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Build Complete: Traxxas Revo 4-Stroke

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Build Complete: Traxxas Revo 4-Stroke

Old 03-25-2020, 02:57 PM
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Default Build Complete: Traxxas Revo 4-Stroke

Man, raising a newborn sure takes up a lot of your time. I bought this truck in early 2018, months before my son was born, and I only just now finished the build.



On the bright side, the year and a half that the truck sat on my RC shelf in various states of disassembly gave me lots of time to think about how to complete this build; I might've abandoned the project if I'd had more time to fret about the difficulties I encountered along the way. And there were plenty of difficulties, mainly "how do I fit an engine 50% larger than the stock engine, with double the displacement, into this truck???" Well, to start with, the engine itself required modification:



For reasons that I will probably never understand, O.S. decided to release a truck engine with an 8mm crankshaft instead of spending a couple minutes per engine to grind-down the crankshaft diameter to 7mm so it could fit a standard flywheel collet. It's not like they don't know the standard diameter for truck engines is 7mm, and it's not like they don't have the equipment do the job; the O.S. 49-PI Wankel engine has a 7mm crankshaft and it's not even a truck engine, it's an airplane engine. So, as shown above, I sent the crankshaft to a machinist to have him grind-down the crankshaft to 7mm in the area where the flywheel collet fits. The machinist in question has charged me for several other jobs I've sent to him in the past, but he did this one for free because it was so quick and easy -- if nothing else makes you question O.S.' judgement in releasing this engine without adjusting the crankshaft for truck use, this fact alone should. But whatever, it's done and it works. I installed a BuKu clutch with a brass flywheel, heavy Aramid shoes, and normal-stiffness springs; this combination gives the flywheel extra mass to keep the engine from stalling at its low idle speed, the heavy shoes combined with the normal-stiffness springs ensure the clutch will engage at lower RPM than a normal 2-stroke clutch, and the BuKu clutch's adjustability means that I can dial-in the clutch engagement without needing to buy a ton of different springs and wasting a ton of time removing the engine over-and-over to swap the springs to find the right combination.

I also modified the pullstart shaft by cutting a matching notch into it on the opposite side from the factory-machined notch -- a trick I learned from a much cheaper engine to make sure the pullstart shaft is balanced when spinning so it doesn't wear-out the backplate prematurely. And I modified the backplate to have the pullstart shaft spin inside a pair of 8x12mm ball bearings, with just a small slice of the original Oilite bushing on the inside to form a seal between the backplate and the pullstart shaft. This is something I've done with other engines that have removable Oilite bushings in their pullstart backplates, and I haven't regretted it yet. I probably could've skipped the Oilite bushing completely on this engine, because it's a 4-stroke so air leaks into the crankcase are irrelevant, but it never hurts to be thorough.

Then I had to deal with actually mounting the engine to the truck:



There was no way I was going to be able to use the stock engine mount, because that's designed for small-block engines, and the O.S. FS-40S-CX is larger than a big-block engine -- even Traxxas' big-block engine mount for the Revo didn't come close to fitting. This was another problem that caused me to let the truck sit on the shelf for months while I raised my baby and occasionally looked over at the truck to think about it. Eventually I realized the supports for the stock engine-mount could be slightly modified to allow a flat plate to attach to them, by cutting off a couple protruding bits of metal used to secure the stock engine-mount to the supports. Then I bought a flat piece of aluminum, eyeballed the basic size and shape the plate would need to be, traced some lines on it with a Sharpie, and took it outside to attack it with my handheld circular saw. The blade I used was a "ripper" blade designed for rough-cutting lumber, but as long as I moved slowly it worked well enough on the aluminum too. Then I spent a few hours cleaning-up the edges with files and emery paper, marked where the holes needed to be, drilled and countersunk them, and attached the engine with a set of standoff posts that I got on eBay. I'd prefer to have solid aluminum engine-mounts attaching the engine to the baseplate, but the standoff posts are doing a good job; maybe someday I'll pay that same machinist to make a pair of solid engine-mounts for me.

After installing the engine, the truck went back on the shelf, as I'd immediately encountered the next problem: How to get the body to fit? This engine has the carburetor dangling off the back, and even after I rotated the carburetor mount so the carburetor would be tilted slightly down instead of up, the engine still stands much taller than any engine the body shell was designed to accommodate. My initial approach of cutting a larger hole in the body quickly proved inadequate; I'd have to cut a truly enormous hole that would cause everything including the carburetor to stick out unprotected. More months passed.



