Will Holoman's MGB V6, with Chevy Semi-Even Fire 3.8L V6

Will Holoman's MGB V6, with Chevy "Semi-Even Fire" 3.8L V6

An Introduction to Odd-Fire V6 Engines

as published in British V8 Newsletter, Volume XV Issue 3, December 2007

by: Will Holoman

So your buddy thinks he's pretty cool - he's just finished rebuilding a stout running V-8 and has it tuned to purr like a kitten. Well, don't let him get too proud of himself - anyone can tune an "even fire" engine because it's impossible to screw it up! For a real challenge, try tuning an "odd-fire" or "semi-even fire" V-6.


General Motors has produced three kinds of 90 degree V-6 engines in the past, the even fire, the odd-fire and the semi-even fire. All are based on the small block V-8 with two cylinders chopped out of the middle. The small block V-8 crankshaft has shared journals for the connecting rods for cylinders opposite one another on the block (1-2, 3-4, 5-6, and 7-8). The odd-fire V-6 also has shared journals, but even and semi-even fire V-6's have offset journals: one per connecting rod.

Even Fire

Almost all engines are even fire. Even fire simply means that the cylinders fire at even intervals with regard to the spinning of the crank. For example, in a typical V-6 engine, the crank spins twice for each revolution of the distributer rotor. Two revolutions of the crank = 360 degrees + 360 degrees = 720 degrees. In these engines the cylinders fire at an even interval: 720 degrees divided by six cylinders equals 120 degrees between cylinders. Every time the crank spins through 120 degrees, a cylinder fires. Look under the distributer cap in these engines and you'll find the terminals for the cylinders placed at an even interval all the way around. Since the rotor turns one revolution for every two crank revolutions, the spacing is 120 degrees divided by two, or rather 60 degrees between terminals. With even spacing, you can drop the distributor in the motor and, as long as the wires are connected to the cap in the correct firing order, you can set the timing on cylinder #1 by simply turning the distributor housing until the motor runs. Dial it in from there... simple!

The Buick Odd-Fire

The Buick 225 is a true odd-fire V-6 engine. The crankshaft journals are shared and the pairs are located at 120 degrees around the crankshaft. This results in each pair of opposing cylinders firing in short sequence with regard to the crank turning. For example: (1) cylinder #6 fires, (2) the crank turns 90 degrees, (3) cylinder #5 fires, (4) the crank turns 150 degrees, (5) cylinder #4 fires... and so on. As you can understand from this, the odd-fire doesn't fire at consistant 120 degree intervals; it fires at 90-150-90-150-90-150. You might like to think of this as three V-2 engines: 6-5-pause-4-3-pause-2-1-pause (from the rear to the front). Interesting, eh? Now think about the distributor. If it fires evenly, the engine will be totally mistimed.

"HEI" V-6 distributors - a Primer

GM's HEI ("high energy ignition") distributors look alike on the outside, but have important differences on the inside which make them specific to the respective applications for which they were designed. The cap is the most obvious difference. The V-6 cap comes in two configurations: the even fire versions have evenly spaced terminals, whereas the odd-fire versions have some short terminals and some long. Upon careful examination of the odd-fire car, it becomes clear what's going on. The distances between terminals corresponds to the 90-150-90 firing sequence. (The terminals are spaced at 45-75-45... remember that the distributor turns once for every two turns of the crank!) The terminals must be located so that when the turning rotor is charged, a terminal is there to make the connection. But what charges the rotor at the proper time?

HEI odd fire distributor cap

Photo of an HEI odd fire distributor cap and diagram of an HEI odd fire pole piece and reluctor.

The rotor is charged by a change in the magnetic field of the reluctor (i.e. magnetic pick up). The reluctor is located in the distributor housing (see the photo) - it looks like a ring with points facing inward when viewed from above. The magnetic field is changed by the passing of the points of the pole piece, which is the three-pointed star-shaped piece mounted on the distributor shaft. As the points of the pole piece pass the points of the reluctor, the change in the magnetic field triggers the rotor to fire. The six points on the reluctor ring are spaced at - you guessed it - alternating 45 and 75 degree intervals for odd fire V-6 engines.

