2016 - HPI Nitro RS4 3 Evo+ #2 3.4
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My HPI Bullet 3.4 is no longer my most overpowered nitro vehicle.
This is:
Hot on the heels of finishing my RS4 SS build, I walked into my local shop and saw an old Team Schumacher touring car being sold on consignment; it was this big beastly thing (for a 1:10-scale touring car, anyway), with a .28 engine with an enormous heatsink, connected to a 3-speed gearbox that drove a belt-drive drivetrain. The guys at the shop told me the car had long since been discontinued, but when it was being sold, the entire marketing line was "It will do 80mph out of the box!" They also said that it did indeed have the speed of a rocketship, as well as the handling of one -- which is to say, it didn't really corner well, if at all, and upon picking the thing up, it was obvious why -- that .28 engine was HEAVY. They also said it had a tendency to trash the 3-speed gearbox and snap the belts. Amazingly, someone bought the thing (not me, I'm not a masochist), but it got me thinking about the idea of getting a big-block touring car, just for the hell of it.
The obvious choice was the Traxxas Nitro 4-Tec. I considered it for a while, but I kept saying to myself, "Self, do you really want to deal with exposed belt-drive connected to a powerful engine, and do you really want yet another engine you'll need to stock spare parts for?" I only have one Traxxas nitro vehicle, a T-Maxx, but it's the older one with the 2.5R engine, not the 3.3 used in the 4-Tec; since I have no intention of shoehorning a 3.3 into my T-Maxx, with its old-school transmission, my choices would either be to buy some 3.3 spare parts for the 4-Tec's engine, or downgrade the 4-Tec to a 2.5R, at which point it would no longer be a big-block touring car.
But I *did* already have a big-block engine in service -- the Losi 3.4. (you can nitpick about whether it's really a big-block if you want, but it's a .21 engine shoehorned into a small-block form factor, so it's functionally a big-block engine anyway.) That presented a possible solution -- I could put a Losi 3.4 into a Nitro 4-Tec and...wait, no, why get another car that I'd need spare parts for? I already have a nitro touring car platform that could do the job, and I have a lot of experience tuning it at this point: the HPI RS4. This is what I came up with.
The Losi 3.4 engine looks like a nuclear reactor compared to the rest of the chassis, towering high above everything else on the chassis. Its appearance is apropos, because it's a stupidy-powerful engine for a car this size. Remember, this was the engine that originally came in the Losi Ten-T monster-truggy.
Fitting the engine was much easier than it might've otherwise been, thanks to the small-block mounting tabs, but it still had its challenges. For starters, I had to get an adjustable motor mount that would fit the RS4's chassis; the RS4 has adjustable-mount holes drilled into it, but they're intended for use with a special RS4-specific flywheel that has a large forward offset built into it. I didn't want to use that flywheel because it only accommodates two clutch shoes, so I had to track down a discontinued motor mount that HPI used to use on the MT2 stadium truck, which shifts the engine forward to allow a normal flywheel to be used. I eventually found one in Germany, so I bought several to reduce the per-unit shipping cost.
As it turns out, the Losi 3.4 engine is slightly wider than a normal small-block engine (because it isn't really a small-block engine, after all), so I had to modify the motor mount to accommodate the wider engine. That wasn't difficult, it just required a steady hand:
With that done, I was able to mount the engine, and I was able to use the same 3-shoe clutch that I use on my Bullet 3.4, which I knew from experience would hold up to the engine's power. (putting all that extra torque through two shoes riding on two pins seemed like a bad idea. Three shoes riding on three pins is better.)
The next challenge was fitting the exhaust. The Losi 3.4 is a rear-exhaust engine, and true to form, HPI has a rear-exhaust header designed for the RS4, but it's discontinued. (the Losi Ten-T header would fit the engine, but it would stick out WAY too far, and probably rub the rear-right wheel too.) I don't remember which foreign country I found them in, but I did find them, so I bought a couple. When I got them, I discovered the gasket that came with the engine was too large to fit into the exhaust header, and the gasket that came with the exhaust header had to stretch to fit the engine and then became too big to fit as well, so I had to improvise -- I made my own exhaust gasket out of a stack of silicone O-rings:
With three separate seals to keep exhaust from leaking, it works like a charm, and you'd never know by looking at it that it wasn't supposed to be that way:
The third challenge with fitting the engine was the carburetor. The Losi 3.4 has a slide-carb, whereas the other engines in my other RS4s all have rotary carbs (or rather, spiral carbs, but the primary motion is rotation), and the forward-shifted position of the engine put the carb slide on a collision course with the brake assembly. I don't generally like to mount carburetors "crooked", but in this case it was a necessity -- and as it turns out, it also pointed the carb slide directly at the special slide-carb-compatible servo horn that I'd never needed before, so that made setting up the throttle linkage easier.