Then one day I happened to be looking at my Summit, and I started contemplating putting a Summit body on my Revo. So I took the bumpers, body posts, and body off my Summit and installed them on my Revo as a test. The Summit body looked terrible with the Revo's smaller wheels and tires, but I really didn't want to upsize the Revo into a "Nitro Summit", because the Revo is huge already. So I took the Summit body off the Revo again, and then I had a thought: Maybe I could use the Summit's body posts and bumpers to raise the Revo's body shell just enough to make it clear the engine. I bought a spare set of Summit body posts and elevated bumper supports, cut the body posts shorter to fit the Revo's body shell, installed everything, and...it worked! The Summit's body posts gave just enough extra clearance, and the Summit's elevated bumper supports kept the raised body shell from looking weird, because the bumpers are still in the same position relative to the body shell -- everything is just a half-inch higher compared to the chassis.

...aaaand then back on the shelf the truck went for several more months, because now that I'd solved the body-fitment problem, I could no longer ignore the much more difficult problem that I'd been able to distract myself from until then: How to fit an exhaust to this engine that would discharge safely behind the truck, like the Revo's original exhaust did? It's not like O.S. ever anticipated this engine being used in this truck, and neither did anyone else, so while there used to be kits for installing FS-40S-CX engines into T-Maxxes, there are no fitment kits for the Revo.



Eventually I did enough research to discover there were flex-pipes made for O.S. 4-stroke engines, and I bought one of them. The flex-pipe I used was intended for a FS-70S, because the FS-40S-CX has a larger exhaust header than the original airplane FS-40S. After some careful bending and test-fitting, I managed to get the flex-pipe to reach from the engine to the opposite-side rear corner of the body, and a muffler for a FS-70S capped-off the exhaust system and provided a place to attach a hose to pressurize the fuel tank. Then the truck sat on the shelf for more months until I had the spare time and energy to buy some mild-steel linkage wires and bend them to make exhaust supports. While I was at it, I also made a throttle linkage to replace the original one, which was absolutely not capable of reaching all the way back to where the carburetor is mounted on the FS-40S-CX:



In the picture above, you can see that the throttle linkage comes perilously close to the pinion gear on the clutch bell; this is unavoidable due to the size of the engine, so I bolted a standoff post into a suspiciously-convenient screw hole on the back of the throttle/brake servo enclosure, to block the throttle linkage from wandering into the path of the pinion gear teeth. Speaking of which, you can also see the higher gearing I had to install, because this engine spins at a relatively-leisurely 19,500rpm compared to the 32,000rpm of the TRX 3.3. The gearing is 22/36, which is 55% higher than the stock gearing, for a top speed of 36mph -- plenty fast for an 11-pound truck that has to dodge trees in my backyard.

With the engine finally installed and connected, I made a couple final mods to the truck, including installing a better single steering servo and an aluminum servo-saver arm, and cutting-up a set of E-Revo rear-center driveshafts to make front-center and rear-center driveshafts to replace the Revo's undersized ones:



Finally, after nearly a year and a half, the truck was ready to run! First came a basic engine test and break-in:


...and then, the very first time I drove it, the flex-pipe I was using for the exhaust broke in half. I replaced it, and the replacement broke in half too. Oh, this was not good. I wondered if I was going to have to buy pipe-bending tools and learn how to make my own exhaust from scratch, in which case the truck would end up sitting on the shelf for yet more months. But after doing some more research, I discovered that O.S. also makes rigid steel headers, and I figured out that I could attach a couple of them together plus a shorter flex-pipe to make an exhaust that would still reach the same location on the chassis. Hopefully the rigid steel headers would absorb most of the thermal stress from the 450F exhaust gases coming out of the engine, and also spare the flex-pipe from needing to be bent near the ends, which is apparently a Very Bad Thing for them in the first place.



Finally, this exhaust setup worked. I've been running the truck for a few weeks now without any problems. However, there was oil spray on the chassis behind the flywheel, meaning oil was leaking out through the front crankshaft bearing. To keep oil from getting into the clutch and fouling the Aramid clutch shoes, I built a PCV system for the engine:



I added a hose nipple to the carburetor to provide suction, another hose nipple to the side of the engine to vent crankcase pressure, and a check-valve in the middle to ensure pressure and excess oil consistently get pumped out of the crankcase. This quite neatly solved the oil-spray problem, though I may relocate the crankcase vent to the camshaft chamber to encourage oil to migrate to the camshaft, and then plug the existing hole in the crankcase with a cut-down screw.

