Of all the modifications we have made to the DR motor, we get some of our best results from bumping up the displacement. As the saying goes, ‘there is no replacement for displacement’. Our first big-bore kit took the displacement from the stock 650cc to 725cc. We really liked that motor and it got us to thinking, if a 725 was that good, what would an even bigger motor be like? So back in July of 2010 we began work on an even larger displacement kit, the big 780. For 2012 it's grown a little bit more to 790cc.
The first question was how big do we go? Our first goal was to make sure that the motor continued to be totally reliable. To keep the sleeve thick enough to remove any durability issues, we figured that a bore of approximately 110mm was what we wanted.
We checked with CP Pistons and they had a 110mm piston blank already engineered and ready to go. All they had to do was revise the final machining to match the specifications and revisions we wanted for the DR and they were ready. This piston has several benefits over the piston in our 725cc kit. The first is that the squish area has been increased allowing for additional piston to valve clearance. As long as the cam timing is correct, there will not be an issue with valve to piston clearance, you should not have to clay your motor during assembly to verify clearances. This will make the assembly process much easier.
Next, the 790 motor will also have 9.5:1 compression which means it will tolerate lower octane gasoline and will run a little leaner with no detonation problems.
With a big piston in hand, we called up our favorite sleeve manufacturer, Northwest Sleeve, and they made us a new custom ductile iron sleeve. You can’t just bore out the stock sleeve to accept the big piston in this kit (or the 725 kit for that matter). Once you have your motor disassembled, you need to take your current cylinder to a shop that can machine the cylinder and drop in the new sleeve. We strongly recommend Northwest Sleeve for this work. They do a great job and turnaround is quick. Visit their website at: http://www.nwsleeve.com/ or give them a call at 877-753-3830 to arrange everything.
All in all, the installation is very straight forward and not that difficult. You are only required to remove the top end, have the new sleeve installed and then put it all back together! Figure a couple of hours to tear it down and a couple more to get it back together. There is no need to split the cases or do any machining to the cases.
Now you probably want to know how well it works. First of all, we wouldn’t sell it if it didn’t meet our expectations, and it does! We have had the kit in our shop bike for awhile and got in a few nice rides before the cold weather invaded, limiting ride time. The extra displacement of the 790 kit allows the big DR to pull smoother down lower than the 725 kit or the stock 650 motor. Do you need any stumps pulled on your property?
Poor weather was no reason to stop running the bike though – that is what they make dyno’s for! This beast has withstood many full-on, wide-open dyno runs with nary a whimper. Never a cough, puff or hiccup. We have abused it about as much as possible short of a full-blown run down the Bonneville salt flats! Click on the images below to see the results of one of those runs compared to an uncorked DR650 and a DR725.
There are a few questions that we get fairly regularly about our 725 big-bore kit, so let us address those right now and get them out of the way. First thing that riders want to know is if the big-bore kit makes the motor run hotter? And the answer is…NO! We have a Trail Tech Vapor unit installed on our shop bike with the cylinder head temperature sensor installed under the outside spark plug and we didn’t see any increase in temperature readings over the 725 kit which also ran temperatures equal to the stock motor.
The next question on the ‘enquiring minds want to know’ top 40 list is: does the bigger piston throw the internal balance of the motor off? No it doesn’t. In fact, the weight of the new 780 piston is within a few grams of the stock piston. There is absolutely no noticeable increase in vibration with the 780 kit over the stock motor. Besides, it is tough to feel any vibration at all when the front wheel is in the air all the time! Just kidding – sort of.
Bottom end durability is another question that many folks want to know about. Again, this is not an issue. The bottom end of the DR650 is real similar to an anvil in durability. It just goes on and on and on and …
The last of our regular questions is about jetting. Most likely you will not have to make significant changes to your jetting after you install the 780 kit. Because the increased displacement pulls air through the carburetor faster, it also pulls more fuel through, balancing out both. You may have to drop one size on the main jet and perhaps drop the needle one clip position, but that should be it. Remember every motor is different, and yours may require further jetting modifications, but that is all we did when going from the 725 to 780 kit.
Click here or call now to get one of the 780 kits.