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questions about porting

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questions about porting

Postby toocheaptosmoke » April 21st, 2006, 4:55 pm

i've got an indy 400 motor that is tore apart right now so i am going to be cleaning up the porting. on the last engine i rebuilt i only worked on smoothing out the imperfections in the aluminum. this time i might want to try to actually change the porting a little. i'm not looking to do anything major, just try and get some experience and maybe a little more power. and i don't want to destroy my engine either, lol. i've been studying the Two Stroke Performance Tuning manual. according to the manual my transfer port duration should be around 120-124 degrees. i found that my transfer ports are open for 116.5 degrees and the xhaust is open for 190 degrees. one thing i don't get is why the bottom of the one transfer port is lower than the other? when the piston is at BDC it never even reaches the bottom of either port. there's maybe a mm to go before the big transfer port is completely open. also, the piston sits about 1 - 1.5 mm below the bottom of the exhaust port at BDC. would it help to lower the bottom of the exhaust port? i have been relying on A.G. Bell for all my info so any tips or help is appreciated. :B


[IMG]http://i20.photobucket.com/albums/b211/toocheaptosmoke/portmap.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 21st, 2006, 11:02 pm

if i were you i would raise the cylinder until the bottom of the lowest transfer was flush with the piston top at BDC. then match the other transfers to BDC and match the exhaust port to the piston at BDC. widen the exhaust port to 70% of bore. if you show some dimensions so i can crunch your port open timings that will help. you want to have between 32-35 degrees of blow down timing.
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questions about porting

Postby toocheaptosmoke » April 22nd, 2006, 8:27 am

well, the cylinder needs to be raised at least 2mm just to get the lower port to line up. i don't really have the money to get the all the machining i assume would be needed to get everything "back to normal." :( if i had to, can you make a cut-out on the piston to increase the duration like on an exhaust port? but i would really hate to do that to the new wisecos that came with the engine, lol.

for the 70% do you use the width of the exhaust port on the port map or from port edge to edge across the cylinder?

is "blow down timing" the time when just the exhaust port is open?

thanks for the help.

[IMG]http://i20.photobucket.com/albums/b211/toocheaptosmoke/997b2ffd.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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Posts: 50
Joined: January 17th, 2005, 3:57 pm
Location: Butler PA


questions about porting

Postby AHood » April 22nd, 2006, 9:53 am

yes the piston top timing edge can be cut to some degree, but not 2 mm. plus like you say with the new wisecos thats unwise. the exhaust port width is measured from edge to edge its called the chordal width. you can find the chordal width from your port map if you know the bore size and the arc width of the port (arc width is just the width of the port as measured on your port map paper).

you are correct blow down timing is the degrees of crank rotation from when the exhaust port opens to when the transfers just start to open.

if you raise the cylinder with a base spacer you would just need to have the bottom of the transfers and exhaust matched to the piston then go to what ever width and height you want for your exhaust port. your cylinder will have to be decked on the top by what ever amount your rasie your cylinder also.
AHood
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Joined: February 1st, 2004, 4:12 pm
Location: Merrimac Ma USA

questions about porting

Postby toocheaptosmoke » April 22nd, 2006, 3:36 pm

i found the chordal length to be 43mm. which is .66 of the the 65mm bore. if i took out a mm off each side the exhaust port would be .69 of the bore.

what do you think of something like this? would rounding the top a little bit help the rings live longer?

[IMG]http://i20.photobucket.com/albums/b211/toocheaptosmoke/exhaust2.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
Member
 
Posts: 50
Joined: January 17th, 2005, 3:57 pm
Location: Butler PA

questions about porting

Postby AHood » April 22nd, 2006, 7:33 pm

i would tend to lean towards something with a squarer shape. at only 45.5 mm 70 % of bore width you shouldnt have any problem with ring snagging as long as you have good port edge chamfers. plus your blow down timing is already suficient and more is un needed so making the port taller even just in the middle is not needed. i came up with this pic for you of what i might do to an engine such as yours. port chrdal width is 70% it has nice large 12 mm corner radiuses to easy ring transition. also the top and bottom edge have 105 mm radiuses to also help prevent snagging. the top of port is at your stock height the bottom of the port will be where your new BDC is.