With the engine running reliably, the transmission geared correctly, the exhaust not breaking during every run, and oil not spraying onto the clutch, dhere was just one last thing to deal with:



...that stupid fuel-cap lifter that the Revo comes with. It's a nice idea to be able to open the fuel cap and re-fuel the truck with the body on, but even if this engine didn't have a pullstarter that requires me to remove the body every time I start the engine, it would still be necessary to remove the body after every run to make sure everything is still working okay. This is a toy monster truck, not a Toyota Camry -- you can't just ignore the moving parts for years and expect things to work okay. The stock fuel-cap lifter interferes with the body every single time the body is reinstalled, which gets irritating super fast, so I removed it and replaced it with a split-ring for a keychain. I threaded a piece of fuel tubing onto the split-ring to keep it from buzzing and rattling against the fuel tank when the engine is running.



...oh, and yes, those are HPI Savage 3.2" wheels and tires, instead of the stock 3.8" Geode wheels and tires. They provide a narrower track width, similar to the two T-Maxx Classics that I own, and more "squish" for driving on rough terrain -- tires are always the first stage of suspension in any vehicle. Also this way I don't need to buy a different size of spare tires just for my Revo.

And that is that. My most ambitious nitro project is finally complete. It's a great truck.
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Last edited by fyrstormer; 03-25-2020 at 09:45 PM.
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Old 03-25-2020, 09:50 PM
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BONUS POST: Here's a photo I took of the engine with my new FLIR camera.



If you've ever wondered what exactly is the effect of RC engines running on alcohol, just look at the carburetor, which is pitch-black in the photo above, despite being bolted onto a very hot engine. You can even see the cold stream of fresh air and vaporized fuel cooling-off the intake runner inside the cylinder head. Very impressive considering the exhaust runner is only 1cm away, and the exhaust temperature on this engine is somewhere around 500F.
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Old 03-26-2020, 01:48 PM
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I came here because you complained. I read through the whole thing because it was an excellent build. Some really impressive solutions. I'm glad I checked it out.
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Old 03-26-2020, 07:10 PM
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Thanks!
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Old 03-26-2020, 07:58 PM
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Now, time to put a wankel into a RC car.....
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Old 03-27-2020, 12:39 AM
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Originally Posted by Sir 51D3WAYS View Post
Now, time to put a wankel into a RC car.....
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Old 03-27-2020, 07:32 AM
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Very nice build.....I like the flex pipe exaust
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Old 03-27-2020, 10:05 AM
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Awesome build.
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Old Yesterday, 11:03 AM
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Man, this inspired me to restore an old 2.5 tmaxx I saved from the garbage. I`m always impressed bu the creative solutions you come up with. Cheers mate and take care of yourself in these hard times.
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Old Yesterday, 07:35 PM
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Thanks! I hope your T-Maxx turns out well. I strongly suggest getting the grey transmission from the T-Maxx 3.3; it will fit the T-Maxx 2.5 with no problems, and it shifts between forward and reverse much smoother. You do need to be careful not to shift it while moving, though, or else you need to buy the OptiDrive module to prevent you from accidentally shifting while moving.

I work from home full-time as a computer programmer and I have no friends who value my company enough to ask to visit me, so the only way my life has changed in the past month is I have to be careful not to waste toilet paper.
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Old Yesterday, 08:32 PM
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Shawn, thank u for the tips. But i think i will just restore it. RCing here is not popular so stuff is expensive. Nitro fuel is pricy, plus the only hobby shop is 40 miles away from me (shipping here is NOT an option). So i will clean, polish and fix it for fun and with what i have. Luckily the engine works fine, it fired up after a few tries.

Here we are in full quarentine. I wish i could go out to do the shopping or something, but i'm asthmatic so being infected by a pneumonia-causing virus is not the best idea.
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Old Yesterday, 08:58 PM
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True, asthma plus pneumonia would be a very bad combination. I got pneumonia in December 2019 from a cold that my kid brought home from daycare. I've never gotten pneumonia before, and I can't remember being that sick in at least 20 years. Everyone who visited my house got sick immediately, too. Honestly I wonder if it was COVID-19 but nobody was testing for it yet.
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Old Yesterday, 11:30 PM
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Originally Posted by fyrstormer View Post
True, asthma plus pneumonia would be a very bad combination. I got pneumonia in December 2019 from a cold that my kid brought home from daycare. I've never gotten pneumonia before, and I can't remember being that sick in at least 20 years. Everyone who visited my house got sick immediately, too. Honestly I wonder if it was COVID-19 but nobody was testing for it yet.
Worst part is, Buenos Aires and the suburbs are basically on a swamp. And it's summer here. So mosquitoes thrive this time of the year, and here we have a disease called Dengue. Mosquitoes transmit this, and summer is Dengue season. So we are fighting both threats at the same time. On top of this, our health system is basically an old witch with sum plants and wierd fruits (joke). God save us.
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