Jazzy

w8ye,

you know, I've been tossing that idea around for a few weeks now. Any pointers for such an undertaking?
I'll consider this in more depth when I see what the actual timing and exhaust port area are.

The "nub" in the intake port is very likely there to keep the piston skirt from careening into the bottom lip of the intake channel. The piston ring is not even visible in the intake opening. It would have to come down another 0.080 - 0.100" to be fully visible.

Thanks,
Jeff

tkg

My Feedback: (1)

No absoulate number. Remember if you cut to much off the cylinder you will expose the bottom of the piston in the exhaust, sorta a diy egr.
WME

Jazzy

av8tor1977,

I just got a good look at the image you posted! Very nice!

Had I seen it sooner I wouldn't have had to make one at work. I used A-cad so it turned out nicely.
I sized it so that it would fit perfectly on an old circular saw blade. The PDF file is actually and 11x17 page.

Thanks,
Jeff

av8tor1977

My Feedback: (6)

Thanks. I made that one up for myself some time ago, and made it just how I like them. Then I got fancy and put my Christen Eagle on it. That pic was taken at an estuary where I used to fly in San Carlos, Mexico. The airplane is a CMP Christen Eagle, with a souped up Homelite 30cc on it. I just post that for anyone to use whenever the question of a degree wheel comes up. I don't care for the layout of a lot of them that are available online, so I made this one up. (Well, modified it from the best one I could find online....)

AV8TOR

Jazzy

tkg,

I haven't yet done the measuring to see how much room I have to play with as far as exposing the piston skirt in either opening. Those will come soon.

The direction I go in with the mods will depend on the timing of the ports in its stock configuration.

In addition to the exhaust port, exposing the bottom of the piston in the intake port at TDC would affect the transfer of fuel charge and, if it were to come to that based on some aggressive timing modifications, I would have to trim the piston skirt an appropriate amount in the affected area.

Regards,
Jeff

Jazzy

Got around to doing some more measuring.

I put the piston at TDC and slowly filled the combustion chamber through the spark plug hole with 2-stroke oil.
I used a syringe-type dispenser that had graduation marks on it. Filled to the bottom spark plug thread the chamber took about 3.4ml (cc) of fluid.
The ring seal is good enough that the leakage past the ring and through the ring gap was negligible.

Compression Ratio (CR):

CR=[(Vol of chamber + piston displacement)/Vol of chamber]:1
CR=[(3.4cc + 27.6cc)/3.4cc]:1
CR=9.12:1

Much better than I expected!! While the method I used allowed for a questionable margin of error, it should be close enough.

Using the above equation replacing CR with 10 and solving for Vol of chamber then using the cylinder volume equation and solving for H the amount required to raise the CR to 10 ended up being around 3.5 thou. Not worth the effort so I won't be skimming any off the head for now.

I think I can attribute the high CR to NOT using the thick gasket material the factory used. After cleaning the head and case - especially the mounting surfaces - I sealed them upon re-assembly with a thin bead of red, RTV (High Temp). Not being a semi-deformable solid, the RTV squished out much thinner than the factory gasket by a few thou. And there you have it. :-)

I did a little more scotch bright and sanding work on the head to clean it up a little.

Waiting for the prop hub to arrive for initial RPM readings!

Jazzy

Okay. Time for another installment.
The resistor spark plug, prop hub, velocity stack, and mounting plate came in from Carr Precision. Very nice!

Cleaning up the flywheel was a time consuming task. It was fun though! The weight came out 4.2 oz less that what I started with.
14.2 oz to 10 oz. Not bad at all! I balanced it with my Du-Bro prop balancer. It did take a while to get it close enough.
If you notice when I broke off (not a recommended method) one of the fins it took a piece of the magnet casing with it. I filled the gap with metal epoxy from Loc-Tite rated for 3200 PSI. That will hold it.

I trimmed the plastic backplate so that I could attach the mounting plate from Carr.

I replaced all of the factory screws with SS 10-24s and attached the velocity stack. To attach the velocity stack I had to get some 8-32 Keps nuts, drill them out and tap them for 10-24. The original nuts were to wide to be of use so I improvised.
When putting the carb adapter (factory part) on I used a very thin bead of the Red RTV instead of the gasket that was there originally.

