BBK installation...the practical side.

FWIW....these BBK cylinders are machined on a production line with very little hand-finishing involved. You can/should do a little finishing yourself. I always take a sharp blade and take the edges off all the casting holes....you want to feel a smooth edge on all the different openings. There were some places on this cylinder where the edge could have fallen loose (like "hanging-chad" . Only takes a few minutes.....



Be sure to clean out all the openings with something like brake cleaner after you do this.....then blow the cylinder clean w/compressed air if possible.

Don't forget to plug the open oil pressure hole with a 6mm short bolt...I use sealer on the threads. You absolutely don't want any leaks here.



I'll get this finished, I swear.

^^^^^^If you look at the top of this page, you'll see that tortoise updated his listing of compression ratios for various engines. None of them, even the liquid cooled models, run a ratio as high as 15/1. That's what I would have been running if I had not modded the stock head.

Slightly more power is what I'm after here.....not a super-high compression race engine with shorter life.

Lots of bad stuff....and nothing good for a street bike.

At 15/1 using the highest octane available (93 here), you might still detonate. This occurs when the mixture ignites in the cylinder even before the spark plug sparks (also called pre-ignition). This is due to the high pressure and resulting heat.

Eventually you'll burn a hole in the piston, break a piston or rings, blow out a rod or main bearing....or worse. And it's not easy to tell that detonation is happening on these noisy engines.....it takes a practiced ear.

Stock compression is about 10/1, maybe a little less. 15/1 is way past what would be considered safe for any street bike...IMO.

Great thread. I'll be checking back to see what kind of performance gains you've achieved.

- Tek

^^^^I'm not worried about that synthetic oil thing from the start. Many autos come with full synthetic oil from the factory.....been that way for quite a while, with no smoking issues.

I've got about 60 miles on this BBK so far....no problems. First thing I noticed was a much quieter engine due to the removal of the Pulse injection. And the sound of the engine/exhaust was much deeper (nicer). Almost sounds like a motorcycle now.

It looks like my gas mileage will remain about the same....I'll calculate it after a few tankfuls. (the stock engine gave 100mpg).

I think the scoot idles better, but that could be my imagination. I know it goes up the smaller hills easier.....I can take the hills at 1/2 throttle and maintain the same speed as at WOT with the stock setup. If that's true I may even better the 100mpg I was getting. ;D

One maybe problem....I'm running at smaller throttle openings now for the same speed. I may have to change the pilot jet...there's a slight bog when accelerating from slow speeds. But I'm still taking it easy for a while, bedding in the new piston/cylinder.

Some actual numbers about compression ratios using a 72cc kit with your stock head.

scootdawg.proboards.com/index.cgi?board=discussion&action=display&thread=34648

With a 47mm bore and a 41.4 stroke things work out to this.

Stock QMB139 head w/base gasket-8.54-1 static compression ratio

Stock QMB 139 head w/RTV for a base gasket-9-1 static compression ratio

NCY 50mm head w/base gasket-8.09-1 static compression ratio

NCY 50mm head w/RTV for a base gasket-8.51 static compression ratio

The rumor that using a stock head with a 72cc has too high of compression has been laid to rest.

MYTH BUSTED!

^^^^^^Not sure I believe that......those figures would indicate the stock compression on a 50cc GY6 is well below 8-1, if by increasing the cylinder capacity 50%, the compression is only raised to 8.54-1.

Without even doing any measuring at all.... the stock compression looks much higher than that. Just where/how were those numbers determined?

Nice writeup Blue. And well illustrated.

RD

If you are worried about too much compression; couldn't you just run 2 base gaskets or make your own single gasket from thick gasket material with an exacto knife ? Robb

Rocket's method of measuring combustion chamber capacity, is the same as I have seen used before. How/why his tests came up with such a low compression ratio.....unknown. If you Google "GY6 50cc compression ratio" you'll get a ton of hits....most are manufacture specs around 10.5:1 (these are for air-cooled engines with standard cylinders).

However....since there are a bunch of different Chinese manufacturers of the OEM pieces, as well as the BBKs, it's quite likely no two tests with pieces acquired from different sources, would yield similar numbers.

Here's what a modified "performance" head from Scrappy looks like....the arrow points to the machined out area, designed to lower the compression (looks almost like what I did to my own head with a Dremel). You could argue this bevel helps the airflow through the head....and it does at the expense of lowered compression.



Here is my stock 50cc piston, with the wrist pin stuck into a 72cc piston. The height of the two pistons is exactly the same.







The stock piston has a very short "dome"....the 72cc piston has a very small "dish" in the center. This difference will lower the combustion chamber volume of a 72cc engine using the stock head....but very slightly. And not nearly enough to make up for the bigger 72cc cylinder volume. Meaning.....Using this particular BBK with the stock head, will give you a compression ratio much higher than stock.

So.....what is the stock compression on a GY6 50cc? Probably varies somewhat depending on the brand/model/parts supplier used that particular day of production/and so on. From looking at the setup I have, I tend to believe what most of the spec sheets say.....around 10:1.

And raising the compression much further than that......is a gamble in my book. Buy a head somewhere with that "bevel" already cut in it......many places sell one. Here's another type...



Either that or drag out your Dremel and do it by hand. Your engine will last longer......IMHO.

For context . . the Tomos A35 is a 2-stroke engine.

So your gonna take Chinese specs over a highly skilled motor builders specs? Whatever floats your boat there Bluefront.

Or, it could be a sign of an underpowered starter?

Mine had no problems with a 83cc BBK and stock head.

Well my 72cc BBK cranks just fine. If it hadn't, I would have hooked up a jumper battery to eliminate any battery issues. All these kits/pistons are not created equal.....presumably. So some kits might give higher compression than others. The starters on some engines may be marginal, when dealing with higher compression.

The self-destruction thing.....hard to point the blame accurately. Blown out crank bearing or main bearings are a possibility, as is a broken piston/rings. Probably a number of pieces could/do fail. As to the exact reason.....take your pick. I'll place the blame on the higher compression, and the rest of the engine pieces being unable to stand the extra strain.

As I've already said.....buy a "performance head" to go with your BBK. It may save you a bunch of headaches.