PDA

View Full Version : 912 vs 914



futureflyer
09-26-2010, 12:17 PM
Aside from the obvious differences of turbo and naturally aspirated. As well as, the horsepower, what are the key real world operational differences between the Rotax 912 and 914? Is the extra cost worth the added HP and altitude? Thanks all

matt

War Eagle
09-26-2010, 09:03 PM
This is a difficult question for anyone to answer. Rather I think it can only really be answered by you because only you know the intended mission of your plane, your personal likes for such things as performance requirements and the financial ability to pay the extra money to buy an engine that will deliver on those things.

Personally, I purchased a 914 because I wanted to fly in high mountain back country that typically has short strips and high density altitude. I wanted the extra margin in performance where I wouldn't have to worry about those issues. the 914 gives you more HP in all modes of operation, more torque, higher climb rate, higher cruise, shorter take off, etc.

I have flown with the 912S in several of the back country locations and sometimes you have to watch the time of day and the load. Sometimes you feel like holding your breath as you clomb to clear the tree tops and sometimes you circle the airport multiple times to gain altitude in order to fly out. With the 914 you don't have to be as selective.

I also like the fun associated with a very fast take off and over 1800 fpm climb. Do I need it? No but I like it. It's fun. But it is just a personal choice. I can cruise at a higher speed or I throttle back to stay with others when they are flying with the 912S.

I originally bought a 912S for my kit but never took delivery due to a Skystar bankruptcy so when I had to purchase a replacement engine I found a 914 for $6K more than the 912S. Well I rationalized for that $6K I would be willing to pay for that extra performance and I bought it.

Everybody that flys my plane loves the performance. Would I have made the same purchase if it had cost me $12K or more instead of just $6K? No. I wouldn't have done it and would have just flown with the 912S.

See the following thread for some earlier discussion on this topic.

http://www.teamkitfox.com/Forums/showthread.php?t=754

Andrew G
09-27-2010, 10:25 AM
I took my 2 boys + Instructor (600 lbs of humans, 2/3 tanks) up on Saturday - 86 degs, windy... and we took off directly into the wind after some landing practice at a tower controlled airport... we went in a 172 with a 160 hp Lyc.

We made... maybe 500 feet per minute climbing out... a far cry from your 1,800... that must be a blast War Eagle... would love to experience that.

Wouldn't it be cool to have a Kitfox SS7 with manual Retract. Gear and a 914? like an old Mooney... one day perhaps...

futureflyer
09-27-2010, 10:36 AM
John
Thanks for the addvice. I like the idea of not worrying about the DA in high altitude back country strips. As well, the added pereformance would be nice. Likewise, I don't think 12K more is really woth it but who knows...I unfortunately haven't flown a KF w/ a 912S. Just the 582 and its the aircraft that got me excited. I wasn't really watching the performance. There something about flying around low with open doors that makes me forget about how fast we're going.
On the other hand, is there any kind operational differences/limitations with the 914? I cannot remeber where I read it, but something about poor oil scavenging at certain attitudes. Anyways, that thread is good reading and again thank you for the knowledge.

Matt

P.S. congrats on KF of the month, she's a beaut!

Pilot4Life
09-28-2010, 05:09 PM
Hello All,
I am currently in a location where some folks operate "airplanes" that run on the 914, and there's a few of them here! I will try and get over to their side of the world this week, get some information from both those that "fly" them and those that maintain them. I'll post their comments ASAP. Don't think anyone's buying a new engine this week, so I think I have a couple days. I'll keep y'all posted.

War Eagle
09-28-2010, 07:23 PM
If you fly with a 914 you have a more complex machine (turbo control unit, waste gate, pressure/temperature sensors in the air box, pressurized carburetors, turbo charger, fuel pressure monitor, electric fuel pumps etc.) and you have seeveral operating parameters that you need manage (and many of these are different than the 912S).

I have never heard anything about poor oil scavenging and certainly haven't experienced any problem in this regard. But I am not flying at FL 25 either.

The 914 creates heat like a furnace so you need to make sure have good airflow. Exhaust temps are higher than the 912S and can operate up above 1600 degrees F. Air box temp has to be monitored and kept below a certain level because that is a control parameter for the TCU. Fuel pressure is also another control parameter. You have to have enough to make the flow work in the pressurized carbs. Too little and it won't run and too much it will overflow the carbs.

You can only use full turbo boost (40 inches) for 5 minutes max because of the temps and stress it puts on the engine so that is also a control parameter that must be monitored. It takes the full 40 inches to generate the 115 hp output. That 15 extra hp doesn't sound like much but I can tell you it is huge difference in performance that you can feel in the seat of your pants and see in the take off roll and the climb out.

The rpm is no higher or lower than the 912S but the 912S generates 100hp at sea level and if you fly at any other altitude than sea level you loose performance at the rate of about 2% per each 1000 feet. So if you are flying with DA of 10000 ft you effectively have 50 hp available now. The 914 can easily generate full horsepower at those DA levels so the comparative performance gap between the 912S and the 914 is widening as the altitude increases.

HighWing
09-29-2010, 10:03 AM
I finally have a take on this discussion. This is not intended to be an argument to either side of the issue, but rather to suggest some things to think about when deciding on an engine.

