EGT Question

[Vanagoner]

Added fuel (rich mixture) makes gassers run cooler, and makes diesels run hotter.  
I think that egt's are most critical to watch on turbocharged engines, because the exhaust outflow restriction (the turbo) also causes heat to pile up inside the engine.  This is never an issue on an N/A, and probably not on a supercharged diesel with a free flowing exhaust either.  Just guessing about that-  the only supercharged diesel I've driven was a '65 GMC two stroke city bus with a two speed automatic.  Ran cold all of the time.  Loved the sound of that one too.

[KTZed]

Hey Gearhead, what kind of EGT's were you seeing on those rotarys...cause they run even hotter than piston gassers dont they?

[saurkraut]

Spark ingition moters have to esentally run a stoichiometric air-fuel ratio.  They can't generate high EGTs with out damageing the pistons and spark plugs first.

If they run significantly leaner than stoichiometric, all kinds of nasty stuff happens: Detonation ( and all the associated joy: broken rings, chunks of piston being blasted loose), spark plug electrodes melting off.

If the run significantly richer than stoichiometric, they belch black stinky smoke, and foul their spark plugs.

Within these limits, I belive the EGT stays relatively low.  Faliure befor the turbo will happen befor any bad things happen to the turbo.  Unless a piston chunk, or ring makes it out of the engine and nukes the turbo.

Mater of fact, EGT is useless in tuning a gasser.  Good and bad happen all within a couple of hundred degrees, and thermocouple accuracy/interchangeability is not good enough to prevent engine damage.  I tried it on super modified two strokes, and its a useless measurement.

[KTZed]

What do you think about this theory? They're speaking about SI not CI engines but I imagine that injection timing would have a similar effect to ignition timing in our diesels.

"Now there is a huge misconception in regards to EGTs and what is and isn’t harmful to your motor and this is where many are confused. We regularly see 975-1000C while road racing and typically the car will be run at these temps for 20-25 minutes. OMG your pistons are going to melt is often what comes to peoples minds but this simply isn’t the case. I tend to be conservative with ignition timing as this puts less stress on the rods, the bearings, results in a smoother torque curve, and it runs the engine cooler. Yes, I said cooler. By running less ignition timing you are giving the engine less time to burn the air/fuel mixture which often results in a less complete combustion process therefore the process continues to burn as the exhaust valves open resulting in a higher EGT reading. More ignition timing starts the combustion process earlier putting more stress on the rods (due to more angle), the bearings, the pistons, and retains more heat in the cylinders even though the EGT reading will be lower. By giving the cylinder more time to burn the air/fuel mixture more heat is retained in the cylinder and therefore more heat is transferred to the pistons. While this does result in more power (up to a point) it’s much harder on your internal engine components. My point is that high EGTs typically mean less heat transferred into your pistons, not more. This is seriously simplified but I hope you get the picture."

[KTZed]

I agree with that except the line "My point is that high EGTs typically mean less heat transferred into your pistons, not more." That statement is only true if the fueling is unchanged and the timing is changed.

Agreed. I think thats what he was trying to get across. All else being equal, with timing as the only variable.

Now, in theory, the more retarded the timing, the hotter the turbine inlet temp (TIT), the more efficient the turbo will be up to the point of turbine failure. Conversely, the more advanced the timing, again up to a point, the more BMEP you will get but it will be harder on the bearings, rods, etc. and you will be putting more heat into the head, valves, pistons, putting them at risk.

Im applying this to my Datsun as well cause I really want to try a big VNT/VGT on it and I dont want to cook the turbo.
Does anyone have EGT records before/after water/meth injection with no other changes?

[saurkraut]

I've had the pump go a whole tooth off once when  bird and grill parts sneaked inside the timing belt cover.  Birdstrike...

Instantly Sky high EGT and boost.

I prefere to leave the timing alone (stock) and putz with the waste gate and pump fueling.

If you want to try something and report your findings here, cool.

Oh by the way, I have a turbocharged '73 240Z sitting in my garage.

[KTZed]

Bird in the timing belt eh  :shock:  thats a new one

I dont imagine its beneficial to play with the static timing on the pump but I imagine the advance curve can be modified throughout the rev range (Im sure thats part of the magic Giles works on these things).

Glad to find a fellow S30 owner on the forum! Mine is fairly stock at this point (susp mods and L28 with SU's) but have been trying to get it together to do the L28ET....or there is always that RB25 in the garage :twisted:  :twisted: