Building an Expansion Chamber

[Deleted member 12676]

yeah but the 1.2:1 compression that the stock engine has is just right for these low reving engines.
I raised the CCR on mine because it rev'd higher than 9000.

 

[LRSimons]

Don't worry Jag, your website is my go to place for tuning information. I honestly don't know where I'd be without your research and compilation.

Steve, did you drill balancing holes as well? How out of true was the crank? Have you tested the higher CCR yet?

My understanding of crankcase compression is as follows. If I'm wrong, please tell me.

Crankcase compression relates directly to the speed of the intake charge as it flows through the transfers. It is important to have the pressure, and therefore speed, of the intake charge at high rpm because the transfers are open for such a short amount of time. With such a short window of operation, very little gas will flow into the combustion chamber normally. This can be mitigated by increasing crankcase compression, therefore placing the crankcase mixture under a greater pressure and blowing a greater amount of intake charge into the combustion chamber. The issue with this is that at low engine speed the increased pressure and speed of the intake charge will cause it to blow out of the exhaust port, limiting power.

I wanted to try raising the CCR to see if it would increase too end power and raise engine speed. I thought that with the addition of the expansion chamber, the pipe would help the charge stay in the chamber and not limit low end power.


I took the bike out for a longer ride yesterday. The only change from the last time I posted was I cut the header off another inch (and angled it away from my leg). My theory about the pipe helping with the downsides of a raised CCR has been disproved. It still could work with a pipe tuned to lug at low rpm, but then it would be defeating the purpose of raising crankcase compression, as the pipe would probably limit top end if it was tuned for lugging.

The bike is faster than it's ever been before. I got it up to 38 mph multiple times. It gladly revs like no tomorrow (until it hits 38, when it seems like it's hitting a limiter or something. Raising the exhaust port may help.) However, any semblance of torque has been checked at the door. It's embarrassing and dangerous to go up hills now. I'm hoping grinding out some of the epoxy will help. Vibration is also not as good as I hoped after drilling the holes. That is probably from the epoxy however. I filled in the newly drilled holes, the four balance holes from China, and the two other holes closer to the center of the crank. The JB weld is only on the surface, I filled in the holes with spray foam first. It's probably added back a good deal of weight on the wrong side of the crank though. I put some epoxy around the center of the crank halves like your picture says Steve.

I may try and put my intake extension back on and see if it makes a difference.

Yesterday also impressed on me the importance of areo on top speed. In as much of a tuck I could do on a bicycle, top speed went up by a few miles an hour every time. I did some searching and a while back somebody found something about a bike fairing. Seems like that and some slippery clothes could boost speed by a good margin, as long as the revs are there.

Anyhow, my long sleeve work shirt seemed like a good idea putting around at spandex racer speeds. Now, hitting the ground at car speed seems much less fun. I may start shopping for a motorcycle jacket. But it's more slippery that flannel anyways.


Logan

 

[LRSimons]

Another question for you Jag. Did raising the CCR boost top end power, allow the engine to rev higher, or allow it to rev higher by boosting power?

I suppose a boost in rpm would have to be from an increase in top end power. The rev limit with no load would probably be dictated by the point where the thing blows up.

I guess I answered my own question. I'll post it anyways I guess.


Logan

 

[Steve Best]

Jag's website and work is a wonderful resource, totally agreed.

This 3rd motor of mine is not running yet, so I could still pull it apart again for photos.
I did not drill the crank because vibes were minimal when it was running unported, but so was rpm (30mph top speed).
With 44t I found the ported cylinder's top speed was in the 35-41 mph range dependent on pipe/head/intake.
This was also no load (downhill) max speed as well.
Since it could be varied with pipe/head/intake it is not CDI related but had to be related to resonance tuning.
This is about 10,000 rpm and very hard on bearings (especially when no-load).
I test against a hill for load and as a true judge of HP.

My crank was within 0.002" at the ends of the crankshaft when supported by the bearings.
The cheeks were within 0.007" (hard to measure, rough spots) side to side as supplied.
I beat it with a brass hammer to try truing it and got it to about 0.001" shaft and 0.005" cheeks.
Truing is more complicated than I can describe, and prone to getting worse, so if within 0.003" at shaft, I'd leave it alone.

