Engine swap Rotax 100 for 80
The grove system is nice, but, it is heavy and, believe it or not, shorter than the bungee gear. This makes the bungee gear actually better in the bush. Weight is the enemy in STOL performance.
Remember, your example was 80 kts. That equates to 92 mph. This is not far from the mark I gave for my plane. Depending on rigging, farings and prop I see nothing outrageously out of line about this speed.
90 mph is a real easy cruise at ~4000-4500 rpm with my yellow plane.
Re: ? Engine swap Rotax 100 for 80
I understand the concerns for wt. Which means, I would be doing myself a favor to lose 20#'s, on so many levels.
I would think if you wanted taller gear, you could just call Grove and have them build you an extended gear, but if it isn't as good, then it doesn't matter anyways.
Also talked with a fella the other day that builds an extended,bungie/spring type gear for the Kitfox.
In regards to the speed issue. I think the fella told me he was getting 80kts.@5400 rpm. And I forgot to mention, w/o whel pants.
Gary
Re: ? Engine swap Rotax 100 for 80
My plane would be a rocket ship if I did the same thing...Originally Posted by shortfielder
I understand the concerns for wt. Which means, I would be doing myself a favor to lose 20#'s, on so many levels.
Was that a model IV? I ask this because the older 1-3 models this would sound more accurate. The under-cambered, earlier, wing is very draggy and therefore much slower. I fly without pants, too.
Re: ? Engine swap Rotax 100 for 80
No,it was a newer mod 4 long wing classic
Last edited by shortfielder; 12-03-2009 at 07:00 AM.
Re: ? Engine swap Rotax 100 for 80
If you want to get the most out of your 80 rotax on your model 4, put the prop that I have for sale on it and you will get up to that 110mph. I know, I had it on my 80 model 4 and it did move right along. I have the IVO patriot ultra light prop in excellant condition for 700 dollars, it's yours if you want it. You pay the shipping.
I did switch to the rotax 100 and I might have gained 3mph on the top end, the biggest change is the take off. I ended up putting on the medium IVO that's why the old one is for sale. It's is a straight across switch out, no suprises. Same engine, just more juice.
steve
slyfox
model IV 1200-flying
912uls
IVO medium in-flight
RV7A-flying
IO-360
constant speed prop
Re: ? Engine swap Rotax 100 for 80
Hi Steve
And thanks for the info on the engine swap. And the prop. The plane I have been talking about, has the long wing, so I woud expect it to be a little slower than your Speedster.
And I would think that the prop may be considered a cruise prop, so you pay a penalty on t/o performance.
Usuall everything is some kind of trade off.
Gary
Re: ? Engine swap Rotax 100 for 80
THe prop is an inflight adjustable, so it is both cruise and take off prop. I originally had the long wing on my aircraft and when I shortened the wings I didn't notice too much gain on cruise with the short wing. I've had the plane for 5 years and about 1300 hrs now.
steve
slyfox
model IV 1200-flying
912uls
IVO medium in-flight
RV7A-flying
IO-360
constant speed prop
Re: ? Engine swap Rotax 100 for 80
Gary,I understand the concerns for wt. Which means, I would be doing myself a favor to lose 20#'s, on so many levels.
I would think if you wanted taller gear, you could just call Grove and have them build you an extended gear, but if it isn't as good, then it doesn't matter anyways.
Also talked with a fella the other day that builds an extended,bungie/spring type gear for the Kitfox.
In regards to the speed issue. I think the fella told me he was getting 80kts.@5400 rpm. And I forgot to mention, w/o whel pants.
Gary
We have a Bush Gear for the IV and earlier models that is taller and has been available for some time. Built to use either .750 axles or the larger 1.25 axles. Although the 80 hp is a great engine and performs very well on the IV's, being in Durango, CO the 100 hp is going to be much better choice for you with the density altitudes you deal with on a regular basis.
Re: ? Engine swap Rotax 100 for 80
Good Afternoon John
Sofar, most that I have talked to feel that the Grove gear is better gear for the off strip landings. Paying a penalty for weight, but more stable on irregular ground, and tougher.
What have you found,or youur thoughts on which gear would work best for my application. Mountain pastres, beaches, gravelbars, etc. Hoping not to encounter anything much bigger than a grapefruit.
And how much is our extended gear?
