Everyone must have an idea how far the throttle blades are open with the throttle closed and the motor just idling, there isnt much of a gap for air/fuel to go through there! I am wondering how you can be driving down the road at say 65mph, your engine is at 3,000rpm's and you completely let off the gas. You will start to slow down of course, but somehow the motor is at nearly 3,000rpm's with the throttle blades practically closed! Is it only running from the rear wheels turning it over or what? I thought with an automatic the converter could just slip, but what about a clutch that's fully engaged? Also, why does an engine have to warm up? Does the fuel "stick" to the intake runners and clyinder head runners, or is it just not atomizing properly because of a lack of heat? Thanks Scott

As for the engine running down the road with the throttle almost closed, when the pistons go down, there's a rather large vacuum, but that vacuum also helps pull the piston up for the compression stroke some. Most of the extra engine speed does come from the wheels turning the rest of the drivetrain. Unless you have a really high stall converter, you won't see much slippage at 3000 rpm. You do tend to lose a good bit of speed, though, it's not like you'll coast on for miles without any throttle. The throttle blades don't open much, but it's just enough to get enough air flowing through when the engine is pulling its idle vacuum.

Engines have to warm up some because the fuel doesn't vaporize right when the engine is cold. That's why carbureted cars have a choke. The choke'll richen up the mixture so the mixture that finally reaches the cylinder is something that'll burn right. If you don't have a choke, the car will run lean when cold, and could detonate if you start loading the engine before it warms up some.

Originally posted by Jonathan:
As for the engine running down the road with the throttle almost closed, when the pistons go down, there's a rather large vacuum, but that vacuum also helps pull the piston up for the compression stroke some. Most of the extra engine speed does come from the wheels turning the rest of the drivetrain. Unless you have a really high stall converter, you won't see much slippage at 3000 rpm. You do tend to lose a good bit of speed, though, it's not like you'll coast on for miles without any throttle. The throttle blades don't open much, but it's just enough to get enough air flowing through when the engine is pulling its idle vacuum.

Engines have to warm up some because the fuel doesn't vaporize right when the engine is cold. That's why carbureted cars have a choke. The choke'll richen up the mixture so the mixture that finally reaches the cylinder is something that'll burn right. If you don't have a choke, the car will run lean when cold, and could detonate if you start loading the engine before it warms up some. =
Scott,
Some more thoughts from the peanut gallery :D

It's as simple as "An object in motion tends to stay in motion",, I believe it's called "kenetic energy" ;)
Goes for air, cars, trains etc. & is a good reason not to run real fast towards a brick wall, the stops a bitch :D :D

Jonathan,
Interesting idea but,,,, once the piston crosses BDC & starts up there is no longer a vacuum & as soon as the intake valve closes you are building pressure.
Also, technically, a vacuum does not "pull" on the air rather it is higher air pressure trying to fill a lower pressure area or "vacuum".
Air (or fuel) flows from hi pressure to low pressure because good old mother nature doesn't seem to like pressure differences smile.gif

Mike, it's true how the vacuum doesn't "pull", I was just using those terms to try and say what I meant a little more clearly. I still think the vacuum will help pull the piston up some. Say you're pulling 20 something inches of vacuum at idle. When the piston is at BDC on the intake stroke, the cylinder is filled with a charge of air and fuel at your intake manifold pressure, probably a little less due to the restriction of the relatively small area of the intake valve as compared to the piston area. Now, on the other side of the piston, there's going to be some pressure above atmospheric due to blowby, even in a brand new engine (not sure of the exact pressures inside a normal crankcase). Wouldn't this difference in pressure somewhat push the piston back up? I know it won't go all the way up, but it is somewhat of a help. Probably you'd see a bigger difference in stroker motors, where you're trying to fill more displacement without being able to run much bigger valves.

I do agree with you about the kinetic energy, there's a good bit of rotating mass in a drivetrain, especially with a manual transmission and flywheel. If I remember my physics right, the kinetic energy increases by the square of the speed increase. (KE = 1/2 M V^2?)

Jonathan,
I think you are trying to "over engineer" this deal :D
No, the only time there is a vacuum in the cylinder is on the intake stroke. The two times the piston is going up are on compression & exhaust & in both cases there would be resistance to the upward travel of the piston. ;)
Also, it's not just the stored energy of all the rotating stuff, it's the mass of the whole car as it is also an "object in motion"