Automatic timer, Cetane Number and Turbos (1 Viewer)


Jan 9, 2008
Moronville, ME
Has anyone here attempted to tune their pump timing curve by altering the springs or shims on the automatic timer (AT)? As the AT wears does timing advance or retard? I'm thinking it advances because there would be less pressure on the springs that limit pump advance. I'm specifically talking about a 3B but I think the principles are the same for any of the inline mechanical injection pumps

A lot of folks here insist that the only way to properly time an inline pump is using the prescribed spill tube method in the FSM. Static pump timing doesn't help determine how far advanced the timing is at higher RPMs.

Here's a snapshot of the advance angle specs for the automatic timer from the B Series FSM. Note that pump RPM is 50% of engine RPM and the pump advance angle would be double the figures in the chart. i.e. 3B engine at 2800RPM should have 7.6 - 9.6 crankshaft degrees of injection advance. A 13B-T would have a max of 1 degree at the same RPM. The 13B-T automatic timer ramps up quickly between 2800 to 3400 engine RPM to a total advance of 7 to 9 crank degrees.


Some boring background info follows:

Back in December I broke the crankshaft in my 3B running only 12 psi of boost measured post-intercooler. The engine was rebuilt with only 25k miles on it since the rebuild. I kept the pre-turbo EGTs at 1100F or less the entire time I owned then engine. Engine ran strong with no strange smoke or vibrations.

Post-mortem diagnosis kinda indicates combustion timing was too advanced. There's no indication of other problems in the engine. Static pump timing was right on the money at 10 degrees BTDC for a normally aspirated 3B.

As I drove across the country from Maine to Texas most of the fuel I purchased had a Cetane Number of 40 on the pump. I was running the grey bottle Power Service cetane booster for all of the trip, at concentrations that likely exceeded the specs because I didn't have a good way to measure it when pouring it into the tank.

In Texas the rating at the Love's biodiesel blend pump was 50. I stupidly continued to add Power Service additive. The crank broke before I finished the 2 tanks of fuel when climbing a long, steep hill at 3k RPM, 12PSI and 1100F EGT.

A higher cetane number means the fuel is much more volatile and will ignite sooner in the compression cycle. Add some boost and the combustion chamber temps will rise faster on the compression stroke enabling fuel to auto-ignite sooner.

I'm thinking the combination of high cetane fuel, boost and pump advance is what contributed to my crankshaft failure.

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