w108 4.5 octane additive

[ambush276]

Currently i am driving this around and running on ~ 85/87 (us spec) petrol. I know that is a little low and the car is still running really hot (had a post a few months back... waiting on new fan clutch at the moment).

Besides changing the timing could this have any affect on the overheating?

Something along the lines of X-1R octane booster or STP octane booster. Or are these a waste of time (or bad for the engine).

Thanks

[fonzi]

I’m not sure about all this, but let me pontificate.

Retarded / late timing apparently helps emissions somehow. I’m not sure, but US spec cars of the period have later timing specs I believe.

Retarded timing makes cars run hotter for some reason. I’m not sure, maybe due to more complete combustion somehow I’m not sure. Doesn’t make sense.

Higher octane is to prevent preignition in higher compression motors. Higher octane fuel is less likely to ignite due to hot spots in the combustion chamber or cylinder wall. If you have to run low octane fuel in a higher compression motor, the instructions are to retard (make it later) the timing to prevent knocking.

Knocking is due to combustion putting pressure on the cylinder while it is still in its compression stroke, basically early detonation.

Early detonation is more likely with lower octane fuel and higher compression motors.

If your timing is set to 5 ATDC with vacuum and 5 BTDC without vacuum (like with AC on or hose disconnected) at idle, then your idle timing is probably correct and set correctly for 87 (US rating of something like (RON + MON) / 2 . That’s not to say that your mechanical advance is working though. I think the specs are written in the manuals, and some say the timing is supposed to advance to something like 30 btdc at 3000 rpm. If you advance is not working, your car will not want to rev high.

Maybe no questions answered here, and some may correct what I’ve written above, but my understanding is that late ignition timing —> hotter running.

If concerned about the heat, odds are it’s your cooling system, but you could just try running some premium which won’t hurt anything. With premium you can advance the timing and just make sure you don’t experience any knocking or clattering. I seem to experience this most often when the timing is advanced beyond spec and I’m at lower RPMs and under load, like with a stick shift going up a hill in too high of a gear.

Please correct me if my BS is untrue.


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[Tom Drew]

According to the research I have done, typical USA Midgrade (89 Octane) is the modern equivalent fuel (in the USA) for the W108 4.5. Mine runs properly with stock timing on this fuel.

Tom

[Graham]

Originally Posted by fonzi

If your timing is set to 5 ATDC with vacuum and 5 BTDC without vacuum (like with AC on or hose disconnected) at idle, then your idle timing is probably correct and set correctly for 87 (US rating of something like (RON + MON) / 2 .

ambush is in SE Asia. Is this a US spec car? If so, those are the numbers. If a "standard" version (ROW?) then manual says TDC +/-2deg at 800rpm and 30deg BTDC at 3000rpm (For normal or low compression engines) Not sure why the difference. Maybe different distributors?)

First of all, I don't know anything about the 108s and the engines they used. But timing figures are usually on plate across front near RAD.

Figures for Standard engine are for 98 RON fuel. 98RON can translate to several different octane ratings, so be careful making conversions. For example, I read that 98RON Shell V-Power 98 in Singapore is 93-94 AKI ((R+M)/2). Going other way, 87 Octane might be about 91-92RON.

Shop manual says to retard the TDC setting by 1-2deg for every 1 RON (Max 6deg). That would mean that timing should be retarded by 6deg. This would mean somewhere between 8ATDC and 4ATDC at idle. But remember, this is for the "standard" M117 4.5L.

If it is a US spec engine, manual says to use 5deg ATDC (and my car manual says to use 91RON fuel (Our Reg 87 octane gas))

As it is, for our US spec cars with this engine, they run cooler and better with timing more advanced - Mine is set at about 5-8deg BTDC at idle with vac. Don't get any pinging using Reg 87 octane fuel.

ambush - I would , as a test, try advancing timing to TDC, then 5deg BTDC and find a hill where engine will be under load, and listen for any pinging. If you do get some, you could use octane booster, buy higher grade fuel (if available) or set timing back a little until pinging goes away. Always test timing with A/C off because it otherwise disable the vacuum retard.

As Fonzi said, retarded timing allows for more complete combustion (for emission reasons), but engine WILL run hotter.

Your problem may be something different!

[berfinroy]

The US spec 4.5 first used in the 72-73 108/109 series cars had amodest 8:1 compression ratio, down from 9.5:1 in the preceeding 3.5. This reduction was due to meet tightening US smog regulations. It will run fine on regular gas, and using higher than required octane fuel is of no benefit to an engine that doesn't need it.
Parenthetically, my experience with "octane boosters" is that they are useless.