Steve Dorling (AMI July 2005)

The brand new Jen .37 from Just Engines is our featured review engine this month and as .37 cubic inches is one of the odd sizes, is it fish or fowl, so to speak?

Introduction

Not too many years ago our engines were pigeon holed into a few firm capacity based classes. A big engine meant a “sixty” descending through “Forty” – “Twenty” – “Fifteen” – “Ten” and “Oh-Four-Nine” – all recognizable discrete sizes. The real tiddlers, like the .5cc Albon Dart, itself twice the size of the positively microscopic Cox Pee Wee’s and Tee Dee’s were too small to haul commercial radio gear back then. Pattern and Scale competition flyers used the standard 10cc unit – ie a .60, pylon racers were tied to a .40 by the rules of the game and the average R/C clubman leaned towards a good sports forty. Gordon Whitehead, champion of a then new “Twentyfive” class, designed a range of superb scale models around those around the period that saw Club Twenty pylon racing evolve, budget racing at its best!

Those “Twentyfives” were bored out versions of the twenty class engines and they soon usurped their smaller cousins, turning out markedly more power from the same sized package. Duke Fox punched his “Sixty” out to .74 cubic inches and the Enya brothers bored theirs out to .80 and so it went on until today where the old established classes have practically merged. Club fliers meanwhile began to progress from pattern aerobatic ships to much lighter ultra agile aeroplanes, due in no small way to that man Foss, previously known for his series of mould breaking slope gliders. His famous Wot 4, introduced us to a completely new flying style, invoking an eventual 3D explosion some time later, which is ironic as these models employ pure control line stunt and combat technology dating back to the 1950’s!

Moving forward to the present day, the once dominant 40’s and 60’s now share the limelight with capacities such as .48 - .68 - .37 .56 – .78 and so on. If there is a gap anywhere now then it’s a very small one but dig a little deeper and you will see that the lineage can be traced back to just three or four popular R/C engine sizes. This Jen .37 from Just Engines is really a “big twenty” rather than a small .40 as you might assume from its capacity and physically it’s about the same size and weight of something like an HB .20 of three decades ago. This .37 is however more powerful than most forty’s of the same period, so here we have a quart of power in the proverbial pint pot. As I sit here writing this, the new Airbus A-380 has just completed its inaugural flight and this huge aeroplane is lighter (pro-rata) than the old Jumbo 747 from Boeing. Whether talking models or full size then, less is definitely more in aviation circles!

Test Subject

The Jen .37 reviewed is the Shnuerle ported two stroke employing ABC piston and cylinder liner technology (nb. JE also offer a ringed version). Schnuerle porting long ago replaced the old crossflow porting both model and full size two strokes. Mr Shnuerle rearranged the transfer ports on his two stroke engines so that the incoming charge assisted cylinder filling and helped prevent part of the unburned mixture whizzing straight out of the exhaust port. ABC refers to an Aluminum piston running in a Chrome plated Brass liner and following its inception by Super Tigre around three decades ago, it is again almost universally employed in this type of engine today. Correctly treated an ABC engine is a very hard wearing and user friendly device. This Jen .37 employs the same exhaust port dimensions as the little OS 32 SX which gives you an idea of the diminutive size of this engine relatively speaking! Interestingly I note that Just Engines are advertising a Diesel version on their website.
Diesels get overlooked rather too often by newer aeromodellers, unsure about their practicality or worth and that is a great shame. A Diesel, or more correctly a “compression ignition” engine will turn huge propellers if need be due to the way in which they enjoy an infinitely variable compression ratio. As the engine is a compression ignition device this means the ignition timing will be infinitely variable too and that’s the model Diesel’s ace card. The biggest propeller you might reasonably expect the Jen .37 glow to run is about 11 x 6 for practical purposes but the Diesel version will probably turn a 14 inch diameter fine pitched prop if need be. Now that Irvine are no longer in the Diesel game, this one should fill the niche that their 20 and 40 Diesels occupied and remember, no plugs to burn out or fail, no starting equipment needed and half the fuel consumption of the glow. Modern Diesels throttle very well and make a lot of sense then for certain model types, in particular scale (ish) types that demand a less frenetic torque orientated performance!

The Jen 37 certainly looks the part, enjoying a smartly anodised distinctive purple cylinder head which compliments the black painted main case. The twin needle “fuel metering” carburettor is once again, a very well proven and almost universally employed design for today’s glow engines. This one is accurate and well made with an accurate barrel to housing fit and O ring seals employed in all the right places to ensure that leakage remains a non issue, something that can ruin slow running and throttling on any engine. This is all good basic solid model engine “stuff” then and none the worse for that. The rest of the internals are all well up to current model engineering standards with fine finishes evident throughout and as most modellers are fliers rather than engineers, let’s dwell rather less on the construction and more on the performance of the engine from heron in.

