Hi all,

I am looking to rebuild my ds engine and add some modifications and I have a few questions:
1. If I want to go to a higher compression, what is the limit a guy can go to on pump premium (91-92 octane)?
2. What weaknesses, if any, should a guy look to fix while he's at it. I have not had any internal engine failures to this point, but if I am going to split the cases, I would like to take care of any inherent issue while I'm in there. I do have a problem with a false neutral between 4th and 5th gear I need to address.
3. I am looking to replace main bearings, tranny bearings, timing chain, timing chain tensioner, gaskets, seals, possibly valves and springs (upgraded), clutch and springs. Is there anything else a guy should look at?
4. I want to add a little extra power, try to reach the 50 horsepower mark. What do you suggest for reaching this goal, keeping in mind the pump gas limitation? I also do not want a drag motor, more for hard trail riding and endurance racing, durability being 90-95% of a stock motor. I also want to limit the extra fuel consumption, so I'm not sure what size of carb to go to. Also, cams, pipe, ported intake were on my list.

My thoughts were: 11:1 piston, cams such as hr2 or hr3, ported intake, Lectron 46hv or Mikuni 45, and either a Top Gun, Big Gun, or HMF slip on pipe. Would this accomplish my goal of reliable 50hp?

 

I bet that would accomplish a goal of having 50 to 55 reliable hp.

 

I am not sure exactly what the top limit is to run 92/93 octane but better to be a bit low than have to be splashing race fuel in to keep the ping/knock down. I think 11.1:1 is around what can be run safely - but ill let someone else say exactly.

If you got a larger carb, pipe (I would reccomend HMF for its price but top gun for its performance) ported intake as well as cams/piston you should pretty easily be above 50hp probably closer to 55 depending on what combination you end up with (cams/piston/pipe).

 

91 is what I'm planning for in a Rotax street bike, BMW F650 same engine as DS650.

To be absolutely safe from hurting my invested dollars, I am thinking 10.5:1. 11:1 seems ok, but marginally ok from previous octane ratings I've seen at 93 (link below - Engine Parts/Pistons ). But I want 91 any elevation and any condition. So yes you probably could run 91 on 11:1 when you absolutely had to, not all out; but I'm thinking maybe running 89 on 10.5:1 when I absolutely have to, not all out.

Look at Engine Parts/Pistons for ratings on link below for high quality 'forged piston'. Assume stock cast AL stock piston would be less robust and be between stock 9.7:1 and 10.5:1, but not sure and would stick with stock 9.7:1 for stock cast AL piston otherwise. These are only ratings I've found on web so far, but should be provided by piston manufacturer.

http://www.hpresearch.net/


Good luck!

 

No one has mentioned shift forks, but since you are getting into the case anyway check them out. The false shift between 4th and 5th could be a result of this and if you are in there you might want to have a look into replacing them.