Big Valve Head Info & 150cc Carb/Intake On A 50cc
Feb 16, 2010 7:43:40 GMT -5
just1artist and dexameth like this
Post by 90GTVert on Feb 16, 2010 7:43:40 GMT -5
50mm combustion chamber with 21mm intake/19mm exhaust valves.
47mm port matched head vs big valve head.
I had planned to drop the head or cylinder off to the machine shop tomorrow to get a little milling done, but I wanted to do some checking first.
Using the stock 49cc head, I came up with about 0.170" of clearance, so I thought I'd be milling off at least 0.030". This time around I got .055" of clearance on the intake valve and 0.080" on the exhaust valve. That's with no head or base gaskets, so I can assume an additional 0.010"-0.020" of clearance when using gaskets/RTV.
I'm not sure why lol, but I made a quick diagram to show why the big change. The piston is flat on the edges and dishes in in the center of the piston. There are also valve reliefs in the dished area. The new valves miss the reliefs and hit in areas that are less recessed. The red marks are basicaly where the valve edges are in relation to the pistion.
I'm used to .080"/.100" of clearance being proper spec for larger engines, now I'm dealing with roughly .070"/.095". I'm thinking those numbers will be fine for this engine, but I sure don't wanna mill anything off.
I cc'd the combustion chamber just for comparison's sake. This thing is huge compared to the stock and 72cc heads. This head is roughly 7cc while the others were 4.5cc and 5cc. I hope this extra flow really works for me, because it sure won't be a high compression engine.
I did some port matching and clean up work and put the engine back together with the big vavle head, 150cc intake, and 150cc carb.
I didn't really do a lot of work because I have been reading and thinking and I don't think this setup is going to be too great anyway. I think that the valve sizes (21mm In/19mm Ex) are pretty well suited for a strong running little GY6, but the intake track is a bit large. Here's part of why I think this...
A. Graham Bell, very famous for his 2T tuning book, wrote a 4T tuning book. In this book he states that intake port area should be 81 to 83% of the intake valve area. I have a 21mm intake valve...
Valve Area = Pi x radius²
Intake VA = 3.1416 x 10.5²
Intake VA = 346.36mm²
Intake port area should be 81-83% of intake valve area so...
Intake Port Area = 346.36 x 0.82
Intake Port Area = 284.04mm²
Now I need to see what size port is needed to achieve that area.
284.04 / 3.1416 = 90.4
√90.4 = 9.51
9.51 x 2 = 19.02mm
So an ideal intake port size would be around 19mm if this is all correct.
Exhaust port area is much more simple, according to Bell. He says that exhaust port area should be 95-100% of the exhaust valve area. I don't need math to figure out that 100% of 19mm (ex. vavle diameter) is 19mm. lol I have seen more modern porting articles that noted that the exhaust port could be slightly larger than the valve. Bell also notes that he does not agree with port matching the exhaust port to the header. He feels that the step is necessary to reduce exhaust gas backflow into the cylinder and believes that port matching here will certainly reduce power, specifically in the midrange.
OK, so go port the intake to 19mm and open the exhaust up to 19mm right? Not that easy. The stock intake port on this head is 21mm, 2mm larger than Bell believes is ideal. The exhaust port is just shy of 19mm, so that could be handled easy enough.
From what I have read and seen, the carburetor should be basically the same size as the intake port. That makes the stock 19mm carburetor nearly ideal (perhaps a slide carb with no butterfly to restrict flow may be a little better?). This also makes a ported and port matched stock intake ideal.
Since the intake port was already 2mm larger than the stock intake, I decided to just port match and use the 150cc stuff and see what happens. This thing might end up being a dog. We'll see.
Knowing all of this, I'd like to see a stock 49cc head setup for a 21mm intake valve, 18-19mm exhaust valve, 19mm ports, 19mm intake, and a good 19mm carburetor. Of course some real port shaping would be cool with that setup too.
I may be way off on all of this, but it makes sense to me anyway.
