Your engine is an air pump. You intake air/fuel mix and expel exhaust. Usually people talk about pumping losses as being when you have resistance to the pistons travelling down the bores and creating a vacuum (lower than atmospheric pressure) in the intake tract. The back sides of the pistons are near atmospheric pressure (in the crankcase), so the vacuum in the intake tract and cylinder resists the pistons moving down to suck air in and this creates drag.

I think this is the only source of pumping losses, besides restrictive exhaust creating ackpressure (pressure on the tops of the pistons resisting them moving upward to expel the exhaust gas). My understanding is that people refer to "pumping losses" as the intake vacuum part and not to the exhaust backpressure, although to me they are just different sides of a coin.

If you have the throttle wide open (lowest vacuum reading), you have the minimum pumping losses. This is one reason why a small engine gets better gas mileage than a big engine in the same car. You hold the throttle open further and create a lower vacuum reading for a given HP and thus to maintain a given speed of cruising.

I do not know how large pumping losses are compared to other frictional factors in an engine, but I have heard this discussed as a problem that engineers worry about, so it must not be too small. I am not an authority on this, but just threw out an idea. In my experience, highest vacuum reading is often a holy grail for tuning and I am not sure how important that is in many cases. For a smooth idle or diagnosing a damaged/worn engine (leaky rings or valves or skipped cam timing), definitely is it useful, but otherwise I'm not a believer.

Lou

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Home of Slant6-powered fun machines since 1988

Wednesday AM,, 2162 miles on the odometer,,
Indy to Fon Du Lac WI, to Waupun WI, to Lacrosse WI and back to Indy
Sunday PM,, 3200 miles on the odometer

traveled 1038 miles, used 50.57 gallons of gas,, equates to 20.52 mpg over the complete trip. there is probably 75 miles of city driving in the total for the three cities visited.
While on the highway, this is running 65 to 70 mph, steady driving in the right lane,, sped up and moved to the left lane to pass when I needed to.

Not bad for that brick of a truck..My best highway mileage is likly to be in the 60 to 62 MPH range.

Ted,, yes the tach does work,, the tach is showing about 1800 RPMs at 62 mph and 1650 at 58. The truck has a 3.21 rear gear, tires are 26.5 diameter P 235 x 75 x 15. I am running 36 lbs of air in the tires,, big tire, lightly loaded,
I am running a hydraulic cam,

Sam,, the cam I have ,,not a lot of duration,, 212 and 206 at .050,,has good low end power, really feels good in the 1800 to 3200 rpm range,, but is ground on a 105 lobe separation angle,, hence has 30+ degrees of overlap and has a lumpy idle,,
nice growl from the pipes on acceleration. Gives the truck the edgy feel I wanted. If you want a smooth idle, you will need a wider LSA.

The knock sensor continues to signal while crusing when going up hills,,, I have not heard audible preignition so I have left the timing alone.

Where did you get your cam? Do have a brand name and model number?

Thanks. Sam

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