hey, does a bigger engine require a higher octane? like a 351W or 351C, could they use 89octane regular? or higher? or big blocks? do they require a higher octane?

 

all stock engines can run on 87 octane, but when you start going for performance and increasing compresion is when you may have to go to higher octane. You should use the lowest octane without detonation or pinging.

 

k thanks, i always wondered that

 

Not true. The 4V engines are 10:1 compression and really need premium. The Hypo's at 10.5:1 definitely need premium.

 

The size of the engine doesn't matter it is all in the compression ratio

 

how does one figure out the compression ratio?

 

If it's stock you can just get on the net or in your owners manual and find it out.

 

If you want to know what compression ratio your engine was built with an owners manual or web site with that information would give that to you, but if you want to know what the exact compression ratio is for your engine is then you need to do a compression test and a leak down (compression test with fuel in the cylinder) test.

 

Try a few gallons of 87. If it pings, bump it up to a few gallons of 89. If it still pings, bump it up to 91 octane. The compression ration on your engine may be rated at 10 to 1, but is it really? Wear and tear may have changed that so that you can burn lower octane fuel. Also, even with a 10 to 1 ratio engine, if you know what you are doing, you can tweak your timing so that it won't ping on lower octane fuel. Now, you aren't going to be able to stomp the accelerator too often, but for a daily driver, and the cost of gas, you can make it work.

 

There are way too many variables that determine the compression ratio of your engine to review in this post. Like others have said, the size of your engine has nothing to do with what grade of gas you run. It has evenything to do with the compression ratio, combustion chamber shape and piston design. Generally speaking, the higher the compression ratio and the more timing advance you run, the higher the grade of fuel you will need to prevent pre-ignition or detonation. (the pining you sometimes hear when you accelerate). To correctly figure compression ration, you need to know your combustion chamber volume at TDC, which is a measure of the cylinder volume, cylinder head volume, whether the piston has valve reliefs, etc. Head gasket thickness is also a factor.

As a basic rule, on a carbed car, if the compression is 9.1:1 or less, regular should be fine. 9.2-9.5 can run super, and 9.6 or higher will need premiuim OR HIGHER!