Allison V-3420

An interesting episode in the development of the Allison V-3420-B10 engine (which powered the Fisher P-75) was its initial inability to meet the contract guarantee for rated power and how it was resolved.

Performance guarantees had been based on testing using an engine configured as a V-3420A to enable its running on the available Allison altitude test stand.

So configured, the test engine used the standard V-3420A firing order with each of the 24 cylinders firing 30 degrees apart.

As designed and delivered the B-10 engine used 60 degree firing, i.e. simultaneous firing of two cylinders (one in each vee), referred to as “zero” phasing by Allison.

When the engines were run in the altitude dynamometer it was found that the B-10 was about 3 percent low on power.

In an effort to restore this power Allison adopted an alternate firing scheme known as 150 degree phasing and utilized a 30 degree interval.

Historically, the details of how they accomplished this have been a mystery.

Allison Competition Engines recently had an opportunity to teardown a B-10 known to have been modified with the 150 degree phasing.

By taking careful notes of the as-found configuration we are now able to provide the details.