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To recap, in the odd fire engine, the three points of the pole piece pass the "even" points of the reluctor, triggering the rotor to fire at the precise moment it passes an "even" terminal on the distributer cap. This terminal must be connected to an "even" cylinder by the spark plug wire. The crank then turns 90 degrees, the rotor turns 45 degrees, and everything hits on an "odd" cylinder, the crank turns 150 degrees, the rotor turns 75 degrees, and we're back to an "even" cylinder, and the cycle continues.

Semi Even Fire

Once you've wrapped your brain around the odd fire engine, consider the semi-even fire engine. In the late '70s, GM decided to combat the vibration of the odd fire V-6 by eliminating the shared crankshaft journals. The resulting engines displaced either 200 or 229 cubic inches. The semi-even fire V-6 was common in Chevrolet cars of this era. The new crankshaft had split connecting rod journals offset by 18 degrees, resulting in a firing sequence of 108-132-108-132-108-132 degrees. This is the same concept of 6-5-pause-4-3-pause-2-1-pause as the odd-fire, but the pauses are shorter. To make matters more confusing, the distributor cap for the even fire and semi even fire V-6's are the same. It seems the terminals under the cap are long enough to pick up the charge from the rotor even if it is early or late by 6 degrees. Therefore, the only way to tell you're looking at a semi-even fire distributer is to examine the reluctor and pole piece. The pole piece will be the now familiar three pointed star and the reluctor will have six points spaced at alternating 54 and 66 degree intervals.

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Setting the Timing

In order to set the timing on an odd-fire or semi-even fire V-6, one must be very careful to align all of the components of the ignition system so that the motor runs properly. The frustration with these engines stems from the fact that a mistake results in a poorly running engine, but a running engine nonetheless. To set the engine up, first set the #1 cylinder to your timing point (~10 deg before TDC). Remove the cap and rotor and all plug wires. Observe the relationship between the points on the pole piece and the points on the reluctor. When the #1 cylinder fires, you want the points to line up on an "odd" cylinder. This means that after the cylinder fires, the rotor must turn 75 (or 66) degrees to fire the #6 cylinder, which corresponds to the wider gap between points on the reluctor. So, when you observe the points on the pole piece and reluctor, you must rotate the distributor housing to align on an "odd" cylinder. Measure the distance between the reluctor points with a caliper if necessary to verify that the next alignment of the points will occur after the "pause," or longer gap. Tighten the housing and replace the cap and rotor, noting which terminal the rotor is pointing toward - it should be an "odd" terminal under the cap for odd fire engines. This terminal must be your #1 cylinder now. You can connect the rest of the plug wires in the firing order after #1 is established (6-5-4-3-2). There is only one way to do this properly.

Chevy 4.3L Even Fire V-6

A quick word about the Chevy 4.3 liter even fire V-6, which replaced the semi even fire 200-229 in the early eighties. This motor's crankshaft has split journals as well, but they are split at 30 degrees offset. This results in even firing at 120 degrees. The early engines are known shakers so GM later added a balance shaft to calm them down. The motor has evolved through the years to include EFI and Vortec cylinder heads, with turbo versions as well. The engine bolts to any transmission with the small block Chevy bellhousing and parts are cheap and plentiful. This motor is great for street use in a British car conversion due to its prodigious bottom end torque. The above discussion on setting the timing does not apply to this motor - the even fire design has six points on the pole piece and six on the reluctor so it is as easy to set up as a V-8! If you read and understand this article - especially if you don't own an odd fire V-6 - congratulations, you now know a secret of automotive mechanic's lore and you are a certified gearhead. You can also challenge your buddies when they tune up their even fire motors and act all proud of themselves - "Yea, but can you tune an odd-fire motor?"

Disclaimer: This page was researched and written by Will Holoman. Views expressed are those of the author, and are provided without warrantee or guarantee. Apply at your own risk.

Photos by Will Holoman. All rights reserved.

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