New Age Boost Pressure

Some important facts about turbo boost and diesel performance chips

Words by Andrew Leimroth

Photography by Caravan & Motorhome

So how does this new diesel produce so much power so cleanly? Well, it has a multivalve engine, common-rail high-pressure diesel injection and all sorts of electronics. But is this really where all the power comes from?

Boost pressure is the pressure left in the inlet manifold. Without a turbo, when the inlet valve opens, the engine has to suck air through all the pipes and air-cleaner to get it. With a turbo installed, the engine now has air supplied by the turbo right to the inlet valve. This means the engine now gets plenty of air whenever it needs it.

Boost pressure in the days of a SWB LandCruiser with 2 L-T engine was about 8PSI. Nowadays the boost in a similar-sized EFI controlled commonrail diesel can be about 18-20 PSI. With all this air available it’s easy to put in more fuel for more power, and burn it more efficiently. The problem is that to generate 20PSI boost pressure, you need more than magic. The trick comes from equalising backpressure on the exhaust side before the turbo.

General rule of thumb for creating boost pressure: 1 PSI inlet boost pressure needs 1-1.5 PSI exhaust gas backpressure to create it.

So you can see that the offset with having a turbo means that the engine has to cope with a lot of exhaust gas backpressure before the turbo can make boost for the engine. This backpressure normally is fine, but it can lead to undue strain on the engine if the turbo boost pressure in a modern EFI diesel is wound up much higher.

Common-rail diesel engines have every part of the engine monitored and managed from the air going in to the exhaust going out. Even the turbo boost is electronically monitored and managed. The exhaust gases required to drive the turbo have been closely controlled by engine design, including the flow of the exhaust system. Changing to a larger exhaust can have effects on the operation and boost level of the turbo.

Some EFI diesel engines can actually have the turbo over-boost because of the gas flow changes of upgrading the exhaust. This can be for better or worse so it is best to record boost operation and levels before you install an exhaust to follow the effects.

It’s advisable when choosing a tuning chip for your new common-rail diesel that you choose one that does not adjust turbo boost, as common-rail diesels already have more than enough turbo boost to cope with. Chips that adjust turbo boost are doing so to offset and hide heavy fuel load increases. Diesel chips that don’t adjust boost are for most of us wanting reliable power gains, as they are not adding excessive load to the engine. Remember, most of us want longevity from our tow vehicle not short-term racing.

Safe Driving

Andrew