Thread: 4BD1T Turbo Sizing and Performance Prediction.

Another note on surge.

Often surge can be avoided if the engine is allowed to accelerate quickly through the lower revs.

However if the engine is labouring somewhat, the heat energy (egt) in the exhaust gasses builds quickly, compared to engine speed, so the turbine spins the compressor faster, while engine revs are still low, and surge can occur.

Remember surge the unstable condition that occurs when the air flow from the compressor is higher than what the engine can swallow.

EFR6255 intercooled plot here: BorgWarner MatchBot

I did this for Jitterbug, this is the same compressor that Flagg used, but a smaller turbine for more low end boost. I've just run the plot to 25psi. The smaller turbine shows drive pressure increasing to above boost as you get closer to 3000rpm.
This shows quite well the tradeoff between a bigger turbine for more top end vs a smaller turbine for more low end. I have also used higher A/F ratio (18:1) and lower EGT since this would be a heavier 6x6 application and running under more boost more often. Especially for 4BD1+T engines which don't have piston squirters. They should run a leaner A/F ratio and lower EGT than the factory T engines with oil squirters cooling their pistons.

Originally Posted by Bush65

...

Regarding turbine A/R (area / radius), it is an important factor. Again Garrett give turbine A/R, but other manufactures don't, and instead give an area (usually in square cm), or a "phi" value. These are all valid indication of how much exhaust gas the turbine can handle.

Because the temperature of diesel exhaust gas needs to be much lower compared to a petrol engine, it doesn't have as much energy for the turbine to convert into mechanical energy to drive the compressor. Particularly when we want boost at low engine rpm (much lower than a petrol engine) the mass flow of exhaust gas is low. So we need a smaller turbine. As the engine accelerates the flow increases and when the turbine capacity (area, or phi) is exceeded, we need a waste gate to bypass the excess.

...

The energy, or more precisely, the enthalpy of the exhaust gas, is a function of the mass flow, temperature and pressure. The more we have the faster the turbine can spin the compressor.

The temperature has to be restricted for material strength purposes, but we can increase the enthalpy, and thereby drive the compressor faster, by increasing the pressure in the exhaust manifold.

This is where the size of the turbine is so important. The turbine is a restriction to exhaust gas flow and the smaller it is the greater the restriction and higher the pressure (and enthalpy).

But the flow of exhaust gas increases as engine speed and boost pressure increase and a turbine that is too small will create excessive pressure in the exhaust manifold. When the pressure in the exhaust manifold is greater than the pressure in the inlet manifold (boost pressure), we have nett pumping losses and increased residual exhaust left in the cylinder, which prevents filling with fresh air (lower VE).

Latest effort.
Borg Warner EFR6255 (smallest in the EFR range) on a 4BD1T with a maxed out pump and intercooler at 3,600rpm using very safe A/F and EGT. Should be good for about 280hp. Possibly more as I've used conservative figures: BorgWarner MatchBot

Originally Posted by Dougal

Latest effort.
Borg Warner EFR6255 (smallest in the EFR range) on a 4BD1T with a maxed out pump and intercooler at 3,600rpm using very safe A/F and EGT. Should be good for about 280hp. Possibly more as I've used conservative figures:

Which mount style do they use??? (I assume not a T2?)

Originally Posted by isuzurover

Which mount style do they use??? (I assume not a T2?)

T25 in that size.

Originally Posted by Dougal

T25 in that size.

How different is that to what I have atm???

Originally Posted by isuzurover

How different is that to what I have atm???

It'll bolt straight up. The port may need a bit of matching, but that's all.