I have flown the mountains of Idaho many times but always with others - flights from six to eight. We always flew as a group so the speeds were all designed to accommodate the slowest airplane. My airplane was clean so it flew fast - the result was incredible fuel economy. One guy initially had a Model IV with 582. He never had problems getting off the ground, and often we flew above him - 912ULs and a couple of ULSs - as he gained altutude. We would coach him up the canyons telling him when to go and when to turn around. I always enjoyed watching him as he was our most exprienced pilot and seeing him work the thermals and ridge lifts was an education in itself. The point: there is very little mountnain flying that requires more than a 912UL in a Kitfox unless your goal is the heavy stuff you see on the extreme U-Tube videos. On the flat lands our typical altitude was less than 100 ft. AGL. - about 5000 ft. altitude in the high desert areas. We have been to 13000 ft. at times to clear some mountains, but you can see them from a long way off so climb is not an issue. The complexity of the 914 blew me away. Even though I flew on an annual trip to idaho with the guys, the other hundred hours I flew each year was more sedate flying. And to carry all that along on every flight to possibly enhance a week a year is not attractive to me regardless of price, not to mention, never being able to really use it because of the group flight.

Regarding density altitude. I believe most mountain flying books will say fly in the mornings and spend the afternoons under with wing with good friends. This is not so much a density altitude issue but the wind picks up in the afternoon and accompanying rollers and turbulance is simply not fun and can be dangerous. We were beat up pretty bad to the point of turning back once on our way to Smiley Creek. It had nothing to do with performance and density altitude - we had been there many times before - but everything to so with comfort and safety.

My new project will have a 912UL. No big time starter, no slipper clutch, no premium fuel, no complexity. Just like the first one; Start the engine - Fly. Land, shut down and roll it into the hangar. No issues, just simple fun.


Lowell

futureflyer
09-29-2010, 01:55 PM
Thank you for this real world knowledge. I am getting a real good handle on the type of rotax I wish to run, when the time comes.

The missions I am looking forward to flying are tight mountain passes and high vallies for camp excursions. Or traveling through alaska and the yukon etc.

John
You say that 40 inches gives you 115HP but for a limited amount of time, correct? I assume this is max. for take off. So when you pull back on the throttle, what is your max. continuous boost/HP?

Matt

Andrew G
09-29-2010, 02:19 PM
I believe strongly in the KISS (Keep It Simple Stupid) principal myself... and I would probably opt for the 100...

But, this 914 has some valuable attributes and possibly the complexity is worth the "safety" (climb out, for ex.).

Have the Kitfox folks performed a real comparison of performance specs?

total installed $ cost/speed/climb/gals per hour/weight diff/TBO, including the turbo TBO, blah ???

As someone who will probably buy or make one of these planes, I would be very interested in the comparison. War Eagle, is yours for sale ? Dorsal won't sell me his :mad:... good string.

War Eagle
10-03-2010, 12:15 PM
Hi All,

Let me try to answer your questions regarding 914 performance.

40 inches of HG is maximum boost available for take off. It's recommended use is for maximum of 5 minutes (this is due to the load and stress on the engine). 40 inches is avaible when the throttle is full opened. At 40 inches the 914 is generating it's max hp of 115. This level of performance (115 hp) is attainable up to 8000 ft.

Maximum continuous boost is 36 inches HG. At this boost the 914 is generating 100 hp. It is not possible to effectively control the boost level between 36 inches and 40 inches because the hp curve is very steep. In otherwords it comes on very fast and with only a small amount of throttle movement. Hence it's wide open and generating 40 inches and then a slight retard of the throttle to 36 inches for max continuous sustained performance of 100 hp. It is not possible to control any boost levels between 36 and 40 inches. 100 HP is max continuous hp and is attainable up to 16000 ft.

Boost is finely controllable at all levels below 36 inches. At about 32 inches your engine is running at 75 %. At about 30 inches your engine is running at 65%. At about 28 inches your engine is running at 55%.

Boost control comes from throttle movement. And as you can see from the details above it is not linear. There is a throttle position senson mounted on one of the carburetors and the signal from the TPS controls the turbo waste gate which ultimately defines the boost that is delivered to the engine.

Hope this all makes sense. Let me know if you have questions.

War Eagle
10-03-2010, 01:11 PM
Forgot to answer all the questions in one post.

No my plane is not for sale at this point in time.

When Skystar was in operation they had a 914 with a Hoffman constant speed prop that they used as one of the demonstartors. I think the plane belonged to one of the principals of the company and may have been leased to Skystar for purposes of demonstrations.

That being said, they flew that airplane a fair amount and used it for a test bed as well. However, I don't know if they conducted a head to head test with the 912s. Maybe some one in the community knows more specific info regarding that particular plane and any specific testing that might have been done.

As you might know about my history (part of the EAA 103 builders group that built 13 S7 kitfoxes where we installed 9-912S engines, 1 Rolls (0-200) and 3-914 engines), this has given us a some practical comparison of the flying characteristics of the 912S and the 914. But we did not make any attempt to do a head to head compare.

Again, hope this helps.