why two carbs

Re: why two carbs

by Roger Lee » 8 years ago

Roger Lee

131 5.38k 5.44k

Hi Karl,
Because you do get more power with two and more efficient. Jabiru with one isn't efficient and could get more power with two, but that would take up more space, more weight and more engineering. Jabiru is just a simple design. When Ultralight aircraft first came out all the engines were single carbs. Then later on to get more power they added a second one.
I used to fly those swing seat, single cylinder 15 hp Yamaha engines. Then 2 cylinder single carbs came to be and then two cylinder twin carbs.

---

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

Re: why two carbs

by Karl Kleimenhagen » 8 years ago

Karl Kleimenhagen

5 10 0

Yes, many race car engines, in the carb era, used multiple carbs. I think it was to reduce air drag from longer runners. Since the 912 series has higher specific outputs, perhaps this was the problem.

Re: why two carbs

by Rhett Mitchell » 8 years ago

Rhett Mitchell

8 99 0

To sell twice as many parts?..... joking.....maybe.

Re: why two carbs

by Karl Kleimenhagen » 8 years ago

Karl Kleimenhagen

5 10 0

I see today Sonex is recommending their "AeroInjector" as a proven, single carb solution for the 912. (I'm not making this up!) The installers wanted to be able to fly inverted. It'd be interesting to see the performance hit from that installation.

I'd stick with the Bings. The Sonex is a very crude carb which requires the pilot to monitor the EGT on changes in power and altitude.

Re: why two carbs

by Karl Kleimenhagen » 8 years ago

Karl Kleimenhagen

5 10 0

I talked with a Sonex builder this morning. He has a friend with the Jabiru six and the single Bing. He said fuel distribution was quite poor in the engine, with wildly varying EGT and CHT. Jabiru's recommendation for evening it out some didn't work.

I'll assume Rotax chose two carbs so they could push the wee engine harder. Once synched, the two should keep the distribution relatively even, and this would allow them to run at higher outputs with less chance of one hot cylinder. Also, according to the hot rod sites, short runners aid in power at higher revs.

Case closed?