Compression ratio increase in CG125, safe or not?

[dualero]

Hello fellow "do-the-toners" I own a Chinese bike that is equipped with a CG125 clone. Engine has around 50k km and I'm up to a rebuild. Living in Venezuela and the streets being flooded with Chinese bikes is easy to find parts. Found a bolt on conversion for my engine to go from 125cc to 150cc (full detailed information and calculations below). After getting the kit, and beginning to prep for doing the job, some interesting and important questions arose in my head, leaving me with doubt.

So as I posted on my build thread (http://www.dotheton.com/forum/index.php?topic=56903) here's the issue:

I've been doing some research about my engine specs before doing the 150cc conversion and have some interesting doubts I want to share with you. And try to solve as well.

The stock compression ratio of the CG125 engine is 9.0:1. Having a 56.5mm piston and 49.5mm stroke, with these numbers I have a 124cc displacement (exactly 124,105771cc). Making calculations the combustion chamber should be the displacement divided by the CR, giving a exact13.7895301cc volume for the combustion chamber.

Now the new piston is 62mm, the stroke remains the same, so the displacement with the new piston will be 149cc (149.443992cc exactly) keeping the stock combustion chamber (cylinder head) the volume of the chamber remains unchanged also (13.7895301cc) calculating the new CR dividing the new displacement by the stock chamber volume, results in a CR of 10.8:1

Now the thing is, will this new CR be too high for the engine to handle? I've been doing some research and it is a high CR but not the highest I could find, but still this is the first time I rebuild an engine, I now the info but never had to apply it.

I found information about the specifications of both CG125 and CG150 clones on the net. Both engines have the same stroke (49.5mm) and the same stock compression ratio (9.0:1) this means the CG150 has a bigger combustion chamber in order to maintain the same CR while there's a difference between displacements.

Although it seems I'm on a dead end, a way to overcome this problem would be to get a CG150 head to install, in order to keep the CR at 9.0:1, BUT, I'd rather rebuild the engine with the high CR to gain in power (the absolute objective of this mod), but prefer not to if doing so will compromise the integrity of the engine. If I have to spend the moneys for safety reasons, I will, there's no doubt about it.

What I'm going to do is:

1st) Get the head out and measure the real volume of the combustion chamber.
2nd) Use that number to make the CR calculations again
3rd) Having the real numbers of the stock and future CR's decide if I really need the new bigger head or not.
4th) Proceed with the rebuild.

That's what's bothering me now. Weigh in, I'll appreciate it. Thanks

DH900sl

 

[dualero]

All right I made a mistake making the calculations for those compression ratios. Below I'll explain the corrections.

V1 = Stock CG125 displacement
V2 = 150cc Barrel/piston kit (CG150) displacement
Vc = Chamber volume
CR1 = Compression ratio stock
CR2 = new compression ratio with stock head. What whew are looking for.

Is it still too high?

Thanks



DH900sl

 

[xb33bsa]

i think you can only get a real comparison by doing trapped volumes with all of the actual pieces
that is the way i would do it
you can ease some metal off the dome or chamber if you need to lower compression
raising the compression may require premium fuel

 

[dualero]

i think you can only get a real comparison by doing trapped volumes with all of the actual pieces
that is the way i would do it
you can ease some metal off the dome or chamber if you need to lower compression
raising the compression may require premium fuel

You are totally right, these are some preliminary numbers I'm roughly calculating before getting into the engine. I almost can assess that the real compression will be lower that 10.62:1, BUT, let's suppose the final CR is 10.62:1, will this be too high for a 150cc? Or can I mod the engine in peace?

My real concern is, how much CR is too much?

DH900sl

 

[xb33bsa]

well i run "10.5" pistons in my 350 twin on premium pump gas and it runs perfect but i have never verified actual compresh
it is a 2 valve head same era of design similar size to yours
you may need premium and should be careful setting timing
if you are doing a lot of stop and go it may be a mistake to have the extra heat of the bigger bore AND higher comp

 

[dualero]

All right.

Could I lower de CR a bit more by using 2 bottom cylinder gaskets? Would it seep oil through the gaskets by doing this? I can also manage to get a thicker head gasket that would help even more.

Too much stop and go is not a problem, the routes I usually use to commute are almost free from stoplights and such.

