Going beyond the typical small-block 55-class two-stroke power-wise for your heli has been covered in the big block article. There, we delved into using an optional BBC kit to permit stuffing an engine with significantly more displacement into a P6 airframe.
What results is termed a pocket rocket. The beauty is how, from the outside, there’s not much to tip off the unwary of what you’re packing in terms of horsepower. Anyway, a P6 with BBC is a yeehaw experience!
However, yet another exciting propulsion opportunity exists for your Pantera P6. This, in the form of a large displacement four-stroke engine, aka a thumper! And instead of a yeehaw experience, flying a big thumper is more like woohoo!
So what’s involved in fitting the engine?
Stuffing a thumper into a P6 works because all the 91-125 class 4-stroke engines we’ve come across share mounting dimensions with the big block OS MAX 61-105 engines. So using the same BBC kit gets you a long way down the road toward fitting the engine.
Disclaimer; physically fitting a big thumper in place is the easy part. Beyond this there are issues with gearing, loading, and cooling the engine. We’ll touch on this within this guide, but remember; this isn’t a factory conversion. Yes, we’ll facilitate it with a few bits and bobs from our parts bin, but the quality of the end result is entirely on you because it’s not a bolt in. No, it’s not hard or particularly tricky, and it’s very rewarding, but because we have no control over the quality of your work, or what use you make of this, we’re not responsible. Bottom line? The end results are 100% on you because we don’t sanction any of this.
First up, let’s touch on gearing.
Gearing
As regards gearing, while the big block OS61SX-H and 70SZ-H work fine with stock gearing, the 91SX-H and 105HZ work best after changing the gear ratio. This, because operating at lower RPMS, these engines respond best to taller gears (same-same as a numerically lower ratio). For that purpose, you’ll perform the same Lerch-conversion as we showed in the Big Bock article.
Basically, you’ll be altering the stock gear ratio from 8.7:1 to 7.08:1 or 7.82:1 by using some Thunder Tiger parts. Refer to the big block article if this is new information. What’s different about stuffing a big thumper under the hood is you’ll be using a Tiger 50 fan hub, part number AUD0037.
Designed for ¼-28 thread engines, this fan hub stacks onto the crankshaft as if it were a prop. However, depending on your engine, you may need to bore and tap it to accommodate 5/16 threads. In our experience this isn’t difficult and it runs quite true.
However, almost to a certainty you may need to shorten the end of the crankshaft by a few threads to keep from nudging the bearing within the clutch bell out of position. If you have any engine warranty concerns, heads up.
Note; because there’s some variability in the physical dimensions amongst these engines, it’s really impossible for us to cover all the scenarios you may encounter. Not kidding when we say this is a guide but you’re responsible for sorting the issues as you roll your own.
However, almost to a certainty, this fan hub is going to be part of the equation for whatever engine you have on hand because these engines are generally assumed by the manufacturer to be for model airplane-use, e.g. sporting a propeller.
Blade loading
You’ll also need a stretch-kit. This consists of a longer than stock carbon fiber tail boom and a longer tail drive belt. This, so you may fit 700-class blades and increase the diameter of the tail rotor blades. Otherwise, you risk interference, which is bad juju. For my four-stroke conversion, I fitted 105mm tail rotor blades, a significant increase from the stock length of 87mm.
Anyway, the principal reason to go from 600mm to 700mm long main rotor blades is to increase the load experienced by the engine. This because thumpers run at comparatively low-RPM (by low meaning 1650RPM head speeds versus 2000RPM). However, because of their torquey nature, then you’ll need more blade to get enough load at lower RPMs. And by more blade we mean longer and with a wider chord.
As regards the term torquey, I use it because these engines have a great big wide and flat torque band. It goes from off-idle all the way through mid to high RPM whilst being broad and very flat in appearance. Not kidding, with these big thumpers the torque curve is practically flat immediately off idle.
Setup
Figure to set up -4°, mid-stick being hover pitch at +4°, but in fact, from 0° pitch you’ll already be at ~90% throttle. From there smoothly go to 100% at max pitch. Speaking of which, +10°/-10° is a good place to start for Idle Up.
But remember, this is a sport conversion, not a 3D kind of thing. Can you do some 3D? Yes, of course, if you have the skills – but – this is NOT what this four-stroke conversion is about. That said, believe me, flying one of these isn’t just different, it’s an awesome experience . . . some say majestic! And the sound is to die for.
