The Thunderbolt siren uses a blower unit consisting of a roots blower and a large electric motor to pump 250 cubic feet of air per minute at 6.5 psi. through the chopper. This gives the siren it's high sound output by "supercharging" the siren unit at the top of the pole.

Here is the blower frame with the motor, on the left, and roots blower, on the right. Air is drawn into the top of the blower through the box shaped thing on top of the blower and the blower outlet air pipe comes out of the bottom of the blower and goes out in between the bottom blower frame rails at the right end of the unit. The thing with the round plates on top of the air pipe just to the right of the the blower is the blower pressure relief valve. This valve releases excess air pressure if the chopper is stopped and closing off the air flow through the horn or if the air flow is blocked for some other reason. The valve has weights that stack on top of it to hold the valve down until the pressure exceeds 6.5 psi. When the pressure exceeds this amount the valve lifts up and releases the excess pressure while the blower is running. This way the blower is protected from overloading. This motor and blower are not the original units that were with this siren. The original roots blower, pictured below, was completely rusted inside. This motor and blower came from a mid-1970s vintage Thunderbolt so it has the smaller type roots blower. The motor is a 7.5 hp 3 phase unit that checked good when I had it tested at a motor shop. I mounted the blower skid on a set of heavy casters to make the siren easy to move around. It actually worked very well. The siren was easier to move around on the casters than my big Matco toolbox. Click Photo To See Larger.

Here is a view of the back side of the blower unit. You can see the air pipe and pressure relief valve to the left.I didn't repaint the blower frame. I just gave it the WD-40 and rag treatment. To see the other types of blowers that came with Thunderbolt sirens see the bottom of this page.

Here is the siren assembled without the blower cover on. I could run the rotator in this arrangement because the weight of the blower and motor was more than enough to keep the thing from tipping as the horn rotates. This is how Thunderbolts were shipped from Federal. They were crated with the horns attached to the blowers in this manner.

Early type and later type Thunderbolt siren blowers.
This blower information is based on the blowers I have presonally dealt with. There may be some other differences out there that I haven't come across yet. There is indeed a gasoline powered Thunderbolt siren but I have never seen any photos of the inner workings of one.
Here's a photo of the early type and later type of Thunderbolt blower units side by side. The difference is obvious from the photo. Federal went to the smaller size blower in the 1970s or maybe late 1960s. I don't know the exact year. The smaller type blower runs at a much higher speed than the larger type. Notice the pulley sizes on each blower. When the blowers are running the smaller one really screams while the larger just putt-putts along. The larger type blower unit is very heavy. I would guess it's well over 700 pounds with the motor on the frame. At the time this photo was taken I had the blower motors at the motor shop for a checkup before sending them to Moss Lake, Texas. Both motors were in good condition. Both of these blowers use 7.5 horsepower 240 volt 3-phase motors.