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Ignition Timing

Henry Boler

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Hi All,


I've been having a play with my ignition timing lately and was just wondering if anyone has any ideas about the topic.


It's a MK3 GT6, All standard stuff, working perfectly. The centrifugal advance, vacuum advance work find and there is no wear on any parts etc...


I run the car on 95 unleaded and I don't use any additives. Where should my engine fire? In the past it's been running at 6 BTDC, now it's on 13 BTDC...


The engine runs like an absolute dream and I do drive it hard, nearly all the time, apart from when there are cops in close proximity! However, sometimes I do wonder how much of a difference it would make if I had the timing spot on...


I've got a timing gun, I have also static timed it with a multimeter just to make sure the gun is working right. In the past I've found the tools to be useless and I've just fired her up and set the timing by ear.


I know it's not actually a question as such, but any ideas appreciated.


Also makes a change from my usual major mechanical failure posts!



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Apart from low compression export territories these cars were never designed to run on 95ron

Whilst it may suit you, performance will be better if you stick to 97+ron and get as close to designed specifications as possible for correct book performance.


95 ron was used in side valves and mowers when these cars were built

in their day these would need 99-101ron fuels 5star not 2 star.


many prefer shell ulitmax


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Yeh! Timing by ear! I've also got a vacuum gauge and you can use your eyes instead of ears! Twist the dizzy for maximum vacuum and then back a gnats.


I've read lately than biscuit is not the spark plug colour to strive for. Greyish light brown is the new colour, due to modern fuel and what's in it, apparently. My own plugs went pink due to fuel additive! I've stopped using it now as I'm told the valves have the memory of lead and will last longer than I will!


I also have a colour tune somewhere but I can never get it right. Maybe that's down to the fuel as well?

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I did see the same comment you made on another thread Pete and it did make me think... I do drive a lot of miles in the GT6, would it be cost effective? I am tempted to try it... Would it increase the mileage by much?


I used to run my old Spitfire on unleaded without the conversion and did about 50,000 miles with absolutely no trouble, I just decided I either do the conversion or see if it breaks as a result of not doing it and then to fix it, I would just do the conversion... It never broke... Ran fine, valve clearances were also fine!

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  • 2 weeks later...



If you a have a Tesco fuel station near you, try their 99 Momentum - very good and I use it in both of my classics. Both purr !!





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I used to fill up at a shell garage as I drove past one for my old job, maybe I'll see if there is another close one.


I also don't know of a Tesco fuel station anywhere near by... I know a Morrisons one!


What actual differences do you notice? Are we talking extra miles to a tank? Faster 0-60? Or just smooth running?


My GT6 has always run like a dream on 95, never misses and it is pretty quick too!

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I'm half tempted to get the old girl on a rolling road to see what's coming out the wheels!


My one always starts first time, even when it was -12'C outside! Doesn't pink, and has never run on, and this is the first time I've ever put anything other than 95 in her.


Richard, what do you mean by 'no lean off'?

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I also have a GT6 mk3 which seems impervious to tweaking or different grades of fuel but, still runs fine and never overheats. I put up a thread on here about domed pistons, I think my engine should have them but when it was rebuilt, 20 years, ago I'm sure it got flat tops. I think this would lower the compression ratio and lead to an inability to improve the engine much by adjusting timing or mixture. Just a thought, might be wrong. :wacko:

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  • 1 month later...

Just an observation about fuels here, back in the 70s when they were new I used to drive several 2.5 P.I s and Rover P6Bs rather quickly on open roads, city streets and heavy traffic. The Rovers required to run 5 Star leaded but the P.Is were 4 Star only, the fuel grade being determined by compression ratio.


Later when the SD1 V8s arrived they ran lower compression engines and were 4 Star as did the MGB V8 and Range Rover. 


As the star rating was a sliding scale of Relative Octane Number (RON) 4 Star could be as low as 96 RON. Given that 95 RON unleaded has ethanol added it is probably better that 4 Star in terms of calorific value.


I run my Vitesse Mk2 on either Elf 95 or Tesco 99 and can't detect any difference in the car's performance between them. 


Most importantly is that as classiclife says, if it runs right that's what matters.

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