Fuel grades and performance issue.

Started by Ascari32, March 09, 2023, 09:50:43 PM

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I still have to fit my 82deg. Thermostat. However the car failed M.O.T. due to high emissions. The garage reported misfiring as a possible cause. The Q4 has a modified MAF body (slatted screen removed).

It was suggested the issue could be associated with coil pack and/or spark plug failures. I still had my original coil packs - known to be good - from the 159 engine which failed. A new set of plugs was bought and the car retested.

It passed this time despite reportedly intermittent misfiring and the garage at a loss as to what the problem might be. One issue that arose was the PSI reading for compression testing - does anyone know the figure and tolerance?

It is true the engine had lost a little sparkle, prior to M.O.T., but I put that down to the low temp of the NTC Sensor after the Man - cat were replaced with Autodelta Headers. However, I have not been doing a lot of miles or long journeys for some weeks now and had been filling up with Ron 95.

Just by way of inkling/suspicion, I topped up with Tesco Ron 99. Immediately, the difference could be felt in performance and wanting to confirm I was not just trying to persuade myself, I savagely floored the throttle on my "Test Track" several times and in 1st through to 4th, though not deliberately straining the engine with high loads and low revs.

Sweet as the proverbial nut with no misfiring and great progression.

I remembered  a while ago, there was quite a discussion about various fuels, grades and content. Is there anyone that knows what the true story is with them? It certainly fits with what little I remember, but is possibly more relevant now given the standard engine is 11.:1 compression ratio, so ought really to be run on the highest grade. However, with the increased duration of my Colombo Bariani camshafts, the compressive load will surely be greater and therefore be more prone to misfires with lower grade fuels, than hitherto.

Any information, technical or empirical/anecdotal would be much appreciated.