whats your idea for best exhaust and transfer degree and duration

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mrfubs

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Jun 13, 2013
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So i know there is quite a few of you high tech rednecks out there that can help me with this one. What would be the highest you can port the exhaust in terms of degrees for high rpm running also what are the advantages of giving the transfers a higher degree aswell?

I have not yet mapped one of these cylinders to figure out what degrees the exhaust or transfers open but would be great to know whats the max you could go.
 

SchwinnStingray

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Apr 1, 2014
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Please excuse my ignorance, but when you refer to degree, are you talking about the dwell point of the piston or the actual angle of the port in relation to the cylinder?
 

mrfubs

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Jun 13, 2013
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yes based on the 360 degree crank rotation, and now that you bring up angle of the port in relation to the cylinder has me ceriuse about that as well.
 

Theon

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Jan 20, 2014
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It all depends on how much low down your prepared to lose, remember there is only one gear unless your running a shift kit.
I don't have as much as experience as others here, but it can be hard to get info out of some of the more experienced.
 

mrfubs

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Jun 13, 2013
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michigan
i posted this video on my other thread and is quite amazing, he has another video of a close up

https://www.youtube.com/watch?v=HOK9DsKZ_KQ

after watching this video has me jealous, ofcourse he is using a different jug but iv seen the ports in the jug and the exhaust port is a wicked dimond shape but then again i believe it had 3 dual transfers, specs where not listed but i found specs on another moped cylinder that is 47mm with a 39.XX mm stroke 185/125 peak power is 8800-9+ rpm. Now the description in that video claims 13k rpm at 14hp not sure how correct that is but sounds pretty close.

et-hr4000162n.jpg

i mostly ride WOT and im building a speed bike so i will not be cruising at much low speeds, mostly high mph speeds where the power is.
 
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Theon

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Raising the ports will also decrease effective displacement of the motor, and unless you have the exhaust tuned perfectly you will be losing charge out the exhast port and creating a narrower power band. But Jugs are cheap, so experimentation is the only real way your going to get an answer, as there are so many other variables in a two stroke.
Best to raise it a little at a time, buy chamfering piston crown till you start to lose top speed, then replace $10 piston and adjust the jug to your findings.
 

Davezilla

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Mar 15, 2014
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The port timing acts the same way cam timing acts in a 4 stroke engine, of course, we don't have cams and in order to change the timing we need to cut at our ports, especially on the exhaust roof and intake floor, this is how we control intake and exhaust duration. Widening the ports has the same effect as having higher lift on a 4 stroke cam, however, it's the duration that has the most effect on an engine's performance and at what rpm range the engine's powere band will be in. This is the rpm range where the volumetric efficiency can exceed 100%, meaning at a certain rpm or rpm range, the engine is actually taking in more air than it's displacement like let's say our 66cc engines would be pulling in 75cc's of air fuel mix on each intake stroke, figuratively speaking.

Now we can't get all this extra power for free, if we add more to the top end, we lose some at the bottom, and if we add more to the bottom end, the top end suffers. It does pay off tho to add more to the top end and gear the bike lower, or add to the bottom end and gear higher. We can also recover some of this lost power by increasing our compression ratio, but there's also a limit to how much we can do this as well, and in some cases, increasing the compression ratio can even cause a performance decrease.

Increasing the exhaust duration has the biggest effect on top end rpm, but also top end power to a certain extent. Adding too much exhaust duration also effects the blowdown, or how many degrees the engine is producing power before the exhaust port opens so also keep this in mind when opening the exhaust port roof.

On the itake side, we lower the floor or cut out part of the bottom of the piston skirt, or cut holes into the piston in order to increase intake duration. We have limits here too.. especially on the piston ported engines as the duration increases past 90 degrees, the piston is actually pushing the air/fuel charge back into the intake manifold, and the carb at lower rpm's, but at higher rpm's where the air is moving toward the intake faster than it can be pushed back out, we can see a gain at a higher rpm. At higher intake durations, we lose idle quality and low end power due to this blowback, but we can usually run up to about 110 to 115 degrees intake duration with decent results and keeping the power at a usable rpm range.
With a reed valve, we can go considerably higher with intake duration because the reed prevents this blowback, and the power is much more noticeable in the lower to mid range with the power tapering down to about the same levels as a piston port at the higher rpms.

