High vs Low compression question

Lapping the head is easy and well worth the effort. Run it a tad rich and it will be fine.
Lean mixtures kill chinagirls, not summer temps.

do what 57 said...
and if you want reliable, to get to school, don't try to run a chinagirl at 35mph!

I got a stock 48 and it cruises at 17-19, tops at 24.
rc

Built right and geard right a 30+mph ride with a china girl can be very reliable, a 36T sprocket on a 26" rear wheel keeps the rpms very comfortable at 28-30mph and with a properly tuned carb and a good flowing exhaust there is good power for moderate hills and it will have a nice cruise speed.

I recommend going with the better balanced engines from thatsdax or motorbicycleracing, less vibration from being better balanced makes for a better performing more reliable china girl.

The right oil mix and a quality oil is also key to best performance and reliability on the china girl engines, I will say from my personal experience and several thousand miles of riding on my bikes since 2009, I have had NO engine failures whatsoever since switching to Opti2 oil and I now run it at 100:1 little to no smoke, excellent performance and I ride hard much of the time on my bikes with many miles of at or near full throttle at speeds of mid 30's to over 40mph, with a good set up a 30+mph china girl bike if a very doable thing.

I also live in Texas, right in the middle of the state and I know well about the 2-3 months at 100+ temps every single day..... my bikes work well in the heat, just do as it has been mentioned and dont run to lean on fuel, keep that plug color at a nice chocolate brown color and you will be fine.

Peace, Map

Map

Just did it to an engine I've had for 5 years- and several others now-

that round ridge on the bottom of the head- when you file that down flat, the compression is increased and the gasket gets a MUCH better seal all accross the gasket and not just on the ring-

thoroghly recommend- don't think it adds much of a heat factor- but really noticibly pops better.

You need a good flat metal file and I also have a smaller finer one I use for final polish

Nashville Kat said:

Just did it to an engine I've had for 5 years- and several others now-

that round ridge on the bottom of the head- when you file that down flat, the compression is increased and the gasket gets a MUCH better seal all accross the gasket and not just on the ring-

thoroghly recommend- don't think it adds much of a heat factor- but really noticibly pops better.

You need a good flat metal file and I also have a smaller finer one I use for final polish

Click to expand...

I have always done this same thing to my stock heads, difference is I went to Home Depot and get a cheap square glass window pane, tape some 320 grit (Black) wet dry sand paper to glass with duct tape and then with a oval motion you can sand the ring down very evenly almost all the way and then I switch to a piece of 600 grit wet dry paper to finish the job so the sanded surface is smoother, makes a nice smooth sealing surface on mine.

Also, I just clamp the glass down with a quick grip clamp so it doesn't slide around while doing the job, just gotta hold good even pressure so it sands down even, but yes it's a fairly quick simple process and it does make a noticeable power increase in my experience if carb is tuned good and the exhaust isn't restricted causing to much back pressure.

Peace, map

The glass and sandpaper helps create a flat surface, even if you don't want to raise compression you want a good seal between the cylinder and head. I also lap the cylinder a little just to make sure it is flat, every little bit you can do to these engines can help them run better.

If a person doesnt want to raise compression but wants a better seal just use two head gaskets after removing the raised ring on the head.

Map

175 psi is the best for allaround power from the pump gas. 150 psi even is ok and higher than factory japanese often. for easiest tune and most durable its 135psi.

