Rotax 915iS vs 914UL: Cost and Technical Differences
Posted On January 16, 2021
By request, here’s a breakdown of the technical and cost differences to equip my Sling 4 with a Rotax 915iS engine instead of the Rotax 914UL engine.
As a quick reminder of why I’m doing this: +26HP at takeoff power (5800 RPM), +35HP at max continuous power (5500 RPM), fuel injection vs. carburetor, service ceiling of 23,000 ft vs. 16,000ft, integral alternator vs. external. I expect to see improved climb and cruise performance compared to a Sling 4 equipped with the 914UL, which I look forward to quantifying during my Phase 1 flight tests.
There are tradeoffs, however. The 915iS is slightly heavier. By the data sheet, the 914UL engine with accessories and external alternator weighs 163.6 lbs; the 915iS weighs 189.1 with the engine accessories plus fuel pumps, a difference of +25.5 lbs. This is consistent with the anecdotal ~30 lbs I read about. The 915iS currently has a 1,200 hrs TBO vs. 2,000 hrs on the 914UL, but Rotax is supposed to raise the TBO to 2,000 hrs once they have enough fleet data.
What are the installation implications for my Sling 4? Well, as it turns out, it’s not as simple as just bolting the 915iS to the engine mount instead of the 914UL. Here’s what else is different.
Firewall Forward (FWF) Kit
You definitely will need a FWF kit designed for the Sling TSi instead of the Sling 4. The FWF kit includes different accessories that are specific to the 915iS, like a larger oil cooler, larger return fuel line, mounting brackets for the fuel pumps, exhaust pipe, K&N air filter housing, etc. I estimate that less than 50% of the Sling 4 FWF kit would be reusable with 915iS.
This one surprised me, but the engine mount is different. The engine mount is included with the fuselage kit, so you either have to modify the Sling 4 engine mount, or buy a TSi engine mount (which is what I did). The difference is in the lower tube; TAF changed the layout a bit to make room for the charge air intercooler the 915iS has; the 914 doesn’t use an intercooler.
The hole pattern on the firewall skin is different due to the different engine accessory arrangement (primarily: Rotax fuse box, fuel pump location, oil tank location). I think the firewall may also be made of stainless steel instead of galvanized steel. Regardless, I decided NOT to buy a new firewall skin since I had already built my fuselage and it would be a lot of work to drill out all those rivets. It would also compromise the structure in the process of swapping the skin, so I’d have to come up with a way to support the fuselage side skins while performing this surgery. TAF has agreed to provide me locations to drill new holes on my existing skin, so I will use that in combination with match-drilling the accessories as needed, and then fill in unneeded holes with sealed rivets.
The 915iS is longer than the 914UL. According to the install manual, the distance between the prop flange (the reference datum) and the mounting points is 51mm further back for the 915iS. Here’s the information directly out of the Rotax installation manuals. The x-axis locations of L2, L3, R2, R3 are the items of interest:
The Sling engine cowling goes right up to the propeller slip ring, so my Sling 4 cowling won’t fit properly, and would require extensive modification just for length.
The 915iS needs a large air inlet for the intercooler. The Sling 4 cowling only has a small inlet for combustion air; the TSi cowling adds a large NACA scoop to supply the cooling air. As previously mentioned, the oil and coolant radiators under the prop are larger for the 915iS compared to the equipment supplied for the 914UL, so those existing inlets are also too small.
This all means the cowling included with my Sling 4 finishing kit is not suitable and would require extensive modification. I hate working with composites. I figured that by the time I bought all the material and made the effort to modify my Sling 4 cowling, I would have come out ahead (in both time and money invested) to just buy a new engine cowling – which is what I’m doing.
The cowling for the TSi was also redesigned to make it easier to remove for engine maintenance. The location of the split is different, making it easier to remove the top half, and exposes more of the engine, particularly towards the front. The new cowling attaches using Camloc fasteners vs. Dzus fasteners. I consider both of these to be worthwhile upgrades.
Due to the use of Camloc fasteners, the cowling attachment strip on the firewall is wider, so TAF is also sending me a new one of those too.
TAF does not use the intercooler supplied by Rotax as-is. TAF modifies the intercooler to change the orientation of the inlet and outlet. I was not able to find a comparable intercooler anywhere else. The used 915iS I bought did not come with an intercooler (it was a different part anyway that wouldn’t work), so I had to ask TAF to supply me with a new intercooler.
TAF deleted the turbo interlock on the throttle for the TSi. It had a small lever that you had to pull to move the throttle position from 100% up to 115% for take-off power. The Sling TSi doesn’t have this interlock, so the throttle lever is slightly different. My guess is that you can leave the throttle at the 100% position as long as the engine speed is reduced to 5500 RPM (via constant speed propeller).
The Sling 4 propeller (Airmaster AP332SCTF with Whirlwind R70W blades) is designed for engines up to 115 HP; it cannot handle the additional power of the 915iS. Therefore I will be using the larger propeller specified for the TSi, which is the Airmaster AP430CTF with Whirlwind R72B blades.
Surprisingly there aren’t any fuselage modifications required to use the 915iS. I did confirm this with TAF – in fact, it was Mike Blyth himself that I corresponded with initially. However, the Sling 4 air frame is airspeed-limited to 135 KIAS, which does not increase with the 915iS engine. Although the Sling TSi uses a very similar fuselage, it has different wings, a different canopy, airfoil-shaped main landing gear legs, and slight differences to the empennage (particularly the elevator), all of which contribute to the TSi’s higher max airspeed of 155 KIAS.
I asked TAF to prepare a quote for a “custom” firewall-forward kit for me, which would consist of a standard TSi FWF kit, with additions and deletions. Here is a summary of what was added and removed:
TSi engine mount
TSi engine cowling set
TSi cowling attachment strip
TSi throttle lever (deletes the turbo detent)
TAF-modified intercooler (this normally comes with the engine)
Andair EFS20-20-D2-TMT extension duplex fuel selector (I already bought this directly from Andair, £354)
Andair gascolator (also bought this from Andair, £119 plus £19 for drain valve)
Battery box (I already bought an EarthX-specific box from Aircraft Spruce)
The standard TSi FWF kit pricing at the time (Nov 2020) was $4,755. For the custom kit as detailed above, I was quoted $8,455, which was a fairly large increase.
I’ve combined this information together along with the cost differences for the engine and propeller. I don’t have current pricing for engines from TAF, so the figures below reflect pricing from California Power Systems as of 16 Jan 2021; other pricing is as shown with notes below the table.
No matter how you look at it, the powerplant for this plane is expensive. The engine and propeller together cost more than the airframe, but that’s not uncommon with kit airplanes. I’m not sure why, but the prices for these engines have skyrocketed in just the last two months. When I checked mid-Nov 2020, it was about $37k for a new 915iS and about $32k for a new 914UL, and now the prices are about 10% higher.
In my situation I didn’t pay list price for the 915iS engine; because I found a used one, I was able to save some $8,000 compared to the price above, so it brought my total cost to a figure that’s much closer to the Sling 4 standard price.
I probably would not have gone forward with a 915iS if I had to pay the list price above. I gave serious consideration to an Edge Performance EP912Ti engine, for which I received a verbal quote of around $30,000. It was an unusual set of circumstances that led me to finding a local seller of a used 915iS engine, and I decided it was an opportunity I wasn’t going to pass up.