2018 - HPI Nitro RS4 Mini
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Oh look, I built another Nitro RS4.
This isn't the new car, though. ;) This is my Nitro RS4 2, which I built a year ago. It is, however, to blame for me building the new car. To build my Nitro RS4 2, I ended up spending a small fortune on 5 different rollers on eBay, completely disassembling them, washing every single filthy part with dish soap and a toothbrush, inspecting every single part for damage, throwing away half the parts I got because of stripped screw-holes, and finally building a functioning car. After that ordeal, I swore off buying any more vintage HPI stuff.
Those filthy old Nitro RS4 2's came with an assortment of engines in an assortment of conditions, but the engines most easily salvaged were all HPI 15FE's. That engine is a repurposed Toki airplane engine, and its power output was pretty bad, because it was designed to run at full-throttle constantly and didn't need to produce low-RPM torque. (it didn't even come with a two-needle carburetor.) As a result, there are a lot of rebuild parts still available, because the engine doesn't wear itself out very quickly and most people don't want to use them anymore anyway. I enjoy a challenge, however, and after almost a year of occasional tinkering and modification, I improved its performance to the point that it propels my Nitro RS4 2 to a top speed of 55mph with without relying on tall gearing.
That improvement had an unintended consequence. My second Nitro RS4, a RS4 3 SS built in 2015, came with a HPI 12R SS engine which had served me well in that car ever since. I'd modified that engine a bit too, getting that car up to a top speed of 50mph (I like round numbers), but after all the work I'd done on my Nitro RS4 2, the 12R SS engine felt kinda slow and boring by comparison. Not only was its power output lower than the modified 15FE, it was also powering a chassis that is a couple hundred grams heavier. I hadn't driven that car in months and I didn't foresee myself driving it ever again, in its current condition. It had been superseded by the Nitro RS4 2.
Meanwhile, I had a Nitro MT2 that I'd converted to electric last year, but it never really lived up to my expectations. I'd paid RC-Monster.com to design a custom electric motor mount for that truck, though, and I wanted to get my money's worth out of it, so I decided to cannibalize it and use the parts to convert the RS4 3 SS to electric instead. That would make the RS4 3 SS interesting again, but it also meant I'd no longer have any use for the 12R SS engine. My go-to engine for all of my nitro vehicles that don't need BONE-CRUSHING POWERRRRR is the HPI T3.0. I could save the 12R SS to use in the Nitro RS4 2 someday, but I knew I wouldn't; I've figured out how to modify the 15FE to work much better than it was designed to, and I have a pile of spare parts for it too, so it was unlikely I'd ever use the 12R SS again. But I didn't want to get rid of it either. It was time to look for ideas to make use of the 12R SS.
That's where eBay came in handy, as it tends to do. I had a thought that maybe I could find a small (less than 1/10 scale) buggy or something like that to put the engine into, but what popped up in the search results was a HPI Nitro RS4 Mini, ready to go with everything included. I wouldn't have considered it otherwise, because of how much trouble it was to build my Nitro RS4 2, but since the car was 100% assembled, clean, and in good working order I decided to bid on it. I was the only bidder, but in the last 5 seconds of the auction, someone guessed my bid and outbid me by $4.50, and I lost the auction. Now I was angry. Now I was committed to building a Nitro RS4 Mini.
I scoped-out other Nitro RS4 Mini auctions, but they were all pretty nasty-looking. Then I remembered that my local hobby shop had a partial roller a year ago, so I went back to see if the owner (Mimi) still had it. It wasn't on display anymore, but some careful snooping revealed it was still there, in the bottom of a large plastic tub full of old short-course trucks. It was dusty as hell, but it wasn't covered in oil, and the chassis plate was flat. She sold it to me for $75. Then I went back to eBay and bought one of the nasty ones to serve as a parts donor to fill-in the gaps as needed, and I got to work. A few weeks later, I ended up with this:
First of all, let me say I love the body shells made by ABC Hobby. They're as detailed as anything Tamiya has ever made, and in some cases the decals are better. They don't get credit for the red pinstripe running along the perimeter of the bumpers, though; that was all my doing, as I was copying a paint scheme on a real CR-X I found while searching images on Google. I spent about an hour slicing 1mm-wide strips of masking tape and sticking them to the inside of the bumpers, before masking the entire upper half of the body for the two-tone paint job. But the front isn't the best view of this body -- the rear view is:
The rear taillights and rear window are a single, full-wrap decal, and it just brings the rear of this car to life. There are even light buckets for all of the lights on this body -- headlights, taillights, foglights, corner signals, and even side signals -- though at present I don't have any lights installed. I've never installed lights on any of my nitro vehicles, because I don't run them at night anyway due to noise concerns, but this body looks so good I might make an exception. You can't see it from this angle, but the rear windshield decal has defrost-heater lines printed on it, though I cut out the left side of the rear windshield decal to make room for the rear vent hole.
