gas mileage

[stormcrow]

Since the rebuild on my 78 911 Sc Targa, I have been checking the gas mileage and it has been averaging about 20 mpg. The last time I checked it was when I took it to Fla.

This past weekend, I did another road trip in my P-car and my gas mileage jumped up to 24 mpg, and 23 mpg when I checked it again. Go figure.

Didn't do anything different to the engine except change out a the fuel lines from the distributor to the injectors. Otherwise, nothing else had been done.

Is it possible that the engine has taken that long to break in since the rebuild. I thought a break in period was about 5000 miles. Any comments?

Steve

[R22tech]

Steve- a couple of things: 1- was this the same route on each check? I mean, it could vary obviously. Quality of gas could have been different and fuel delivery from new lines could change it as well. As far as break-in period, didn't you tell me on our 1st drive after carsncoffee that you hadn't driven it like that since the rebuild? That, plus the mountain drive, could have finished off the break-in. Does your car feel and sound a little smoother since those drives? Hey, I wouldn't complain ,though!

[Jim Sims]

I once doubled the gas mileage on a pick-up by replacing a leaking fuel line. Perhaps you fixed a leak when you replaced fuel lines. Besides that possible change there are numerous things which can affect gas mileage such as external conditions: wind, road grade (on a "loop trip" this should even out), air temperature, altitude (air pressure; normally aspirated cars and turbo's without a waste gate lose about 3% of their power for every 1000 ft. above sea level), road surfaces; vehicle condition and configuration: oil type and level, vehicle weight and weight distribution, tire pressure, tire type and tread design, wheel alignment, state of brakes (dragging pad or parking brake shoe), state of tune, state of engine break-in (yours is at 5000 miles? I have always considered an engine to need 10,000 miles for a full break-in unless it was run in on an engine test stand), windows open or shut etc.; then there are the deceiving things like speedometer/odometer error due to tire size changes (different tires, effective rolling radius changes due to inflation and/or wear), filling station pump variability (not all the automatic shutoffs will fill one's tank to the same level). Then one must consider auxilaries such as A/C or big electrical power drains such as lights or high power music systems. These will absorb engine power through the alternator and use up fuel. Also one's driving habits. Long road trips with fewer bouts of accelerating and braking (which use energy and hence fuel) usually significantly increase fuel mileage. I've heard claims of some of the later SC's attaining close to 30 MPG on long steady road trips. Jim

[stormcrow]

Well, both trips were taken over the same route - 85 South to 300 and south to Tallahassee. I have started using Havoline (10W-30W) instead of valvoline (20W-50W) and that may have made the difference.

But, I am not complaining. The engine does sound better - little less valve noise. There were no gas leaks on the fuel lines - replaced the original with braided lines.

Anyway, I am happy with the results. Thanks for the replies.

Steve