The vast majority of pumping loss is caused by compression followed by friction. To even consider vacuum resisting the piston going down during intake as a significant loss factor just defies logic and common sense IMHO. I'm not saying its not part of the loss, just not a significant part.


Kevin

It's not vacuum it's fighting compression also the fact that you are not feeding it as much fuel and therefore getting less combustion and so no power to push the pistons crank etc. Plus the fact you have changed the gear ratio. Vacuum has very little to do with how fast the car slows down.

If the throttle is closed, or mostly closed like it is at cruise then there is nothing or very little to compress, therefor it is working against the vaccume. A gasoline engine has vaccume brakeing and diesel engines have compression brakeing.

If you have the throttle closed you are still pulling air and fuel into the engine through the idle circuit. You aren't closing off the cylinders to outside air. You do not create a vacuum in the cylinders. You still have compression. Sorry not a vacuum brake.

Hook a vaccume gauge to your engine go out for a drive and come back and report that you had no vaccume at cruise

What has that got to do with it. When you let off the gas the compression of the pistons in the cylinder still produce 150lbs of pressure. The 13" (6psi)of vacuum the engine makes will not over come that. It is compression slowing the car not vacuum. The vacuum created is in the intake not the cylinders.

How can you have compression if there is virtually nothing going into the cylinders? Ever do a compression test and see how much differance there is with the throttle open VS closed? And that is only mabey 50 RPM not 2000 or so.

Because it's not nothing in the cylinders. You still have air being compressed. just not as much. The compression is only a few lbs different. It's still 100+psi. What do you think causes the vacuum in the manifold. It's the pistons sucking the air in. You aren't sealing the cylinders off when the throttle closes. There is still some air and fuel being introduced into the cylinders which still gets compressed. Where is tremendous anmount of vacuum that's pulling the engine down located. If that much vacuum was being created it would suck the carb open which defeats your purpose. You are only making a very small amount of vacuum in the intake.

Again I say, hook up a vaccume gauge, go for a drive then report back what your vaccume was at cruise.

My 318 held 17 inches at 70 mph cruise. A piston area of 10.65716 pulling 17 inches of vaccume is 181.1717 inches of vaccume on each cylinder X 8 almost 1500 inches vaccume being pulled every other revolution of the crank, that is far from nothing. Again I am not saying the fight against compression is insignificant but the vaccume loss is huge also. Compressing air that is under 17plus inches vaccume into an area 1/10 the size it was is not hard when you have 14 PSI (atmospheric preasure)or so pushing the bottom of the piston up. Someone a little more mathmatically gifted than me can convert it to LBS for you so you can see how much it really takes to pull those pistons down, you WILL be suprised. I know when someone showed me I was totally shocked

The problem with that is that you aren't pulling vacuum into the cylinders you are pulling air through the intake at 17" of vacuum. That air is being compressed and burned at 10:1 not vacuum. Not only have you increased the drag by droping a gear you are still fighting the fact the the cylinders are still firing even though it's a lot less fuel and air. Your example would only work if there was a perfect vacuum in the intake (no fuel or air)which is not the case. Want to give the numbers on 8 cylinders compressing the same volume at 125psi in one revolution of the crank?. I think you'll find it's a lot higher. 17" vacuum = 5.8 psi.

Even if the compressed mix did not light off that preassure that resisted the pistons upward movement will still push it down on the power stroke, this is why a Diesel engine will not slow you down much going down a hill unless you have a jake brake, a diesel has very high compression and NO SIGNIFICANT VACCUME in the intake like a gasoline engine.

I came up with some experiments for you to do stumpy. Get in your car, prefferably manual trans, go to the top of a big hill and let your car start coasting down the hill, engine off with the clutch in and the trans in 2nd or 3rd then once you get rolling real good let the clutch out, you will feel the engine rev up and the car will begin slowing drastically, now with the engine still off stomp the gas so the vaccume goes away and full compression can now be achieved, you will notice the car will not be brakeing near as hard as it did with the throttle closed and very little compression but high vaccume. That should prove beyond a shadow of doubt to you that vaccume has a much larger loss of energy than compression.

Hope this helps

I regularly drive between 1600 and 2000 RPM now. Used to be hard to maintain anything under 1800 in 5th gear before (I have a tremec with .64 5th gear and 3.55 gears), but now I have a properly curved distributor, lots of initial advance [24°], and absolutely no knocking in the driveline down to almost idle (I can coast to 1200 and slowly ramp up again). Before this distributor, I could not maintain 90 km/h (56mph, the posted limit on backroads here) in 5th, I had to stay in 4th.

Of course, my car is much lighter, around 3000 pounds.

I drive to and from racetracks, usually between 2000 and 2500 RPM, at which point I'm slightly above the highway speed limit. I get 14 to 16 mpg.

Is a picture worth a thousand words?

This graph is from a combination of many
{well-adjusted}
carburetor equipped engines as tested by Taylor on the MIT shaft dynos

Let me see who I can pi$$ off.
TKO Keisler APPROX 2150 @ 70 in 5th. 354 Dana.
I was wanting a 6th gear, but not after reading all this.

You can get HP and MPG estimations for the Imperial at different speeds on the road from here:

http://www.bgsoflex.com/mpg.html

I would guess an Imperial would have a Cd of approximately 0.50
and a frontal area of approximately 25 square feet.

Taylor is using lbs of fuel divided by horsepower-hours in his graph, getting a best of about 0.5 lbs/hp-hr in the little circle in the upper left hand side.

Bowling and Grippo want to use a slightly different BSFC in the units of gallons of fuel divided by horsepower-hours.

Most gasoline blends are between 6.5 and 7.5 lbs per gallon density,
so maybe use 7 lbs/gallon as a guess
unless you want to weigh your local gasoline.

Possible Correction:
a Sept 1981 article by Jim Dunne
in Popular Science magazine
titled 'Low Drag Cars' lists the
Chrysler Imperial of that year as having a
Aerodynamic Cd of 0.41
pretty good... considering that the
Porsche 924 of that year was 0.34
and a Corvette 0.45
The Eldorado of 1981 is listed as a whopping 0.55

shape at:
http://www.oldcarmemories.com/content/view/51/114/

sample quote
"Interesting to note the aerodynamics on the 1981 Imperial was so good for its era that some NASCAR teams used the Imperial as race cars."