Oh, and that chrome exhaust tip? It's functional.
I'm super proud of how the exhaust turned out. Every build has its challenges, and the biggest challenge with this build was figuring out how to route the exhaust in a way that didn't require cutting a hole in the middle of the rear bumper. I originally got a tuned-pipe exhaust made by Jet Racing, which had the outlet positioned over the rear-right wheel where a silicone deflector tube could route the exhaust out under the lip of the body, but that exhaust didn't work with the chassis in the short-wheelbase configuration. (as with all RS4's, the rear suspension arms are reversible to change the wheelbase length -- but unlike normal 1/10-scale RS4's, it's actually a useful feature on the RS4 Mini, because bodies for "Tamiya M" size cars are available in 210mm or 225mm lengths.) So I was stuck with the ancient airplane-style swirl-chamber exhaust. I took a trip to the auto parts store to look for tubing I could use to make a proper exhaust pipe for this car, and I settled on a package of hard-nylon fuel tubing. It came in two different diameters, which conveniently fit together so I could use the narrower diameter for the main run and the wider diameter for the end pieces. The hanger is just a piece of mild-steel wire from my local hobby shop, bent around my hex drivers to make nice round eyelets at each end. The chrome exhaust tip is actually a tire valve stem protector, which fit onto the larger-diameter fuel tubing with the assistance of a small bit of heatshrink to make it fit snugly. (every Honda Civic needs a chrome exhaust tip, right?)
Speaking of bending things, when I bent the hard-nylon fuel tubing, I wanted to make sure it wouldn't collapse, because I knew I'd have to bend it pretty severely and I didn't want the bends to restrict the exhaust flow. (the exhaust already has a hard enough job just because of that swirl-chamber, which isn't tuned to do anything useful at all.) So I made a mandrel that I could stuff inside the fuel tubing, so when I heated it up using my heat-gun and bent it, the outer surface of the bend would be forced to stretch instead of collapsing. That mandrel is made of a piece of silicone fuel tubing with a narrow spring from the auto parts store stuffed inside of it; the two of them combined provided enough support to force the hard-nylon fuel tubing to maintain a round cross-section as I bent it, and I was still able to pull the mandrel out afterwards because the silicone stretched and got slightly thinner when I pulled on it. And in case you're wondering -- yes, it was incredibly hard to stuff a flexible spring into a flexible silicone tube! I won't do that again, but I only needed to do it once, because the mandrel is reusable forever.
Other than the (literal) contortions I went through to fit a functional and non-ugly exhaust to this car, there's not a lot to say. I took apart two rollers, cleaned everything, replaced all the bushings with ball-bearings, and put it all back together according to the manual. I sprung for a couple upgrades, like OEM universal-joint axles, GPM ball-bearing caster blocks on the front hubs (which I soaked in Drain-O to dissolve the purple coloring, since nothing else on the chassis was purple) as seen above, and an OEM side-belt brace as seen below. Oh, and I also replaced the original shocks with TRF shocks, fitted with silver accents made by Max Speed Technology, but I put those same shocks on all of my touring cars, so it's not really noteworthy at this point. I suppose it's worth mentioning that I mounted the rear shocks further inboard than the manual specifies, because they were WAY too stiff in their original mounting position, even with the soft springs that the Nitro RS4 Mini is equipped with.
As you can see, I stuck with a single-speed transmission on this car. I have parts for a 2-speed transmission, but this car can already hit 40mph, and that's plenty fast for a car this small. Remember, shorter wheelbase equals sharper turns, and I don't want this car to hit an acorn, lose control, and crash into a curb.
