Diesel Tweaks: Performance and Fuel Economy

With a few inches of snow last night and crews that don’t seem too interested in actually cleaning road surfaces, the Suburban was in its element today.  It was very slippery early this morning, and it wasn’t much better when I came home late in the day.

I decided that if I’m going to be driving this machine, I should take some time to make a couple quick adjustments.  I decided to adjust the injection timing to tune for better fuel economy and to adjust the wastegate pushrod for more boost.

Injection Timing Markings

The marks show how the injection pump has been moved relative to the engine block.

When I bought this engine, the injection timing was set at approximately 8 degrees before top dead center.  To find this out, I disconnected the output connection for the #1 cylinder on the injection pump, primed the system, and slowly rotated the engine while looking for the point where the fuel would be released from the #1 port.  The 8 degree timing was an indication that this engine was originally retarded from the normal Isuzu specification of 13 degrees in order to meet California emissions standards.  Of course, I fixed that.  In the attached photo, you’ll see a row of three punch marks I made on the pump on the left side of the vertical joint shown.  On the right side, you’ll see a row of three marks which were in alignment with the three marks on the pump when it was still retarded.  When I set the timing to the normal Isuzu specification, I made a single new marking in that location.

In the photo, you’ll note that the pump has been rotated even further recently.  I thought I’d see if further advancement of the timing would help the engine’s fuel economy.  Given that that two marks on the right are about 5 degrees apart, I believe I advanced the timing another 4 degrees; taking the engine to approximately 17 degrees BTDC. It actually hurt the fuel economy by several miles per gallon, so I pulled all but about one degree of this additional advance out.  I’ll top off the tank, again, and watch my fuel economy for a few weeks, again, under a variety of conditions.

17mm Gear Wrench with Flex

This 17mm Gear Wrench with a flex joint near the end allows me to reach the top injection pump mounting bolt between the pump and the engine block.

When viewed from the front, rotating the pump counter-clockwise advances the timing, while moving it clockwise retards it.  The injection pump is mounted with four nuts on studs that have to be loosened to rotate the pump relative to the engine case.  These nuts aren’t easy to reach, especially the bolt at the top of the pump between the pump and the engine block.  To reach this, I use a 17mm Gear Wrench with a flex end.  When I get the flex angle just right I can use it to loosen and tighten that bolt.  I should find a ratcheting wrench with a smaller angle between clicks, because it’s so tight in this area that I can only get 1-2 clicks of ratchet between tightening or loosening movements.

I’ve been reading Dougal Hiscock’s thread on turbo sizing and performance predictions on the 4BTswaps website.  Dougal also provided performance predictions that made it into my newly-released e-book The Art of Diesel.  Looking at these, Dougal predicts that the maximum horsepower can be extracted from these engines by allowing the turbo to produce around 26 psi of boost.  Because I once experimented with disconnecting my wastegate, I know that

Threaded Wastegate Actuator Rod

The rod on the wastegate was cut and threaded. With this threaded coupler installed, I can now adjust the length to tweak my maximum boost setting.

my GT2259 will produces approximately 25 psi of boost measured at the manifold, after an intercooler that has about a 1 psi pressure drop (based on further experimentation).  When I previously tried increasing the boost above the wastegate’s 15 psi setting, I didn’t actually increase the performance, but I hadn’t backed out the fueling screw or removed the aneroid pushrod.  I have more fueling available, now, so perhaps the additional boost would be put to good use.  This shouldn’t hurt normal fuel economy, as 15 psi is rarely seen and the wastegate isn’t exercised that often.

I’ve threaded my wastegate’s arm and installed a sleeve nut to make the length adjustable.  Tonight I considerably shortened the length, and tomorrow I’ll see how much boost the turbo will provide.  Soon I should have some butt-dyno and mpg results to report.  I’ll get back on this soon!

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