Displacement vs Compression

[YTZ drew]

OK engine guys, need an opinion on this. I am putting together an '84 200X, and have two top ends on the shelf to choose from. The original 200X with stock compression, or a '79 XR185 jug and its stock 10:1 piston. I will be using the XR cam, 28mm carb, and head (so the compression release can aid my silly 200X kicker in longevity). So what do you think will be worth more power, the 10:1 180cc top end, or the 9:1 200cc top end?

[fabiodriven]

If you find out their exact sizes you can better determine. 200X's are only 192cc's or something, don't quote that, that's just for example. Regardless, 10cc's of difference isn't going to matter but one point of compression might matter slightly.

[prometheus]

The power between stock atc 185 and 200 engines is negligible. The higher compression will be what your looking for. Also the trans will be geared differently than the stock 200 trans. Cams make a huge difference. Webcams makes some nice power building cams You might want to check them out.

[YTZ drew]

Its a 12 cc difference (180 vs 192). The bottom end is a stock 200X with the crank bearings, lower rod bearing, and kicker gears replaced. If I had it to do over, I would build the bottom end using the left half of a 200S crank so I could run a recoil but keep the manual trans, and forget about the 200X's inherent kicker issues. Oh well, next time.

[Dave8338]

I would go with the 185 jug and 10:1 compression. It is my experience that with +- 10cc, you're going to find your best power with the higher compression. Also, it allows for a higher cam profile to be used if you find that you don't have the punch that you are looking for. It will make better use of your 28mm carb, too.

The manual compression release was one of Honda's better ideas...

My .02

[pcs]

doesnt the xr jug have smaller fins on the lower portion of the cylinder, wonder if that would cause heating issues with the added weight of the atc. i would use the 200x jug and run the xr cam and 28mm carb. why not buy a 10.0 compression piston for the 200x jug?

[thcowboy]

If you find out their exact sizes you can better determine. 200X's are only 192cc's or something, don't quote that, that's just for example. Regardless, 10cc's of difference isn't going to matter but one point of compression might matter slightly.

It is 192cc for the auto 200s and manual 200x. In 1986 it turned into a 197cc or smothin like that

[YTZ drew]

doesnt the xr jug have smaller fins on the lower portion of the cylinder, wonder if that would cause heating issues with the added weight of the atc. i would use the 200x jug and run the xr cam and 28mm carb. why not buy a 10.0 compression piston for the 200x jug?
The jug does have smaller fins, but I don't think it will be a huge issue, since the utility models all used the small fin jug, and many of those idled around farms all day for years without any issues. The 200 jug is only 9:1, so I don't want the longer duration cam to bleed off any cylinder pressure and end up at a loss for low end torque. I think at this point I'm going to measure both cylinders really carefully, and use whichever one is tighter. If I go with the 200X jug, I'm going to use an xL185 cam I have, its specs are in between the stock 200X cam and the XR cam. I feel like that will be a better match for the lower compression piston. As far as why I'm not buying a new piston, well, its a budget build. The only reason I opened the engine up was to fix the kicker gears.

[Dave8338]

GREAT thread. I agree with your setup. I know that in the trials bikes, the 200x jug is highly sought after, simply for the larger fins. Guess they think it helps in the over heating game. The smaller fined jugs are great little numbers. Tough and as you mentioned, they've been doing it for the past 30+ years. The XL cam should put you right in the middle for everything. Again... good choices.

Dave