I also removed the spring from the throttle lever on the carb. Not hard just unhook and unwind using some pliers.

Hopefully I will get some initial RPM readings today.

tkg

My Feedback: (1)

Most folks use the closing of the exhaust port as the starting point to measure the actual compression ratio

spaceworm

Current think seems to be to either leave the spring in place to act in case the throttle servo gives out, or just slip the spring hook off of the arm and leave the spring in place as a spacer to keep the throttle plate from moving sideways and wearing the venturi. Just a thought. Good luck, keep up the nice work. I have several Mac weedies I hope to convert when I get a round tuit.

Jazzy

Spaceworm,
I tried just slipping the catch off the throttle lever but it was not conducive to what I deemed acceptable "spring" behavior. There IS some lateral play in the throttle arm so I may end up replacing a spring of some sort to compensate... Thanks for making me look a little harder.

TKG,
Referencing the following:
I am using the "uncorrected" version of determining CR.

I am also curious how I would go about determining the CR in the "Corrected Method" above without having to guess at more measurements and a larger possibility of error? Seriously. I would like to know how I would go about this as I am not familiar with this method. The way I see it I would have to fill the cylinder again with oil (or other) at the moment of exhaust port closed and take measurements. Honestly, it is too tedious for me at this time.

Sorry folks! I did get some things accomplished as far as my improvised test-bed goes but not everything. I may be able to test RPM readings tomorrow. Or at least someday soon.

Thanks and Best Regards,
Jeff

av8tor1977

My Feedback: (6)

You can figure the compression from exhaust port closing mathematically. However, I never have particularly believed in this method of doing it. I go by the "old fashioned" full stroke method, knowing that a high exhaust port timing is going to alter the tune of the engine accordingly; same as we do with four stroke engines with larger than stock camshafts, and late exhaust valve closings.

AV8TOR

Jazzy

Ah, yes. Finally had some time to tinker more.
Unfortunately, the remote primer bulb bought the farm while trying to start the engine yesterday... Or was it the day before?
Anyway, I ended up plugging the primer line and went with a single line to the carb.
Using a Zinger 18 X 6 (the only pitch I had on hand) I managed to get some tach readings today. I used my old GloBee tach and didn't feel like digging out my lab tach. Maybe another time.
Reliable idle: ~2000 RPM
Max : 6500 RPM

While I'm not thrilled with the RPM readings, I have to also consider that the only mods are: a resistor spark plug, RTV in place of the head-case paper stock gasket.

The muffler is still stock as well as all of the porting.

I did notice while running the engine that even with the choke at the 1/2 position the max RPMs didn't really change. This tells me (I could be wrong though!) that it isn't the intake that is restrictive, it is the exhaust whether it is porting or the muffler.

Next thing to try is removing the muffler entirely. Yeah, the neighbors are gonna love me! Hahaha

There seems to be a problem with the file upload service... I'll post when I can.

Jazzy

I took the spark plug out and the muffler off.

The top of the piston, after about a tank and a half of 40:1 mix, is already caking with crud.

The exhaust port and side of the piston are discoloring/staining as well.

First task will be to run it without the muffler.

Still have to do the timing... That would be good information to share so that anyone doing this conversion already has the groundwork done. (It may already be done by someone... somewhere...)

Until later,
Cheers!

Jazzy

Ran it without the muffler today after work.

IT IS IMPERATIVE THAT HEARING PROTECTION IS WORN!!!

I actually had foam plugs in as well as my shooting muffs. It was bearable.

Well, I got a little over 7K RPM with the Zinger 18X6.
Idle and transition were much better.

I did notice that when holding my hand in front of the velocity stack (blocking prop-wash), the RPMs increased and were more stable.
I rotated the angled portion of the stack to the rear. I'll have to check to see if it made a difference.

The results of this experiment are:
- The direction of the angle of the velocity stack DOES make a difference.
- The muffler is too restrictive.
- Exhaust porting will need some work.

