Mac Racing TRX500R Review:
By: Dan Wade, Contributing Editor

The TRX500R Testing is Complete. The Results are Phenomenal.
Our Findings: What a Brute.

Team Source has completed the testing and review of the Mac Racing Inc, TRX500R. Mac Racing Inc, from Salem Oregon, has created the latest in the escalation of drag racing engines. With their hybrid combination of a Honda TRX250R and a Honda CR500 engine - Mac Racing has set a new standard to gage from. The Mac Racing TRX500R is a 500cc hybrid engine using the standard 250R bottom end assembly that is transformed to accept the enormous CR500 cylinder. The benefits of such a hybrid are many, but first the details of how this engine came about.

Initially, Mac Racing Inc owners, Brent McMillan and Tim Pettit, wished to create the ultimate in drag racing prowess. Mac Racing has been crafting high output engines for many years, but had not yet created a hybrid such as the 500R. The 500R was created with the intention of being the ultimate for drag racing, however along the way it was discovered that the 500R’s available power was suitable for other applications as well.

To create such a hybrid as the 500R it requires extensive welding and machining. The 250R engine cases are disassembled and inspected. The welding process begins as the existing cylinder studs are removed and the holes welded up. The cylinder base and gasket-mating surface is built-up through successive welding passes. After the welding build-up is complete, the cases undergo machining by Mac Racing’s partner Northwest Precision Machine (NWPM). Mike, the machinist at NWPM, takes the rough looking cases, and machines them to precision dimensions to ensure the critical surfaces are within tolerance, as well as providing for a quality finish on the exterior. In applications where the crank’s stroke will be extended more than 4 millimeters, the cases must be “trenched” to provide sufficient clearance for the connecting rod, this operation is performed in conjunction with all other machining done to the cases. In inspecting the exterior of the cases, the craftsmanship and quality are very apparent.

Once the cases have been welded and machined for the larger cylinder, the work begins on completing the cases. New case stud inserts are installed along with new studs. The water pump outlet must be modified to allow for the larger cylinder. Additionally the clutch cable mounting location is moved onto the stator cover to provide clearance for the larger cylinder.

The remaining modifications are traditional processes to crafting high output engines. To begin with, a Hinson clutch basket is required for its stronger design, and then is modified to accept 8 friction plates, providing for sufficient clutch friction area to transfer the power. The flywheel is lightened and re-timed for ignition advance as well. Further, the crank is stroked, in the case of the test engine, the crank was stroked 7mm's to a total 79mm's stroke. A new custom rod is polished to increase its strength, then installed, followed by truing and welding of the crank assembly. No work is required to the cylinder to allow it to fit to the bottom end assembly.

Mac Racing then, relying upon years of experience, begins to methodically perform the porting alterations to net the desired power characteristics. The cylinder head is modified, then cut for o-rings to eliminate any head gasket and to increase the thermal transfer from the cylinder to the head. The cylinder and the cases are matched to provide a seamless transition between the two components. A new Wiseco piston is fitted, the cylinder is bored, honed, and the ports chamfered. The engine is assembled, a leak down test performed, and then installed into the chassis.

The intake side is handled from a Moto Tassarini V-Force reed cage, as designed for the CR500. A custom CNC billet intake and boot are used to mate up the carburetor. The exhaust side is handled currently by a Mac Racing modified FTZ fat in-frame drag pipe. Mac Racing has modified the pipe with a larger diameter stinger to account for the increased exhaust gas volume generated from this massive engine, also to accommodate the different outlet angle on the exhaust spigot the head pipe must be modified as well. Mac Racing has also created a custom silencer, due to Oregon Dunes National Recreation Area requirements; all ATV’s must meet a sound check of fewer than 93 decibels. While Team Source did not actually perform a sound check on the silencer, the sound levels were greatly reduced given that this was a big bore high output engine.

In using the 250R cases several chief benefits are presented. First, the chassis need not be modified to accept a different configuration. The engine bolts right up. Further, with the 250R having been originally designed with a counter-balancer system, the vibrations created by this massive engine is greatly reduced. Team Source compares the vibration to that similar of the larger Pro-X and Polonda engines (350-400cc). Mac Racing is in work developing and incorporating innovative crank modifications to further reduce vibrations, and in doing so will also free up some available power in reduction of rotating mass. Finally of the chief benefits is a 6-speed transmission to provide for a wide range of gear selection.

As tested the cooling system was that of a stock unmodified 250R system. During all of our test rides, never did we experience any over-heating conditions. With plans to go to methanol, the engine will run even cooler, making cooling firmly a non-issue.

