Lower octane: Will it hurt?

[Tomguy]

I was low on gas so I figured I'd put 3 gallons of regular in to see how she would run, I figured if it was horrible I would just fill with 93. Well, it wasn't too bad, but she did run a lot louder while accelerating, especially on inclines. I figured it was engine knock, so I tried 89 now since it wasn't too bad on 87 (or at least I didn't think so, but I don't want to damage my engine). Well, on 89 it still knocks, but not quite as bad as on 87, and not as quiet as 93 (I'm actually beginning to think some of the noise I complained about earlier was engine knock, because it got louder on 87/89).

Will running on 89 damage my engine, or just make it run a bit louder?

[j9fd3s]

as long as its not knocking its fine. mine seems ok on 89, when its really hot it will ping at part throttle a little

mike

[rgnprof]

I run my 72 250 on both 91 and 89 octane and while it doesn't seem to make any difference, it sometimes 'feels' like the car runs better with the higher octane. I really don't notice any difference in the gas mileage. Generally, I alternate octanes and have been doing so for the past year and regardless, I get about 12-13 mpg around town.

Ryan

[mzsmbs]

I run mine on 87oct. without any adverse effects.

[Mockles]

Hmmm, and here I though that the feeling of it driving better at a higher octane came about in order to justify the $2++ that some of us are paying per gallon..................

I know I have been using the credit card instead of cash, just because you don't notice it for a few weeks.

[tosbt]

Mix the high and the low, yoiu will be running 90. Do not let the engine ping, kind of tough on the pistions. Gas prices will soon come down, maybe 6 months or so. Don't harm you car over a few or more bucks.

[Mark DiSilvestro]

Most cars sold in the US reduced compression in '72 to be able to run unleaded gas, which was only widely availible in regular at that time. I know this was true of Mercedes sixes but there was some argument about this, concerning Mercedes V8s on a previous thread, so I'm not sure about the 4.5. The US octane rating method was also changed in the early '70s, so this adds to the confusion.
In any case, older engines with high miles and carbon build-up may experience pinging, even if they were designed for regular fuel.
Ignition timing, temperature, air/fuel ratio and altitude will also affect pinging. Mercedes engines of the '50s and '60s even had a second adjustment on the distributor, called an 'octane selector'. In the case of 'Miss Daisy', my '60 Fintail, it's a lever on a notched scale that can be used to retard timing and allow driving in countries with poor fuel, then easily reset it upon return to 'civilisation'.
Conversly, by the early '70s, many cars were running retarded timing in the US , for reduced emissions.

For many advanced tinkerers, choice of ignition timing may come down to retarding it for regular fuel, with a corresponding loss in power and gas mileage or advancing it for better performance, using higher octane gas.
My practice has been to adjust ignition timing for maximum power and gas mileage, with the octane fuel recommended by the car manufacturer. Typically, I advance timing to the point of borderline pinging, under load on a hot day, then back it off slightly.
I've found that any savings I might get with cheaper gas doesn't make up for the loss in performance and gas mileage I get running retarded timing.

I recently noticed that the octane selector on Miss Daisy was set 2 notches retarded. Perhaps her previous owner had it adjusted to run regular gas. As I've been using premium gas since I got the car, I reset it to 'normal' (full advance) and have noticed a slight increase in power (and hopefully, gas mileage) without any pinging yet.

Happy Motoring, Mark

[Tomguy]

I don't know if this is related to the gas or not...

Today I drove it a little hard (not nearly as hard as I've done before) up the Morgan Highway (mountain road). When I got to the manifold pipes felt a little warm (as did the valve covers) and I figured it had plenty of power, so I leaned it out a bit.

On the way back, on the expressway, I was like "Holy *****, is it quieter?" I muted my music, and I already had my windows lowered. It sure sounded a LOT quieter! I went to the left lane, normally whenI accelerate a bit there (just a little pressure on the throttle) I can hear tick-tick-tick whether it is/was the exhaust leak or not... I didn't hear it! The engine sounded completely quiet!

I have yet to see if it was missing a bit from running too lean, or if it was just the wind (pretty damn windy) that was pushing my car. But it sure ran a LOT quieter! Idle sounds the same, but no more tick-tick-tick!

[autozen]

Mark pretty well summed it up. The 4.5 liter engine was low compression and designed to run on regular. You should stick an infrared up the tailpipe and adjust the knob on the side of the computer for about 1.5% CO and look at HC also. I would also remove the cap and rotor and slide the folded edge of a clean piece of paper through the points to remove that blue crap that builds up there. Also remember that service on those cars calls for a valve adjustment every 10,000 miles.

The six cylinder engines came with a set of Zenith 35/40 Inat carbs until 1972 when they went to 32/40 Zeniths. The numbers indicate that the main bore was reduced in size by 3mm. They were also designed to run on regular gas. If you look at a cylinder head from a 72, the combustion chamber is totally diferent than previous year models. As an aside I think the Zenith carbs ware amongst the best ever made. Everyone condemns them and wants to replace them with Webers, because someone has probably screwed them up so bad they cause all kinds of problems. Once those Zeniths are set up properly, you can't beat em.

Peter

Tom, if you don't already, I would run a few bottles of fuel system cleaner containing Techron through the old girl, in order to clean up any carbon deposits in the combustion chambers. Might make it more tolerant of lower-octane gas. Just remember, as ambient temperatures rise, so does engine operating temperature, and therefore octane demand. What may fly in May could have a crash landing in August.

[Tomguy]

Paul: I've run so much stuff through it to try to increase economy, it's ridiculous. I know Techron made its rounds through at some point.

autozen: Mark said he wasn't too sure about the 4.5's. I dunno, but isn't 8:1 pretty high compresson? All I know is that mine pings really loud on 87, at 5°ATDC, and I don't want to back off timing to be able to run it, because my economy is **** to begin with.

[j9fd3s]

Originally posted by Tomguy
Paul: I've run so much stuff through it to try to increase economy, it's ridiculous. I know Techron made its rounds through at some point.

autozen: Mark said he wasn't too sure about the 4.5's. I dunno, but isn't 8:1 pretty high compresson? All I know is that mine pings really loud on 87, at 5°ATDC, and I don't want to back off timing to be able to run it, because my economy is **** to begin with.

in the scheme of things 8:1 is very low, 11:1 is high. it depends on the combustion chamber design as to what you can run with what fuel.

for example the hotter honda engines run 11:1, turbo motors usually around 9ish

[autozen]

Tomguy,
Again I suggest you try my suggestions in my earlier post. I also wouldn't worry about the coming hot summer, because the oil companies wiil change the blend to accomodate the heat. Unlike single malt scotch, which I love, gasoline is one product where blends are better. To emphasize my point about not worrying so much about octane, I'll tell you what I do. I have a 1990 300SEL, an 88 300SEL, a 91 300E, and a 99 230SLK that I run on midgrade although they all call for supreme. I sold the 560, but also ran it on midgrage. Of course my diesels all run on diesel for the moment until I can get my waste vegetable oil gear going. These cars don't ping and really scoot on midgrade. If I take a trip at high speeds and high temps on the super slab or go to the mountains, I will switch to supreme.

Peter

[Tomguy]

When I got home from a normal run tonight, headers were cherry red! Ran fine but hot. Set her a bit richer (around 3/8 turn). Still sounded quiet on the way back!

[autozen]

Tom guy,
You're not listening to a word anyone is telling you. I did the new car preps on these cars at the dealership and feel that I know them pretty well. I'm outa here.

Peter