Spring time- so busy now!

It sucks I haven't been able to update this in awhile, but sadly, most of what I'm working on falls under the umbrella of semi-classified. I'm back at my old job doing product testing at MSOE, so my free time has completely disappeared, I'm spending a lot of time doing some repair work that has been haunting me all winter, we're getting ready to move, so i'm packing up the shop and getting rid of junk, and most importantly I'm finally getting more parts off the ground and into production. All my cash and time have been tied up with product testing and development lately, but it should start paying off.

If you're waiting on machine work, I'm sorry, things are crazy busy right now, and the heads, kits, etc. are on the back burner while i try to get some products developed. Its amazing how much time and effort goes into producing something from an idea, to prototyping, testing, refinement, production, etc etc. I'm committed to producing the finest quality, made-in-america high performance moped parts, and its very time consuming, but hang in there folks. Some really great stuff is on its way.

Cylinder compression ratio in two stroke engines

I wrote this post a few weeks back, but its currently about 2 pages and only halfway done. Even i dont want to read it, so i'll try to condense the knowledge and answer most of the questions i've been getting lately about combustion dynamics and compression, without boring the heck out of everyone.

Despite the fact that it contains no valves, and could basically be replaced with a flat slab of aluminum, the cylinder head in a two stroke engine has had an incredible amount of research done on it. The head geometry controls of the major factors influencing efficiency (power) of the engine: combustion pressure, and combustion geometry. The burning of fuel and air in the cylinder is basically a very fast chemical reaction. The laws governing speed of a reaction, imply a faster and more thorough burning of the fuel + air in the head, with a) more air velocity and b) more pressure=temperature=decrease in volume.

Quite simply put, the compression ratio creates a higher pressure in the cylinder when the spark plug fires, causing the flame to 'explode' with a lot more force, pushing down on the piston. This creates higher torque because of more force pushing on piston, but also requires more force to push the piston back up and compress the fuel air mix.

We calculate CR by taking the total volume (displacement plus head) and dividing that by the head volume alone. For a 70cc kit with a 7 cc head (stock 50cc) + gaskets ( 1-2 cc) it works out like (70+8)/8= 9.75:1.

PSI can be an indicator of compression ratio, but requires much tricker math to actually figure out CR. Other factors like ring sealing, port dimensions, pipes, intakes, etc, effect the PSI reading.

The 'squish band' affects the 'v' or velocity part of the equation. Air moving quickly has more energy and explodes faster. Imagine having a bowl of vinegar and dumping in baking soda. If you stir the heck out of it, it will fizz up much more violently. That is what is happening as the piston approaches the squish band. The quick decrease in volume will blast all the gasses into the hemispherical chamber around the spark plug at the same time, the violence of moving air speeds the reaction. Jennings and Bell reccomend the 40% of area- 7 deg taper method of sizing a squish band.

Because of the 'faster' flame in a high compression setup, ignition timing will have to be retarded. The ignition triggers and begins igniting the fumes before top dead center, to account for the time it takes to burn the air. When the everything burns faster, the max intensity of the explosion is at the wrong time and pushes the piston down as it comes up (knocking or pinging). This is exacerbated by a pipe, (or supercharger) which increases the pressure in the cylinder.

Sometimes the compression is just too high, the fuel ignightes without any spark, this is also problematic. Going to a higher octane fuel (only necessary in this condiditon) will allow you to continue to run the higher CR and pipe, otherwise you have to drop CR or go to a different pipe.

The 2 stroke gets more complicated also, because there is a huge hole (exhaust port) in the side of the cylinder. So even though on paper the compression ratio for identical 70cc cylinders, with different exhaust timings, will be the same, the holes in the side, and efficiency of exhaust porting will make their actual combustion pressure drastically different.

Finally, what are you looking for with your bike? The mods that mostly effect the final combustion pressure are: More aggressive intake porting= higher compression, more aggressive exhaust porting = lower compression, more radical pipe= higher compression at high rpm's when pipe hits.

If you switch to a higher compression head, and think you are detonating (sounds like rocks rattling in a can... and isn't your crankshaft) try to retard your timing a bit, see if your power improves. Some kits require so many base gaskets to clear a stock head, or small HC, that by the time all that gets put on there they actually have crummy compression, or a lack of bottom end from having all the ports shifted up. When the compression comes back, a bike that was tuned for different setup, can get thrown off. The detonating sound will be nastiest when you are really cramming on the pipe hard. If the timing doesn't fix it, and you're jetted correctly, and running premium gas and synthetic oil, then try putting in more head gaskets to drop compression a bit.

The cylinders that i've already made a lot of heads for (DR hobbit, MK65, and Polini) the compression ratios are just about figured out, but there is a lot of improvement for specific setups. Knowing what to look for makes it a lot easier to fine tune compression with head gaskets, and play with squishin' dynamics.

Mid-winter update

As indicated by my previous post, i've been busy as heck. I'm trying to get a bunch of projects finished up over my 'quarter break' coming up at the end of the month here, because next quarter i'm headed back to MSOE more-fuller-time with 2 or 3 classes, and it looks like i'm headed back to my other day job too, so my moped time will be shut down for about 3 months.
I've got three major projects I'd like to get into 'production' state by then, so I can sit back and coast, but its put my personal bikes on hold. I figured, in the spirit of Terry Dean's helming of MotoMatic Blog, i'd cruise around the shop today with a camera and take some teaser photos of what i've been up to.

