Thats what I said a shorter stroke will rev faster, . A 250x with a 80mm piston will be around 288cc , a 300ex is a 282 cc motor . Both are around the same cc's but both will have different power curves . The 250x would have more torque but the 300ex would rev faster . Because Honda stroked the crank instead of installing a larger piston .

ok i think the question of big bore 350X's is one of the most popular. it seems like people always ask how big to go and what kits are out there. while this is not a 100% complete list as there will always be a engine builder doing his own thing. it is rather extensive. first i will give the basic displacements. then i will discuse options and differances

piston...................68mm crank (stock).............74mm (6mm)stroker



81mm (stock)................350.4cc.................... ....381.3cc
81.5mm ........................354.7cc................... .....386cc
82mm............................359.1cc........... .............390.7cc
83mm............................367.9cc........... .............400.3cc
84mm............................376.8cc........... .............410cc
84.5mm.........................381.3cc............ ............414.9cc
85mm............................385.8cc........... .............419.9cc
85.5mm.........................390.4cc............ ............424.8cc
86mm............................394.9cc........... .............429.9cc
87mm............................404.2cc........... .............439.9cc
89mm............................423cc............. ..............460.3cc
89.5mm.........................427.8cc............ ............465.5cc
90mm............................432.5cc........... .............470.7cc
90.93mm.......................441.5cc............. ...........480.5cc
92mm............................452cc............. ..............491.9cc
95mm............................481.9cc........... .............524.5cc


as you can see thier are many different sizes to choose from.

one other thing you may notice is there are two ways to achieve the same displacement. however although they will be the same cc and have the same peak H.P. they would be completely different in there charecteristics. let me explain by using the stock 81mm piston with a 6mm stroker that achieves 381.3cc vs just boring the cylinder for a 84.5mm piston to get the exact same displacement.

the 81mm/74mm bore/stroke combo is a more "square" design. by that i mean the bore and stroke are closer to the same size as the 84.5mm/68mm bigger bore set up. what does all that mean? a squarer motor will produce quicker and more revs and have a broader H.P. range. meaning it will produce more H.P. at the lower RPMs then the 84.5mm bore only motor. this translates into a more usable powerband with the bore/stroke combo. wereas the bore only will have a slower reving powerband.

this is why all the new four strokes have faster reving motors then the older designs. this produces a better "race" motor. before four strokes were mostly trail bikes/quads and a slower reving motor was more desirable for "play" use.

 

My goal is not really to out run a 300ex,thats just what I was trying to use for comparison..

My 250x is jetted, k&n filter and crappy full cobra exhaust.. we also have a 300ex with pipe,jets and filter...

Now the 300ex seems WAY better down low(torque) but my 250x seems to be a little better up top..

The 300ex revs through the power band really fast..

I have 2 250x bikes and just to want to have some fun with one..

 

O.K. I didnt know you had both bikes I thought you were trying to outrun your buddy. You see the difference then. I still think the bigborekit would be slower but you never know . Depending on how much you want to spend a 350x motor is a very fun option. I bought mine for $500 on craigs list . If your 250x motor runs good you can sell it and put the money towards a 350x . Mine will run with a 400ex and out runs most 300ex ,s . I say most because I got outrun last year . But a 350x /250x will be faster than say a 330ex
But I also like the 250x motor they are fun bikes and the 250x has reverse . Good luck with your bike if you buy the bigbore kit post the results and let us know how it rides

 

Thanks for all the advice.. I think I will just take the money I was gonna spend on the big bore and put it into a 350x fund..Now if I can just find a 350x at a good price..

 

Sounds like a good plan. And a fun project. Good luck.

 

The 350x project is a great one. I did this swap three years ago rode it for two and sold it last year. I totally regret selling it. I had the 86 with 6 spd transmission. This thing would walk away from STOCK 400exs (notice I put stock) A custom exhaust, 450r muffler, 400ex carb and a uni filter was all I had on it too. If you need any help with building it, Im sure I can find my old notes on what i did. It was a blast to ride. 350x rear swingarm 300ex frame 400ex front shocks, it was a great heinz 57 quad

 

I got a 1987 Honda TRX250X how do I or what is a cheap way to get more speed

 

I have an 87 honda trx250x that I don't get to go faster but with a cheaper way

 

will 330cc big bore kit fit on 1987 honda 250x

 

I have a 1987 250X and I also have the 330 jet the 3:30 should I have the cylinder will not fit down in the bottom of the jug what is my problem it is too big