I say "easier", not "easy", because if you look closely at the picture above, you can see the throttle linkage and the brake linkage come perilously close to each other. The two linkages have to cross paths, and because of the way the throttle and brake servo horns move, I ended up having to carefully bend the brake linkage at specific points so it would arc up-and-over the throttle linkage just enough for the two to clear each other.
The last challenge was the transmission gearing. I had recently fitted a long-discontinued OFNA clutch-type 2-speed gearbox to my RS4 SS, and I briefly considered fitting another one to my RS4 3.4, but I ultimately decided against it; as I've mentioned already, the Losi 3.4 is a stupidly-powerful engine for a vehicle this size -- it's capable of making my Bullet pull rolling wheelies -- so if my RS4 rally car could get away with a single-speed transmission with a .18 engine, then this car *definitely* could. Making the transmission simpler means there's fewer things to fail when the engine is running full-tilt and there's no way to stop the car before it careens out of control.
Having already converted my RS4 rally car to single-speed, I knew how to do it, I just had to figure out what gearing to use. I searched high and low for a Mod 1 clutch bell that had more than 20 teeth, and I found none; my RS4 rally car has a 19-tooth clutch bell, so I knew to make this car able to use the full potential of its oversized engine, I would need to go significantly higher than that. Ultimately I settled on using the stock 2-speed clutch bell, with the 1st-gear position cut off with a Dremel, and the 22-tooth 2nd-gear pinion held in place with Loctite. This also had the benefit of matching the color of many other aluminum parts, including the engine's heatsink. (color coordination is important; don't judge me.)
Then I slid the engine as close to the center gearbox as I could get it, and I figured out that with minor modifications to the adjustable-mount holes, I could move it close enough to use a 39-tooth spur gear. SURPRISE! Those are discontinued too. I think I found some in Norway, or maybe France, I don't remember anymore. A 22/39 gear ratio is 36.6% higher than the 19/46 gear ratio my RS4 rally car uses, and 10.3% higher than the 2nd gear in my RS4 SS, so it should put the Losi 3.4's torque to good use. (this is a truck engine, remember.) So this car has a 4.56:1 final drive ratio, which is approaching the FDRs used by stock-class electric touring cars.
The last thing I did to accommodate the larger engine (not really a challenge, because there was nothing to figure out) was to drill-out the exhaust stinger. I use the stock RS4 tuned-pipe with the tuning cone removed on all my builds, because it works fine for my needs; I'm not racing these things, and I prefer a wide powerband vs. achieving the maximum theoretical power the engine can produce. However, because this engine has significantly more displacement than the .18 engine that came with the car, I decided the exhaust needed to be less restrictive. The stinger has a 3/16" inner diameter (except for a minor manufacturing flaw in this specific pipe, which has since been drilled out anyway), and I widened it to 7/32" -- 16.67% wider, which neatly corresponds to the percentage increase in engine displacement as well. The engine was definitely happier about revving to high RPMs after I did this.
Having fitted the engine, linkages, and exhaust, the rest of the build basically consisted of things I've done before:
- Steel universal-joint center axles
- Steel brake disc from GPM
- Sintered brake pads from the Hot Bodies R10
- A diff locker from the Crawler King inserted into the front diff
- The rear diff filled with silicone grease to give some limited-slip action without leaking because the diff isn't fully sealed
- Tamiya TRF shocks with color-coordinated accents (this time provided by Yeah Racing instead of Max Speed Technology)
- Hitec digital metal-gear servos
- HPI stainless-steel suspension hinge pins and rear toe-block brace (discontinued, of course)
- MIP CVDs (also discontinued)
- A HPI primerless racing fuel tank (yep, you guessed it! it's discontinued!)
- Aluminum shock towers
- HPI titanium turnbuckles (do I even need to tell you they're discontinued?) with Tamiya ball-ends
- A mixture of Traxxas and HexHeads steel hex screws
- Exotek orange aluminum clamping wheel hexes
- Heavy-duty steel drive cups
- HPI X-pattern Pro Compound belted tires (you know what I'm going to say)
- HPI mesh wheels +1mm front / +3mm rear (miraculously, not discontinued yet!)
