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Author Topic: 350 cc big bore conversion  (Read 3528 times)

Mansour_Zianour

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350 cc big bore conversion
« on: December 11, 2016, 09:56:47 AM »

I have a question for motorcycle mechanics out there who have enough experience with Hyosung 250. I have 2012 gtr 250 and have increased the displacement to 306ccs. The bike is running flawlessly with a lot more power. I have about 30,000 kilometers on this pistons. I like to increase the displacement to around 350 ccs using Honda CBR600RR(2007 up) which is a perfect match.
 I already know everything about compression height and figuring out everything as far as fuel delivery and so on. I like to know, can the crankshaft, specially the pin handle the extra displacment? I know the bearing have no problem. I am not modifying anything else like bigger valves and stuff.

Update:
The engine is finished with great results. I have about 1000 kilometers on it with no over heating issues. It runs awesome with 344 cc displacement.
I had to do some modifications to CBR's pistons. The top crown had to be machined flat, the valve groves had to be machined since the piston pin is off center and on the Hyosung twin you have to rotate the rear piston and some machining was required since the Hyosung connecting rod is a little wider. The sleeve was removed and a bigger sleeve with lots of boring was replaced for 67mm ID bore. I used  Daelim 250 cc injector with a 10K potentiometer at the MAP sensor. I am also using 50K Potentiometer at the IAP sensor to advance the timing and enriching the fuel a little . If you use all the original Head and below cylinder gasket the compression ratio will be slightly lower than original. I might change the rear sprocket to 41 teeth now since the power is greatly increased. I don't have a lot of pictures. The white clay on the picture is showing the valve indentation. Just to make sure the valve don't hit the piston after machining.




« Last Edit: April 06, 2017, 05:27:49 AM by Mansour_Zianour »
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Re: 350 cc big bore conversion
« Reply #1 on: December 11, 2016, 10:21:11 AM »

Mansour,

As far as I know, you are the first person to try.

I hope you are successful!
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serp

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Re: 350 cc big bore conversion
« Reply #2 on: December 11, 2016, 01:31:42 PM »

I would be more concerned about the effect of heat on the thinner cylinder walls.
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Mansour_Zianour

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Re: 350 cc big bore conversion
« Reply #3 on: December 11, 2016, 03:17:03 PM »

I would be more concerned about the effect of heat on the thinner cylinder walls.

I haven't  bored out the original sleeves. I bored out the cylinders and fit larger sleeves. there is plenty of room there still. According to Dave at LA Sleeves, minimum is 2 mm as long as you don't go higher than 10 to 1 compression ratio. I am running 9.5 to 1, putting out about 34 horsepower and the engine is producing less heat than the stock. I some times run regular gas with no pre ignition issues. With the increased power, I have changed the rear sprocket to smaller for better balance and the top speed is raised to about 195 km. 
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Joss

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Re: 350 cc big bore conversion
« Reply #4 on: December 12, 2016, 11:45:34 AM »

Hey Mansour_Zianour,

I'd be interested to see some pics of the process if you have any?

I was going along the same lines as Serp with wall thickness but I must admit it's not something I've dabbled in much. If LA Sleeves are willing to put their name to it then I'd guess it would be fine.
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garuda

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Re: 350 cc big bore conversion
« Reply #5 on: January 07, 2017, 10:06:18 AM »

Bump for this. I'd love an update.
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albertinhouston

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Re: 350 cc big bore conversion
« Reply #6 on: January 13, 2017, 02:12:57 PM »

give us details about the 306
 upgrade?
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Dojy_biker

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Re: 350 cc big bore conversion
« Reply #7 on: January 14, 2017, 02:46:57 AM »

i would think the rod and bearings would be fine. personally id use slightly thicker oil to prevent "oil squish out" on the bearing surfaces due to more force (the other option is to get slightly higher pressure oil pump, but probably not possible). Thicker oil may affect clutch friction, but with more cc's id be upgrading the clutch springs anyway.
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Death666wish_

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Re: 350 cc big bore conversion
« Reply #8 on: January 14, 2017, 09:51:26 AM »

Hyosung should learn a thing or two from you, instead of their measly 275cc GT300R!
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Mansour_Zianour

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Re: 350 cc big bore conversion
« Reply #9 on: January 16, 2017, 08:04:12 PM »

i would think the rod and bearings would be fine. personally id use slightly thicker oil to prevent "oil squish out" on the bearing surfaces due to more force (the other option is to get slightly higher pressure oil pump, but probably not possible). Thicker oil may affect clutch friction, but with more cc's id be upgrading the clutch springs anyway.

I have done modifications to the oil pumps on cars pumping a lot more horsepower, but that applies to shell type bearings. Hyosung has roller crank bearing and there is no pumping action there. Using more viscous oil will actually slow and limit the flow of oil to the bearings that are not force fed, like rollers. I am sure there will be some starvation at very high RPM's which I am not planning to run for long periods.
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