Valve timing

dennis · Post by **dennis** » Mon May 05, 2008 1:12 pm

82 rabbit, 276,000 miles. I have fairly new (6 Mo.) injectors and just reset the injection timing to .86mm. The motor starts great, very little light smoke at full load, but only getting about 40 MPG. Now that I have access to the proper tools, I am considering checking the cam timing. My previous method may not have been spot on. Is there much tolerance in the cam timing? What's the chance of it being the cause of poor smilege?
Dennis

Krank · Post by **Krank** » Mon May 05, 2008 4:57 pm

That MPG is not out of line for a NA engine with the miles you're describing and (most likely) a 4 speed tranny. If you're looking for more, get a turbo and a fresh engine, then read Hagar's thread.

tawney · Post by **tawney** » Mon May 05, 2008 6:42 pm

I can't really answer your question about how much tolerance there is in the cam timing relative to mpg, but it's relatively easy to get it right on, so why not. You might do a compression check on the engine just to see how worn the rings and valves are, but if it starts easily when cold, must be that the compression isn't too bad. If you're looking for improved mileage, check out the topic relating to internal pump pressure which is a 'sticky' toward the top of the list of topics. Some of us have experienced a 15 to 20% increase in mpg by adjusting the pump pressure properly. I would also suggest advancing your static timing closer to 1mm with your new injectors. Do you know what pressure they were popping at? If your compression is in the mid 400 range, and if your injection pump isn't too worn, you should probably be able to get closer to 50 mpg.

Steve

tylernt · Post by **tylernt** » Mon May 05, 2008 11:59 pm

Is 40MPG city or highway? Winter or summer? I have an engine with low compression (375PSI, wear limit 400PSI) and I get 44MPG city in the summer and 38MPG city in the winter (never really take it on the highway). I have my timing at 0.98 if I remember right and have new injectors too.

Cam timing probably does affect mileage but not by much I suspect. If it was way off you'd be smashing valves into pistons so checking and adjusting your pump's internal pressure is probably a good place to check. One of these days Real Soon Now (tm) I need to solder a pipe fitting onto a spare OUT bolt so I can test mine...