What determines if your engine needs higher octane?

 

Compression ratio. Up to 9.5-1 works on todays regular. 10-1 could start to need premium depending on chamber and piston dome design. Around 11-1 to 14-1 then you need octane around 110-125. Above 14-1 you're in diesel territory.

 

Well I have to retard my ignition alot or the engine seems like it ignites before it reaches the top of the cylinder, the higher i rev it the more i have to retard it, it's retarded so much that it's stalls really easy when i give it gas. i just had the heads shaved but not much .010 also would the msd ignition cause it to detonate with 87 octane fuel?

 

dmc, You have left out a lot of information and that makes it difficult to help you. What engine? What year? What was the c/r before shaving the heads? What heat range on the plugs? What plugs? What gap? and so on, and so on.

Help us out here.

 

I think a less technical answer is if your engine is properly tuned and is experiencing pinging while under load, then you may need a higher octane fuel. Don't buy all the marketing crap from the oil companies about the higher octane fuels - use what the car manufacturer recommends or what your car will run on without the ping. Anything more is wasted money.

If you do need a higher octane and you can run on the mid grade fuel, buy half regular and half top grade - you get better octane than the midgrade for less money. (EX: $1.79 for 87. $1.89 for 89 and $1.99 for 93) If you buy half 87 and half 93 you get 90 octane for $1.89 average. Pet peeve of mine that they get away with this scam............

 

302 out of an 87 grand marquis, don't know specs on heads, autolite plugs at .44 gap

 

Well, you still left out a lot. So, I'll just have to say what Nitramjr says. He hit the nail right on the head.

There are so many things that can cause pinging. Hot spots in the cylinders (carbon build up), wrong sparkplugs, wrong gap, lean A/F ratio, crappy gasoline, etc.

I doubt the MSD is the cause, but why not remove it and see? Also, doesn't MSD boost the firing voltage? If so, open that gap to .50