Turbo talk

In this article I’ll try to explain some of the names, acronyms and some basic turbo theory.

This article is very much a work in progress so if you have any suggestions or comments then let me know in the forum area.

Common Acronyms and what they mean.

Boost Pressure

This gets talked about a lot. The boost pressure produced by a turbo depends on a lot of factors. The figure that most people will quote is the maximum regulated boost pressure that the turbo is set to. The maximum boost pressure is only reached under full load at reasonably high revs. An engine revved in neutral will rarely hit its regulated boost pressure as there is no engine load. Boost pressure should always be measured in as high as gear as practical. The L series are artificially limited in the lower gears so boost is best measured in 3rd gear or above. Typically a standard L series will be producing full boost at 3000rpm (often sooner), but usually always by this point in the rev range.


This is a simple flap inside the turbo that allows the exhaust gases to bypass the turbine and take a short cut down the exhaust. This regulates the boost level produced by the turbo.


This is a little device that uses the pressure of the air compressed by the turbo to open and close the wastegate regulating the air pressure generated. The actuator as the boost pressure increases extends a rod to open the wastegate. The most common/simple way of altering the boost pressure is to adjust the length of the actuator rod which alters the spring pressure holding the valve closed and therefore the pressure that the valve opens at

Dump Valve / BOV

This isn’t needed on a diesel. Petrol turbos may need them (most don’t), but the L series diesel doesn’t need one so don’t bother spending your money on something that at best sounds like a bus releasing its brakes and at worst will decrease performance. As a brief explanation it is a valve that opens (dumps) the pressure in the air intake system after the turbo under specific conditions. These should be when the throttle is closed (and therefore air flow is VASTLY reduced and excess pressure is undesirable). As the L series diesel doesn’t have a throttle you don’t need one. A throttle is a flap that controls airflow – it isn’t the same as the accelerator pedal, but in most petrol cars the two are connected via a cable. Diesels don’t generally have a throttle.

EGR – Exhaust gas recirculation

This is the process by which a small controlled amount of exhaust gas back into the air intake. Why do this? Well its purpose is twofold.

Firstly and primarily it is there to reduce emissions. The emissions it targets specifically are NOx emissions, these aren’t tested as part of the UK MOT test which is why many people suggest removing the EGR valve or blanking it off.

Secondly it is there to help speed up the engine warm up cycle. recycling hot exhaust gases back into a cold engine makes it warm up faster. The later models further enhance this feature by putting a heat exchanger around the EGR connected to the engines cooling system to extract more heat from the gases.

EGT – Exhaust gas temperature

This is well what is says really. You want to avoid too high an EGT as this can damage the turbo or melt the pistons. Avoiding high EGT’s in diesel tuning is VERY different to how you avoid them in petrol tuning. Generally a diesel will have far lower EGTs than a petrol engine, and this is why you can’t generally fit a diesel turbo to a petrol engine. High EGT’s are in the order of 800-850 deg C in my view.

EMP – Exhaust Manifold Pressure

This again is fairly self explanatory. Its the gas pressure inside the exhaust manifold. This is NOT the same as boost pressure and it can very significantly from boost pressure. EMP is a necessary evil as is some is needed to give good boost response, however too much and you start getting issues like head lift. Anyone with a stage 2 turbo running high boost will know about this, or will do in a short time!