By the way, as with the BBCs, you have an interesting alternative, load-wise. Instead of fitting 700mm blades, you may elect to fit a Quattro kit fitted with 620mm blades.
Point being, there’s more than one way to skin the cat, so this is another way of adding more load. And of course, it’s the same formula used by Hughes back when they released their original hot rod, the Hughes 500. Simple formula for harnessing scads of horsepower, smaller diameter and more blades!
Note; regardless of how you chose to load it, stretch kit with a set of 700mm blades, or a Quattro, it’s not our decision; this is 100% about how you roll.
Engine selection
Basically, you can select from a host of large four-strokes. And not just engines from OS MAX and Yamada are on tap, but also from Saito. Plus the Chinese clones like ASP and Magnum. They all fit our BBC mount and are the foundation for opening the door to a new and exciting possibility of flight based on a thumper for power.
And you can give into your artistic side. The urge to paint this OS91FS Surpass was strong, and presto, a few minutes with a spray can and this is the result.
Note; you’re not limited to a big block, either. Ken fitted and flew one of these with an ASP61 four stroke fitted to the stock engine mount. Ran 550mm blades and said it was fun.
What's more, half the fun is shoehorning it in there as you mod bits never intended for large displacement four-strokes and bend them to your will. And to be honest, we never envisioned using big thumpers in the Pantera airframe.
Folks who did the heavy lifting
Getting to the point of using four-stroke engines in your P6 came about not out of a factory effort but by enthusiasts like you. Straight up, we never dreamed of this so it took visionaries, e.g. folks other than us, to envision the possibilities.
So whom do we have to thank for bringing this concept to life? There are several players but principally, it’s Ken Bailey, aka online as grwarlock. Ken is who played around with various gear ratios before settling on the same ratios that worked for the big blocks, 7.08:1 and 7.82:1.
Let me be clear, it’s Ken who largely developed ideas for cooling, loading, and everything. In short, if you build out one of these, it’s his baby.
Heck as I write I still marvel how my mind was blown when Ken and other folks began sharing photos of what they were up to. Honestly, being queer for engines meant my eyes immediately lit up and I rushed to the workshop to snag a YS91 to confirm it would fit. So that very evening after work, I set to rolling my own!
Proof of concept being this next photo, which is of a Saito 125. Like just look at how sweetly it fits! However, note this engine is fitted with our AUD0037 Tiger 50 hub, and modifying it to fit may require a lathe to increase the original 1/4" bore slightly larger to 5/16" with precision, or better still, just tap it 5/16".
Anyway, it's easy enough and the clutch shoes end up in the perfect position within the clutch hub making this a straightforward and genuinely lovely installation!
Note; we offer two alternative engine hubs (AUD0037, Hub, Short and AUD0037L, Hub, Long). Both are bored 1/4", which makes them great foundation pieces for adapting one of these engines. After all, being too small is a whole lot easier than too big and because some of these engines come with ¼-28 threads, going the other way around (smaller) would be harder!
And by the way, while we usually discourage use of a remote glow plug connection, with these engines there's precious little choice.
Cooling
So in the above photo, did you take note of the bit of alloy, which extends beyond the cooling shroud? The raw stock was sourced from the hardware store where rolls are sold cheaply for the purpose of roof flashing (as valley material). But plastic works, e.g. the cover of a school binder. So whatever you lay hands on will work.
Oh, and added bits of shrouding can be attached with automotive grade double-stick tape, glued in place, or secured with screws like this setup. Anyway, and obviously, the basic idea is to make the cooling air flow more effective by forcing it through the cooling fins instead of letting it be wasted bypassing the head. Especially since, like anything else, air will always take the path of least resistance unless forced to go where you want it.
So here's the thing, 4-stroke engines run hot and because the stock fan isn't up to the job, you'll delete it. Then, in addition to better ducting of the cooling air flow, we call in reinforcements . . . an electric fan to replace the mechanical fan.
Electric fans
In general, we’ve been unsuccessful in cooling these engines with the stock fan. Some have fitted 10-blade fans off a T-REX but once summer approaches (temps in the 90°), the engine begins to overheat. So what we, and others have turned to are electric fans.
And using an electric fan is surprisingly easy. But once again, there’s more than one way to skin the cat. While I favor using inexpensive bobbin-type computer fans, this next photo shows a 40mm fan liberated off an Odin. But heads up, 60mm fans fit! And FWIW, I've seen 10mm thick fans on eBay for about $8 so we're not exactly talking big money.
