Engine buffs: why do our engines need to warm up?

Hey folks,

When I start my daily driver, it fires right up, ready to drive off. Now I don't know about your 7123 but mine needs nearly a minute of warming up time before it can idle properly.

Why is this? I'm looking for technical explanations

Cheers,

Vincent.

its a @Don C question! So i'm sure he'll come up with a tech answer!

for my part i'd say its all about atomisation.

The carbs on these v8 spray a significant amount of fuel and when cold, its lots of fuel in liquid state.

Loads goes wasted too and mix in chamber isn't well balanced.

That's another plus for the EFI. No more holding the engine with the pedal.

Hey folks,

When I start my daily driver, it fires right up, ready to drive off. Now I don't know about your 7123 but mine needs nearly a minute of warming up time before it can idle properly.

Why is this? I'm looking for technical explanations

Cheers,

Vincent.

Click to expand...

Because your engine does not burn liquid fuel. It burns vaporized fuel. It takes heat to change a liquid to a gas.

Hey folks,

When I start my daily driver, it fires right up, ready to drive off. Now I don't know about your 7123 but mine needs nearly a minute of warming up time before it can idle properly.

Why is this? I'm looking for technical explanations

Cheers,

Vincent.

Click to expand...

Because your engine does not burn liquid fuel. It burns vaporized fuel. It takes heat to change a liquid to a gas.

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Bingo. Carbs don't vaporize fuel well without heat. Fuel injection does a much better job cold and direct injection even better still.

So where does the vaporizing happen then? In the carb? Does that get warm enough? What if you have a thick spacer? Or in the intake?

That is why you have the heat stove on the exhaust manifold with the tube to the air cleaner snorkel in a closed system.

So where does the vaporizing happen then? In the carb?

Mainly yes, but when warm, the intake helps with the inside heat. The carbs also have a special shape that helps accelerating the air to create myst.

Another problem on v8's is the distance from the fuel source and the chambers. When cold in winter even if the myst is "ok"

there is some condensing of the myst on intake wall returning liquid before it can reach one cyl chamber. As Hemikiller pointed out, a direct injection removes this distance.

A cold engine needs a higher fuel to air ratio than a warm engine, this is because it cannot vaporize the fuel as fast as a warm engine.

A carburetor cannot easily adjust for this, most have a choke that can restrict the incoming air to create more vacuum and allow the engine to take in more fuel. Because of this the car cannot perform well until it has warmed up and the choke has been opened back up.

In a Fuel injected car the fuel injectors are just set to add the extra fuel as necessary, this allows the engine to be ready almost immediately.

 

I am using an EFI setup and don't have to wait for the motor to warm, acts like a modern day car.

Good explanations. Fabrice is right on about the fuel condensing on the walls of the intake manifold, one of the biggest problems with carburetors.

Which raises the question, why do throttle body fuel injection systems work so much better than carburetors? It's all about fuel atomization. Carburetors work by the air flow through the carburetor pulling fuel out of the carburetor nozzles in droplets, not really well atomized. The throttle body injectors spray the fuel into the air stream in a fine mist. As John said, the F.I. system adds extra fuel, when cold temperatures require it. This compensates for the condensation on the walls of the intake manifold, without restricting the air flow. Closed circuit systems, with O2 sensors, work better, more accurate at adjusting the fuel quantity. Injectors right before the intake valves work even better, with direct injection into the cylinders being best.

That all makes it very clear guys, thanks for explaining! Didn’t think that snorkel is important for starting up. Many cars don’t have it. Does the snorkel make a big difference for starting up or was it more an environmental thing?

it does help on start up, just like closing a throttle plate chokes the carb, the snorkel being closed also reduces the airflow. How much is debatable.

TBI systems operate at a higher pressure than a carburetor which makes atomizing fuel easier with a cold engine.

The snorkel, when connected to the exhaust manifold heat exchanger, is designed to improve cold engine driveability. The exhaust manifold heats up quickly and the warm air from it helps to atomize the fuel until the engine warms up enough.

The heat riser tube does restrict the air flow some, but less than the choke. When everything is operating correctly the flapper in the snorkel should open about the same time the choke is fully open. Just the open snorkel is somewhat restrictive, which is why so many original air cleaners are replaced with open style air cleaners. Removing just the snorkel probably worsens the air flow due to air passing over the sharp edges of the air cleaner housing.

That all makes it very clear guys, thanks for explaining! Didn’t think that snorkel is important for starting up. Many cars don’t have it. Does the snorkel make a big difference for starting up or was it more an environmental thing?

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For my car at the back of the air cleaner there is a temp switch in the base that is used to control the vacuum motor on the snorkel.  The snorkel would direct air from the outside air or air directly from around the exhaust manifold.  When you do a cold start the air would be ported from around the exhaust manifold which would get heated from the exhaust to help atomize the fuel.  When the air temp rose the switch would cut off the vacuum to the vacuum motor and it would close off the exhaust manifold heat and fresh air would then be directed to the air cleaner because the engine block heat would provide the heat needed to atomize the fuel going into the intake and you would want cold air entering the motor for a denser air/fuel mixture. 

Hope this makes sense.

I see Don answered this too.

I understand. I do have the original aircleaner and remember seeing some vacume parts. But do I understand correctly that most original aircleaners are replaced with others without snorkel becauserhey deliver better/more air flow? And were ALL 73’s delivered with a snorkel and tube to theheat shield?

I understand. I do have the original aircleaner and remember seeing some vacume parts. But do I understand correctly that most original aircleaners are replaced with others without snorkel becauserhey deliver better/more air flow? And were ALL 73’s delivered with a snorkel and tube to theheat shield?

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Yes, I believe they were. As much due to emissions as to operation, to contain has many fuel vapors as possible. I'm sure there are jurisdictions where they are anal about such things that still require the stock air cleaner.

These old cars are like women. If she is cold, you aren't going to get anywhere with her. When she's hot and raring to go, you'll go places you only fantasize about!

These old cars are like women.  If she is cold, you aren't going to get anywhere with her.  When she's hot and raring to go, you'll go places you only fantasize about!

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lollerz

I knew when I saw you had responded to this thread that it would be a good one.

But do I understand correctly that most original aircleaners are replaced with others without snorkel because those chrome round ones, and other aftermarkets looked HOT under the hood

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Fix't :whistling:

Chrome open element air cleaner assemblies, wow. There was a time when everyone had one and the factory air cleaner assemblies were ditched asap. It amazes me now to see factory air cleaner assemblies on so many musclecars. Where did everyone find them years later to reinstall? When I bought my '68 GTO 400 4 speed in 1980 it had a very small chrome open element air cleaner on the Rochester Quadrajet. I swear it was the size of those air cleaners you'd see on the Pontiac Tri-Power carbs. Had to be restricting air into the carb. I replaced it with a normal-sized Moroso chrome air cleaner assembly. Obviously the chrome open element air cleaner had no heat from the exhaust manifold to close off a snorkel, and my choke wasn't working very well. That car was my daily driver for a few years even in the northeast winters. Many winter mornings I have no idea how I kept that engine running, but it sounded like it had the most radical cam known to mankind when it started, and idle speed was set with my right foot. hehe Yep, now you just turn the key and you're on your way, regardless of weather.