The only other thing I can think to mention is the chassis plate itself. If you're not aware, the Nitro RS4 Mini is a truly ancient platform at this point -- it came out in 1997, so this car is old enough to drink. It was made before HPI discovered the color purple, and apparently also before HPI discovered countersunk screw-holes. The original screws were all scraped up from hitting road debris, since they all stuck out below the chassis plate, and I had no interest in fighting with damaged screws, so while the car was in pieces I countersunk every single screw hole on the chassis. (except the engine-mount screws, which need to be able to move side-to-side. For those I just used some flat pan-head screws from Axial Racing; they're wide enough that the hex sockets are protected from impacts against the sides of the screw-heads.)
That's about it, really. It was a pretty straightforward build, compared to some builds I've done anyway. The hardest part was fitting the exhaust, but I feel like I made some pretty damn good lemonade from that batch of lemons. This will be one of my last RC projects for a long time; it's only 2.5 months until D-Day, and I have to start getting ready for that. I have a TRX-4 to finish up, and another project that I honestly might not get done in time. But if this ends up being my last build for the foreseeable future, I think it will be a good high-note to end on.
The baby has been taking up most of my time the past couple months, no surprise, but I finally got around to doing winter maintenance on this car. I had previously modified this engine to work with a resonant exhaust, but since the RS4 Mini can only fit a non-resonant exhaust, the increased exhaust port timing I'd cut into the cylinder sleeve was no longer helpful. So I replaced the piston and sleeve, which were starting to develop a loose fit anyway, and I was more modest with my mods to the new sleeve:
The biggest modification was hollowing-out the portion of the sleeve directly under the main intake port, to produce a channel for the intake charge to travel straight up and into the combustion chamber. There is no channel cut into the engine block for the main intake port; instead, the intake charge is forced to travel the convoluted path of passing through the underside of the piston, out through the hole in the side of the piston, and up-and-around the lip of the piston. This allows the intake charge to cool the piston from the underside, which is helpful in an airplane engine, because airplane engines are tuned very lean to ensure they heat-up sufficiently despite the massive airflow inherent to being installed on a moving aircraft. (the 12R SS was made by Toki, a defunct Japanese RC airplane engine manufacturer.) However, cooling the piston is not especially important for a car engine, because they are tuned richer and aren't running at full-throttle constantly anyway. On the original cylinder sleeve I just cut that portion of the sleeve completely off, but this time I left a thin bridge connecting the two sides so it can resist thermal deformation and avoid pinching the bottom edge of the piston skirt. Is this really necessary? Hell if I know, but it can't hurt.
I also radiused the lower edges of the side intake ports a little, but that's all I did to them; the lower edge of the piston sleeve is already knife-edged for better airflow around the lower edge, and the channels in the engine block connecting the crankcase to the side intake ports are already plenty large enough, so I didn't see any reason to make further modifications to the side intake ports. Likewise, I didn't change the internal dimensions of the exhaust port this time, though I did modify the external dimensions of the exhaust port to port-match it to the exhaust header like how new engines are constructed. Having a lip around the external dimensions of the exhaust port is sometimes useful in 4-stroke engines to restrict backflow into the cylinder, but since 2-stroke engines need backflow to prevent too much intake charge from escaping, I deemed it worthwhile to remove the lip. But again, the internal dimensions of the exhaust port are unmodified, so the internal lip at the bottom of the exhaust port can still prevent excessive outflow during the beginning of the intake cycle to reduce wasted fuel.
I also modified the transmission a bit:
2-speed transmissions use one-way bearings to allow the low gear(s) to freewheel on the transmission shaft when the high gear is engaged. This has a secondary effect of allowing the engine to idle-down while coasting, which changes the handling of the chassis by eliminating engine-braking, which may or may not be useful depending on your driving conditions, but it has two big advantages as well: 1) it eliminates a minor-but-nonzero source of heat buildup, which makes the engine easier to tune, and 2) it reduces the risk of oil-starving the engine when coasting because it isn't being forced to rev at high RPMs without adequate fuel/oil supply from the carburetor. The few 2-stroke automobiles that have existed throughout the years have always employed one-way bearings in their transmissions to allow freewheeling when coasting for these exact reasons.
