tony-qld Posted May 29, 2011 Report Share Posted May 29, 2011 I have been having all sorts of problems with the timing on my 4 since dragging it out of Hibernation after our wet season. My distributor was re- bushed and the timing fettled when the engine was rebuilt and all was going well up to a couple of months ago. I was blaming problems with pinging on erratic quality of the 98 octane petrol at my local station but it seems that all that cursing of the petrol suppliers was all wasted effort. The problem is a vacuum advance that is giving anything up to 15 degrees advance and doing it in a totally erratic manner. I have disconnected it and set my timing at 32 degrees full advance and the beast is running like a dream. My question is "will I miss having vacuum advance" My Lotus elan didnt used to have it and to be honest I am not sure if I will be missing out on anything by leaving it disconnected? Quote Link to post Share on other sites
Steve Brabazon Posted May 29, 2011 Report Share Posted May 29, 2011 Hi, As you have noted some engines do not have vacuum advance , Old Cooper S, Lotus etc. One of the reasons these engines do not use vacuum advance is the manufactures didn’t think the owners would be interested in the feature vacuum advance provides which is fuel economy when driving on a light throttle. So depending on your driving style you may have to fill up slightly more often! Steve Click on thumbnail Quote Link to post Share on other sites
tony-qld Posted May 29, 2011 Author Report Share Posted May 29, 2011 Fuel economy isnt a concern as I "drive it like I stole it". I am more interested in performance. Is fuel economy at cruise the only reason for vacuum advance? Quote Link to post Share on other sites
Steve Brabazon Posted May 30, 2011 Report Share Posted May 30, 2011 Hi, Yes the vacuum advance is only there to give better fuel economy, and this is mostly if not always only when you are cruising on a part throttle. It can also advance the ignition on idle so you may find the idle speed goes down a little. I would however keep an eye on the engine temperature, plugs etc. as you are effectively changing the amount of advance the engine is running at part throttle. Steve Click on thumbnail Quote Link to post Share on other sites
67_gt6 Posted June 1, 2011 Report Share Posted June 1, 2011 (edited) Vacuum advance allows you to use the torquey characteristic of the engine to respond to light throttle adjustments when you are in high gear. for example if you are in 4th at 30mph the engine is only doing about 1500 rpm. with a light throttle you can bring it up to 40mph using the vacuum advance rather than using heavier throttle. difficult to describe but it does work. I also drive mine like I stole it but I would reckon I am still at light throttle cruise 90% of the time. andy Edited June 1, 2011 by 67_gt6 Quote Link to post Share on other sites
Ian Vincent Posted June 1, 2011 Report Share Posted June 1, 2011 If you think about it, the vacuum advance can only advance the ignition when there is a vacuum in the manifold. If you are old enough to remember old Ford Prefects and the like, they had vacuum powered windscreen wipers that virtually stopped when you were accelerating because you can't generate much of a manifold vacuum when the throttle is wide open. Therefore in effect, the loss of vacuum when you open the throttle retards the ignition to prevent pinking. It allows you to cruise with the ignition further advanced than normal - hence the more complete burn and the better economy. If you wanted to, you could in part compensate for the lack of vacuum advance by using slightly softer springs for the centrifugal advance. Rgds Ian Quote Link to post Share on other sites
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