Cylinder Porting for Dummies....

[scotclayshooter]

If you don't want to grind on a cylinder, or lathe a pop-up.


I saw a pretty interesting way to raise compression - take out the decomp valve and wind in a bolt that protudes even slightly into the cylidner. Sure make a difference. I suspect it messes with gas flow though... However.. the saw ran remarkably well....

How about building up the top of the chamber with a tig, then grinding it smooth with a big ball cutter?

Would the exposed threads not make hot spots and cause pre ignition?
I remember Doug Miller tried somthing similar for variable compression but dismissed it eventually.

 

[Carlyle]

Don't know if this will help but TW has a pic of a pop-up in this thread, I think that it is on page 2. Timberwolf is over my head on the whole porting thing, so I will just leave it up to him.

hey guys,
since this is the 'dummies' thread...
would you mind explaining the pop-up?
you machine off a ring of material around the top edge of the piston right?
how does this raise compresion?
thanks,

one of the dummies.

 

[scotclayshooter]

Neat idea! Just a slightly over length plug hmmmm...


Has anyone tried cutting the jug into separate head pieces?

Maybe with some careful measuring and a nice large thin kerf slitting saw, you could make it happen on a mill?

Ahh probably too much headache, but it would allow some more tinkering with compression/domes without messing with your port timing.

Look up Timberwolfs old posts he seems to have done everything you can think of and seperate heads is one of them.

 

[Lakeside53]

Would the exposed threads not make hot spots and cause pre ignition?
I remember Doug Miller tried somthing similar for variable compression but dismissed it eventually.

Possibly, but I didn't notice any. It was a saw in for service so I don't know the history. I can't even remember if the threads has been removed on the end.

There was an old tech note on the 075 where they discussed removing the decomp valve and plugging the hole to raise compression and HP. I'll need to dig it out to refresh my old memory what they actually said though!

 

[Tree Sling'r]

Possibly, but I didn't notice any. It was a saw in for service so I don't know the history. I can't even remember if the threads has been removed on the end.

There was an old tech note on the 075 where they discussed removing the decomp valve and plugging the hole to raise compression and HP. I'll need to dig it out to refresh my old memory what they actually said though!

I plug all of my saws as well as others, and I do think it helps compression.

 

[04ultra]

I had a kart guy (kt100) tell me that in a few classes that they put longer reach spark plugs in to take up space in the combustion chambers to raise the comp a little ...



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[scotclayshooter]

I plug all of my saws as well as others, and I do think it helps compression.

With those saws over 195 psi they must take a fair old tug to get them going without a de-comp:jawdrop:

 

[04ultra]

Good write up Brad...

I differ on one small point - don't just use "anything" for a gasket. A gasket must not compress over time or (particularly on bigger saws) you'll end up with loose cylinder bolts and maybe cracked cylinders. Sure, almost anything will work for some period of time, but will it last?

If you have to use paper (verses a coated metal gasket), use real gasket material.

3120 P/C's are spendy when the cylinder bolts come loose .........



.

 

[epicklein22]

This is a fun thread. Lots of information being tossed around. Thanks to all the guys answering them. I am definitely gonna try porting a saw this year. Probably around summer when I have more cash flow and time. Now to find a test subject and a dremel, plus buy some bits and I should be in business.

 

[blsnelling]

would you mind explaining the pop-up?
how does this raise compresion?

When you cut the crown of the piston around the edges, you can then lower the cylinder even farther. Then the raised center section of the piston sticks up into the combustion chamber displacing volume. The less volume there is in the combustion chamber, the higher the compression is.

How do you go about raising the bottom of an exhaust port. If I were to mill down the base of my jug and turn the piston to make a pop up, I'd have a problem with the bottom of the piston skirt clearing the bottom of the exhaust port on my 066BB kit (Is that the sub piston induction term?). It already does a very small amount.

Is JB weld or something else used to fill in stuff like that?

A longer piston would do the trick too, but I'm not sure what would be compatible in 56mm.

This is a factor you always have to keep in mind when lowering the cylinder. A longer piston skirt is the only answer I know of. I seriously doubt JBWeld would hold up in those kinds of temps. My 066BB slightly free ports at TDC as well. Turning a popup on the piston and lowering the jug more is simply not an option. It's a limitation of that kit. I have seen the crown of a 066BB piston build up with TIG welding and then turned down, resulting in a .060" popup. Last I heard it was running fine. I hate adding mass to the piston, but you've got to do what you've got to do.

I saw a pretty interesting way to raise compression - take out the decomp valve and wind in a bolt that protudes even slightly into the cylidner. Sure make a difference. I suspect it messes with gas flow though... However.. the saw ran remarkably well....

I'm going to give this a try!

Has anyone tried cutting the jug into separate head pieces?

Racers do it all the time. They machine the head off of the jug and build an entirely new head from billet aluminum. That way they can create the combustion chamber any size they want and get whatever compression they want.

I plug all of my saws as well as others, and I do think it helps compression.

My 076 Super has no decomp and it starts easily with an Elastostart. My 084 is a little more demanding.

 

[volks-man]

When you cut the crown of the piston around the edges, you can then lower the cylinder even farther. Then the raised center section of the piston sticks up into the combustion chamber displacing volume. The less volume there is in the combustion chamber, the higher the compression is.

thanks. gotcha!
i had missed the part where you lower the cylinder.