Vespa cylinders!




100SL2 yamaha project, for a customer, pretty rad bike





Ewwww... four stroke... my CL100 racebike for the Lake Erie Loop this spring. thats got a big 'in progress' sticker on it.




What's this all about?


Exiting things to come..

Busy

Holy cow, this blog is going down in flames. I'm sorry, i've been too busy to post, but there are a lot of fancy things in the works. Sometimes it seems like the amount of real mopeds v. internet mopeds is inversely proportional.

Ciao FAIL

On wednesday, i was bored, and still frustrated with not having fixed the leakage issue on the ciao, so Alex, Joel, Jimmy and I tore it apart again (I can get the whole engine out in about 7 minutes now, most of that is removing the pipe) And I was finally able to figure out what was going on. In addition to the Decomp valve still leaking quite a bit, It appeared that when I used the face mill on the cylinder, It cut unevenly where it caught on the edge of the broken off fin. I should have broken or ground that fin off to start with, but you live and learn, eh.

Back in the machine shop, I took the cylinder and lined it up in the lathe, clamping the cylinder skirt. I was in a bit of a hurry to finish it up, and forgot my rule about doing this sort of thing... you have to run a piston inside the skirt or it will break the skirt off. I just had to take a few .001's off, so i wasn't too concerned, but when i started up the mill, some boner had left the feed engaged and right away the tool ground into the face of the cylinder and did this:


What a buzzkill.

I also went ahead and took the head down a couple .001's, which was probably a bad idea because now the extra tall piston i made is hitting, so i'll have to machine that stupid thing down again.


Yeesh, look at that nasty decomp, no wonder it was leaking, carbon and gunk all over...


Oh well, thats mopeds, should have left well enough alone when I had the chance. It looks like the cylinder will be fine with the skirt broken off, but if it eats it, I'm on the lookout for a new stock vespa cylinder. Anybody got one they'd like to get rid of? I've got a parts grande lined up, but i'm pretty sure thats a 10mm pin bike, so i'd rather not back track and rebuild another whole engine.

The good news is, break-in appears to be going well. I was concerned about re-using the old rings, but i cleaned them up with a little sand paper, and re-honed, and it looks like they are sealing really well. Given the wierd compression issues, i was worried they hadn't sealed, but this pic says it all:


The transfers also seem to be pretty well matched, the burn pattern on the piston is a little bit dark, but i'm 99% sure thats because of the wicked air leak causing things to run a bit hot.


You can see on the piston the hot spot from the decomp valve leaking (also perhaps because of the wierd head shape around the decomp) and there is a big black mark where the head wasn't sealing that well. Hopefully cleaning the decomp again and fixing the sealing issue will take care of my shitty low-end and maybe even 'fix' the misfire situation.

Mysteries of the universe. of mopeds.

Recently I discovered the library.

Rediscovered, is more like it really, I used to spend a lot of time in Eau Claire's LE Phillips Memorial Library reading and perusing all sorts of interesting things, but since coming to MSOE and signing my life over to the forces of homework, lab reports, and other such nonsense, I haven't really had time to waste on fiction. My dynamometer research got me back in the MSOE library where I found the amazing and mysterious TJ section. Titles like 'The design and development of 2-stroke engines' have completely rocked my world and dominated the last few weeks with knowledge I had, until lately, just grasped at.

In the upcoming weeks, I'll be spending as much free time as i can spare reading and distilling all this crazy knowledge, for you, my lovely readers. I just had the bomb of knowledge dropped on me in a big way, not to mention a lot of orders for machine work, and some interest in some of my other lesser-discussed projects, so it might take me awhile to dig out, but trust me, it will be well worth it.

Also, in the world of the ciao... massive head leakage, i think it has a lot to do with the decomp valve which i suspected was leaky-weeky, top speed so far has been a shy over 42, but low end is miserable, and compression is very low. I'll have this written up also as i get a 'round tuit'

Moped tuner/machinist for hire.

After a few recent conversations, It has occurred to me that I'm not the best self-promoter or businessman. Its apparently unclear here and other places I post my knowledge, that I do moped work for-hire. I spent the summer doing repair work trying to pay the bills, and since I'm still under-employed, I've got a lot of time to hang out in the workshop and make things faster.

My standard shop rate is $30/hr, for just about anything. Fabrication, welding, custom parts, adapters, manifolds, exhausts, if you've read the blog you know what I can do.

In addition to that, some services I commonly perform have flat-rate pricing:

Cylinder head milling: $25/ on-center spark plugs, $35/ off center spark plugs, $30/ Garelli and Minarelli heads with recessed sealing faces. All heads are custom machined for a specific bore and compression ratio, with 2-angle squish band.

From Machined Heads

Stock-cylinder porting: $50, you specify performance characteristics, I compute ideal port map. Jobs requiring additional machining extra.

Dellorto SHA or Bing carbureator boring, up to 7/8" (15.8mm): $20, plus any time spent cleaning/disassembling carb. This is performed to a mirror finish with a boring reamer, I will also remove and modify dellorto emulsion tubes.

Case-matching: $40 + gaskets. Shown is metrakit E50 with additional transfer porting.

From Metrakit Nate B

My gmail address works well for contacting me. I turn stuff around pretty quick, usually a week or less, but it depends on how busy I am. Email and discuss things with me, everything is custom, so Its my priority that its done just right for your build.