We are often asked questions about the similarities and differences between the different Jacobs engine models. This article is written to clarify those questions. We will begin with the earlier Jacobs models and come forward to the present.
Jacobs L4
This engine was granted its Type Certificate on February 27, 1934 and was rated at 225 hp. @ 2000 rpm. The earliest of these engines had either dual Bosch or Scintilla battery distributors. A later version (L4M) had dual Scintilla magnetos and the WWII military version (L4MB) had a battery distributor on the right side for easy starting and a magneto on the left side. The early L4s were utilized by Waco for their open cockpit and cabin models, and by Beech for their cabin biplanes. This L4MB was rated at 245 hp. @ 2200 rpm. And was produced by the thousands for the Cessna UC-78/AT-17 “Bamboo Bomber”
Jacobs L5
The L5 was the next engine produced by Jacobs, being released in January 0f 1936. The same ignition systems and designations were used for the L5 as had been used for the L4. The L5 was rated at 285 hp. @ 2000 but was never a successful design. Nearly all the L5s in service have been replaced by other Jacobs models.
Jacobs L6
The L-6 was approved on March 26, 1938 and again was utilized primarily by Waco and Beech prior to WWII. During the War the L6MB was produced in quantity for the Canadian Avro Anson. The L6 was rated for 330 hp. @ 2200 rpm. and 300 hp. @ 2100 rpm. Though never originally installed by Cessna, after the War the L6 was used as a higher horsepower engine for the Cessna 195.
Jacobs R-755A2
The R-755A2 is a post-war evolution of the L4 engine, having the same bore and stroke as that earlier engine but with higher compression pistons and a power case that allows better air flow to the cylinders. The A2 is rated at 300 hp. @ 2200 rpm. and was originally designed for the Cessna 195. It is often utilized today as a replacement for the L4, L5, and L5 engines in many Waco and Staggerwing installations. It is also a popular engine for the Boeing Stearmans.
Jacobs R-755B2
After the War, Jacobs found themselves with hundreds of new, war surplus L4MB engines. The demand for those engines was not great, so someone at Jacobs had the idea of putting a set of the higher compression pistons from the R-755A2 in the L4MB, and voila, the R-755B2 was born (this is actually an oversimplification, as there were other minor internal changes made to the L4MB power section as well as the legal issues related to moving the engine from one Type Certificate to another). The R-755B2 is probably the most popular and most successful of the Jacobs engine designs and along with the A2, is used as a replacement engine for many aircraft.
Jacobs R-755S
The R-755S engine was a development of Page Aircraft Industries while they owned the Jacobs Type Certificate in the late 1960s. Initially, the S engine was just a L4MB with a turbocharger installed in the exhaust (without a wastegate but with a pressure relief valve in the airbox). Later versions of the engine used a different piston and cylinder combination, finally settling on a piston with the same ring arrangement as the A2 and B2 engines, and with a cylinder identical to the R-755A2. The engine was rated at 350 hp. At 2200 rpm. and was used on both the Cessna 195 and the Grumman G-164 Ag Cat. Through the years the S engine has gotten mixed reviews, with some operators loving its performance and others choosing to replace it with a R-755B2 or A2.
We are often asked questions about the similarities and differences between the different Jacobs engine models. This article is written to clarify those questions. We will begin with the earlier Jacobs models and come forward to the present.
Jacobs L4
This engine was granted its Type Certificate on February 27, 1934 and was rated at 225 hp. @ 2000 rpm. The earliest of these engines had either dual Bosch or Scintilla battery distributors. A later version (L4M) had dual Scintilla magnetos and the WWII military version (L4MB) had a battery distributor on the right side for easy starting and a magneto on the left side. The early L4s were utilized by Waco for their open cockpit and cabin models, and by Beech for their cabin biplanes. This L4MB was rated at 245 hp. @ 2200 rpm. And was produced by the thousands for the Cessna UC-78/AT-17 “Bamboo Bomber”
Jacobs L5
The L5 was the next engine produced by Jacobs, being released in January 0f 1936. The same ignition systems and designations were used for the L5 as had been used for the L4. The L5 was rated at 285 hp. @ 2000 but was never a successful design. Nearly all the L5s in service have been replaced by other Jacobs models.
Jacobs L6
The L-6 was approved on March 26, 1938 and again was utilized primarily by Waco and Beech prior to WWII. During the War the L6MB was produced in quantity for the Canadian Avro Anson. The L6 was rated for 330 hp. @ 2200 rpm. and 300 hp. @ 2100 rpm. Though never originally installed by Cessna, after the War the L6 was used as a higher horsepower engine for the Cessna 195.
Jacobs R-755A2
The R-755A2 is a post-war evolution of the L4 engine, having the same bore and stroke as that earlier engine but with higher compression pistons and a power case that allows better air flow to the cylinders. The A2 is rated at 300 hp. @ 2200 rpm. and was originally designed for the Cessna 195. It is often utilized today as a replacement for the L4, L5, and L5 engines in many Waco and Staggerwing installations. It is also a popular engine for the Boeing Stearmans.
Jacobs R-755B2
After the War, Jacobs found themselves with hundreds of new, war surplus L4MB engines. The demand for those engines was not great, so someone at Jacobs had the idea of putting a set of the higher compression pistons from the R-755A2 in the L4MB, and voila, the R-755B2 was born (this is actually an oversimplification, as there were other minor internal changes made to the L4MB power section as well as the legal issues related to moving the engine from one Type Certificate to another). The R-755B2 is probably the most popular and most successful of the Jacobs engine designs and along with the A2, is used as a replacement engine for many aircraft.
Jacobs R-755S
The R-755S engine was a development of Page Aircraft Industries while they owned the Jacobs Type Certificate in the late 1960s. Initially, the S engine was just a L4MB with a turbocharger installed in the exhaust (without a wastegate but with a pressure relief valve in the airbox). Later versions of the engine used a different piston and cylinder combination, finally settling on a piston with the same ring arrangement as the A2 and B2 engines, and with a cylinder identical to the R-755A2. The engine was rated at 350 hp. At 2200 rpm. and was used on both the Cessna 195 and the Grumman G-164 Ag Cat. Through the years the S engine has gotten mixed reviews, with some operators loving its performance and others choosing to replace it with a R-755B2 or A2.
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