Tech Section - 4-Stroke FAQ
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Tech Section - 4-Stroke FAQ |
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Intro - Why 4-stroke?
R/C Four Stroke engines, such as the O.S. .40, are a bit more complicated to set up than a normal 2 stroke powerplant, but once you get one running well, the rewards are worth it if you are looking for:
 | Insane bottom end torque. |
| Durability (Steel sleeve+steel ring+lower revs=over 10 gallons between rebuilds) |
| Unique sound |
| More consistent tuning (or the
complete lack thereof
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| Easy Starting |
| Easy on Parts |
| Forgiving Nature |
Top speed may or may not suffer depending on your gearing, but don't expect
to be besting .21 powered trucks. It's a fairly low revving engine, so to
get high top speeds out of it, you need to really work with the gearing.
In a truck, it's a downside that isn't as important as say, a touring car,
but understand that if you want insane speed out of your truck, not just
wheelie poppin power, you might want to look elsewhere. Not to say that your
FS powered truck will be any slower than stock, just that stock top speeds
are about all you can expect without going insane with gearing.
Engines
Currently there are 2 different four-stroke engines suitable for use in 1/10 - 1/8 scale trucks, both from OS Engines. Both come in a pullstart and a non-pullstart version. Unfortunatly both are now listed as "discontinued" in the US, however there does seem to be a few available at different shops. Hopefully OS will release some other 4-stroke versions for cars/trucks.
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O.S. FS-26S-C
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Parts list: Click HERE |
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O.S FS-26S-CX
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O.S FS-40S-C
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Parts list: Click HERE Exploded view: Click HERE
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O.S FS-40S-CX
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Parts list: Click HERE Exploded view: Click HERE
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Gearing & Flywheel
A FS engine also has special needs, that you will have to meet in order
to have any success with one. Without creating yourself some oddball voodoo
setup (which isn't inherently bad, just more difficult) you need to
understand some of the engine's inherent quirks:
It only revs to about 20,000 RPM's. Your gearing has to compensate for this.
A 2 stroke engine revs to approximately 30,000+ RPM's.
The engine must have a decent load on it. This falls into the gearing
category also. When selecting your gearing (I will give a baseline setup
later in this post), understand that with the added complexity of the
valvetrain, not putting enough load on the engine will cause the engine to
overrev, and float the valves. Valve Float can be noticed as a curious bog,
and a truck with a truly crummy top speed that no amount of tuning can
remove.
Translation - Don't use your stock gearing, and make absolutely certain that
your 2 speed is shifting into second gear. If you don't have a working 2
speed, you will be forever complaining about how horrible your truck runs,
blaming it on the engine, get fed up, and throw it out the window.
Hoping it will be run over by a large truck I am sure.
You will need a heavy flywheel. No getting around this.
A MIP Heavy flywheel will work fine. Also make sure to get a FS collet.
You will need one. New Era makes them, as do others.
Suitable 4-stroke flywheels:
| MIP Flywheel T-Maxx
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P/N: 3073 | Tower Hobbies Link | |
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Kyosho 4-stroke Flywheel
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P/N: FZW035 | Tower Hobbies Link |
For gearing, make sure to run something like 22/66 for starting out
(Note: T-Maxx gearing). You can alter it later, but consider this
a good baseline to start out at.
You don't really need anything special in the driveline department, except
CVD's.
If you can't tell if the two speed is working, do this. Set the carb
needle to 1.5 turns out from closed (yes, this is normal for a FS). Make
sure you are running gearing in the 22/66 range
(Note: T-Maxx gearing).
Run the truck. Does it run like absolute crap? No
top speed? Bad acceleration?
Then your two speed isn't working. Your valves are floating due to the
engine revving too high. Lower the 2 speed shift point until it shifts
(consult your truck's owner's manual. You know, the dust covered thing in
the plastic bag with the stock air filter. You may have to dig in the trash
for it.)
It's that simple.
Fuel System &
Exhaust
Another quirk, is the amount of vibration caused by a FS. Since it fires
only half as often as your old two stroke, the vibrations are effectively
doubled over a stock engine. In some cases, this can cause foaming in the
fuel tank that can usually be solved by isolating the tank from the chassis,
be it with o-rings, or short pieces of fuel tubing under the stand offs (my
preferred choice).
Fuel delivery is another issue, but a really simple one to get right. Just
emulate a 2 stroke exhaust. That's right, get out your trusty .21 sized
tuned pipe (for a .40, use a .15 pipe for a .26) and bend a custom header
out of some 5/16th's inch brake line. It's not that hard, all you have to do
is pick up some brake line for 2 bucks at the auto parts store, along with a
$10 pipe bender. Use a bench grinder or something to grind the flare on the
end of the pipe down enough to fit in the exhaust port, bend up the pipe,
and cut it to length.
I use
white
Aerotrend coupler for my engines, ask for it at the hobby shop. To make
it fit the tiny diameter of the header, take a piece of cut up coupler
material, and roll it up, then stuff it inside the coupler to shrink the
diameter of the inside hole. Don't just go off expecting to hoseclamp the
huge opening of the coupler to the tiny header and not have it leak.
Standard silicone couplers will not do. You will toast and destroy one
within a day. FS Exhausts are FAR hotter than 2 strokes.
My Aerotrend coupler material has held up for going on 2 years.
Run a pressure line too.
A lot of people experiment with different exhausts (I run open header) and
you can feel free to do so, but when you are trying to get the engine
running, please take a variable out of the problem and just run an exhaust
to get the truck running.
Once it is running the way you want, then feel free to do as you wish with
your exhaust. Nothing is more irritating than trying to help someone get
their engine running right, only to find out that they are trying to run
open header before they even tuned the darn thing.
Chassis
(T-Maxx spesific)
Now, as for chassis', well, you have a lot of
options. On a .40 you will need a .21 chassis, along with some .40 engine
mounts. New Era makes a .40 kit, RCAlloys' .21 kit will also work, but you
need to use New Era mounts with it. I've also used an AdrenalineRC .21 kit
w/ no mods.
Don't expect a direct bolt together either.
You may have to massage parts to get stuff working, or it may just fall
together perfectly. It depends on your setup. I, for instance, have a .40
running in a Stock Chassis. It's a pain to do, but my truck can wheelie
better than yours because of it
Final Thoughts
Above all, be patient. A FS engine is still a relative novelty in a gas
vehicle, so expect some pains in getting it running at first. Once running,
it is the easiest, most pain free way to enjoy your truck.
The fact that it kicks ass in hillclimbing, and will absolutely walk over
terrain that would get a stock truck stuck (like, say, mud) is just a bonus.
You will not be able to prevent the wheels from spinning by sinking it in
mud or anything. It is the perfect "truck" engine. If you use your truck
like the majority of people, as a street going touring car, you don't need a
FS, and in all honesty, should trade in your T-Maxx for a nice Nitro 4-Tec.
Those are designed for street use.
Many - MANY thanks to Don "Komodro" Modro who did all the work on this FAQ!!
| Last update 08.05.2006 |
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