My results have been very similar, although around 155 psi.
The angle fire head does not cut it at those power levels/compression levels.
And that is after removing quite a bit of metal from the dome, and getting a squish of less than 1mm.
My old Raleigh motor was getting 160 Psi, without issues, and running a shaved straight fire head, shaved all the way to the fins. Which is now in Henshooters Kitty, And I hope he enjoys.
If you want hi compression buy an aftermarket head, or shave a straight fire and set your squish.
My thought after testing a few different compression ratios, is there's nothing much to be gained from the angle fire head over about 145 Psi. top end wise.
But low down torque is improved with a little more.
I can't see why you couldn't get the compression up to 175PSI, with the right head/fuel.
But I suggest aiming a little lower for engine life/reliability ease of standard Gas, and cost of purchasing an expensive head.
And remember if you set your squish correctly with a standard angle fire head, you will have too much compression!
The shaved straight fire head and correct squish are the go,
Bolting a straight fire head on to an undecked barrel with a 38mm crank, will not give better compression.
Moral, Go the 40mm Crank if you want performance.

Theon said:

My results have been very similar, although around 155 psi.
The angle fire head does not cut it at those power levels/compression levels.
And that is after removing quite a bit of metal from the dome, and getting a squish of less than 1mm.
My old Raleigh motor was getting 160 Psi, without issues, and running a shaved straight fire head, shaved all the way to the fins. Which is now in Henshooters Kitty, And I hope he enjoys.
If you want hi compression buy an aftermarket head, or shave a straight fire and set your squish.
My thought after testing a few different compression ratios, is there's nothing much to be gained from the angle fire head over about 145 Psi. top end wise.
But low down torque is improved with a little more.
I can't see why you couldn't get the compression up to 175PSI, with the right head/fuel.
But I suggest aiming a little lower for engine life/reliability ease of standard Gas, and cost of purchasing an expensive head.
And remember if you set your squish correctly with a standard angle fire head, you will have too much compression!
The shaved straight fire head and correct squish are the go,
Bolting a straight fire head on to an undecked barrel with a 38mm crank, will not give better compression.
Moral, Go the 40mm Crank if you want performance.

Click to expand...

Great advise I have one straight fire head I plan to shave down more, the engine it is on has always ran very smooth, I bought it second hand and its currently just sitting in shop, but I have a bike I plan to put it on and I may do some extra shaving on it just to see what I can do with it.

I know bairdco has always had good running fast bikes and he like the straight plug heads also..... proof is in the pudding and obviously he has had great results with them.

map

Gordon Jennings wrote: "no [mass produced] 2 stroke engine in my experience had ever tolerated cranking pressures above 175psi".
and he was talking about motorcycles with better quality parts than what we have with these engines.
If you increase the compression then it is advised to get an aftermarket CDI that allows selectable ignition retard at high RPM, especially if your engine is ported to rev higher than normal. Read up on the subject and you will see that very high compression is not for most high reving engines. Those usually don't have more than 150psi. But of course these engines aren't high reving but that gives an idea of the part of the powerband most affected by increasing compression. Anyway, without ignition retard the peak combustion pressure becomes too great as a result of increased compression.

I always aim between 150 and 160 Psi.
And get 9000 RPM, keeping her cool in tropical summer can be a problem, hoping a better Cdi will help.

A two stroke has problems getting all fresh charge into the cylinder compared to a four stroke, and this pollution causes detonation much sooner than in all fresh mix. The pollution of the charge with burnt exhaust is the reason high end motors can tolerate more than a stocker, because they remove exhaust and fill up the cylinder better The better the ratio of fresh mix to burnt at high rpms, the more power. Better porting and proper cylinder head design can improve this substantially. The cylinder head design also has a lot to do with how much compression a given engine will tolerate. A good, turbulent mixture with properly aimed transfer ports will make more power and take more compression as a general rule.

I always port my jugs too.
Agreed Maniac,
And as I see it the only way you can get more than about 70 Kph out of one of these is to get a proper pipe.
But by then the angle fire head don't have enough cooling (in the tropics).
Each of us will have there own favorite compression ratio, mine for a 'daily' is a 150 Psi.
Once you go over that I expect problems, increased engine wear, hard starting, overheating, bent axles (on the pedal side!), increased tyre wear from starting lock ups, increased chain stretch and wear, and it goes on.
But, for a racer, well I guess it depends on the fuel you can give her.