Test

My test Jen 37 came with two alternative silencers, one being the quiet option and most club fliers with a responsible attitude to field protection, will no doubt go for that one. The standard silencer didn’t prove unduly rowdy but I’m afraid my decibel meter sits on the shelf and is rarely used today because I don’t subscribe to the theory that measured and perceived noise are one and the same thing (unlike some clubs…)! Your idea of a noisy engine and your local village action committee’s won’t be remotely similar, so there seems little point in my quoting you figures to bolster your case if they are measured in my back yard!

Just Engines also supplied a couple of manifolds with the engine, these being intended for use with a tuned pipe. Going back to something like an old HB .20 or the like, this Jen .37 on a tuned pipe will slot neatly into place where that engine once resided, but performance will just about be doubled! Now where did I stow that vintage Club Twenty racer?

Before running a new engine your first port of call must be its manual and as Paul at Just Engines takes the time and trouble to produce meaningful instructions to help you get the best from your engine, I am sure he would like you to read them. On the flip side, ignoring their content could cost you dearly, so take particular note of fuel, propeller and plug recommendations and ensure that you at least comply with the warranty requirements for peace of mind and safety. If you run the engine on a bench rig then make sure that nothing can be ingested into the propeller, like a rag for instance or your shirt sleeve! Damping down and sweeping the floor in front of the engine is not a bad idea as this reduces the chances of small foreign bodies jumping into the works and it greatly pains me to see engines being run up to full power on the edge of dusty WWII runways at shows and the like! Paul advocates bench running for familiarity and initial set up purposes and will sell you a neat little universal stand for the purpose if you don’t have one.

With an APC 10 x 4 propeller selected for the first start, I filled the fuel tank with Paul's recommended 5% nitro brew and fully choked the engine on a wide open throttle to thoroughly wet the innards. Closing the throttle to about a third I turned the engine over slowly with a gloved hand with the plug energised and was rewarded with a reassuring “bump” as the piston reached top dead centre. A few flicks and we were in business and whilst some ABC engines are very tight from new but this one was less so than most and it started quickly. From thereon in it needed little in the way of further running in before it willingly turned at a solid 18k plus RPM, sounding strong and very even. I ran a few more props against the tachometer, confirming the RPM of each of them before switching to the quiet silencer to repeat the exercise. The figures show the difference twixt the two and as you see, there isn’t a marked difference in power throughout the range!

This engine is definitely a candidate for the front end of a competition Fun Fly machine where it will turn out responsive and snappy performance on something like an APC 11 x 4 with never a hint of stopping until the tank runs dry. On the other hand, if fitted with a 9 x 7 for instance, then it will endow your Quickie 500 pylon machine or maybe that fast delta that you hankered after with impressive speed. Given its smooth willing ability to rev it should go really well on a pipe. By the time you read this I am sure that Paul will have a few combinations to offer you if you choose to go for a pipe option. Remember, running a pipe does not necessarily mean more noise, in fact with a well designed quiet pipe, quite the reverse can be true.

Conclusions

This is the second Jen engine to cross my test bench, the first being the now well established .56 which performed very well. Paul Landels wanted to extract the most power from the least bulk with the 56, allied of course to good reliability and long life and this .37 follows the trend with the same aims in mind. This is a sweet running, beautifully responsive engine and as seems increasingly to be the case with today’s most excellent model engines, there is little to criticise about it, if anything at all. It starts easily, goes well, has great transition and it’s very smooth. It’s a quart of power in a pint pot and if it came in a tin then I am sure Just Engines would say “it does exactly what it says on the can!

Technical Data

SPECIFICATIONS
Weight 300g inc carb
Height 64mm
Width across bearers 30mm (same as OS25FX)
Length from Prop driver face to rear of last bolt 75mm

PERFORMANCE
Fuel 5% Nitromethane content – warm humid day
Plug Weston PP5
Propeller RPM quiet silencer / RPM standard silencer

Bolly 10.5 x 6 = 12100 / 12500
Master 10 x 6 = 13000 / 13300
APC 10 x 4 = 15000 / 15200
APC 9 x 7 = 14400 / 14800
APC 9 x 4 = 16200 / 16300

More Reviews

You can read a customers views on a JEN .37 Stunt Version Here

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