how do I get cruising rpms down
 

picked up a 94 940 sedan for my mom to putt around in, it is a b230f naturally aspirated version with regina ignition system. Did a full tune on it, water pump, timing belt, cleaned flame trap/oil trap, throttle plates ect. runs great, only thing I don't like is that at hwy. cruising speed 65-70 mph it runs 2700-2900rpms is this common on these cars? anything I can change to make it better, gear ratio in rear end? tires make much difference? they look smaller than my 760s. thanks.

its a 4 cylinder engine. those are pretty typical RPMs for those speeds.

is it going into overdrive properly? at highway speed, if you push the OD button and the OD disabled up arrow light comes on, the RPMs should go up about 20%, so instead of 2700 it will be like 3500 or something (totally safe for the motor). press it again so the light goes out and it should shift back into OD and the RPMs should go down again to what they were.


bigger than stock diameter tires will increase your speedometer error (when it says 65, you might be going 70 instead). the RPMs at indicated 65 would remain the same.

around 3k rpm at 70 is what it is, normal for the car.
changing the gear ratio in the rear end to do whatever is cost prohibitive.
760s come with 15" wheels like the 940. i believe the stock 760 wheels are 1/2 inch wider.

Taller rear tires are the easiest way to change your final drive ratio. Use a GPS that tells you how fast you are going instead of your speedometer.

First though, you may first want to load up your trunk to see how much clearance you have in your wheel wells before running something like 225/65/R15's. Otherwise you may scrub the tires in the wheel wells going over a bump with a load in the trunk.

Thanks for the advice, the od does function properly. I'm going to crawl under it tonight to drop the trans pan and clean/check filter screen, replace fluid(kinda brown, slightly overfilled) and to locate an exhaust rattle. I'll identify the trans, thought the non turbos had lock up torque converters so I'm curious. I'll also compare tire sizes to factory specs.

Gold, are you going for MPG improvement?

Reason I'm asking is this is a hobby of mine.

Assuming you are, naturally the first thing you want to do is get the car running right stock.

I'm guessing you're curious about the torque converter lockup, because if that's not working it will kill your highway MPGs.

I have a 1994 940 wagon with the NA B230F and auto trans. Everything works properly and it sits right about 3000 rpms at 70 mph.

the car does have the aw 71L, I didn't drop the pan because of the way the exhaust hanger tied into the mount so i just drained and refilled 2x with a drive in between.

I am interested in fuel mileage, my wife is always griping about filling up the 760 turbo 18-20mpg average, I was hoping the lighter non turbo sedan would be a big improvement, my mom is on a tight budget.

I believe the car is running well, I've replaced all the usual tune up parts except waiting on a fuel filter from FCP. This car has 157,000Kms (Canadian car) if you have any tips for increases in mileage I'll try them out, thanks.

Tire size on this car is 185/65R15

the NA should be getting around 25 mpg average running around town and maybe 28-29 MPG on long highway trips if you drive very smoothly and don't speed. leadfoot it and all bets are off

MPG tips:

Run your tires 5lbs over. You'll get better MPGs and the tires will last longer.

Tune up, naturally.

Don't run that car over 65MPH on the highway. Teach your mom to let the car coast in to stop signs/lights (don't race up to them). Slower highway driving and 'pulse and glide' whenever possible will give the single biggest increase.

These cars are supposed to have splash guards, which cover the first 1/5th of the belly. Yours is probably gone. Get one, install it. It will improve aerodynamics. A mild chin spoiler will also improve aerodynamics, assuming you don't drop it so low it causes downforce. A 4" drop from the existing spoiler is probably enough (I used black garden edging, it worked). You could probably skip the splash guard if you do this, although that will keep crap from going into the engine bay.

Chinese water torture it each year Chinese Water Torture (a SeaFoam recipe) - Turbobricks Forums

Probably too much trouble for your mom, but 3oz of acetone per 10gal of gasoline will give you 10-20% increase in MPGs. I've done this for years, on several cars, only good effects. Do not exceed 4oz per 10gal, that will degrade economy.

I did all this on a minivan (along with replacing the huge driver's side rear view mirror with a peanut concave mirror and using DIY octane booster) and got an EPA rated 24MPG (@55MPH) van up to 31.5MPG @55MPH. For real.

I'm going to try some vortex generators this spring for drag reduction, let me know if you want the example I dug up (same type used on ultralight experimental aircraft).

the engine 'guard' heavily improves cooling by creating additional suction behind the radiator

thats its real function more than protecting the engine.

If you live in Cali, anyway.:cool:

guess I'm lucky, this car still had the engine guard/vacuum creator device. I'm going to look for one for the 760 as well. Although I cut one coil out of the front so it sits an inch lower which probably helps too.

actually, where I live in California, summer temps are 70s/80s, winter are 40s/50s. it very rarely gets hotter or colder than that. now, sure, inland a few dozen miles, the temps can either skyrocket or plummet (hotter summers and colder winters, go figger)

I live in NH, before that NY. Lots of slop here in the Winter/Spring. Going to do some rustproofing this Spring and put the shield back on mine...