6.5L Turbo Diesel Blazer Project

[More Power]

As mentioned earlier in this forum thread, Sarah's 1994 electronic 6.5TD is equipped with an aftermarket mechanical "Turbo-Master" wastegate actuator (shown here), which can be adjusted to manage maximum boost pressure. Currently, the Turbo-Master is adjusted to allow the factory GM-4 turbocharger to produce about 12-psi during a full-pedal run, and that boost pressure is relatively constant throughout the rpm range - assuming a more or less constant full-pedal accelerator position is maintained.

When originally stock, the electronic engine management system controlled boost pressure using the OE vacuum controlled wastegate to produce a boost pressure that varied across the rpm range even with a constant acc pedal position - with boost pressure maxing out at about 2400-rpm with around 7-psi max and then trailing off as engine speed increases above 2400-rpm. Various theories exist for why that's so... I suspect boost pressure, when stock, trails off as engine speed rises above 2400-rpm for a couple of reasons... 1- These electronic DS fuel injection pumps decrease fueling as engine speed increases due to both limitations in mechanical design and the EFI programming being used, thereby reducing the need for more boost pressure as fueling decreases. 2- And, reducing engine power/torque as rpm rises may help with emissions control and engine durability.



The above photo was taken by Sarah just last week while cruising on the Interstate at 75-mph - showing 7-psi boost pressure and 550-degrees F exhaust temperature. The EGT pyrometer is located in the crossover pipe a couple of inches below the flange where it connects to the driver's side exhaust manifold.

Not surprisingly, with the current Turbo-Master adjustment, there is almost no smoke produced by the 6.5 - under nearly all driving conditions. It's easy to experiment with boost pressures when using a Turbo-Master, and I'd like to experiment with a lower boost pressure for three reasons - say around a 7-psi max. 1- The current fuel rate doesn't need that much boost pressure to burn clean. For example, a stock Duramax when driven at these speeds/loads only produces about 3-psi boost pressure. 2- These factory GM-series turbochargers are not very efficient above about 7-psi - and it really needs an intercooler at 12-psi boost pressure. 3- The engine may be more fuel efficient when the wastegate is allowed to open more - to reduce turbine drive pressure while in a light load cruise.

Incidentally... with more than 5K miles since the rebuild (it's on its 3rd engine oil service), the engine oil still darkens at around the usual 3000 mile mark (this is typical for the 6.5TD), which indicates that a higher than stock boost pressure may not have a lot to do with how soon the engine oil darkens from soot. I thought it would have made more of a difference... I do know that there is no visible blow-by at the oil fill pipe (when the cap is removed) while the engine is idling.

Did I miss anything?

Jim