MPG Enhancements

[Volvoist]

With the rising (yet again) cost of fuel, I figured I would ask the age old question of how to get better MPG. I have worked on TDI's and on older 1.6 diesels, and it is said that adjusting the timing (advancing it slightly) helps with fuel economy. Is the same to be said on the D24/D24T engine? I am more focused on doing this with a D24 engine. European spec vehicles (just about any car made) got, or gets better MPG than their American spec counterparts, which is rather annoying. Did the D24/D24T get better MPG in Europe? If so, what made this happen? In all of my D24/D24T engines, my averages were as following:

1980 200 M46 series all averaged between 38-41 MPG on the highway depending on how I drove and the temperature, and got the best mileage out of any other year Volvo diesel I owned. I owned 3 of these, and the result was the same. Not quite sure why. Perhaps it was the different cylinder head design and the different timing spec for the 1980 model year.
1982-1984 200 M46 all averaged around 35-36 MPG on the highway depending on how I drove and the ambient temperature.
1984 200 BW automatic all averaged 30 MPG on the highway depending on how I drove and the ambient temperature.
1985-1986 700 ZF22 all averaged 30 MPG on the highway depending on how I drove and the ambient temperature.

These were my findings over the past 10 years or so, and after owning many D24/D24T equipped Volvos. Has anyone achieved better results with simple modifications such as advancing timing slightly or injector modification?

[Volvoist]

I just noticed an older post with the same questions, but timing mods were not mentioned... that is what I am most interested in learning about. These are for a D24 engine in a 200 series.

[nick]

In my 1983 760 GLE D24T M46 I get 38-39 if I drive 70-71 mph, and 32 mpg if I drive 78-80 mph. I've conducted this test a couple of different times and the results have been consistant. I have the overall timing advanced about 5 degrees from the factory spec, so about 1.00 mm.

I want the timing to be advanced further at about 3000 rpm, so I've been considering replacing the spring beneath the motor side timing plate of the IP with a lighter duty one. If I don't do this, then I'll probally advance the overall timing another 3-5 degrees. I put more time on my engine at 2900-3000 rpm than any other speed.

[Aidan]

I would be very reluctant to advance it any more than what you have already, you could start burning things out, plus when I got my D24TDic 940, the timing was a bit too advanced (1.2mm) and it rattled like a box of bolts, the noise was unbelievable. It's now at 0.95 and a lot quieter. You can safely take a US engine up to Euro specs but I wouldn't want to go too much further as the engine will start fighting itself and you will end up robbing power instead of gaining it. Reading thru several forums about the D24T, 0.95mm seems to be the optimum setting for these engines. Regardless of what modifications you do to any engine, diesel or petrol, it still requires the same amount of fuel to drive the car at a given speed. Making an engine go faster is relatively easy, and contrary to popular belief, at a steady speed it wont make any difference to the fuel consumption whatsoever, it's acceleration that kills fuel consumption. Most people who have beefed up engines complain that the fuel consumption is worse, it's only appears worse because your more inclined to use the extra acceleration. You can do loads of things to improve the power output (which usually means quicker acceleration) but sod all to improve fuel consumption unless you spend serious money on methods to burn the fuel more completely. I cant remember the exact figure's but if we say for arguments sake a diesel will burn say 75 per cent of the actual fuel that goes into the cylanders, if you can modify the engine to burn say 90 per cent of the fuel, then you will get an improvement in fuel economy. Without modifying the engine, the easiest way to improve fuel consumtion is to fit a taller geared axle, thus reducing the rpm at a given speed, the downside is acceleration will suffer because the engine now has to work a bit harder to get up to speed, which will generally negate any advantage a taller axle will give unless your real easy on the pedal, or alternatively, reduce weight (get rid of all the crap in the trunk and make the wife and kids walk), improve airodynamics (that aint gonna happen in a hurry) and reduce rolling resistance (fit good quality tyre's and increase the pressure's, it will improve rolling resistance but the tyre's will wear out prematurely). Unfortunately there is no "magic wand" method of gaining a significant improvement in fuel consumption (apart from loads of money to spend) the best way is to optimise what you have by regular maintenence, keep your air filter clean, use good engine oil, remove roof racks etc, throw out all the junk in the trunk etc, that kind of stuff will make a practical difference. There are also a few phsycological things you can do that do work, move your seat a couple of inches back further than normal so that you have to stretch further to press the pedal, you find you'll drive more sedately, also, you can fit a stronger throttle return spring, that will tend to push your foot back off the pedal giving the effect that it is harder work to go fast so you tend to slow down, you can also fit one of those bloody annoying "speed warning" buzzer's, they do make you back off the pedal. At the end of the day it's your right foot that has the biggest effect on fuel consumption. I have a 940 wagon with a D24TDic auto, if I go flying around with a load of tools etc in the back, I struggle to get 30 mpg, when I dont carry anything and take it nice and steady I can easily get 45 mpg (about 36mpg in US gallons). Going by the mpg figure's your getting your doing damn good, I'd leave your engine well alone as 38/39 mpg works out to practically 50 mpg in Euro gallons which believe me, is damn good for a D24T and you'll be very hard pushed to improve on that.