port. the smaller ones seem to aim up, while the bigger ones are more horizontal. maybe this engine was designed this way? my drawings are pretty bad, but hopefully you can see what i'm trying to say. the place where the ports are cut through the sleeve and meet the aluminum is also pretty abrupt. you can kind of see that in the drawing also. i tried to show the aim of the ports too.

[IMG]http://i20.photobucket.com/albums/b211/toocheaptosmoke/trans.jpg[/IMG]


the manual said that if the aim of the transfer ports was changed for the worse it could ruin the cylinder, i don't want to drastically change something the factory designed. but, by the "cheap" look of the casting, i wouldn't be surprised if they screwed something up a little bit. but i really don't have an idea if it is right or wrong.

do you guys think extending the smaller transfer ports out to the side will give any benefit? i could adjust the aim a little more towards the back of the cylinder to get more of the exhaust out.

something like this?

[IMG]http://i20.photobucket.com/albums/b211/toocheaptosmoke/transfer.jpg[/IMG]



i really appreciate you guys help.
1978 Ski Doo RV 340
1979 freestyle Blizzard 5500
1980 Polaris TX 440
1980 Ski Doo Blizzard 5500
1980 Arctic Cat panther 440- WIDENED.
1981 Ski Doo Blizzard MX 5500 (parts)
1983 Polaris indy 400
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questions about porting

Postby AHood » April 26th, 2006, 5:23 pm

so much to reply to here it goes.

looking at your proposed port it looks like its too wide. if 52.8 is the arc width that makes your chordal width almost 73% of bore. thats not uncommon on race engines and they can last with proper radii and edge chamfer but i wouldnt recomend it for your engine. a 70% wide port has a 50.4 mm arc width for your engine. also will the bottom of that port extend to your true BDC? it looks like its too short. the corner and top and bottom radii look good though.

definatly get a flex shaft. i have the craftsman version and it has worked great for me for like 7 years for anything but production porting it will serve you well. the dremel right angle setup sucks A$$ dont waste your money on it. once you put a bit in it the head will be too long to fit easily in the bore and do any work without hitting the bore walls and being a general pain in the A$$. if you want a quality compact right angle tool get this one [url=http://www.wttool.com/p/30728708p/3072-8708]angle tool [/url]
its what i use and it is by far the best bang for the buck in angle grinders i have found yet. its small and takes 1/8 inch shank cutters. fits easily in any medium size bore. its air driven though so you must have a compressor with a regged output though. I think the price went up a bit it used to be about 125 i could have sworn.

your transfer ports look typical for an older mass produced sled engine. where the main transfers are flatter and the secondaries aim up more. if you get the right angle tool i would just clean up the port windows where the sleeve meets the casting. unless you have a great understanding of scavenge flow patterns from experience i wouldnt change any of the roof angle or entry angles. you could make the secondary windows wider but i wouldnt angle them toward the back of the bore. heres an interesting diagram that shows the general effect of different port angles. and the effect of the hooked secondary port. [url=http://www.hpt-sport.com/images/120.jpg]port angles [/url] granted these engines use reeds and intake boost ports.
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questions about porting

Postby toocheaptosmoke » April 27th, 2006, 4:21 pm

alright, i'll stay away from the right angle attachment for the dremel. if i get a decent job this summer i might have to invest in that right angle grinder.


you're right, i screwed up a dimension when i was figuring the arc length. when i use a chordal length of 45.5 i get an arc length of 51.6, what are we doing different? i keep getting 45.5 as 70% of the bore. my exhaust ports are about 28mm tall right now, they're 29 in the drawing i made.

that link to the port angles is cool. is that a general trend for "high strung" engines to have the ports aiming more towards the center? i remember the manual saying something about lower speed engines needing more flow towards the back to keep the gas in the cylinder.

i think i'll leave the angles on mine alone and just clean them up. if i was to extend the secondary a little, do you think i should just try and keep all the angles the same, but just extend them back a mm or 2?

and i have another question. on the bottom of the cylinder where the transfer ports start, the smaller secondary ports have a bigger opening than the larger mains. is that normal? i could have swore my rotax 503 was setup the opposite way. when i sharpen those "fins"(don't know of the proper term) i'm thinking i should take the most material off the main openings to make them less restrictive, right?

[IMG]http://i20.photobucket.com/albums/b211/toocheaptosmoke/ports001.jpg[/IMG]
1978 Ski Doo RV 340
1979 freestyle Blizzard 5500
1980 Polaris TX 440
1980 Ski Doo Blizzard 5500
1980 Arctic Cat panther 440- WIDENED.
1981 Ski Doo Blizzard MX 5500 (parts)
1983 Polaris indy 400
toocheaptosmoke
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Location: Butler PA

questions about porting

Postby AHood » April 27th, 2006, 5:58 pm

the engines with ports aiming toward the center dont necisarily need to be more "high strung"they just simply tend to make more midrange and top end power.

as for your port width i use an excell sheet to calculate arc and port width its all set up just input the bore and the port width and it tells you the % of bore and the arc width. Pm me and ill send it to you i need your email addy. it has a bunch of other useful engine building formula programs that are great too.

yes i dont like that those engines have more port volume to the secondary than the main transfer. so yes if it were to thin the divider i would take the material from the main side.

if you extend the secondary port width i would leave the angles alone. some of my engines get a hooked secondary transfer wall like in the diagram i posted but these engines have intake boost ports to take advantage of that.

if you thin the port divider dont make it knife edged keep a smooth flowing radius like a rain drop. i only knife edge the divider if the divider gets really thin.
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questions about porting

Postby j_85 » August 13th, 2006, 4:33 pm

slightly off topic

could anyone tell me the exhaust and transfer port duration in degrees for the rotax 503?

thanks
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questions about porting

Postby AHood » August 19th, 2006, 4:24 pm

if you can tell me the stroke. the port heights from the top of the cylinder and weather the piston comes right to the top of the bore at the top of its stroke i can tell you your durations. con rod length would help to but isnt critical.
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