There is one other method of controlling the intake while preventing blowback, which is the rotary valve, this is basically a disc that spins with the crank and has an opening in it to allow air in only at a certain crank position and it's closed of during most of the down stroke, this configuration does make the most power, but at a much narrower range than reed valves or piston ports. Rotary disc valves are typically on the side of the case and need the carb to be mounted on the side of the case as well, but some engines will have the valve behind the cylinder and spinning off a bevel gear on the crank, but in it's simplest form, you'll usually see the carb mounted to the side of the case. You can run up to around 200+ degrees intake duration with this setup, but the engine won't be as strong at the bottom end or midrange, then it hits Hard once it comes up to it's narrow powerband. Great for road racing where the engine speed is pretty much constant and running in a very narrow rpm range.

I'm not going to recommend you any timing specs to follow because every engine is different and there are too many variations in gearing, tire size, bike weight, rider weight, outside temperature, and elevation... Basically, what works for me may not work for you or someone else. Read the 2 stroke Tuner's guide by Gordon Jennings, and also Graham Bell's 2 stroke tuning book to get a good idea of what all is going on when you rais the exhaust roof or lower the intake floor, or even widen a port etc.
Start by cutting a small amount at a time, this requires taking the jug off and putting it back on several times, but it'll be worth it when you hit that sweet spot.
Start by indexing the piston, cutting off about 1mm at the exhaust port, and 1mm at the transfer openings on the piston crown and about 5mm from the bottom of the skirt at the intake port. Use a degree wheel and record all your opening times so you can refer to them later. Once you got a good port timing by indexing the piston, you can map out the ports on the cylinder and cut out the intake and exhaust ports accordingly.
An easy way to see what you're doing is to remove the piston rings and coat the inside of the jug with Dykem, a dark blue dye for machinists, this stuff stays put pretty well, but you can scratch scribe lines and be able to see these lines really easily. Once you paint around your ports with the Dykem, you can assemble the engine with a NEW UN CUT piston without the rings on the piston (the rings can scrape off the dykem so that's why they're removed for this step) Install the base gasket or gaskets you'll be using, put spacers on the cylinder studs (we're not installing the head so we need spacers so you can torque the jug down)and torque the nuts down. Now connect a protractor or degree wheel to the crank and find a good reference point like the rear corner of the mag cover or make a pointer out of a piece of wire. Bring the piston to TDC, set your degree wheel so the pointer is pointing at zero, or rotate the wheel so zero is lined up with a good reference point and tighten up the degree wheel enough so it won't slip. Remember those numbers you recorded when you cut on your piston? We need them now. Turn your crank to the same degrees you wrote down for the exhaust cut on the piston and mark this on the inside of the jug with a scribe using the new piston as your ruler. Now rotate the crank to where the transfers were working their best and mark a scribe line here above the transfer ports. Once you got the lines scribed in (you could use a sharpie with no Dykem, but the sharpie marks may wipe off when you least expect, and the sharpie is no where near as accurate as a scribe line when dealing with fractions of a milimeter) now it's time to disassemble and remove the jug. Cut out everything under the scribe lines and chamfer all edges to prevent ring snags. Use a pencil grinder if you got one, a dremel with a flex shaft is second best, but pencil grinders are under $20 at Harbor Freight, but need a 30 gallon compressor minimum. Use carbide rotary files to do the big cuts, then switch to diamond coated burrs for the more precise work, sanding rolls and drums work really well too, but the diamond coated burrs are the most precise. Run the grinder rpm as fast as it'll run nad use a very light touch, don't try to force the cut and let the tool do the work. Remember the pencil grinder? these guys spin at 54000 rpm so they can cut pretty fast, even with a fine diamond burr just by barely touching the work piece, they cut fast, but very controlable at the same time... I can't recommend the daimond burr set enough when it comes to precise work. The pencil grinder will also reach into those transfer openings you'll never get close to with a dremel too.

