Post by rocketdog on Nov 12, 2010 20:27:01 GMT -5
As I said in another post, about whether to run Hi Octane fuel in a standard 72cc BBK, I've been measuring combustion chamber and piston volumes. I've been trying to find a good combination for a race scooter I've been working on. It's a Generic 47mm BBK w/ a dished piston. Just like most of the kits I've seen.
So far I have measured the chamber volume of a stock 49cc Chinese head, a NCY 50mm head and the piston-cylinder volume of a 47mm BBK with the stock dished BBK piston. Here's what I've come up with.
Stock QMB139 49cc Head-5.4ml=5.4cc
NCY 50mm Head w/.20 inch Shaved off-6.0ml=6.0cc
It's something like 6.5cc Stock
I would shave more off the head but you run into piston to valve problems with much more than that. And as 90GT has stated, one of the problems is the dished piston valve reliefs don't line up with the big valves on the NCY head.
QMB139 47mm Bore Head Gasket Thickness Compressed-.32mm-.0125 inches X 49.62mm .62cc
47mm bore piston to Cylinder Deck Volume Stock Dished @tdc-2.7ml=2.7cc
QMB139 Base Gasket Thickness Compressed-.5mm=.020inch
Base Gasket Thickness effect on a 47mm Bore @ TDC =.87cc
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
I've seen where some folks are worried that a stock head, on one of these kits, would have too much compression. Well nothing could be further from the truth. The stock head is actually a pretty good design and running a comp. ratio of 8.54-9-1 would run regular 87 octane fuel.
As far as the NCY head goes, my friend 90GT points out that the larger valves really aren't necessary for that size of engine. That they might be detrimental to the intake and exhaust velocitys. Plus you lose a .5 a point of compression. Leave that head to bigger engines.
Here's a few shots of my ccing and a piston I'm thinking of using. With a bit of minor machining and a stroker plate I think I can run my compression up to where I might actually need Hi Test Fuel. As long as the valves will clear.
Stock BBK 72cc piston on the right
I intend to measure the Stock 49cc QMB139 and see what that comes up with in Comp. Ratio and a P 49cc engine I have lying around. I'll get back to you. ]
RD
So far I have measured the chamber volume of a stock 49cc Chinese head, a NCY 50mm head and the piston-cylinder volume of a 47mm BBK with the stock dished BBK piston. Here's what I've come up with.
Stock QMB139 49cc Head-5.4ml=5.4cc
NCY 50mm Head w/.20 inch Shaved off-6.0ml=6.0cc
It's something like 6.5cc Stock
I would shave more off the head but you run into piston to valve problems with much more than that. And as 90GT has stated, one of the problems is the dished piston valve reliefs don't line up with the big valves on the NCY head.
QMB139 47mm Bore Head Gasket Thickness Compressed-.32mm-.0125 inches X 49.62mm .62cc
47mm bore piston to Cylinder Deck Volume Stock Dished @tdc-2.7ml=2.7cc
QMB139 Base Gasket Thickness Compressed-.5mm=.020inch
Base Gasket Thickness effect on a 47mm Bore @ TDC =.87cc
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
I've seen where some folks are worried that a stock head, on one of these kits, would have too much compression. Well nothing could be further from the truth. The stock head is actually a pretty good design and running a comp. ratio of 8.54-9-1 would run regular 87 octane fuel.
As far as the NCY head goes, my friend 90GT points out that the larger valves really aren't necessary for that size of engine. That they might be detrimental to the intake and exhaust velocitys. Plus you lose a .5 a point of compression. Leave that head to bigger engines.
Here's a few shots of my ccing and a piston I'm thinking of using. With a bit of minor machining and a stroker plate I think I can run my compression up to where I might actually need Hi Test Fuel. As long as the valves will clear.
Stock BBK 72cc piston on the right
I intend to measure the Stock 49cc QMB139 and see what that comes up with in Comp. Ratio and a P 49cc engine I have lying around. I'll get back to you. ]
RD
.
