The brand new Jen 91AR follows hot on the heels of the much acclaimed Jen 56 (the 40 that thinks it’s a 60…) and the splendid little 37, both of which I have reviewed this past year. The 91AR looks all muscle and go and indeed, speaking to Paul from Just Engines about it he simply smiled and said, tell me what you think of it, passing me the very first one in the country to review exclusively on your behalf.

Reviewing engines is a thankless task these days in many ways because for the most part, nearly all engines are good engines now, albeit with some falling firmly into the very good or excellent classes. Few fall below my expectations now, something that certainly wouldn’t have been true a decade or two back! I have had engines through my hands over the years that wouldn’t start until they had taken lumps of flesh out of you and when they did eventually bark into life things either broke or fell off in between bouts of indifferent running and lousy performance. Thankfully those days are well behind us although just occasionally you see a bad one, witness not that long ago a very large two stroke that refused to run in the air for an entire flight despite performing to perfection on the bench. Bench running usually shows up any problems but it has to be said, not always and for that reason I like to fly the engines I receive whenever feasible. I usually follow up a bench test with a comment or two in the column at a later juncture to give readers follow up concerning long term in the field behaviour.

What colour would you like your engine then, red – purple – black – blue – plain aluminium or any combination of those and more? These days the average engine has a painted or anodised finish to set it apart from the crowd and the Jen range are certainly distinctive in their black and purple detailing – very muscle bound and purposeful the range looks and more importantly for the suppliers – distinctive and recognisable.

Engine Strip

So how do you like your engines tested then – do you like the engine to be taken apart when it’s all brand spanking new and shiny so that you can see all of those tiny little machining marks on the components, or would you rather I ran the poor thing to the limit of mechanical restraint to show you how the innards stood up to the test? The real answer of course is that both would be nice and as Paul Landels sent me the wrong engine (for him) to start with, literally sending me the first one off the production line, which he wanted back for obvious reasons as it was his “001 baby” I got to take one apart unmolested as it were and run the other one – marvellous.

With the first engine dismantled for photography and a good once over in the workshop, I set about bolting the other into the test rig to see what it could do for me (and you)! A 90 two stroke is a formidable beast if given its head and I had little doubt that this one would measure up – let’s face it – there would be little point in Just Engines going to the cost and trouble of putting one into production if it wasn’t going to work well.

The Jen 91AR is a twin ball raced single ringed two stroke employing the usual (for today) Schnuerle ported cylinder liner– in this case of steel construction allied to its single cast iron ringed aluminium piston. The exhaust stub dimensions are (deliberately no doubt) identical to the OS 61 FX’s 42mm port meaning that silencer and pipe options are virtually endless. Various manifold and pipe options are available from the distributor should you need to go that way but for the purposes of the review, two silencers (ie standard and quiet) were supplied and these will suffice for most applications. Silencers are pretty effective these days, certainly when compared with the pathetic little castings that we used to strap over our exhaust efflux’s as a token gesture a few years back but it’s nice to have the option of even less noise and Just Engines are to be applauded for taking on the need to protect sensitive flying sites in this way. That the proprietor is a model flyer himself will have a great deal to do with this policy no doubt.

Looking at the “bits and bobs” (technical terminology here) laid out on the bench, the fits and finishes looked well up to current engineering standards and indeed, why wouldn’t they be. Everything looked pretty hefty, as befits a fifteen cc two stroke likely to deliver a healthy two or more horsepower and I looked forward to harnessing it to a propeller or three to see what it could deliver. Having tested a few other engines of similar capacity and type over the past year or so I had plenty of material on hand to make direct comparisons with and this always makes the tests fascinating. Most would expect the most powerful engines to produce the highest propeller figures, logically speaking but that isn’t always the case and as often as not an engine that produces very high horsepower for its capacity will prove pretty uncompromising and deliver disappointing RPM’s on larger propellers. In the case of a two-stroke, which tends to produce power at high RPM, tuning for torque is the answer and as you don’t get something for nothing in the real world, this will harness the engine in question back at the top end of the scale. It’s horses for courses then – so what will your Jen 91AR be used for? I would venture that an aerobatic ship is the most popular home for a two stroke like the 91AR these days and for that we want to be able to swing a mid range prop and useful speeds whilst enjoying really pleasant throttling characteristics and so, outright power is not our ultimate in this instance.

Test

Setting up the 91AR on the test rig involved little more than grabbing a handful of 12-15 inch propellers and my fuel caddy before hooking it all up for the initial start. Undoubtedly this engine would be perfectly happy on "straight" ie no Nitromethane brew but for the purpose of this test (and my general flying) I used a 5% nitro/synthetic which proved fine. RPM’s will drop a little on straight fuels and the quiet silencer will likewise knock a few RPM off but for sports fliers this will be of small consequence. I merely include the information for clarity here!

