Rover SD1 Forum Snippet #5 - Use High Octane Fuel (or Not) in RV8?
Question: In the UK we have two or three basic types of unleaded fuel. Their price increases with octane rating. Is it worth using the high octane fuel and does the Rover SD1 V8 need it or any of the various available boosters?
Answers to this may depend upon the Rover SD1 V8 variant which have mostly two compression ratios, 9.35:1 or 9.75:1, to consider.
As for the fuel, when we see names such as Regular, Premium and Super Premium, along with different octane numbers, this defines petrol's ability to limit premature (pre) detonation, commonly called knocking or pinking.
Knocking is noise and if allowed to persist, damages the engine. A fuel's ability to prevent knocking is based on its ratios of various hydro-carbons. Higher rating numbers are more efficient for higher compression engines. Suppliers also put additives in petrol variously, to limit knocking, as well.
Aligning the question with the actualitierre of today's green revolution, prevailing road congestion, speed restrictions, punitive laws, fuel cost and general low/non availability of leaded and high octane fuels, brings me to think that my following analysis and solution will suit most needs and most wallets, regarding performance and cost.
Initially, consider the known universe in which our engines must operate, telling me the following:
O Either low or high compression ratio Rover SD1 V8 engines will handle lowest octane unleaded fuel without damage providing they are not thrashed and also allowed to rev freely rather than inducing unnecessary low rpm loads.
O The Engines have to be correctly adjusted.
O If the user needs to improve performance, then higher compression ratio engines benefit most from higher octane fuel.
O There is variation in performance between different suppliers' fuel of the same octane rating due to various additives, that might only be discovered empirically .
O The variation between high and low octane levels needs some adjustment of ignition timing to get the best performance commensurate with minimum potential damage due to pre-ignition.
O The above ignition timing issue exists between fuels of the same octane from different suppliers.
The solution to all these variables for me, an average Rover SD1 V8 enthusiast, was to decide which convenient source of lowest octane fuel seems to run best in my engine (*), then set the ignition timing for that fuel during a series of short runs between trunk road lay-bys with the distributor securing bolt slightly loose, and subtlely adjusting the timing to just eliminate pre-ignition under really heavy load. i.e.: Accelerating in high gear, uphill or with brake applied.
Then, with the timing secured in the best position and always using the same fuel source and type, this gives me long term, consistent low risk, low damage potential, motoring.
I dont use additives, but if one thought that a suppliment such as lead, or octane booster was desirable or indeed necessary, then ignition timing must be similarly re-adjusted as described, and that/those suppliment(s)) must then be used consistently (as per advices on the container) to ensure engine performance is always at it's best with that fuel/additive combo.
Given all the above presumptions and decisions, it would then become a complete waste of time and effort in getting the engine running just tickety-boo, to subsequently chop and change fuel source, octane rating and adding/subtracting suppliments willy nilly because what ever changes are made, the engine will the suffer the ongoing random vageries of incorrect ignition timing.
Finally, what about performance? First of all it goes without saying (doesn't it?) that the engine must be correctly set-up (tuned) with timely replacement of oil, spark plugs/leads, filters and other servicing such as coolant temperature control, tyres, brakes and exhaust system. Thus, whatever the choice of fuel and ignition timing, the engine will, accordingly, always be at it's best.
Then, on the rare (?) occasion when my latent boy-racer instincts get the better of me (even tho' I'm nearly 70) when I bury the loud pedal of my heavy ol' Twin Plenum SD1 Vitesse, the launch makes me feel like I'm riding a Saturn 5b Apollo Moon Rocket, part of the upholstery and making 0-60 in a tad over 7 seconds. Yes! It still seems to drive like poo off a shovel!
(*) Currently Tesco Regular 95 Unleaded because it suits the engine and earns points.
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