139qmb 72cc BBK with BVH or stock head

[stenen]

I've got a 139qmb engine and I'm getting a 72cc big bore kit. The thing is I don't know if it's worth getting a big valve head along with the big bore kit or if I should just stick with the stock head. From what I can read in forums the big valve head allows for more airflow, but the thing is that some say that the 72cc kit can't utilize the added airflow while some says that it will make a big difference. Then there are some that say that the stock head will make up for the difference in airflow because of higher compression due to the head's diameter being smaller than the cylinder's.

So what is best? The added airflow or higher compression?

If the big valve head is that much better I might just get it but if it isn't that much of a difference I might still stick to the stock head due to the big valve head being much more expensive. Where I live I can get a 72cc cylinder and piston for about 18 Euros while the BBK with the BVH costs me about 48 Euros. These are probably chinese made and poor quality kits but it's a really cheap scooter and I don't really want to spend that much on it.

I'm also thinking about getting a bigger cam and want to know if it's a good idea running it with either the BVH or the stock head. The cam in question has no specifications so it really is a gamble if it will work but if it can work and will make a big difference if it does work I will probably buy it because it's only 9 Euros.

Thank you!

Scooter: QINGQI 50QT-6A

BVH: www.motor-x.com/cylinder-kit-with-cylinder-head-4700mm-80cm-pin-13mm-with-sls-large-valves-139qmb-qma-gy6-50-4t,p.html

BBK: www.motor-x.com/cylinder-kit-4700mm-80cm-iron-pin-13mm-139qmb-qma-gy6-50-4t,p.html

Cam: www.motor-x.com/the-camshaft-80cm-139qmb-qma-gy6-50-4t,p.html

[sinfull]

Big valve heads are kinda not really big valve, check valve dimensions and both of mine measured exactly same valve dimensions. Iirc 17mm both on the stock head and the taida heads. Difference is in the ports both intake and exhaust are definitely larger, but the other thing is the taida is probably lower compression. Mine had more of bowl shape than my stock head, same gasket area just radius more. So not sure of which head was actually better, just swapping heads gave me no different in top speed.

[stenen]

From what I can read in the product description’s on the website I’m going to buy the BBK from the stock valve heads are 18.5 mm intake and 15.5mm exhaust. The big valve head is 46.5mm (72cc) and has 23mm intake and 20mm exhaust valves. It’s quite a big difference but I don’t know if I will be benefiting from the added airflow.

[GrumpyUnk]

FWIW, the BVH link goes 404.

I was going to check the component costs. Thinking you could try the BBK with the original, and then add the BVH if it was not enough. I think, no stats, the original will do fine with a 47mm bore.
Everyone seems to want higher top speed. My take is that good acceleration and keeping up with traffic is more usable daily than top speed. Additionally, no way do I want to be on one of these going much over 40. Life is too short as it is, and they are really not designed(most) for running at higher speeds safely. IMO.
tom