Secrets of Speed: Don Holmboe Interview

Meet engine builder Don Holmboe of Competition Motor Corp. For the past 30 years this man from Tigard, Oregon has been tinkering with motors of all shapes and sizes, and that has made him one of the most-respected tuners in the world of karting. He's also the man hand-picked by Rod Falkner of Dumonde Tech as the primary engine builder for our assault at the famous Bonneville Salt Flats.

Together, we transformed a Honda CR125R into a machine capable of going twice the speed limit in most states. We're surpassed 115mph, which was a challenge considering we encountered temperatures ranging from 60-112 degrees, strong crosswinds and several rain delays. Oh yea, we also ran at a power-robbing adjusted altitude of 7300 feet!

Holmboe and Falkner spent the better part of four months developing the CR125R engine for this unique application. They have the thing running at 13,000 rpm, and have used a wide range of parts from various Honda machines. It has parts from '99 motocross and new road race bikes and even a turbo charger that Holmboe helped us debut although it was not his project.

KF: How did you get involved in engine building?
DH: I started racing karts in the late '60s. My brother actually got me started, and like most people, I was addicted real soon. I started building engines in 1967. I was a little mechanically inclined and as a kid I started playing around with my dad's lawnmower. In the '60s, everything was pretty much modified. I had to learn by trial and error, but overtime I got to be pretty good at it. Then in 1987, opened up my own shop and we do a couple hundred motors a year.

KF: What's been the most-challenging aspect with this land speed record project?
DH: Trying to make a CR as fast as an RS (purpose-built Honda road race engine). The CR isn't designed for this type of application. The RS is designed for 14,000 rpm and it has stronger cases and stronger components. We've been able to get 13,000 rpm out of the CR, and that's stretching things quite a bit. We have to make our power through torque rather than rpm, and that can be really difficult. Remember, we're chasing streetbike records with a dirt bike and we're proud of what we accomplished.

KF: What special challenges does Bonneville pose for an engine builder?
DH: Well, I've really never run under those conditions before now. The altitude and the heat were two really big factors. They say you lose three percent of your horsepower for every 1000 feet in elevation you gain. That means we lost about 20 percent or more! That's a big loss, but our testing at El Mirage produced some very promising numbers. There were a lot of unknowns with a project like this. I've tried to make comparisons between my karting experience and this project, but they are too different.

KF: What's the biggest change you made to the motor?
DH: Everything. We've studied everything from TM motors to an RS street bike. We've played with all sorts of combinations and have spent a lot of time on the dyno. This was and will continue to be a really big challenge. If I had to narrow the item with the single biggest change . . . well, it would have to be the RLV pipe. We've worked on all types of different designs, and the challenge is wrapping it around the motorcycle. We have all sorts of space limitations, but we've come up with some pretty good stuff.

KF: What do you think about Rod Falkner's idea of putting a turbo on a stock '04 Honda engine?
DH: (laughs) That was tough, but I knew if anyone could do it, it would be him. Crankcase pressure poses a lot of difficulties and there are a lot of competing interests. The turbo was his baby, but he got some really good speeds out of that. It impressed a lot of people, but there's still a lot of development work that needs to be done.

KF: What have you learned form this project?
DH: I've learned a lot. We've added a lot of material to the head to reshape it like the RS, and the exhaust pipe in this project really was everything. Horsepower is made and lost in the pipe, and RLV has done a great job for us. Several years ago I started building TM engines, and then I heard that Honda motors were putting up some really big horsepower motors in our sport. I played with a lot of things, and that's when I realized that the pipe was the key to making a Honda run. I even took the Honda pipe, modified the connection to the cylinder, and than put that on the TM, and there was a huge horsepower increase.
For this project, we've had to work with a lot of pipe configurations, and the most difficult part is figuring out the bends for the motorcycle. We're used to karts, and there are different space issues. In engine building, we actually figure out the cylinder and headwork first, then build the pipe, then figure out ignition timing. Only then do we determine jetting. Tuning is the real key for this project. Unfortunately, we had a lot of unusual weather. We also got less than a third of the runs we expected. Working with three bikes, this made it extremely difficult to tune, but we learned a lot. I'm confident our bikes can beat the records, and we're already looking at coming back to the World Finals in October.

KF: Are the goals realistic?
DH: Yes. Now that I've experienced the unique qualities of Bonneville, we should be able to go at least 120 mph.

KF: What's the future for 125cc two-strokes?
DH: It's all up to the manufacturers. I know they can build cleaner-burning two-strokes, but I don't think that will affect things that much. There are a lot of people out there who want four-strokes, but there are a lot of people who still want two-strokes . . . all of that is too hard to predict.

KF: How does performance affect durability on the 125?
DH: We're pushing these engines to the max. They weren't designed for this type of horsepower and speed. Even with the increase in power and RPM we're able to get about a year out of the lower-end and we're getting some really big horsepower numbers. Overall, these things are pretty good!