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900-specific? Posted by Saana88 [Email]  (#207)  [Profile/Gallery]
(more from Saana88) on Fri, 23 May 2008 08:00:41
In Reply to: Re: Question about coasting, JeffR  [Profile/Gallery]
, Wed, 21 May 2008 20:19:31
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First of all, I do this, almost all of it, in my '88. When one of the Centric-rebuilt brake calipers are not hung up or dragging, in nice summer weather (8 valvers get significantly worse mileage below freezing, moreso than LH) I've gotten around 33-34 in light-on-the-gas non-expressway commuting.
I definitely won't do anything unsafe, but when was the last time you turned the key on a 900 off and had the steering wheel lock on you? If this happens, you have serious problems.
My driving has become a blend of coasting, light acceleration, cruising, and engine braking. On 8 valve 900s with manual transmission, there is a solenoid valve for an air bypass around the air metering plate that locks the lambda modulating valve at 60% so the mixture doesn't go too very lean and shuts off the fuel flow to the four main injectors. This valve will engage if you begin engine braking at engine speeds over 1900 RPM or so. The valve won't cut out until the engine speed reaches 1100 RPM. LH computers do this as well, though I do not know exactly how, whether through cutting down the injector pulse times or what. The thing is, when your engine does this, it creates drag and slows the car. As a result, I use engine braking only at times when I need to slow down, such as approaching a speed restriction, traffic signal, or slow traffic. For other times when I may need to accelerate again soon, I will only coast, in order to maintain as much speed as possible.
That's really what good mileage boils down to- maintain your speed as much as possible. Don't be so concerned about maintaining your place in line at the next traffic light. If I approach a traffic light that is about to turn green again, I will either coast or use engine braking in the above-mentioned decel zone until I reach a point where the traffic queue ahead has started moving at my incoming speed, typically as I approach the nearest car in the line. This is as opposed to the way everyone else drives, on the gas until the last possible second until making heavy brake applications to be sure that they are there, ready and waiting for traffic to move again. Every time you use your brakes, you are wasting away your kinetic energy.
As for cruising, use cruise control if you don't have a '92-'93 900 with cruise control. (Within those years, the control modules are junk and it won't work anyway.) However, when you come to a hill, charge it a little, let your speed scrub off on the way up, and regain your cruising speed on the way down. My '88 base car doesn't have cruise, and I don't miss it much. On the '92, surprisingly enough, it doesn't work yet. Other factors that help you be as efficient as possible at a steady speed are keeping that steady speed down to 65 or less (fine aerodynamics they are, but you can't argue with physics too much by that time), choosing a 900 turbo or base car with high final drive like my '89 nonturbo transmissions, shedding excess weight from the car, and installing the turbo airdam under the front bumper. I'm not kidding. $15 for someone with a parts car on the classifieds, $3 in new screws to hold it on (you're threading into plastic so even my '88 didn't have rust issues here) and you get about a half a mile per gallon all the time. I couldn't believe how much of a difference these three chunks of plastic made. The rest of the work has been done for you- the C900 floorpan is extremely aerodynamic for its day. There aren't many sudden depressions or things jutting into the slipstream. Smart. The fun part of maintaining your speed is going around corners smoothly without scrubbing off excess speed. Usually I end up coasting in very early because the traffic ahead of me has no idea how to drive around corners and making it so I catch up with them just after the curve ends. That's the best case. Unfortunately, the usual way ends up just accelerating moderately in high gear on the way out after getting a little too close around the corner.
And now for acceleration, the fun part. If you've got a turbo gauge (or a vacuum gauge or a turbo gauge on a nonturbo car), use it. Keep yourself out of the boost whenever possible. Once that turbine kicks in, your 900 can only get about fifteen miles per gallon tops (estimated, I've never owned a turbo). The trick with acceleration is to be as light on the throttle in as high a gear as possible and still get you up to speed. The green range on the tach is a good guide, but it can't represent what you are doing with the throttle. A vacuum gauge (the white range on the turbo gauge) cannot necessarily represent how fast the engine is spinning. Your job as the operator is to find the optimum combination of the two.
Keeping your car tuned is an ongoing battle, but it's going to be that way no matter what you do. Your brakes should not drag enough to noticeably slow the wheels. The same applies for wheel bearings. That being said, after 227,000 miles, my '88 still rides on her original bearings. The engine should be in good shape. Replace your oxygen sensor after 60k. Yes, yes, it still works, but it can no longer react quickly enough to work well. Keep your air filter and air mass/airflow meter and idle regulating valve clean. Clean out the throttle on your car every six months or more often on a high-mileage engine. On cars with settable timing, check it once a year when all is well after setting the mixture and idle speeds. Vacuum leaks are bad news, including vacuum advance diaphragms, manifold seals, and old hoses. So is failing wiring and connectors and crank position/hall sensors.
The EPA is biased. The original numbers for my '88 8 valve 900 with manual transmission are 22 and 27. Using their new, improved formulae (which obviously suck) the numbers are 20 and 23. Manual shift! What a crock! (Judging from their insitence on upshift lights and continual downgrading of their mileage estimates beyond belief, the EPA still hates manual transmissions.)
Back in the day, when other members of the family drove normally (no excess coasting) they got between 25 and 27 MPG, including AC. Stuck in traffic in the middle of winter they got 23.
Driving as I described above with the dead weight of the AC removed from the car, I get 31-35 in the summer (last year, and my mechanic tells me I was running a little rich) and 25-27 in the middle of winter with everyone else driving 25 miles an hour. Even better, last winter I put a block heater on the car, and using it every night the temperature went below 40 degrees, the worst mileage I got was 27.2. That's the best 46 bucks I ever spent.
My car used to be an automatic, and even when I took it out of gear to coast (not recommended, especially with these autoboxes) the best mileage I got in the summer was 24.4. My 2003 transmission swap paid for itself in a little over four months in projected fuel savings back when gas was $1.75 a gallon. Nowadays, the payback period would be closer to two months. Fine, it's another slam against automatics, but it should be obvious how I feel by now.
This advice is for when I have to drive. I get nearly infinite mileage when I ride my bike to work, plus I get to bypass all the traffic congestion on an uncongested shoulder and be in decent shape at the same time. No more fighting for a parking spot, worrying about the bums redecorating your car in the garage, and sometimes there are girls on the bike path. Don't get me wrong, I love driving my 900s, but sometimes I'd rather be on a bike ride.
You're more than welcome to pass me on the Thruway next Tuesday as I'll be cruising at 65 on the way back. Maybe I'll be able to put the top down.
, Fri, 23 May 2008 06:53:35, Wed, 21 May 2008 20:19:31, Fri, 23 May 2008 08:00:41 <-- Viewing This Message
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