Finding orange parts was harder than I thought it would be, so I had to settle for some silver-colored parts. However, that worked out well, because a few parts from HPI were only available in purple (like the rear toe-block brace and the single-speed spur gear hub), so I had to de-anodize them so they wouldn't clash with the orange parts. The titanium turnbuckles were also anodized purple, but due to a strange quirk of titanium, the oxide layer that forms the anodizing is impervious to even the most horribly caustic chemicals, so soaking them in drain cleaner wouldn't work like it did for the aluminum parts -- yet the oxide layer is soft enough that it was easily buffed away with a motorized steel brush. (titanium is weird -- extremely useful, but weird.)
So in the end the car ended up with a nice two-tone mix of orange and silver-colored aluminum parts, which complements the two-tone appearance of the engine itself. I think this picture does a nice job of showing the two-tone effect:
So...having done all that work to the chassis, what body did I choose to put on it? In my mind, there was only one body suitable for a high-speed sports car with a truck engine:
Yes, the Dodge Viper. It's the perfect body for a big-block touring car -- from a conceptual point of view, anyway. As far as actually fitting it...well, that was a challenge. The body is low-slung and the hood covers half the length of the chassis.
Remember how tall the Losi 3.4 engine is? Yeah, I had to do some extra trimming.
Not only did I have to cut a hole in the roof for the engine's heatsink to poke out, I also had to cut a hole in the hood so the fuel filler cap wouldn't scrape against the underside of the body shell. Add in the ventilation holes in the front and rear windshields, and this body has twice as many holes that needed careful cutting (as opposed to simply reaming a circular hole) as my other nitro touring car bodies. All that ventilation works wonders, though; the engine barely hits 210°F after several full-throttle passes.
The +1mm front wheels and +3mm rear wheels perfectly fill-out the wheelwells, being neither inset nor sticking out too far. Those X-Pattern Pro Compound belted tires stick really well, too; someday I'll have to switch to the V-Groove Pro Compound non-belted tires, but I have a couple spare sets of the X-Patterns so it'll be a while before I have to downgrade the rubber.
All the work to make the body fit was worth it, though. Between the front air-dam, the side skirts, the rear spoiler, and the hollowed-out rear bumper, this body generates a ton of downforce. You can see it hug the road tighter as the car accelerates. I hope I don't have to replace the body anytime in the foreseeable future, but I don't regret doing all the work to make it fit, because it's a great body for a fast car.
Speaking of which: How fast does it go?
This was the result of a couple tanks of engine-tuning and speed runs along a deserted stretch of roadway in an office park this past Sunday morning, while all the good people of the world were at church. The full-throttle runs were all done on a slight incline, so the car may actually be able to eke out 1-2mph more than this, but 61mph is more than good enough to make me feel like this project was worth the effort. It beats my next-fastest car by 36%, it has no stability control so it's properly nerve-wracking to drive on unswept pavement, and despite having super-high gearing the engine makes so much torque that it's easy to drive at low speed and takes off like a rocket when I mash the throttle. When I was initially testing the chassis with some cheap D-compound tires, the engine would spin all four wheels on takeoff and make the chassis powerslide diagonally.
I suppose it's famous last words to say "I'm done building touring cars now", but if it's any indication how much my hands were shaking -- and how much I was grinning -- by the time I was done speed-testing this car, it will keep my need for speed satisfied for a good long time. And what a wonderful sound it makes in the process.
So, fun facts about what the drivetrain was experiencing at top speed:
Linear speed: 1,636.164m/min
Tire circumference: 21cm
Tire RPM: 7,791.26rpm
Tire radius: 3.34cm
Tire angular acceleration: 2,268.77gees
Final Drive Ratio: 4.56:1
Engine RPM: 35,515.99rpm
Update: I got lucky and managed to track down the parts needed to install a carbon-fiber upper deck sooner than I thought:
I ordered old Axial orange standoff posts from Japan (the only place left I could find them) to match half the aluminum parts on the car; the special posts that the servo mounts to were stripped of their anodizing to match the other half of the aluminum parts on the car:
I also finished fitting the bumper to the body shell, and installed a (probably fake) carbon-fiber bumper brace, using more old Axial orange standoff posts:
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