Port timing and size (area)

Bartw...I dont think i know enough about these motors to tackle somthing like that. Neat idea though.


Cagaroni........who was that directed at??

DK

just adding my bit of ancient chinese secret

So it would make sense that you would want to keep the ports as low as possible and make them wide enough to get your area. This way you can keep all your compression, blow down and still feed to motor at high rpm. Cool!!

Now, what is blow down area?? How would you find that out?!?!

cagaroni.......confusus say.....man who go thru airport door side ways going to bang........never mind.

DK

pasted from my forum

Starting with the piston at top dead center (TDC 0 degrees) ignition has occurred and the gasses in the combustion chamber are expanding and pushing down the piston. This pressurizes the crankcase causing the reed valve to close. At about 90 degrees after TDC the exhaust port opens ending the power stroke. A pressure wave of hot expanding gasses flows down the exhaust pipe. The blow-down phase has started and will end when the transfer ports open. The pressure in the cylinder must blow-down to below the pressure in the crankcase in order for the unburned mixture gasses to flow out the transfer ports during the scavenging phase.

Blow down time is from the top of the exhaust port to the top of the transfers as the piston is traveling down.

Here is a part of the ancient Chinese secret demonstrated on a midbike (X1 style bike) cylinder. Let's hear your thoughts on what you see here and what is different about that cylinder and how it can help you create the "Perfect" Cag cylinder. D. King, since I already told you what it was please hold off until the others chime in.

did you put the steel liner in there, or did the cylinder come like that?

r

It's just a windowed liner. That will improve the flow pattern and flow rate. That cylinder must be mega weak tho. The wall is only 3mm thick to start with so to bore it out and stress the wall by pressing the lining in must put the thing right on the edge of staying together, especially given the putty like quality of the chinese ally. It is a good idea tho, but not really suited to the cag barrel.
I hate to be a hypocrite but this post is about Port area and Timing. This an engine mod and although related to the topic (but then what cag chat isn't?) this isn't going aid in working out area or timing.

I thought you guys would be a little more creative. As far as the weak cylinder goes, see the whole trick is to use a big bore cylinder (44mm) with a sleeve in it bored out to 40mm (standard bore) It is easy to reshape the ports, or fill them in altogether and recut new ones if you don't have to worry about the piston riding on them. The sleeve takes care of that. The sleeve starts off blank, and you can cut the ports in it anywhere you would like and as many of them as you want. This is the only way that you can truly CORRECT ALL THE PORT TIMING PROBLEMS (thought I would stress that since my idea was accused of being off topic). It also helps the ports velocity when they are not exposed. My buddy Boosted306 is going to stop at my shop today or tomorrow and we are going to attempt to make the best Cag engine possible with his balanced stroker crank and a custom cylinder sleeve with a Wiseco PW50 piston (40mm).

Well how did it go?

I have been swamped building 4 stroke super pocket bike engines and restoring a couple of bikes and Boosted is swamped at his job and has not made it down here to get started. All in all it is a bust unless someone wants to donate some serious time for bragging rights as it would be too expensive to sell.

Okay pullin up a bit of an old thread on this issue.. Read it all, very informative!!!! Learnt alot..

However i have the two piece cylinder head setup with crank etc (as per pic below):

Now i have come up with the following figures:

Exhaust : 100degrees ATDC
Transfers: 125degrees ATDC

Blowdown: 25 degrees..

Can any one guide me to what i should do to improve this.. It has the Port in the Piston which runs a seperate port down the rear of the cylinder

I had measured the exhaust from the top of the port, as there is a small wedge going up the cylinder for about 4mm..

If it helps i had also measure the above without the lower cylinder gasket, which changed it about 5 degrees (105 / 125)

If it helps i am running 109 ELF fuel, and im going to setup a small dry NOS setup on it in the near future..

I am new to this, so i am trying to get my head around this all..


I also wanting to raise compression in this setup to..

Any help would be great!!!!! thanks in advance...

Someones brought my baby back up

that is correct. Just ignore the small slot/recess when measuring ports.

Your tranfers are good. Your blow down is a bit low but your exhaust is opening late also.
So you should raise the top of the exhaust up 5 degress.
This will give you the magic 30 degress of blowdown and and exhaust opening at 95 degrees - which is a pretty sweet place to be

That will be your durations sorted but then you need to turn your attentions to port area. Your ports will be opening at the right times but may not be wide enough. To work this out you will need to do a port map and transfer it onto graph paper so you can calculate the area of each port. (See macdizzy.com/cyl_primer.htm for top info on this) Then you will know how much wider to make them. Use my spreadsheet (and other sources of port area calcs) to give you rough guide/feel for what area's you need. If you don't already have one, a boost port will most likely be needed to get a good match of transfer and exhaust port area's.

As for compression: i'd aim for 15:1, but this is as high as you should really go, between 12:1 and 15:1 is cool.
Get a good squish - aim for 0.5mm. Remove or use thinner head gasket/s to get this, or you may need to get the amount required machined off the top of the barrel (not something thats hard for a machine shop to do).
If your CR is still low once your squish is correct then you need to get a smaller dish/chamber in the head. I can't see from your pic if you have a removable dish or not. If not the only way to raise the CR is to get a head with a smaller dish in it or get material added (welded) to your head's dish.

im jumping in a little late but i have to ask some stuff. i got my ADA big bore headkit w/ 13:1 compression dome. i got the wrong kit was supposed to be for a 40mm but mine was already 44mm so the piston pin that came w/ the kit was too small so i had to use the stock piston therefore i had to raise the head b'cuz the stock piston was taller than the other one. I have to port the exhaust port a little to level the bottom of it w/ the piston. Do i just grind it down till it's level or does it have to be technical a certain way?

why dont you send it back & get the "right" kit before you use it?

i think its 2 late everythings out of the package and ive put it together a couple of times.

MR CAG - Wow.. This is like getting my prayers answered!!!! I have found this thread absolutley informative...

The Chamber in the head is one piece.. Cant remove it.. If i was to raise the comp, could i raise the floor in the head to get the right comp, thus making the chamber smaller??? (I hope this makes sense) I can do all this here at work on the lathe etc...

I will look at doing the port drawings and see how i go..

A few more q's..

How do you figure out the squish???

Should i look at modifying the port in the cylinder where the "power port" runs in the piston.. It is about a 4-5mm half round port there.. (can show a pic if needed)

Is it worth lightning the piston for better response (around the skirts etc) like in the porting thread, and taper the section up under the gudgeon pin??

Thanks again... This site i have found to be the most informative yet..

I can see that your Q's are a natual progression on from your last but this is getting off topic now.
I have started a new thread for you here

any update on this project?

this is getting off topic now.

geez you're extremely anal retentive, almost like those cops that are "by the book" and would write their own grandmother a citation for j-walking, anyway bartw can start a new thread on the result of his project so put your mind at ease my friend. I bet you must buy preparation H by the case lol.