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AFR vs Exhaust temps

Practical Diesel Tuning

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Mechanical fuel pump Tunning.

On older mechanically driven fuel pump engines, with relatively lean AFR’s you get higher exhaust temperatures, is this due to fuel injector efficiency.

is it safe to go to richer AFR’s despite these temps or should the EGT’s be treated as your limit.

I run a Toyota Landcruiser with a 4.2 liter 1hd-ft, boost pressure is at 32psi with AFR’s between 20.5 to 21:1 I see max exhaust temps pre turbo of about 730 Celsius at 3700 rpm in high gear under full load acceleration.


In the OEM world 1350*F is often treated as the abort temp pre-turbine for sustained output such as towing up a grade for minutes on end. Realistically you'd likely pick up significant power by turning the pump up and running a slightly richer AFR (18.5 for instance). You would just want to make sure that your use at that power level is intermittent to avoid part degradation.

The curveball in this equation is injection timing. I'm not familiar with the pump used in your application. It may be possible to advance timing 2-4 degrees which should lower EGT significantly compared to the stock timing spec.

Hope this is helpful,


Thanks for the above Nick,

the pump is actually an upgraded pump already running advanced.

unfortunately being mechanical the advance of the pump is set for the entire Rev range. This causes some diesel knock at lower rpms.

that being the reason why I wouldn’t want to advance timing further.

the internals of the engine are stock with upgraded turbo, intercooler, exhaust, air box, injectors and high pressure pump.

Your pump is a Bosch VE style and has a timing advance mechanism built in, which is controlled by transfer pressure (which is dictated by rpm).

You could try advancing your static timing just to test what effect it had on full load power. If you decide you want that extra timing but only at full load, pull your static back down to where it was and work out how to adjust your mechanical advance. From memory there is a little cover on the side you can remove and make an adjustment with an allen key or screwdriver to adjust spring pressure for more or less timing as rpm rises.

Edit: come to think of it, leave static timing, just adjust the timing advance and see what it likes. Will be significantly easier.

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