Reliability is in the eye of the beholder.
Any power increase is going to lower the life of the engine.
I don't know the exact numbers but for example if a stock 45bhp NSR lasts about 20'000kms before requiring a rebuild, a worked 60bhp might do 10'000-15'000kms before requiring a rebuild. Some of us including me consider that an acceptable drop in engine life for the extra power gained.
Raising the compression on an NSR just makes the motor peaky with a loss of midrange and in most cases a loss of peak power.
The NSR does't have the most compression of all the 250's but it does have one of the best designs. _________________ If I have to take the carbs off once more...
Which model do you have? _________________ MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball
If you are using H cylinders with L heads then you're already on HRC SP spec, which is an avgas spec.
We have found volumes that work for unleaded 95 here in Thailand, and it's not that radical. Only if you have a high exhaust port should you consider a decrease in volume.
You'll find that a relatively low comp NSR revs nicely and makes smooth power. Up the comp and although it'll start building midrange at first, you'll then hit a wall where it won't rev.
Tightening the squish works, as does reducing the squish area, which slows down the MSV meaning less chance of the dreaded detonation at higher rpm.
If you're after more power then look at the areas that a production line doesn't take care of such as casting flashing, sharp edges etc.
When it comes down to it, the compression ratio is very much like the ignition timing. It doesn't need to be right on the edge to be good, but on the flip side and erring on the safe side can yield a nicer ride. It's also very easy to go too far and undo all your hard work in a pool of molten aluminium!
Mine's made just under 68hp with standard heads and port timing* on pump 97 unleaded. I've seen NSR's with increased compression fall flat on their asses a number of times... and fail to even make 60hp!
Fiddling with compression isn't something to undertake lightly, and like I said in my first post, there are better ways to reliable power.
*My ports and deck heights are "optimised", and while it ran extremely strong for 3 years, even then it still eventually eroded away the tops of the barrels due to sporadic detonation. (I do tend to run the jetting rather close to the edge much of the time though.) I am confident that had the compression been increased beyond standard, there is no way it would've suffered the abuse it did for 3 years! I've seen bikes with fresh motors and increased compression fail to even finish a track day! _________________ Andy.
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