This setup ran very similar to a ported 50cc stock head with a modified 50cc stock carb. I think with the right work done to increase compression, this could be a very good setup for 70cc+ 139QMB engines.
47mm port matched head vs big valve head.
I had planned to drop the head or cylinder off to the machine shop tomorrow to get a little milling done, but I wanted to do some checking first.
Using the stock 49cc head, I came up with about 0.170" of clearance, so I thought I'd be milling off at least 0.030". This time around I got .055" of clearance on the intake valve and 0.080" on the exhaust valve. That's with no head or base gaskets, so I can assume an additional 0.010"-0.020" of clearance when using gaskets/RTV.
I'm not sure why lol, but I made a quick diagram to show why the big change. The piston is flat on the edges and dishes in in the center of the piston. There are also valve reliefs in the dished area. The new valves miss the reliefs and hit in areas that are less recessed. The red marks are basicaly where the valve edges are in relation to the pistion.
I'm used to .080"/.100" of clearance being proper spec for larger engines, now I'm dealing with roughly .070"/.095". I'm thinking those numbers will be fine for this engine, but I sure don't wanna mill anything off.
I cc'd the combustion chamber just for comparison's sake. This thing is huge compared to the stock and 72cc heads. This head is roughly 7cc while the others were 4.5cc and 5cc. I hope this extra flow really works for me, because it sure won't be a high compression engine.
I did some port matching and clean up work and put the engine back together with the big vavle head, 150cc intake, and 150cc carb.
I didn't really do a lot of work because I have been reading and thinking and I don't think this setup is going to be too great anyway. I think that the valve sizes (21mm In/19mm Ex) are pretty well suited for a strong running little GY6, but the intake track is a bit large. Here's part of why I think this...
A. Graham Bell, very famous for his 2T tuning book, wrote a 4T tuning book. In this book he states that intake port area should be 81 to 83% of the intake valve area. I have a 21mm intake valve...
Valve Area = Pi x radius²
Intake VA = 3.1416 x 10.5²
Intake VA = 346.36mm²
Intake port area should be 81-83% of intake valve area so...
Intake Port Area = 346.36 x 0.82
Intake Port Area = 284.04mm²
Now I need to see what size port is needed to achieve that area.
284.04 / 3.1416 = 90.4
√90.4 = 9.51
9.51 x 2 = 19.02mm
So an ideal intake port size would be around 19mm if this is all correct.
Exhaust port area is much more simple, according to Bell. He says that exhaust port area should be 95-100% of the exhaust valve area. I don't need math to figure out that 100% of 19mm (ex. vavle diameter) is 19mm. lol I have seen more modern porting articles that noted that the exhaust port could be slightly larger than the valve. Bell also notes that he does not agree with port matching the exhaust port to the header. He feels that the step is necessary to reduce exhaust gas backflow into the cylinder and believes that port matching here will certainly reduce power, specifically in the midrange.
OK, so go port the intake to 19mm and open the exhaust up to 19mm right? Not that easy. The stock intake port on this head is 21mm, 2mm larger than Bell believes is ideal. The exhaust port is just shy of 19mm, so that could be handled easy enough.
From what I have read and seen, the carburetor should be basically the same size as the intake port. That makes the stock 19mm carburetor nearly ideal (perhaps a slide carb with no butterfly to restrict flow may be a little better?). This also makes a ported and port matched stock intake ideal.
Since the intake port was already 2mm larger than the stock intake, I decided to just port match and use the 150cc stuff and see what happens. This thing might end up being a dog. We'll see.
Knowing all of this, I'd like to see a stock 49cc head setup for a 21mm intake valve, 18-19mm exhaust valve, 19mm ports, 19mm intake, and a good 19mm carburetor. Of course some real port shaping would be cool with that setup too.
I may be way off on all of this, but it makes sense to me anyway.
This setup ran very similar to a ported 50cc stock head with a modified 50cc stock carb. I think with the right work done to increase compression, this could be a very good setup for 70cc+ 139QMB engines.