Once you got all your ports cut out, go back to the intake port and rough up the inside with a 100 grit sanding drum, just get that smooth sanded texture, that's planty good enough to disrupt the flow and create the turbulance to keep the fuel well atomized.
You can do the same up the transfer openings going up from the bottom if you wish.

Go back to the exhaust port and sand it with the same 100 grit drum, then switch to a kratex cone and polish it out. Kratex is that fine grit impregnated rubber polishing cones that come in some of the dremel polishing kits, this'll leave an almost mirror smooth texture so the exhaust can flow out slightly better and carbon deposits can't stick to the nside of the port.

Once you're ported and polished AND happy with your work, go back in and chamfer all your port openings with a diamond burr, needle files, or fine sandpaper to take the edges down, this is very important or all your work can go down the drain in a matter of minutes by either snagging the ring or the nikasil can start peeling off. A good smooth chamfer will prevent this.

Sorry for being long winded but there's just so much to porting and changing your intake and exhaust durations that it's difficult to explain quickly and simply... hope this all helps someone.
 

rogergendron1

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Sep 18, 2013
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The above post is the best one i have seen on port mapping made simple in a long time.

Now i run at high rpms at most of the time and i want my power made between 5000 and 9000 rpms, i want a high mid to top end motor then run a 40t rear, this gives me power and torque through gering and high rpms and the 40 mph magic number i want ... To get this i have a port map i follow in my mind and its what has been working best for all my motors.

first i make a thicker 1.5 mm base gasket to raise the jug up a half mm then i deck it down for a zero deck hight, this raises the transfer ports and adds duration to the exhaust. Then i cut a lil bit of the roof of the exhaust and widen it a LOT so the stock exhaust is raised up a lil bit the roof has been rounded up a lil bit and it has been widend a lol.

Now the intake i grind down as far as i can and create a d shaped port and then widen it as far as i can and smooth it out, i dont touxh the top as it has already been raised by spacing up the jug with the thicker gasket.

The transfers should be close to where they should be if using a .5mm thicker gasket at the base so all is needed is a good clean up and smoothing out that step in the casting .... Of course best results are had if you port the crank case to but you do not have to for good gains. Also try to angle them so the blow the charge into the center of the jug.

Now the piston ... Cut a smooth ramp in the exhaust side and steep sharp ramps in the transfer sides and this with further correct port timing. I also lightly round off the rim or edge of the piston to prevent hangups and scraping .

I run this setup on piston port motors with long large bore intakes and a large 16mm carb and it is a very good top end power and speed setup . I do however have to set the idle quite high because it does not like to idle low with this type of port job .... But thats no big deal its worth it for the crazy top end power you get and the super high rpms achivable woth this setup
 

rogergendron1

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Look if your looking for the numbers for that broad powerband high flow torqey motor you wont find them lol

But there is a basic plan to get you close to there and its this ...
Port the intake a little bit lower and quite a bit wider
Port the exhaust a lot wider and just a tad higher not mutch
Just clean up the transfers dont cut into them
Now grind ramps in your piston for the exhaust and transfers and cut the intake skirt off so it is not visable through the intake port at TDC
Run a larger bore intake manifold and port match it, the intake manifold ID should be the same size as the bore of the carbs venturi to prevent any restriction after all whats the point of a 16mm high flow carb if you run a 13mm bore intake ? The flow will be restricted to 13mm
 

Theon

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I have found.
For revs.
I have Ex opening at 102 ATDC (156 Dur.)
Transfers open at 118 ATDC (124 Dur.).
inlet closes at 62 ATDC (124 Dur.).
14 Deg. blow down.
You lose a bit of low end but has got me 80km/hr.

For more Low down but still >8000RPM.
Ex 105 (150 Dur), Tr 123 (114), inlet 60 (120 Dur). 18 Deg Blow down.
This will pull from a stop and cruise at >60Km/hr with a mildly modded standard exhaust.
This is what most probably want from their motor with even torque all the way.