Choking the inlet with the throttle open wide I flipped the propeller over a few times before hooking up the glow battery. An (as advised by the suppliers) OS number 8 plug was fitted and with a single reverse flick on the propeller the engine started first attempt. From then on all starts were of the single flick affair and with no bad manners to catch you unawares, the 91 is a delight to handle and should prove reliable on the field, which is important. My initial propeller choice was a Bolly 13.5 x 8 which proved pretty much on the mark as the 91AR whizzed (technical terminology again) this one around at a healthy 10300 RPM which is about right for a 15cc engine of this type. Working through my propeller selection I ran the tachometer against these until I had enough figures to work out where the horsepower peak was and that was that. In fact the engine quite liked to rev and had I pushed things then no doubt it would have continued to produce useful power up around the 15K mark but this would have been an exercise in academia given that no user in his right mind is ever going to prop the 91AR that small. I reckon that 13 x 6 is about as small as you would wish to go and that on something like a fast delta but even then, 13 x 8 would be better (and quieter) and for sports/aerobatic flying something like a 14 x 6 -15 x 6. Propellers are of course chosen to fit the engine but the model in question should also determine your eventual fitment and experimentation is always worthwhile. Likewise a particular model/motor and propeller combination may be exactly right for your flying style but conversely, could be wrong for someone else who handles the sticks in a different way. Nothing I write here is going to be a substitute for your own experimentation but at least you have a starting point to work from.

Conclusions

So how much BHP does the 91AR make then and does it matter anyway? Well the short answer to those questions respectively are "enough" and "no"but as most of you will want to know, sticking the figures into my computer and pressing the appropriate keys induced the program to spit out a figure of around 2BHP+ at a nominal 12k which is entirely in the ball park. Idle speeds on all tested propellers was so slow to be of little consequence and throttling up was instantaneous always so what more could one want? There was once a time when every engine test carried a set of idle figures on propellers various but those days seem to be over as nearly every engine I handle these days keeps running until you can count the blades going round when throttled right back and the 91AR was certainly of that ilk. I also used to get the old dB meter fired up to give you meaningful noise test results, until that was, I came to realise that these are not very meaningful at all because not only will your 91AR make less or more noise than mine depending on its location and surrounding objects, all of which reflect or absorb sound waves, but even a weather change makes an appreciable difference. Noise is very easily determined - if your field is attracting the wrong sort of publicity when you are flying then your model is too noisy - period. Just engines can supply all silencing solutions to suit your needs.

I ran a few figures on the quiet silencer option to give comparisons before deciding that, on average this one knocks of at the most - a couple of hundred RPM in the worst case scenario which again, certainly isn’t going to hamper many of you and particularly as this engine is a big 60 anyway (ie it’s a 15cc engine in a 10cc crankcase size). In fact consider how much quieter this option is I think I would go for this one regardless. Using this pipe the 91AR had the distinct sound of my Moki 135 and it has to be said, felt and handled very similarly (and that IS a very definite compliment).

The largest propeller tried was an APC 15 x 6 and running this on the quiet pipe the impressive 91AR managed a cool 9k RPM and for those of you with a noise issue, this is a very quiet and useful combination. 9000 RPM on a 15 inch propeller is a lot of air moved and when you take into consideration that the engine is physically only a 60 the possibilities are pretty encouraging. This engine is happy whether revving hard or lugging a big propeller on the knee of its torque curve and when you add into the mix its excellent throttling and starting behaviour, there really isn’t anything lacking. With the previous Jens seen here behaving impeccably I would like to see the company boldly go forward and produce a really big one - say something around the 25cc mark or more because this is where the serious scale aerobatic ARTF’s are at their most prolific these days and for the power hungry pilots amongst us, a "one forty" or there abouts would be just the ticket. How about it Paul - any thoughts?

Tests were executed on a warm sunny summers afternoon - fuel used 10% nitro content - plug OS 8

Technical Data

Propeller figures;
APC 15 x 6 – 9000 RPM – QUIET silencer
APC 15 x 6 – 9100 RPM – STANDARD silencer
Bolly 13.5 x 8 – 10300 RPM – STANDARD silencer
Bolly 13.5 x 6 – 11500 RPM – STANDARD silencer
Master 13 x 6 – 11600 RPM – STANDARD silencer
Master 13 x 6 – 11400 RPM – QUIET silencer

Picture shows Standard Silencer bottom right, Super Quiet silencer top left

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