Latest Work Completed Mantua, New Jersey
Original Site:
September 2004
E-mail: usav8or@yahoo.com
Converting your Jacobs R755-9 to the R755-B2... Moving from 245 to 275hp
We're (read that as "I'm") always looking for more hp. Makes those take-offs breath taking. If you're worried about your gas milage you can always pull the throttle back and sip gas at a lower rpm.
As mentioned several times before, in several different areas on the site, I wanted to power my Skybolt with 360 hp. Not wanting to purchase a new, or used Ven at $22,500 (as of 2008) or more, I had to seek other options. When I discovered that the Jake was just about as bullet proof as a tank
and the 245 engine could easily be upped to 275 hp I was all over it.
OK.. great, now to find out how.
Sent Mr. Jake himself an email, Dale Murphy, better known as dsnaproll. A reply within a day and Dale sends me a link to a pdf file explaining how it's done.
A quick overview...
The best part of reading this was that, not only does it increase the hp to 275, it does it one better. The increase it more like 282 hp. Wooohooo !
To understand the conversion it's helps to know a little history of the L4, aka R755-9 engine.
During World War II, Jacobs produced thousands of the Jacobs R755-9 (245 hp) engine for the Bamboo Bomber. At the end of the war Jacobs modified/redesigned the 245 hp engine by increasing the compression ratio
with high compression pistons, rejetting the carburetor and enlarging the interior of the induction housing. This resulted in an engine that produced 300 hp; the R755-A2.
By the early 50's Jacobs had tons of surplus military R755-9 parts without much of a demand for that engine (lucky for us.) They asked themselves how they could create a demand for these parts. And with that question,
the question "What if we use the higher compression pistons and chrome cylinders of the A2 with the lower end of the R755-9?" The end result was the R755-B2. For whatever reason they
chose to rate the engine at 275 hp, when in actuality it produced 282 hp at 2200 rpm.
Enough of the background...
Now for the conversion...
1. All parts are to be checked to the production drawings and specifications.
2. All interchangeable parts must be re-numbered to the current numbering system.
3. All cylinders are to be porous chrome plated.
4. Install new style pistons.
5. Install the new piston ring configurations.
6. Install the new front case oil flange.
Now you may ask... Can I convert my R755-9 to a R755-B2 and be legal? The FAA's answer to this is... Anyone could certainly read an overhaul and parts manual and could
install the correct parts to convert it from the -9 to the -B2. BUT, the R755-9 was built under Type Certificate 121 and the R755-B2 under Type Certificate 237. By what authority
would a person move an engine from one Type Certificate to another and what will they do for a new data plate ?
As a homebuilder, hanging it on a homebuilt biplane, I can "legally" do this on my own. Well, not on my own. I'll have the help of someone knowing about radial engines looking over my shoulder.
September 2008 update:
It turns out that the Jacob I located, and bought, was an R755-A2, which is the 300 hp Jake. All I need to do is get her up and running then fuel inject her to bring her up to 330 hp.