[IMG]http://i3.tinypic.com/w8q8o3.jpg[/IMG]
AHood
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Joined: February 1st, 2004, 4:12 pm
Location: Merrimac Ma USA

questions about porting

Postby toocheaptosmoke » April 24th, 2006, 4:09 pm

that port you drew up is sweet, thanks! what is the best way to cut a port out to the shape you want? i was thinking of making a template out of some thin metal, then putting it over the port and filing up to the edges?

the inlet port seems to be plenty big enough already. it's supposed to be 115% of the carb area, right? i have a set of 38mm carbs i'll be putting on this engine. i also noticed that the skirts on the wisecos are about 1 mm shorter than the OEM ones. about how much would that increase the intake duration? do you think it will hurt the midrange at all?

do you think i should just leave the transfer ports alone since i won't be raising the cylinder? i don't want to ruin the block. i'll still have a lot of work to do smoothing out all the casting imperfections. this engine is the worst i've taken apart so far.
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
Member
 
Posts: 50
Joined: January 17th, 2005, 3:57 pm
Location: Butler PA

questions about porting

Postby AHood » April 24th, 2006, 10:10 pm

my prefered method of cutting ports to shape. is by making a whole computer generated port map much like your port rubbing but it has all the final dimensions for port sizes location ect on it. i then use a razor and cut out the port windows in the paper. making a metal template is overkill. after i have the new port map cut out i put the paper in the cylinder and align it to the top of the bore and match up the port edges. once i am satisfied i have the paper in the proper place i tape it in securely. no i can see all of what i need to cut to achieve my new port shapes. i then grind to with in about 1 mm of the paper template. i try and do all my major grinding and cutting in one step to prevent alignment mistakes if i have to make another template and realign it and cut some more you can have flaws. i use the last mm or so that i didnt cut to allow a good radius chamfer on all the port edges. you should allow about 2 mm to radius the top and bottom of the exhaust port. your intake port looks plenty large for 38 mm carbs. the slightly shorter skirt on the piston should not have any noticable drawback. its common to have a high intake duartion with out much deacreas in midrange performance.

if your not going to rasie the cylinder i would leave the transfers alone. one a smallish single exhaust port you need all the blow down time you can get.

make sure when you smooth the imperfections you dont make the ports polished smooth. this is a very common mistake and its a personal pet peave of mine when people call it porting and polishing cause there should never be any polishing. i like to finish the surface with a fine small carbide cutter. i let the cutter kind of chatter and skip on the surface. this will produce an even textured surface.

if you want i can make you a port map for your exhaust port that is 1:1 scale so you should be able to save it and print it cut it out and use if for your cylinders.

final note out of curiosity what type of tools are you using to do this work. a dremel with flex shaft will work wonders for the home porter. also you should start saving up an assortment of carbide and stone cutters and well as cratex( abraisive rubber). the more cutters you have the better suited to any particular job you will be. different cutters work better in different areas. cratex for example is what i use to finish radius my port edges. it cuts vert little and is good for taking out light rough marks left by the stone and carbide cutters.

hope it works out good for you and have fun. feel free to ask if you have any other questions ill help you out any way i can.
AHood
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Joined: February 1st, 2004, 4:12 pm
Location: Merrimac Ma USA

questions about porting

Postby Wheeler550 » April 25th, 2006, 4:36 am

This is one of the best posts I've seen in a long time. Thanks Toocheap for asking all the questions I needed answered.
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questions about porting

Postby ZR800XC » April 25th, 2006, 3:18 pm

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!


[img]http://my.core.com/~twister76/twister1.jpg">[/img]

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