2021 - HPI Nitro Bullet MT 4-Stroke
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Last year I finally finished a build I started 3 years earlier, resulting in this truck:
While I was proud to have gotten it to work, the truck was just too heavy for the OS 4-stroke engine I put in it -- even with a massive 6.5cc displacement, it actually produces power more comparable to a 3.0cc 2-stroke engine, which is just not enough for the big heavy Traxxas Revo. You might be inclined to say the FS-40's monster torque makes up for that, but not really -- the monster torque gets cancelled-out by the taller gearing needed for a slower-running 4-stroke engine to run the truck at an acceptable speed. So, before I managed to destroy the engine, I decided to build a new, smaller truck for it to power.
Yep, another HPI Bullet. This makes my third Bullet in operation, all of which are nitro. (hmm. should I get a brushless Bullet while I still can? but I digress.) This allows a very direct comparison between the 3.0cc 2-stroke engine in my other Bullet MT, and the 6.5cc 4-stroke engine in this truck. The 3.0cc 2-stroke engine in my other Bullet MT uses a 12-tooth pinion gear, while the 6.5cc 4-stroke engine in this truck needs a 20-tooth pinion gear to achieve the same top speed. The 4-stroke engine needs a pinion gear 67% larger to compensate for its lower RPMs. The FS-40's monster torque vanishes in a puff of mathematics.
I still like the engine, though. It has a very unique sound, and it's extremely rare too. That's why I kept it and found a more suitable truck to put it in. I had also done a bunch of work to the engine to adapt it for use in a truck, because OS never finished converting it from an airplane engine. (modification details here.) I was in too deep to sell this engine without trying again.
Given the Bullet's smaller size, I anticipated severe difficulties fitting the enormous OS FS-40 into it, and indeed it would've been impossible -- BUT I won the eBay lottery and found a vintage Kyosho right-angle carburetor mount, direct from Japan, that made the whole project possible.
Technically the right-angle adapter was made for the smaller FS-26 engine, which was marketed as a touring car engine, but luckily the bore inside the adapter matches the bore inside the FS-40's intake port, so it works without restricting intake airflow in any meaningful way. In the picture above you can see how the right-angle adapter makes it possible for the carburetor to just barely clear the rear shock tower. You can also see some orange spacers that I used to move the rear mounting posts forward on the shock tower by 3mm, to improve fitment of the stock Bullet MT body, which comes from the factory with slightly-misplaced rear body-post holes. I have since applied that fix to my other Bullet MT, but again, I digress.
With the carburetor situation taken care of, there were 4 other problems to deal with: engine mounting, center gearbox positioning, throttle linkage connection, and exhaust routing. Much to my surprise, they were all pretty easy to resolve. The engine's extra length was within the limits of what I could compensate for by flipping the center gearbox around "backwards", and my prior experience making an engine mounting plate for the Traxxas Revo came in handy again on this truck. (I have also made an engine mounting plate for yet another RC, but I haven't finished that project yet; suffice it to say I'm glad I finally gave it a try and figured out how to make engine mounting plates.)
I did have to enlarge the spur-gear clearance cutout in the chassis plate, extending it forwards to allow the center transmission to mount "backwards" without needing to be lifted-up on spacers. Technically that was the last thing I did to finish this build, but if I'd planned the build it would've been done at this point in the process, so now is when I'm going to talk about it. 😋
With the engine mounted, the two remaining problems were exhaust routing and throttle linkage connection. The FS-40's exhaust port is way up on the top of the engine, and not even remotely similar to 2-stroke exhaust ports, and unlike on the Traxxas Revo build I didn't have very much room to work with, because the exhaust port is also right next to the body shell. There was no way around it; this time I was going to have to make a custom exhaust header. So I bought some 11mm copper pipe on Amazon, a thread-cutting die, and a pipe bender, and came up with this:
Is it perfect? No, but it works and it doesn't look terrible. Good enough; problem solved.
That just left the throttle linkage to sort out. Much like the exhaust port, the intake port on the FS-40 is in a totally different position than it is on any RC 2-stroke engine, and the right-angle carburetor mount could only partially solve that problem. I examined my other RCs for inspiration, and when I looked at my T-Maxx, I knew I'd found the solution:
Here's a little video of the linkage in motion, because it's a little complex and hard to appreciate in a 2D still photograph:
Of everything I did to make this build work, the throttle linkage is the part I'm the happiest about. Despite using parts from HPI, Traxxas, and Losi to make this linkage, it somehow looks like it was built this way from the factory.
Let's see...other than the engine design, this truck is built almost identically to my first Bullet MT, because over the years I got that one dialed-in exactly the way I want it. Why change what works, right? The stock brake pads are replaced with sintered brake pads from a Hot Bodies buggy, the brake linkage has a stiff spring inserted in place of the stock rubber tubing for stronger and more progressive braking action, the entire brake assembly was relocated to the front of the gearbox so it wouldn't wiggle and loosen the drive cup mounting screws by transferring braking force all the way from the rear of the gearbox, the plastic bulkhead braces were replaced with factory-upgrade aluminum braces, the plastic shocks were replaced with factory-upgrade aluminum shocks that are damn near impossible to find anymore, and the suspension arms were modified with extra shock-mounting screw holes further inboard to increase suspension travel:
Oh, and the center driveshaft dogbones have longer pins installed into the center-facing ends, to make use of the extra material on the gearbox drive-cups. The gearbox drive cups aren't hardened, so they wear-out fast, but having longer pins that contact more of the drive-cup material should significantly improve that problem in the future:
I guess that's all there is to say about this build. It's a variation on a theme I'd already worked-out, but the very different engine made for some unique challenges that fortunately were all cleanly resolved.
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