What else will you need? A small ESC for controlling the fan. An el cheapo 10A generic unit will work fine. Also, while some just plug the fan into the receiver (letting it come on full blast when you switch on the avionics), we feel a better approach is to treat it like a servo, and plug the tiny ESC into a spare channel and use a program mix. Yup, just slave the ESC to throttle and map fan speed (RPMs) and Bob's your uncle! Once done, as you open the throttle, the fan speed increases, and vice versa, low throttle means less fan speed.
How much less? Honestly? Dunno, every fan and engine combination will be a bit different so you'll need to experiment . . . but that's half the fun!
More cooling system modding
In this next photo we see yet another approach to engine cooling. This time using a free-flight ducted fan unit. The whole thing aims downward at the engine!
And another view is straight down the gullet of the fan unit. Me? I’m not convinced there’s a need for the entire ductwork, because the air is going to become turbulent as all get out surrounding the engine so I’d be game for shortening the works.
Anyway, I'm sharing this next picture just to whet your appetite for a low buck approach to 4-stroke installation. This lovely setup showcases an inexpensive Magnum 91XL installed within the P6 model. I've seen these on eBay for $100 and as you can see, it fits nicely. And see how the needle valve just clears? Sweet!
Exhaust
The exhaust is another area to touch on. One school of thought has you using a stock four-stroke engine type muffler. For example, Helifreak user ‘helihole’ fashioned a very high quality workmanship accessory for adding support to the muffler of his Magnum 91XL installation. Otherwise, he says, the threads tend to bugger in the head due to engine vibration.
Another approach uses flex pipe. Braze up an adapter, then use a hose clamp to secure it, and route the fumes aft and away from the engine. Works well, is remarkably quiet, and works well.
Wrap up
The thing to bear in mind about these four-stroke engine is they love an RPM range that’s a few thousand lower than the typical 2-stroke engine. Point being, you’re going to need to change the gear ratio. They also bring more torque to bear and thus, need more blade to be properly loaded. Mechanically, the installation is a doodle, but cooling can be a bear.
This next photo shows the level of detail some guys bring to the game.
Note; another benefit of running these engines, besides the fact they can be bought cheaply on the 2nd hand market because so many modelers have switched to electric is they burn less fuel.
Anyway, fitting the engines is easy, cooling can be dealt with, and running more blades results in a fun experience with a great sound. Further to cooling, eyeball another shroud extension, this one fashioned with a bit of scrap plastic to fit the existing cooling shroud.
Also, while a set of 700mm rotor blades work on the 600-class head as long as you limit RPM to 1650 maximum, an alternative approach to loading is the Quattro head fitted with 600mm to 620mm blades. And this looks sweet, too, whilst creating a butt load of thrust. Saying this actually works very nicely.
In fact, you can even do decent 3D with the model (although many say this is kind of pointless as the engine masses much more than a 2-stroke so it's never going to pop quite the same). Anyway, because the model ends up heavier, plan on doing this for fun instead of for 3D!
Especially because where these 4-stroke conversions shine are for just farting around performing low RPM aerobatics. They excel at this and sound different. Enough so everybody stops what they're doing and watches, added to which, with a butt load of torque on tap, they're an absolute hoot to fly!
Anyway, as it happens, I dumb thumbed my YS FZ115S equipped P6 BBC conversion into the ground the other day. I have yet to resurrect it due to the press of work. However, with a 7.08:1 gearset, because the engine happily turns to 13,000RPM, this means turning the 4-blade rotor head at 1850RPM all day long and with 620mm blades, it was an utter hoot to fly! And yes, it'll get repaired, I just need a bit of spare time. Point being, it’s my opinion this is THE engine for one of these conversions.
Closing
Major point being, when modding this bird you have many, many ways of skinning the cat 'if' you're an engine guy. And one thing is certain, do you know of any other heli that opens the doors to fun quite as widely as a P6? After all, the whole purpose of these conversions is having a blast!
And when fitted with the Quattro main rotor head, it not only sounds different but looks different too. However, before pulling the trigger on a four-stroke conversion, next, let's touch on yet another type of big block engine, a gasser. Yes, the type of engine, which burns 2-cycle premix consisting of gasoline and oil instead of nitro fuel.
