rogson
9th March 2007, 16:40
I have two VX800s. I also have a spare motor from a wrecked bike. The VXs are great bikes, but I think they could be better - and I've got all this VX stuff to play with. So, what about a big-bore VX?
The cylinders have steel liners with a bore of 83mm. I have looked at the barrel and also inspected a cross-section of it in the shop manual, and I don't think it will be possible to remove and replace the sleeve with a bigger one. So, to keep it (relatively) simple I thought I would try just getting the maximum capacity out of the existing liner.
The OD of the bottom (and smallest) portion of the liner that protudes into the crankcase is 90.7mm. I have contacted a shop that does overbores/resleeving, etc., who tell me it is safe to bore-out to a minimum wall-thickness of 2mm. Doing this would give me a bore diameter of 86.7mm. The stock motor has a capacity of 805cc, so the new capacity would be 878cc - about a 9% increase.
I have contacted a pistion manufacturer in the US who will supply custom made (forged) pistons and ring sets.
I have also thought about increasing the compression ratio when I do this, but the stock compression ratio is 10:1 and I have read that going much higher than this runs the risk of detonation occuring when using regular fuel - which I want to continue to do.
I'm pretty sure the air-box and carbs can handle an extra 9% of air/fuel (some rejetting may be required) without restricting the flow, and same with the exhaust. So, I'm hoping I can get close to a 9% increase in torque and power throughout the rev range.
So, are there are any pitfalls to doing an overbore job as planned (apart from the cost which I estimate will be about $1000 if I do all the disassembly/assembly work myself)?
The cylinders have steel liners with a bore of 83mm. I have looked at the barrel and also inspected a cross-section of it in the shop manual, and I don't think it will be possible to remove and replace the sleeve with a bigger one. So, to keep it (relatively) simple I thought I would try just getting the maximum capacity out of the existing liner.
The OD of the bottom (and smallest) portion of the liner that protudes into the crankcase is 90.7mm. I have contacted a shop that does overbores/resleeving, etc., who tell me it is safe to bore-out to a minimum wall-thickness of 2mm. Doing this would give me a bore diameter of 86.7mm. The stock motor has a capacity of 805cc, so the new capacity would be 878cc - about a 9% increase.
I have contacted a pistion manufacturer in the US who will supply custom made (forged) pistons and ring sets.
I have also thought about increasing the compression ratio when I do this, but the stock compression ratio is 10:1 and I have read that going much higher than this runs the risk of detonation occuring when using regular fuel - which I want to continue to do.
I'm pretty sure the air-box and carbs can handle an extra 9% of air/fuel (some rejetting may be required) without restricting the flow, and same with the exhaust. So, I'm hoping I can get close to a 9% increase in torque and power throughout the rev range.
So, are there are any pitfalls to doing an overbore job as planned (apart from the cost which I estimate will be about $1000 if I do all the disassembly/assembly work myself)?