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Classic Engines, Modern Fuel

Comments on Topic: idle timing advance

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Submitted by Mart


I am from France and i read your book (with kindle) with great enthusiasm, i have a Triumph TR6 US model with carburetor stromberg 175cd2 and low compression engine 7.75:1

the engine run great and my idle is very smooth, but i have a question .

Why when i advance the timing at idle the engine rpm increase?

The advance @idle is 12°BTDC, 850rpm (for year1973), actually i run with 14°BTDC with fuel 98 octane and when i increase the advance 14° to 20° BTDC my rpm increase , (at 21°BTDC the engine run poor and the rpm decrease), what is the sweet spot accordind to your knowledge?

From what i have understood if the rpm increase with the distributor advance ,the fuel burns better and gives more power ??

Thanks for your answers

Best regards


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Submitted by The Author

Hi Martin,

Thanks for your comment.

You are right when you say that if the ignition advance is correct, the fuel burns better and the engine produces more power. At tickover this will mean the engine will run at higher RPM. Unfortunately, the engine advance at tickover is not a good measure of the correct setting for the engine.

The correct ignition advance depends on both engine RPM and LOAD (i.e. how hard you are pressing the accelerator). At tickover (850 RPM and with no load) is not how the engine normally runs when you are driving the car. As the RPM increases, the distributor needs to advance the ignition. This is called the ADVANCE CURVE. If you register on the website under the IGN ADVANCE tab, there are instructions on how to measure your advance curve and you can plot it and compare it with other cars. The ignition advance also depends on engine LOAD, this is what the vacuum advance does if your car is fitted with one.

The best way to test if you have the correct advance is to take your car onto a ROLLING ROAD. The engine is run at full throttle and they measure how much power it is producing and can adjust the ingnition advance to give maximum power.

I hope this answers your question.


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Submitted by Mart

Thanks Paul

Please wait .....