 

[PatrickIreland]

Should be stickied or at least linked to, IMO as a newbie. 4-strokes I can tune up all day long, but for the past 20 years the only 2-stroke things I've had to mess with are outboards and lawnmowers - either of which I'd dump at a shop to be fixed. I don't want mt outboard quitting on me, and my lawnmower? hell, I only use it about once a year anyway, I don't expect it to start!

I actually talk about most of that stuff in that thread. The only way to learn is to get in there and start doing it. It'll start to make sense once you have the cylinder in your hand and reread the thread.

Here's what I recommend for starters.

Don't do anything to the transfers, period. You can make tremendous gains without modding them.

Do not raise or lower the intake or exhaust ports. Widen them only. Mark the ports on the piston skirt and then widen them so that there's only about 1/16" left to seal the port. DON'T go any farther though. Keep in mind you have to rebevel the port too. You also have to watch out for where the ring ends are. They are often right at the edges of the port. In that case you may not be able to go as close to the edge of the skirt. You MUST leave cylinder wall for the rign to ride on as it goes past the port window! I wouldn't leave less than 1/8".

Remove the casting lines from the windows in the piston. Also angle them on the inside as I have shown in the 361BB porting thread.

Don't make the roof or floor of the ports too flat. The flatter they are, the more area you open the fastest. But go too flat and you're likely to catch a ring even after beveling.

Make the walls of the ports straight up and down. If you leave them curved, you're not opening the port area as qickly as possible at a given point in the rotation of the crank. You want maximum area as soon as possible. A totally square port would be the max but won't work as described above. It's a balancing act.

Use a round stone to rebevel the ports. You don't need a huge angle. It only needs to be a few thousanths deep. The ring doesn't go into the port window that far. Just lightly round off the edges of the port so that the ring won't catch on it.

Reassemble the engine using a gasket made of any paper or soft metal that will give you a squish of about .020". I've used get well card, pop can cases, etc.. Make sure to use a fuel resistant sealer like Threebond 1104/1194.

A little about port timing. Port timing on the exhaust port is measured off of the roof, closest to the combustion chamber. The critical point is where the port window first opens as it travels from TDC. Do not lower the floor. You may have the port open with the piston at TDC. That's not desireable and lowering it will give you no performance gains.

Intake timing is measured from the floor of the port. That's why some "port" the intake by removing material from the bottom of the skirt of the piston. It's best to lower the port when needed. Again, you don't need to do this on your first saw. Do not raise the roof or the rings may drop below the top of the port and catch. Some actually drop below by design, but there's no gain to be had by raising the roof of the intake.

Correct me where I may be wrong guys. I've only been doing this a couple years.

 

[Wet1]

Excellent basic info on porting Brad!

 

[volks-man]

ok porting gurus, sorry if i am doing this >>>>>>:deadhorse:


lets say you nix the base gasket to get your squish and you have raised your exhaust port by the same amount the cyl has dropped....

i asume the intake timing has been altered but this drop in the intake floor is o.k. as it is just like porting the floor lower when using a base gasket?

also, what do you do with the transfers after lowering the cylinder?
do you raise the tops to recover your original timing on them?

 

[blsnelling]

Your assumptions are all correct. If you're just starting at this, don't raise or lower anything other than setting the squish. The .018-.020 you lower the jug by removing the gasket only amounts to about 1* of port timing. It's not enough to worry about. Concentrate on port width and shape and you'll get the gains you're looking for.

 

[volks-man]

thanks DARTH!
i will not be fooling with the timing if i can help it!

out of curiousity, how many degrees difference does it take before you need to start raising and lowering ports/transfers to recover your original timing?

 

[blsnelling]

thanks DARTH!
i will not be fooling with the timing if i can help it!

out of curiousity, how many degrees difference does it take before you need to start raising and lowering ports/transfers to recover your original timing?

I don't have enough experience to answer that. I lowered the jug about .060" on my 084, but the popup was .042". That did alter the intake timing that much though. I did not have to change the factory timing at all though. I measured the timing on everything and the number were pretty good, actually a little high on the exhaust and transfers. It still has good compression though, about 170-175.

 

[Bluemaxpilot]

You guys buying or milling out these domed pistons? If buying, then where?

 

[blsnelling]

You guys buying or milling out these domed pistons? If buying, then where?

Custom made on a mill/lathe.

 

[Martinm210]

Has anyone read through this Gordon Jennings book?

I just started in on it and there's alot of really good information there.
http://www.edj.net/2stroke/jennings/

Rather than guessing, there's some tools in there that'll let you figure things out. Not sure if I'll be successful, but I'm going to try and write a spreadsheet using the tools he's provided within, seems like it would be worthwhile.

There's some other really good stuff on muffler/pipe design which is giving me some idea to improving the internal baffle system on our mufflers. I don't think we can complete the necessary length internally for a fully tuned pipe, but I would think a short megaphone style internal baffle at the appropriate angle would help draw exhaust out of the cylinder. Again rather than simply cutting a larger hole for exhaust, maybe something additional to simulate some of the properties?

Anyhow the porting information in his book is really good and generalized as well. It goes into figuring out port area and time and some recommended values for intake/transfer/exhaust and how different shapes of ports affect the power curves, etc. Anyway, there's tons of good stuff in there. It's alot to absorb, but pretty neat.