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 much better combustion efficiency.
We determined that the 790 motor should have a true 9.5:1 compression which means it will tolerate lower octane gasoline and will run a little leaner with no detonation problems. Note: stock engines we have measured actually came in a 8.7:1 compression. That's a long way from the factory specification so a true CR of 9.5 is a nice boost while still remaining well in the safe zone. The dished piston dome allows a big increase in valve to piston clearance.
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. 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, then bore the resleeved cylinder to fit the new piston.
All in all, the installation is very straight forward and not 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 remove the motor from the frame, 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. Check out the dyno comparison below.
There are a few questions that we get fairly regularly about our 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 stock motor.
The next question on the 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 790 piston is within a few grams of the stock piston. There is no noticeable increase in vibration with the 790 kit over the stock motor.
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. The test mule has been running the big bore for several years and well over 30,000 miles. It has done close to 100 dyno runs and is still going strong.
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 790 kit. Because the increased displacement pulls air through the carburetor faster, it also pulls more fuel through, balancing out both. You may even 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 stock to the 725 and to 790.
Click here or call now to get one of the 790 Big Bore kits.