Two question for you:

1) By removing material from the head, which area I'm supposed to trim first and in what way?

2) what octane grade is the premium gas you use?

Thanks for your input man, a lot of light has been shed already.

DH900sl

 

[Sonreir]

My 360 ran fine on premium fuel at 11.2:1.

 

[dualero]

My 360 ran fine on premium fuel at 11.2:1.

How many octanes for the premium gas? Thanks

DH900sl

 

[xb33bsa]

yeah i think it may be just fine,i think i would run it at the higher comp
92 octane here i use the ethanol free
adding a base gasket will advance cam timing slightly which could change the power a bit, in theory it would help a bit in the lower rpm ranges with a possible slight loss at the upper rpm's
if you need to reduce the volume the easiest way is some very careful removal of metal from the top ares of the piston you must be careful to not really make it much thinner just rounding some corners is safe
if removing metal from the chamber well as long as you stay away from the squish you can remove a general layer from most ares but i would not remove any of the taint between the valves

 

[dualero]

Here in Venezuela we have 91 and 95 octane gas, so I wold always go with 95 and be on the safe side, regarding auto-ignition.

Seeing as those 360's are practically two CG125 engine welded together (apart from the OHC/OHV differences) if Sonreir, has got un to 11.2, I think I will be fine with the (rough) 10.63.

Anyway all this previous talk is for guidance, I'll make the necessary measurements to get the volumes of all the parts involved and then get the real CR.

A friend of mine told me to get all the calculations on matlab (mathematical software), so I could vary the values of the variables (Vc, bottom gasket volume, head gasket volume, piston volume) and see how much each variable varistion inputs on the final CR. This will allow me to "feel" which of the parts involved in the calculations for the CR, are the ones that make the most influential change.

I'll keep you posted on the subject as I work forward. This week I'm in midterms. Next week, the game is on. Thanks guys.

DH900sl

 

[xb33bsa]

you will want to get a graduated burrette to measure trapped volume of the assembled engine at tdc
the other calcs are simple math

 

[dualero]

Yes! Just have thought of that! Luckily for me, my girlfriend works as a technician in a good microbiology lab, so getting those instruments is a piece of cake! Hurray!

DH900sl

 

[dualero]

Big updates my friends!

Some bad news, some good news.

150cc conversion came sooner than expected, the reason why, explained below.

On Thursday, I went to work. There I stopped the engine for a moment and then when I tried to start it again, the bike wouldn't fire up.

Some fiddling on the carb and the engine surroundings did nothing. I went to push the bike to get it started, released the clutch lever and the bike intended to fire but without any success, accompanied with a metallic sound coming from the engine.

Did I hear right? Ohhhh no connecting rod bearing exploded and destroyed my engine! Or so I thought. Called a friend who picked me up and the bike. Leave it at his house. Today presented my last exam for this exam streak, and went inside my little 125 engine.

Began cleaning the engine a little, as it had years of oil dripping catching every little particle the road had to offer. Here's my endogenous work bench, beside a bottle of my trusty degreaser. Gasoline. I know I'm an heretic for using gas as degreaser, but here it is cheaper than water BY FAR.

These are the journeys I embark on, by working on my parking space hahahaha.


A lot cleaner engine:

Exhaust and inlet ports:

Removed valve cover then tappet assembly:

Removed Head. Notice the big amount of carbon in the combustion chamber and the top of the piston:

Removed the piston from Con-Rod:

After a lot of fiddling (without going into the lower part of the engine. Yet.) I found the Con-Rod, does not have any sort of play in any of its ends, other than axially.

Taking a look back at what happened got me guessing why would the bike sound like that and unable to fire up. Looking deeper at the piston and chamber and I found little pieces of what looked like some sort of drop-like shaped object, of aluminum or another soft metal. This lead me to inspect further.

Spots where "intruder object" made impact with the piston:

Another prove that something entered the cylinder, was this. Bent spark plug electrode messing completely the spark plug gap, reason why it did not star again:

And last but not least, organization:

So this is the progress I have until now. Tomorrow I'll clean the head and valves, and will lap the valves too. I have time y will break into the lower part of the engine, though I'm lacking a extractor for the rotor, I will fins some way to get it out.

Hope you enjoy the reading, this project is really really getting interesting.

Cheers buds!