With reed valved motorcycle and piston port sled engines I've never seen a downside to stuffing. Always had positive results pretty much based on the volume of epoxy I put in. Have never felt it shifted the powerband, just supplied more power throughout the band. It often broadened the powerband actually, as in this stuffed reed box I did on a Blaster:

From previous experience (KTM, Honda CR, Yamaha YT, Blaster, and DT200) raising the exhaust port will raise the no-load and light-load max rpm, as will a shallower combustion chamber, more timing advance, and leaner mixtures. Torque at any rpm is usually built in the other direction with any of these parameters. You can experiment with raised exhaust port by shaving the exhaust side off the top of the piston (cheaper to replace a piston than a ported cylinder) like this:

As for aerodynamics, I mostly don't worry about it. I am not racing, just traveling. At my age comfort trumps small speed gains!

 

[butre]

I haven't gone in and experimented much, but I think there's no such thing as too much case compression, just not enough case volume. for case stuffing, I tend to go to about 1.4 or 1.5 (depending on static compression ratio and how high an octane requirement I'm willing to put up with) and leave it there.

 

[LRSimons]

Time for an update. I disassembled the engine and took out the epoxy in the balance holes and lower holes, hoping to get some torque back and better the vibration.

Took it out for a test ride today. My initial observation was that the exhaust seemed to be louder. I doubt the CCR actually had an effect on this, was probably just me not riding for a few days. Either way, I finally made a muffler out of a dog food can with some steel wool inside. It's not fantastic, but definitely quieter than before. The torque seems to be about the same honestly. The biggest change, however, is in the engine's top rpm. Today, the engine felt like it was hitting a limiter at 34 mph or so. The exhaust note changed from the screamy sound to something like it was gasping for breath. Could lowering the CCR really limit fueling at high rpm to that extent? Am I missing something else that would act as a rev limiter?

On a funny note, my ride was hampered somewhat by, er, a rapid and unintentional raising of the exhaust port. That's what I get for reusing an unreuseable teflon gasket


Thanks for the input everyone. It seems like a higher CCR is the way to go in this application. The only real downside I could find after doing some research is the increase in "pumping losses" as the crankcase mixture is compressed on the downstroke.

Steve, glad to hear you had some success with the crank. I'm having a crank rebuilt currently from a Kreidler moped. Not something to get wrong for sure!


Logan

 

[Deleted member 12676]

another reason not to raise the CCR on these engines is that it wears out the cheap crummy crankshaft seals faster.
Unless some change makes a huge difference in power (10% or more) then it's hard to feel the difference because these are low power engines. But I don't remember raising the CCR as having any benefit in my engine. Probably because the transfer port flow is too vertical compared to a well designed engine. That causes the intake charge to shoot too upwards which can then more easily wind up out the exhaust port before it closes. A higher CCR adds to that problem. I wound up removing a good portion of the JBWeld I had put in there.

 

[Deleted member 12676]

ie: your mentioned rev limit.
here's what affects that limit:
1. the expansion chamber. They do that at the end of their powerband. Shorten the header for more top RPM
2. the stock CDI because it doesn't retard the ignition at high RPM
3. the limited port durations in the cylinder. increase all three for higher RPM.

 

[LRSimons]

Can the transfer angle be improved any? I remember reading your page on partially filling the back of the transfer opening. I'm assuming that is what you're talking about. The next time I do a tear down that will be on my list of items to do.

The only thing I can think of limiting rpm besides the CCR would be the addition of the muffler. Those were the only changes I made. It's hard to imagine a can full of steel wool could cut off the powerband, but I'm going to take it off and see if any difference is made.

I do have a filed ignition key, which seemed to made the engine run smoother at lower rpm. I may have to start saving for one of your CDIs Jag.


Off to the garage!


Logan

 

[Deleted member 12676]

Read my page again about how to correct the transfer flow.
The filed key should of helped a lot at high rpm also. Are you sure you installed it so the magnet was more CCW?