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Octane rating and compression ratio

O

One_fast67

Member
I am going to freshen up the 351W for my ‘67. This will include new pistons, aluminum heads, and a new cam utilizing the stock crankshaft. The highest octane rating in my area is 91. While I am looking for a boost in performance over stock, I’m not chasing crazy horsepower numbers or revving the engine to 7000 RPM. What would be a safe compression ratio to shoot for?
 
A lot of factors will influence the answer, but I think around 10:1 should be ok for 91 as long as it's not a wild cam beast.

Horseplay can probably give you an exact ratio.
 
Aussie67 said:
A lot of factors will influence the answer, but I think around 10:1 should be ok for 91 as long as it's not a wild cam beast.

Horseplay can probably give you an exact ratio.
Click to expand...
Agreed. My Shelby is factory 10:1 and runs fine on 91. That is the lowest recommend by Ford.

Mel

Sent from my Pixel 2 using Tapatalk
 
If it's good gas 91 should be fine. That is of course expecting the timing to stay within usual range. Make sure to use a quality, properly tuned carb and well designed intake. It's possible to run into individual cylinder issues that when riding on the edge (I don't think with 91 you would be) with fuel octane level and detonation if a cylinder is seeing lean condition, for example.

A/F ratios are pretty cheaply monitorable these days. I don't think I'd do a build without one. Peace of mind being able to monitor it especially with a new engine set-up. This is how new cars can run crazy high compression rations without fear. They have the ability to constantly monitor A/F and adjust everything from valve timing to fuel mixture to accommodate any change necessary to keep things safe.
 
Ultimately, I plan to go with a Pro Flo 4 system. I believe A/F is one parameter that can be monitored on the tablet/APP.
 
Any EFI system will monitor the engine A/F. You can then set up parameters for it that the system will follow which will prevent it from getting too lean or rich. This will all but assure you of preventing detonation, for the most part.
 
I used to rum my old 289 engine with 10.5:1 on 91 octane when driving on the street. Don't remember ever any pinging. I did run 110 at the track.
 
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