So, maybe I'll get the porting angles and durations tonight.
I went back to the garage an hour or so later (we just can't help ourselves!) and put the timing wheel on the prop hub. Upon removing the spark plug I gazed carefully into the opening with a flashlight. The outer edge of the piston is a light gray color - not shiny but not coated with carbon. This tells me the compression is pretty good if not darn close to just right. After running without the muffler the center area of the piston cleaned up a little.

Side note: The Zama carb appears to be big enough so far. I was able to richen it to a 4-stroke cough at full throttle.

Cheers

Jazzy

The porting for this engine is as follows:

Exhaust Port:
-open @ 114 deg
-close @ 254 deg
-duration = 140 deg
-ideal range: 140 - 150 deg

Transfer Ports:
- open @ 132 deg
- close @ 232 deg
- duration = 100 deg
- ideal range: 105 - 125 deg

Intake Port:
- open @ 300 deg
- close @ 425 deg
- duration = 125 deg
- ideal range: 115 - 140 deg

Looks like I'll be opening the exhaust port a little and extending the transfer ports.
I'll shoot for 10 deg additional on the exhaust port and 15 deg on the transfer ports.
I'll take a sharp tool and scribe lines on the cylinder wall at the appropriate degree marks and have at it with a dremel.

av8tor1977

My Feedback: (6)

Make sure that you make the transfer ports open perfectly equally, and at the same directional angle as original. Transfer ports are the most difficult to rework.

I coat the cylinder wall with Dyekem, and then scribe the Dyekem. Magic Markers can also be used if you don't have Dyekem....

AV8TOR

Jazzy

AV8TOR,

Thanks for the tips. I haven't figured out yet how I intend to get a tool to the top of the transfer ports.

av8tor1977

My Feedback: (6)

I use an "end mill" bit, but once again, it's tough to keep the exact original angles in place. The ports are usually aimed to direct the charge upward and across the cylinder. Another "trick" is to relieve (grind) the top of the piston at the port location to change the port timing instead of the port itself. Just don't get too close to the top ring when doing this....

Since raising a port changes both it's opening and closing times, a little change makes a big difference. Grind a little and double check, grind and check, etc.....

Also be sure to put a nice bevel on the newly ground port edges to protect the piston and rings from damage.

AV8TOR

Jazzy

Yeah, that's what I was figuring on using.

I considered modifying the piston crown but that would affect CR. It seems to be the one thing that is just about perfect so I don't want to mess with it.

I'm reluctant to do the port time area and the brake mean effective pressure calcs. They would tell me exactly what I needed to do get this engine cranking over 8k and running like a top.

av8tor1977

My Feedback: (6)

The time/area calcs are labor intensive and probably not worth it for a sport engine. If you shoot for the timing numbers you stated, along with an 11 to 12.7mm carb and a free breathing exhaust it should run quite well. If you really want to make it "sing", raise your exhaust numbers to over 170 degrees, and put a tuned pipe on it....

AV8TOR

rangerfredbob

My Feedback: (13)

put two more flights on my Mac 32 powered Ultra Stick Light 120 today, that is a fun plane... Would have flown more but my nerves were driving me nuts, first time I've flown in like 3 months but it came back quick, two 3 point landings and I called it a day... That plane handles like it's on rails, other than the rudder that is

If I remember right I just increased the exhaust duration to around 150* (I'll try to search for my old post on that tomorrow) on this thing and maybe pulled out the jug gasket and put a 1/2" venturi carb off a G230 car engine on it, but it takes that 10lb plane and will hover and slowly pull out with a 17.25x8W APC (it got shortened and reshaped after a minor prop strike, works well), I don't think I've ever got an accurate tach reading on it, it still 4 strokes a little but I've taken that out before and it didn't like it so I'm leaving it where it's happiest

Jazzy

Sweet!! The thrill never goes away...

Whatever info you can share would be great.

Thanks,
Jeff

rangerfredbob

My Feedback: (13)

Apparently I din't report on the actual duration I ended up with on the Mac... I know I used a goal of 150 on the exhaust and mostly a little widening on the intake, and found a note that I used aviation seal on the head...