Riding The 500R Beast

Team Source met up with Mac Racing in Florence Oregon for some initial testing. Thankfully the weather was nice for the Oregon Coast. As later and most recent testing sessions shows, the weather can get down right nasty. Mac Racing had worked diligently to get ready and had received continued help from Bill Stewart at Shorty’s Part Warehouse with getting parts and accessories. Mac Racing would like to thank Bill for all his help with this project and everything else.

Starting the 500R can be difficult when cold. Even with spot-on tuning and good technique, the fact remains that you are kicking over a +7mm stroke big bore monster. Mac Racing is working on different ignition systems with initial ignition retard to aid starting. Additionally this engine had moderate head modifications resulting in higher cranking compression leading to more difficulty in getting an effective kick in to start the engine. Mac Racing knows this and is working on improving it near-term.

We headed out to the dunes, after getting out to the sand flats and having performed a good warm-up of the engine, some test passes were in order. We made a couple of passes up and down the strip to begin to get a feel for the shift points. Initially we noted how wide the power spread was. Literally the power comes on at idle and runs well up to and over peak RPM. This engine behaved expectantly as a big bore would, but was able to rev surprisingly well.

We lined up to make some solo drag race passes. BRUTAL. The ability of the 500R to launch out of the hole was simply impressive. Keep in mind this engine has what would be considered “recreational” mild porting alterations: we were able to easily pull very hard and fast launches in 2nd gear, with a 275 pound rider, 22X11-8 10 paddle Ultra-lite Haulers, and 15-38 gearing (that’s TWO teeth UP from the stock 13-38) in WET sand (Launch Video). A 10 over swing-arm was barely enough to keep the front end from pointing skyward. The power of the 500R is mind-bending. It truly is not only a brute, but at the same time it is very rider friendly. Literally lug it in any gear, wick the throttle and it was able to torque its way out and rev up and out. 4th and 5th gear wheelies with a 10 over swing-arm only require a blip of the throttle to get it pointed skyward. While making hard fast wide open passes, you can feel the quad torque-steer, and requires differential air pressure to keep it going straight, this is a testament to the massive power this engine is creating!

After getting a good feel of the engines personality, we lined it up against several other very respectable drag race bikes. The results were equally impressive, we were able to out pull, out launch, out run them all. The 500R truly owned the drag strip that day. This all from an engine that had “recreational” porting and an experimental drag pipe.

We wrapped up testing for the day and headed home. We had made plans with Mac Racing to meet up again for continued testing, as they wanted to have the engine tested with the modified FTZ fat in-frame drag pipe. Some coordination finally resulted in us meeting up again with Mac Racing at Sand Lake Dunes, near Tillamook Oregon.

This time the weather was NOT as cooperative. Heavy rains and gusts to 40-50 mph made for quite an interesting and WET day. During the time since our last test ride, Mac Racing had their good friend and customer Marc Hernandez, from Salem Oregon; do up a very nice custom paint job on the 500R. Quite a change from the plain white Fullbore fenders from the previous test ride. Marc can be reached at 503.365.7828, and provides full service custom painting.

Please note that in several of the pictures shown there are raindrops on the camera lens making blotchy spots on them.

The weather relented slightly, and we had Brent make a couple passes for us before taking over the controls. Again the 500R really demonstrated how hard this engine pulls. From the previous test ride with the experimental (and not very well suited) drag pipe, to this test ride with the modified FTZ fat in-frame drag pipe, was a noted difference. The power spread became even wider and amazingly more potent. Again it launched super hard and fast, and the speeds it ran at were no less than impressive.

In summary, Mac Racing Inc, has truly created a very potent, yet very ride-able brute of an engine. The engine does have its drawbacks, however Mac Racing is diligently working on resolving the issues involved with starting and vibration, and do have several promising solutions to them. Price wise this engine is within the current line of bigger bore 250R based engines, though on the upper end of that price range. However the performance potential is much greater than that of the available set-ups on the market making the additional cost acceptable. The fact that the engine that we test rode was in a very mild state of tune with only minimal porting and head alterations only show that this engine is and will become even faster and more potent. That is truly mind-blowing as to how fast and how hard this engine pulls already. With the addition of more aggressive porting and head alterations and perhaps the addition of methanol fuel the 500R will truly become a monster. To date this is the most potent and fastest engine that Team Source has ridden, a testament to Mac Racing’s innovation and skills in crafting high output engines.

Mac Racing
5137 Salem-Dallas Highway
Salem, OR 97304
Phone: 503.390.0094
Fax: 503.391.5512