For anyone reading this, get your self a cheap protractor/degree wheel and give it a go.
It can make a big difference to your motor if you do it right.
Combine This with a squish of less than 1mm and a shaved centre fire head for best results!
Others may have there own magic numbers, I welcome them to put theirs forward.
I don't claim to be an expert, but I do take in what I read.
Of course doing a neat match port, cleaning the transfers and widening the exhaust all play an important part.
 

rogergendron1

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i have got to get myself a deg wheel and see what my favorite port map measures out to be, i asume th exhaust duration on my favorite port is close to 115 deg
 

Davezilla

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Just do a google images search for degree wheel and you'll find plenty enough printable ones you can print out on card stock or you can just print on paper then glue to an old cd or a piece of scrap pastic or aluminum sheet. Make a pointer out of some rigid wire or even a paper clip. You can even use a 50 cent protractor on the end of the crank and get accurate readings, no need to spend money on a wheel.

You'll need a dial indicator to get TDC accurate and set the wheel to zero, if you don't have a dial indicator to find TDC, you can use a piston stop set at a predetermined depth, then rock the crank to the stop and record your number from the wheel, rock it the other way and count the degrees. Your actual zero point will be half the number of degrees between the stop. The dial indicator on the piston crown is easier and quicker way to find your exact TDC tho.

You can't really eyeball where TDC is because your crank can move about 3 degrees in either direction at TDC before the piston drops .001" on a 40mm stroke crank.

Also, the simplest formula for duration is your crank degrees subtracted from 180 and multiplied by 2.
 

Theon

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Patience and a squish test using solder gets me a fairly accurate TDC.
But a Dial indicator is much better.
I use 1mm solder, looking for a squish of around .8mm.
But if you leave the head gasket out while doing your squish test you will get a solid 'bump' as the solder compressors. rock the crank back and forth over the bump and pick the middle.
 

Davezilla

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I agree, solder makes for a good piston stop as it won't hurt the aluminum, but patience is definitely the key when setting up the wheel because it can get frustrating when you get it just perfect, lock it in place and it's reading a few degrees off again... lol. I do frequently double check that the wheel didn't slip when I'm using it to record my opening and closing events since it's possible to get an inaccurate reading if you bump the pointer or the wheel slips even the slightest bit.

My durations came out a bit higher on my latest build because I used my old jug on the new engine and had to raise it up about 1.25mm to get my squish right but it doesn't seem to hurt the low end that much, it sounds agressive as all get out and revs sky high tho for sure...
 

sub66

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Apr 25, 2014
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Just do a google images search for degree wheel and you'll find plenty enough printable ones you can print out on card stock or you can just print on paper then glue to an old cd or a piece of scrap pastic or aluminum sheet. Make a pointer out of some rigid wire or even a paper clip. You can even use a 50 cent protractor on the end of the crank and get accurate readings, no need to spend money on a wheel.

You'll need a dial indicator to get TDC accurate and set the wheel to zero, if you don't have a dial indicator to find TDC, you can use a piston stop set at a predetermined depth, then rock the crank to the stop and record your number from the wheel, rock it the other way and count the degrees. Your actual zero point will be half the number of degrees between the stop. The dial indicator on the piston crown is easier and quicker way to find your exact TDC tho.

You can't really eyeball where TDC is because your crank can move about 3 degrees in either direction at TDC before the piston drops .001" on a 40mm stroke crank.

Also, the simplest formula for duration is your crank degrees subtracted from 180 and multiplied by 2.
incorrect way to find tdc. piston stop anywhere then split the results.

--------
ex port duration is the only variable worth discusing. in whatever form of measurement u prefer. blowdown is a target powerband only.
 

Davezilla

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Yes I know I can set the piston stop anywhere in the bore that I want to and predetermined means just that. I'm sure you read a few books and what I say doesn't match the books word for word, but that's because I talk from experience and not what I read in some book last week.
I still haven't seen any pics or videos...
 

sub66

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Apr 25, 2014
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now it makes sense but you have heard it elsewhere but the information was given to confuse not to teach. its not your fault. google my words then google rubber rainbow pants > lol :) more likely to get a match with the rainbow pants.