Thanks Gary
Re: ? Engine swap Rotax 100 for 80
Gary,
I’ve swapped an 80 for 100 hp Rotax in my Model IV 1200. While it’s not a “big deal”, it was much bigger deal than I had turned up in my investigation. So hear are the potential issues (in order of certainty);
1) The bigger starter will require you to modify the starter (cut off tabs) and/or modifying your firewall (i.e. its longer). For me it was both, but I’ve heard others get away with just the starter mod.
2) New radiator. The bigger engine requires a larger radiator than was typically sold with most Model IV/912 setups. No problem, but likely (as in my case) the holes won’t line up, hence requiring shifting the position of radiator (and so on…).
3) Oil cooler. This was new to me, and while not such a big deal when building, to modify and accommodate an oil cooler is not the same dealing with it during building. “Back in the day” when I bought my 912ul, I never heard of the need for oil cooler (nor in my opinion does a 912ul need it in Montana) , but they are highly recommend of the 912uls.
4) Related to oil; at some point, the oil tank changed size. I don’t know, but I assume the current oil tank dimensions are the same for the ul and uls. Anyhow earlier 912ul engines came with a shorter and fatter oil tank that I received with my new 912uls. Well these minor changes in dimension can be a big problem. This was perhaps my most perplexing problem to solve and I know at least another converter whom encountered the same issue. There was not enough room under the cowling for the height of the new tank. The other person I talked with solved this by keeping the short/fat tank and selling the taller version with the used 80hp engine. Rotax (R.O.A.N.) advised me that the inlet and outlet fixtures of the tank were also changed and were larger in diameter than the older tanks. They suggested this redesign was not without reason. I’ve taken a class from these folks and one of the first things they do is impress you with how highly engineered (exacting) these engines are. My solution was to re-route some engine wiring and modify the tank by eliminating the quick drain plug on the bottom (requires some high-tech welding) to gain space. It barely fits, but works, the tank essentially rests on the engine. I’ve put anti-chaff on the bottom of the tank and check it regularly. This may or may not be an issue with other cowling installations that might provide more room.
5) This is the final issue and perhaps the most unusual. At some point Rotax changed the distance between the two bolt holes used by the mounting system provided by Skystar and Kitfox Aircraft LLC for the 912. This affects only the top holes on the mounting brackets that go back to the “donut” (one hole is on the gear box, the other on the engine case). I was surprised to fine that the distance between the two holes for each bracket were not the same as my old brackets. This seemed to surprise Kitfox as well. The solution is simple; welding or purchase new brackets. I mention it here because it’s just another potential issue, hence cost and time. I talked to one other converter whom dealt with this as well.
Again, I don’t know when they made these changes (oil tank, bolt hole distances) so these last two issues may or may not be applicable. My ul was a 92-93 version (i.e. very early) and my uls is a 2008 version. My reasons (personal rational) for the upgrade are complicated and would take too much ink to explain, but I agree with many of the other posts here: A 912ul is a great match for a Model VI and is likely sufficient for most purposes, however, I’ll fly circles around you with these extra ponies.
As for your performance question (expected gain): This science and data are not exacting because of a number of different inputs (temp. & alt.). I’m in Montana and generally flying at 4000 and up. I’d guess I gained an average of 200-300 fpm climb and can get 1100 fpm out of my home base of 3500ft. The difference seems more noticeable at higher altitudes. As for cruise; I was formally a 100mph plane; now 110-115mph is pretty easy. I’ve started to worry about my duct tape. Mine is a pure to design Model IV-1200 (long wing) with 8” wheels and ATV tires (627 lbs empty). These numbers are difficult because of the biggest variable; a new prop, which goes to your second question; props.
I’ve always run three blade props and was a fan of the GSC wood prop on my 912ul. I generally only adjusted it for long x-country trips for better speed. While GSC makes a prop for the 912uls, the one I had on my 912ul was inappropriate and I had to change. I now run a IVO Medium in-flight adjustable prop. Lets just say that I definitely adjust it to fit my needs and this accounts for some of the gain I mention above.
As for your questions about engine values: One of my many reasons for the conversion was the high cost of rebuilding these engines and their high re-sale value as used engines. Verifiably good used engines (mid-life or lower) command good prices (ul or uls). My ul was over the hump, and I had other good reasons so I made the conversion. If your going to buy a new engine, it makes sense to go 100hp, even in a Model VI were it might be considered excessive. But I was happy with both engines. I just grin a little more with the new one.
I didn’t intend to write a detailed novel or "how-to" but perhaps this will be useful to others who might research this conversion now or in the future. This may become more common as older 912ul engines run their time out. I wish had more information to draw on when I made the jump.
Les
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