Can too much boost pressure really cause long engine damage?

[Krusty70]

Before I launch into buying a Bighorn I've been reading and learning a lot about turbo boosted direct injected diesel engines (4JB1T mainly) from the many and varied topics on this (awesome) forum.

From what I understand, and lacking any real technical knowledge:
1. The volume of diesel in the cylinder at the time of ignition will burn until all the oxygen is used. Overfuelling beyond this produces sooty black smoke.
2. The volume of burned diesel and therefore energy (and heat) is proportional to the power generated. More burned diesel = more power = more heat.
3. The cylinder pressure from the gaseous expansion of the burned diesel must be far greater than the 17:1 compression of 20psi worth of boost pressure.
4. The temperature of the air at TDC of the compression stroke is what ignites the diesel.
5. The temperature of the compressed air must be greater when running 20psi boost than 10psi, but by how much and is it significant in terms of possible engine damage when the diesel is burned?
6. The factory setting of the 4JB1T is for 10psi boost with a 15psi limiter.
7. The timing of diesel injection also important in using the generated power. Too advanced and there are too many stresses on the internals and mechanical damage will result. Too late and the generated power is less useable due to crank position, leverage/torque, etc.
8. Fuelling is tweaked/increased by boost pressure, but this must be relative.
9. EGTs are good to monitor to prevent piston meltdown. Below 600 degC is good. More air has a cooling effect.

Correct me if I'm wrong but from what I understand as long as there is plenty of oxygen available, it's the fuel volume and timing that determines power and combustion chamber temperature and not so much the boost pressure.

So if the boost pressure is set to 15psi and the fuel is adjusted to just below the over-fuelling point, then the boost is increased to 20psi so there is an excess of air/oxygen and the EGTs are low, would this extra boost really reduce the life expectancy of the engine??

If you haven't fallen asleep or moved on by now then any knowledge and experiences would be great.

I'm considering buying a 4JB1T that's been boosted to 20psi and runs low EGTs, and it's got some great power, but I'd prefer to drop it back to 15psi and not kill it than enjoy it for a short time.

Cheers.

[geeves]

1 true sort of. Fuel atomisation is never good enough to allow all the oxygen to be used Overfueling and unburnt diesel happens well before all the oxygen is gone. This is the real advantage of common rail. The higher fuel pressure allows better atomisation so better power and economy. Computer control also helps. power is controlled by the amount of diesel not air. Extra boost without extra fuel does give a tiny amount extra power but not enough to make a difference.
2 generally true
3 You can calculate this. Take the torque in foot pounds then divide by the stroke (roughly 4 inch or 1/3 foot stroke so divide by 1/3 (multiply by 3)) then divide by the square inch area of the piston top. You will end up with a figure in the hundreds of psi.
4 true
5 this is nothing compared to the temperature of the burning diesel
6 10 and 16
7 true Both retarded and advanced timing cause the motor to run hot as well and advanced causes a loud knock. Its not uncommon for engines running higher fuel and boost to also advance the timing slightly to get more power but its a fine line between a raging demon of an engine and a hand grenade. Leave playing with this to an expert.
8 boost compensator in the top of the fuel pump
9 More useful to save the turbo and head but does give an idea of whats going on in the engine. The actual center of the combustion chamber does momentarily hit temperatures well over 1000c which is plenty hot enough to melt pistons if they were not adequately cooled. Piston cooling on the 4jb1 is by oil spray on the underside. A poorly maintained engine can have a few mm of sludge build up on the underside of the piston. This could be the reason for many an engine failure.

Standard turbo wont boost a 4jb1 to 20psi so this engine has a different turbo Messing with the boost could cause other issues. Max boost is controled indirectly by throttle setting. If you dont plant boot you dont get full boost. My wagon runs 14 at full noise but cruising along the motorway is only showing 5

EGT can be misleading. If the sensor isnt in the exhaust flow as close as possible to the exhaust valve it will read low.