Hi Chris,
You'll get lots of opinions on this one....
First, even though the factory said the engine is 10.75:1 it probably isn't. The only way to know for sure is to measure everything. You can sometimes approximate by measuring "cranking pressure" but that's heavily influenced by the camshaft's timing of valve open/close events, so not all that reliable.

The lead additive in fuel used to be there to do two things: raise the octane rating (knock resistance) and to lubricate the exhaust valve seats.

Detonation (knock) is influenced by lots of things - fuel octane rating, actual cylinder compression (pressure), temperature, combustion chamber condition and shape, turbulence, spark timing, even the material the heads are made out of (iron or aluminum).

It's a paradox: heat and pressure in the cylinder are your friends when it comes to making power, but they are your enemies when it comes to detonation. As to which grade of fuel to use, the "correct" answer is to use the "minimum" grade of fuel requred to keep your engine from going into detonation under any of the conditions/situations you drive it. Anything "more" than that is wasted money. However, considering the consequences of not having "enough" octane vs. the consequences of having "too much" octane, I believe I'd prefer to err on the side of "too much". I've been told there is such a thing as "silent detonation" - detonation that you can't hear. One of the ways to detect it is to check for tiny flecks of shiny metal on your spark plugs. Those flecks are pieces of melted aluminum from the pistion - a sure sign that the cylinder has experienced detonation.

The other fuel issue is exhaust valve seat lubrication. There are two camps on this. One says that not having lead in fuel is always a bad thing and it will eventually cause the valve seats to get hammered into the head. The other says that this is possible, but won't always happen - it depends on how the engine is used. If it's frequently used in a high load, high temperature situation (pulling a heavy trailer) then it's likely to happen, but otherwise won't be. Note these concerns only apply to engines from the pre-unleaded fuel days. When the switch to unleaded fuel happened, manufacturers began using processes to harden the exhaust valve seats to avoid the problem.

Personally, the second view is the one that sounds right to me.
(I hope so, because I'm going to be running a set of very rare cast iron heads without hardened seats on my engine).


If it were me with your car, I'd probably run the best fuel I could get. If fuel cost is a concern, then I'd cautiously "work my way down" in quality paying close attention to what I could hear from the engine and also frequently examining all the spark plugs for any signs of detonation.

Bear

 

Thank you Bear, this is a very helpful answer!

The only thing that makes me think of using higher octane rating now is the so called silent detonation. With 95 (91 US) octane, I can't hear the enigne knocking.
But maybe I should use the 100 octane to be on the "save side". ($ 7,7 a gallon instead of $ 6,5).

I will never pull a trailer with my GTO and drive it very long distances on the highway at high speeds, so the fuel additive won't be necessary. Could it do any harm to my engine if I try it just to check for any differences in how the engine sounds?

 

I wouldn't think so. Detonation is most likely to happen at low rpm, under heavy load, with a slightly lean mixture, with a lot of heat in the motor. If your cooling system is working very well and you've got your fuel mixture dialed in over the entire rpm range (lean is bad, just a tiny bit rich is "safer"), try it and see how it does. If you hear obvious knocking then you know you've got a problem. Otherwise, drive it enough to get what you think is a good 'test' and then check the plugs. I don't know how to be any safer than that

Bear

 

I try to run a 50-50 mix of premium and Race fuel whenever possable.

 

That's what I will do
I've onl been driving at low outside temps yet (30-50) and the temp gauge is always between 175 and 185 constant, in traffic and when cruising around. I think in summer it will raise up a litte. Whats the average temp for my engine? 180-210?
I have a big aluminium radiator (ron davis is written on it) and a electric fan.. didn't hear of the brand "ron davis" before, but I hope it's a good quality radiator.

 

I found their web site, and their stuff looks pretty good to me. If your radiator is up to the task then temps should stay reasonably close to your thermostat rating. If you find it still gets too warm for your liking you might consider going to a dual fan setup (if you haven't already).

Sounds to me like you're going about it the right way.

Bear

 

I've only one fan.. in summer I'll see if it is good enough.
I also will install a new 180° thermostat, should I worry about heat if the water temp in summer times will stay at about 200-210°? Is there something like a deadline where I should begin to worry?

I can only tell my BMW had a 180° thermostat and the temp was always (winter and summer) constantly at 200°.

 

Sounds to me like you're already intending to monitor everything pretty closely, and that's probably the best approach. As long as you're paying close attention to what's going on with things, you should be in a great position to detect a problem and then deal with it before something "ugly" happens.

Enjoy!

Bear

 

That's the point, I want to solve the problems before they appear
At what temp should I start to worry?

 

I'd start getting concerned above about 220-230, but remember the reason you're paying close attention is so that you don't encounter detonation. If your engine seems happy running a little on the warmish side, and isn't knocking (and not turning the plugs silver), then don't worry too much about it. It may get a little hotter than that under a heavy load (or after a "pass" at the track) but as long as it cools off well afterwards you should be fine.

As relates to detonation, staying cool is important because having "hot spots" in the combustion chamber, especially on edges or sharp corners, can cause detonation.

Except for that, having heat in the chamber is actually a good thing for making power.

Bear