I didn't want to install a 2-speed transmission in my RS4 Mini because I don't intend for this to be a really fast car, but I did want it to freewheel when coasting, partly for the reasons mentioned above and partly because I find the sound of a nitro engine that is being dragged along by the wheels at high RPM to be very irritating. So, I dug out my stash of OFNA 2-speed transmission parts (the same ones I've used in all my other HPI RS4 builds), flipped the low-gear carrier around backwards, made a little cylindrical shim from a piece of aluminum tubing to extend the portion of the transmission shaft that the OWB rides on, and make a one-speed freewheeling transmission. I had already done this in my RS4 3 Evo rally car, so I knew it would work.
Oh, and one last thing: I bought a set of aluminum supports for the intermediate driveshaft just in front of the engine, scuffed them up with Scotch-Brite and scored the edges with a hobby knife, then soaked them in Liquid Plumr alternating with Tarn-X silver cleaner to strip off the purple anodizing, and installed them on the car. You can see them a little bit in the second picture above. This will support the intermediate driveshaft more securely than the stock plastic part, and also help the chassis plate resist bending. I won't actually be able to drive the car until spring, but it should be much nicer to drive now.
I recently managed to source an aluminum upper-rear brace for this car. I needed to de-anodize it to remove the champagne-colored anodizing so it would match the other aluminum parts, so I decided to de-anodize the engine mounts, engine heatsink, and carb dial too:
The markings on the carb dial were no longer clearly visible after the de-anodizing process, so I took a triangular diamond-file and made new markings.
I'm starting to really like the ghostly whitish-silver appearance of de-anodized aluminum. I can't get the same look on the exhaust unfortunately, because it was never anodized in the first place, so oven-cleaner spray doesn't etch the surface the same way. It just turns the surface a dull gray, and unlike the dull finish left behind after de-anodizing, it can't be brightened again using Tarn-X jewelry cleaner. I'm contemplating painting it black with high-temperature BBQ grill paint, so it will match the engine crankcase instead. Or maybe I'll just leave it as-is, since exhaust headers get really hot and are bound to discolor eventually -- they're generally the ugliest part of any engine that has been run for more than 5 minutes in its life.
In the interest of reducing the number of engines I have to buy spare parts for, I decided to swap the engine in my Nitro RS4 Mini to a HPI 15FE -- the same engine I'm running in my Nitro RS4 2. That reduced the number of ancient HPI engine designs in my RC fleet from 2 to 1, so I could sell all of my 12R SS engines and rebuild kits.
Like my Nitro RS4 2, I modified the 15FE engine to correct some of its deficiencies from the factory. I cut chunks out of the crankshaft to make the counterweight the proper shape to reduce vibration...
...and I lightened the piston a little to further reduce vibration...
...and I cut a notch into the bottom of the cylinder sleeve to make a proper main-transfer-port that doesn't have to pass through the side of the piston, and I reshaped the outside perimeter of the exhaust port to match it to the exhaust header in the crankcase.
Unlike my Nitro RS4 2, however, I didn't change any of the port timings (especially not the exhaust timing, which would make a huge difference on a side-exhaust engine), I didn't smooth the edges of the intake port on the crankshaft, I didn't fill the end of the crankshaft borehole with silicone to make a smooth ramp, and I didn't lower the shim-stack to increase compression. Also I'm running the original single-needle carburetor, which only has a main fuel adjustment, no additional adjustment for low RPM. And of course I'm running the original swirl-chamber exhaust on this car, instead of a tuned resonator exhaust. But despite intentionally hobbling the 15FE by not implementing the same improvements that I already know made a huge improvement for my Nitro RS4 2, the 15FE in my Nitro RS4 Mini still reaches close to 33,000rpm because the car is so small. It's funny to hear an engine that ought to be struggling for air rev so high -- the sound is high-pitched like a racing engine, but without that distinctive clear ringing sound that makes racing engines sound so exciting.
Then, to celebrate, I bought a different .12 engine and installed it in a different car, thus accomplishing nothing to actually reduce the spare parts I need to keep on-hand. ![]("http://i.imgur.com/o85q4F3.gif")
But, at least I finally have a Picco engine in my fleet again.
That's a different topic, though.
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