Posted by R.Davis on July 18, 2000 at 00:32:17:
In Reply to: Help needed posted by Dan on July 17, 2000 at 16:38:56:
What is octane? Nothing but a rating of the way a given batch of fuel burns in an internal combustion engine with a predetemined compression ratio.A test engines is used to test freshly refined gasoline.This engine has adjustments to vary compression and fuel mixtures.The fuel is run in this eng.and the results are recorded.Then two test fuels,heptane-which is easily ignited and isoctane-which is resistant to ignition knock. The mixture of these test fuels are then adjusted until the test eng. runs with the same results as the batch of refined gosoline.If it takes 84% isoctane to 14% heptane,the refined fuel is rated as 84 octane fuel.Low compression engines will function on low octane rated fuel but since the 50's and higher compression engines,higher octane rated fuel became a must.For some unexplainable reason,the lead additive they used raised the octane rating of fuel without affecting its burn rate but did affect its resistance to pre-ignition which is the knock or ping you hear because you are using too low a octane rated fuel for that particular eng.Did the lead also act as a cushioning or protecting agent for the valves or valve seats?My book does not mention it,but that doesn't mean it isn't true.The octane rate boosting agents used today to replace the lead of yesterday is just as deadly as lead I've read,but disipates from the atmosphere much faster.So no you do not need a lead sub. in your fuel since you have a 251 with hardened ex.valve set inserts and with 7:1 compression ratio you can even get away with sorry 87 octane fuel but use 89 or higher,it couldn't hurt. Use .035 gap on the plugs.