engine efficiency

Hey all you XJ rocket scientists,

How does one calculate (without the use of a dyno) the volumetric efficiency of one's engine? i.e. best gas mpg at given speed/rpm/gear/atmospheric conditons/ambient temperatures, etc.? Or am I asking the impossible?
I've made simple mods (intake/exhaust) to my XJ and have installed synthetic lubricants in all drivetrain cases..just trying to get the best mpg possible. I'm averaging 25 mpg on the highway. Can it get better? Or am I pissing in the wind here?

I would think your pissing in the wind.
25 mpg's is great. I'll trade you any day of the week, and on Sunday. I only get about 13-15 on the highway. I did the K&N, exhaust, Hi-Flow cat. Bored throttle body, 4.11 gears. But I have never changed my speedo gear (coming soon) That should help. But 25 is a great number. These things are like boxes driving down the highway, but as we lift them and larger tires, roof racks and the like at weight and mass to that package. Thus the mpg's go to crap. Juice

25mpg? Damn!

I'm getting 18 or thereabouts on average...

Figure the average VE (per Desktop Dyno) as 70-75%, with a peak at about 78% for the Renix setup. Improving flow rates thru the head will increase that number, but that's a port/polish job that takes patience or a good machinist. If you can get a street engine up to 85%, you are doing good.

You are, however, more worried (from teh sound of your post) about Brake Specific Fuel Consumption (BSFC) and Air/Fuel use rates, which are easier to calculate. I'll dig up my notes on auto maths and post later to-night or to-morrow...

5-90

Juice,

Don't go through too much work on this one. I just can't justify why I can average 25 mpg on the highway and only do 15-17 in the city. Its obvious the differences in rpm constants between stop and go and highway, but why such a big difference? In theroy, if I drove around town at 2000 rpm's (same average as highway) I should obtain simlilar results. But unfortunately thats just wasting fuel. If a honda civic with a four banger can average 30 mpg in the city but only do 34-35 on the highway, what gives? Not enough cci's (engine cubes)to push that small car at given rpm's (load/ wind drag/cargo) or just excessive rpm's cranking too much too fast? Got me! I'm lost! Oh well, gotta think of things to think of while dickin' aroind at work!

Be happy. The new XJs don't give as good economy as the non-HOs. My '88 usually gets 23 MPG highway, with a one-time best ever of 28 MPG when it was new and I ran 93 octane to bump the timing.

25 MPG with a 99 is unheard of.