no knock sensor: car requires 93, runs better on 89?

[JHZR2]

Car is a 1991 BMW 318i, with the M42 1.8L 4-cyl engine. I guess it has the best of the late 80s technology (distributorless ignition, etc.) but no knock sensor. I have replaced the ECU chip with a genuine dinan chip, and it is explained in the chip instruction manual that the car will require at least 92 octane.

I have always run it with 93, but have been kind of curious about what would happen if I ran it with less. I believe all stock e30 (3 series) BMWs reccomend premium octane gasoline, but some folks apparently run on midgrade and some have even run on regular with decent results, so I figured Id just give it a shot and try 89 octane in my nearly empty tank. Worst case scenario I could top with some octane booster or sunoco 94 Ultra.

I filled up, and added ~2.5 oz of lucas UCL, per my usual routine. Started the car up, idled it not knowing what to expect. I moved the car slowly from the gas station, and after about 10 seconds of moving, I heard a slight knocking sound. It quickly went away, and so I kept accelerating gently... It never came back, despite accelerating to about 40 mph.

I merged onto the highway, when getting heavy on the gas in 3rd gear or higher, it knocked for a short time rather slightly, then went away. After I got up to speed, I tried flooring it for a time in 5th gear, and it knocked again shortly, but then it went away.

Now the engine seems to run a bit smoother on the highway, its hard to get it to knock at all regardless of how hard I try to accelerate, in any gear, etc. Further, the MPG gage (instantaneous MPG, but not very accurate) sits higher now while cruising. This is good.

Could running on lower octane gas in an engine with no knock sensor help to heat things up and clean out the chambers? I am actually quite pleased on how it runs on the 89. Because I did hear knock on a few occasions, I will use 93 as usual from here on, but I am curious as to the possible beneficial effects that may have occurred by me running on the 89. I really feel that the engine runs smoother (but injector tick is still present), has at least the same power, and is running more efficiently.

Also, after sitting and idling at the station for about 30 seconds, then movinmg the car for about 50 feet, I got out and a strong odor seemed to be coming from the exhaust. It may be because I got the fuel at a conoco station, and conoco gas is 'top tier' gasoline, meaning that it has significantly more detergents and cleaners that are guaranteed to be in all grades at all stations across the country. Maybe it was doing some cleaning from the gas, or maybe it was the 89 octane fuel burning hotter???

Any ideas?

Thanks very much,

JMH

 

[unDummy]

Simple rules, use the lowest octane that doesn't knock. Any knocking is bad.

You might be able to mix the two octanes by doing 1/2 tank fillups alternating between the 89 and 93.

I would never tolerate even a hint of preignition or detonation in any vehicle I own. An extra 10 cents per gallon on fuel beats a headgasket or engine rebuild.
Some of the knocking might've knocked some carbon out of the engine. Benefits? not if you beat the snot out of the pistons, heads, or valves!

Try to improve engine cooling if you really want the 89 octane. Cooler intake charge(like cold air intake or coolant bypassing the TB) might help. A lower temp thermostat or new OEM thermostat along with an antifreeze flush(quality coolant and distilled water) might be all that you need. Also, cleaning the bugs out of the radiator(or larger replacement) might curb the pinging on cheaper gas.

 

[JHZR2]

nah, I don't really care to run 89 again, I will not routinely, if ever, chance it by running a gas that may at all ping/knock. It was more of an experiment...

I'm actually going to top off with 93 or sunoco ultra 94 to get the average back. Unfortunately I cant do much of my own mixing, in NJ, that makes the gas pump atendants unhappy, but I appreciate their jobs when its 5F out and windy

If I could get 91 octane around here Id try it. Doubt Id notice any difference from 93 in terms of power. I dont want to push it with the 89, but I wonderr if I'd get noticably more power running the 89 with the chip. Im not brave enough to dare try.

I'm hoping any carbon, if there was any in there got knocked out, all is clean and emissions are clean as well. I'll be starting using fuel power soon...

JMH

 

[GT Mike]

Are you 100% sure it doesn't have a knock sensor? My old '83 528e did to the best of my knowledge have one. If I remember correctly that was a Bosch Motronic mamagement system on that model.

The chip manufacturer is most likely telling you to burn premium as a C.Y.A. maneuver. If someone ran really el cheapo gasoline in the car with a performance chip and detonated the engine to death there's always the possibility that the chip manufacturer could be held liable. It's probably safe to run lower octane. The chip manufacturers tend to err on the side of cautious most the time.
I know in the Pontiac Grand Prix world, the reprogrammed PCMs all demand premium fuel for N/A engines when it's not always necessary, nor optimum for maximum power. They do it to protect themselves against stupidity.
Stupidity is a GTP owner that has to have his engine rebuilt because he installed a 3.0" (replacing the stock 3.8") supercharger pulley on his car with no supporting mods and ran 87 octane fuel in it.
You'd have to know our cars to realize that you only install a 3.0" pulley on the car if you're running race gas. It can produce boost pressure high enough to render 94 octane premium insufficient.