Thread: new exhaust

Can someone tell me if I'm on the right track?Turbo and intercooler is fitted,time for new exhaust,so 3 inch mandrel bent one fitted,from the turbo back,EGT has dropped by 100-150 deg.(after turbo)same long hill was rising to 550 now 420,I know some say to fit the piro before the turbo,and one day I might,but for now its where it is,as yet I don't have a boost gauge.How can the temp drop so much?Are the EGT reading lower because the sender is in a larger pipe?Would the bigger exhaust have increased the boost enough to lower the temp?Is up to 500 deg a safe limit?
One other thing,the red service thing on the air filter has now poped up after a hard run.
Jim.

Originally Posted by yt110

Yes its a "freefloating"turbo,some numbers off the turbo are
comp-T28 60 trim AR.48
turbine-T25 62 trim 53.8 0.89
466898-6
I dont really know what they all mean,I get a bit lost with all the different turbos others have fitted.
Jim.

The pic below shows where the inducer (inlet) and exducer (outlet) diameters are measured on the compressor and turbine wheels (impellers).

I think of T25 and T28 as family or housing sizes - larger number denotes larger housing and range of air/exhaust flow capacity.

Within a particular housing size, the A/R (area/radius), inducer and exducer diameters can vary to further optimise the flow (within the range of the housing) to suit different engine requirements.

For the compressor:

• Air flow capacity is most dependent on the inducer diameter (actually the area determined from diameter, i.e. proportional to inducer diameter squared).
• Boost pressure (actually PR, i.e. the ratio outlet/inlet pressures) is most dependent on the impeller tip speed squared, so it is a function of impeller rpm and exducer diameter squared. Poor efficiency limits small compressor impellers to low PR.

Compressor Trim = (inducer^2 / exducer^2) x 100 and gives a good indication of compressor efficiency - smaller trim gives better efficiency.

Better compressor efficiency means the compressor will create lower temperature in the charge air (desirable).

Increasing any of; compressor inducer diameter (greater air flow), exducer diameter (greater boost pressure), and impeller rpm (greater boost pressure), will require more torque from the turbine.

Larger impellers, both compressor and turbine will have a higher moment of inertia. This results in more time taken to change impeller rpm. Manufactures endeavour to reduce it as much as expense and reliability permits - even with cost constraints, exotic materials are often used.

For the turbine:
Smaller impeller will produce more torque at low engine speed (when there is less drive pressure from exhaust gas flow and temperature), but can over-speed and create too high back pressure at higher engine speed and load. A waste gate is commonly used to control turbine speed and reduce back pressure.

For the turbine size, alternative A/R housings are used to optimise the turbine to the lower or upper range of performance. Because diesel engines have lower exhaust gas temperatures than petrol engines, they nearly always use lower A/R housings.

Turbine Trim = (exducer^2 / inducer^2) x 100 . Unlike compressors, changing turbine trim does not have a great affect.

See 2nd pic for meaning of A and R. A/R is the resulting value from dividing area A by radius R.

Variable nozzle geometry has become common for turbos used on diesel engines. Changing the nozzle geometry gives results similar changing the A/R of the housing. Closing the nozzle area (small A/R) will spin the compressor faster, resulting in more boost at lower engine speed. Opening nozzle area reduces back pressure and controls turbine rpm, giving better performance at high engine speed.

Small turbines by necessity, have small shafts, which have small torsional strength. This lack of strength is highlighted in high performance VW Tdi engines. Being small engines they have small turbos, typically GT15V or GT17V. These turbos often break shafts at high boost pressures or when the compressor surges.