Re: Is driving a cold engine bad?
I have the hardest time drilling that into the heads of customers and new employees in the engine shop...
Everyone wants to machine parts while they are cold with no load on them, and expect them to operate correctly when they are warm with a load...
When you bore and hone a cylinder on a stripped block, you get a nice round cylinder while the engine is cold with no head bolted on.
Cast iron is SOFT, and distorts enough you can actually see it deflecting/ distorting under load.
A. If you heat the block, the bore expands, and the head bolt holes actually move farther apart.
B. If you have a head bolted on the cylinder, the cylinder top will distort.
C. Heat that block and head up, and you have even more distortion.
D. Heat that block and head up a different rates (like when an engine is first started) and you will have shifting as well as two rates of distortion going on....
E. Heat will affect the top of the deck to cylinder top connection at a different rate than the cylinder walls them selves.
The deck will transfer heat away from the top of the cylinder faster in the beginning, the stress of the head bolts will move that cylinder 'mouth' around in strange shapes, and if you have connected cylinder walls between cylinders (Siamese cylinders), you can add oval cylinders to your problems...
F. Now add in the expansion of the piston, and piston rings...
The piston is aluminum, and will expand at roughly twice the rate and twice as much as cast iron.
The piston also has the distinction of having several different amounts of mass in it's design.
The bosses around the wrist pin will expand more because there is more material, but they will expand slower because it takes extra mass longer to heat up.
May makers cut the tops of the pistons (makes the head thinner in places) or drill on the bottom sides of the piston heads to 'balance' the pistons (again, making the head thinner in places).
Piston rings take a REAL a$$ whoopin' through all of this! Their piston grooves expand and contract, the piston diameter expands and contracts, and it expands in odd shapes, the cylinder expands and contracts in odd shapes around the ring, and the ring it's self expands and contracts in the middle of all of this chaos...
The top of the cylinder is where your compression pressure is highest, so any small gap in the piston ring to cylinder wall fit will be a MAJOR leak.
When we do an engine here, we always heat the block to appx. 180 Degrees, Use heated lubrication for the hone, and use a torque plate to simulate the load of the heads, and have the main caps on and torqued to simulate crankshaft main cap distortion.
This way we get a nice round cylinder when the block is hot and has heads bolted on.
On some V-8 types, we even bolt the two torque plates together to simulate the intake manifold and the way the intake will cause the heads to move when the block heats up.
On top of all of this, you have to realize the engine actually grows taller, and in the case of the V-8's, wider because the two banks of 4 cylinders each of the 'V' are growing 'UP' in different directions
Then figure the gallery end plates that hold those cylinder banks together are pulling in on the cylinder banks as they try and grow 'UP' .....
The cylinder banks are literally being pulled in a 'U' shape as they heat up and expand upward.
I almost forgot, NO OIL TO THE METAL ON METAL PARTS AT START UP!
It takes as long as 1 minute after start up to get a reliable oil supply to the top end of older engines.
Bearings can go as long as 30 seconds without oil on startup.
All of this is slow death to your engine.
By the estimations of SAE, ASE, EAC, GM, FoMoCo, Ect. 75% to 90% of all wear on engine parts occurs at cold start ups....
The bearings rely on an oil film being held in place by OIL PRESSURE. There is no oil pressure at startup.
You are grinding metal on metal at the main bearings to crank, crank to connecting rod, connecting rod to piston, cam to block, cam lobe to lifter, ect...
Pretty destructive stuff... Dry starts are the absolute worst thing you can do to your engine, and on most stock vehicles, unavoidable.
If you have a turbo charger, the turbo is the first thing to stop getting oil (at the top end of the oiling system) and last thing to get oil on start up (again, at the tail end of the oiling system).
Some turbos will go for a minute or more before they get a reliable oil supply, and will spin VERY HOT for up to five minutes after the engine is shut down with no oil...
So, lets recap...
Cylinder tops that are star shaped and expand and contract spreading the head bolts at angles, heads that expand at a different rate, and crawl around on the block deck surfaces like a snake in a frying pan.
A Block that is growing taller and shorter, and in the case of a 'V' cylinder configuration, cylinder banks that are 'U' shaped.
Pistons that expand and contract barrel shaped, and at two to three times the size and rate of the cylinders.
Rings that have to try and seal the entire mess up enough to keep the engine not only running, but make horsepower and torque.
And do all of this with little or no oil protection.
And people still wonder if they should allow the temperature to stabilize before driving ANY engine...?
I'm always stunned that any engine runs at all!!!
So many cats, so few recipes...