Fuels

Gold TX Sleeper · backyard mechanic Joined: 05 Feb 2002 Posts: 509

Can anybody tell me what the best commercially available fuel i can buy, would be good to run in my Gemmy ? I've had a few ideas chucked at me, but want to know for certain what i can & can't run, and for how long. eg: I've been told to use Shell Optimax, but how long can an unleaded fuel be run B4 damage is done - or does it even give the car a boost 4 that matter. I don't particularly want to travel the 200 odd kilometers to get av-gas from an airport, so if anyone has better ideas i'm happy to hear them. There has 2 be something i can use as i'm getting sick of paying almost $20 for bottles of pro strength octane booster. I've also been told by countless people to mix 50:50 leaded with a good premium (Vortex, Optimax etc.) but this sounds a little fishy to me, as i've never seen a person actually do this and encourage it... all help is appreciated.

Racegem · backyard mechanic Joined: 30 Jan 2002 Posts: 987

Depend on what you've done to the motor as to why you need a higher octane fuel?

For the most part, older cars that had cast iron heads, had their valve seats machined into the heads. Fine for leaded fuel because the lead acted as a lubricant to stop the valves 'chewing' into the head. Cars that have alloy heads already have a steel insert that the valve seats on (the alloy's a bit soft, even for lead). What this means for us Gemini owners is that we can run unleaded fuel without much of a problem.
Now the Octane rating is a different matter. Stock motors have less than 9:1 compression ratio so should run fine on normal unleaded. Modifications done to a motor that then increase that compression, ie: reconditioning, shaved heads or decked blocks, advanced timing, Turbos, etc. all can lead to detonation or pre-ignition. The higher the Octane of the fuel, the more compression you can run. Premium unleaded has a higher Octane rating than standard, obviously, Avgas is higher again. An Octane booster can also increase the Octane of the fuel you add it to but dont beleive all the claims you read on the bottles about how high they really take it. Personally, I've had nothing but good reports/experiences when using Optimax in all the cars I own. I even get more kms per tank in my 308 VH towcar.

Pete · backyard mechanic Joined: 05 Oct 2001 Posts: 502

my brother and dad both work at shell refinery in sydney, so i can tell you from an experts perspective that using optimax in a gem (with stock motor) will give trivial results. anything you 'feel' ie. smoother running etc is all in you head. the octane difference is 98-optimax vs
96-leaded. unleaded is only 92 octane.

Woz · tinkerer Joined: 05 Oct 2001 Posts: 126

ive noticed a fair amount of difference between leaded vs optimax or bp premium in my gem with extractors/webber. no difference with a standard engine? seems hard to believe...

martin · backyard mechanic Joined: 04 Oct 2001 Posts: 900

the performance boost may be bullshit but my uel gauge dosn't lie, neither does my odometer

i get almost 100k's more per tank of premium and it ends up being way better economy to get premium, specially when i drive 160k's per day in city traffic!

but then my engine is meant to run on unleaded not leaded so i dunno how much difference those two will make