Increased Compression Ratio

[Butterfinger]

When raising the compression ratio (new heads) from the stock 9.0:1 (actually 8.8:1) to 9.44:1, what tuning considerations are there? Will I have to run higher than 91 octane fuel? Do I leave my timing set at 14 degrees? Does the spark plug gap have to be smaller? I'm not planning on using a power adder.

Thanks for the help, this is the first time I'm changing the engine's CR.

 

[Mustang5L5]

Back timing down to 10 degrees and run higher octane fuel. Detonation will come easier with higher compressoion so you want to play it safe here.

 

[90stang5speed]

I would run 93 octane set the base timing back to 10 and then try to advance timing back up 1 or 2 degrees at a time. I ran as much as 17 degrees base before I got my TWEECER RT but thats just me. be careful though. I once listened to a guy that said run a max timing of 38 degrees. that didnt work out real well as I cracked 3 spark plugs from that mistake.

 

[NIKwoaC]

I would run 93 octane set the base timing back to 10 and then try to advance timing back up 1 or 2 degrees at a time. I ran as much as 17 degrees base before I got my TWEECER RT but thats just me. be careful though. I once listened to a guy that said run a max timing of 38 degrees. that didnt work out real well as I cracked 3 spark plugs from that mistake.

He was probably talking about total timing advance... Not initial.

 

[90lxcoupe]

thats not a question you could really answer without just going out and trying it, like someone else said, just advance the timing slowly to 14 and see if it detonates. With a NA motor, even if the motor pings, it will not hurt anything if you dont do it for extended periods of time

 

[FastDriver]

Yeah, but that's still way too much timing, IMO

Tuning considerations: You were talking about switching to efficient aluminum cylinder heads, but you didn't mention that. With the 58cc AFRs you were talking about the other day, I'd personally leave it at 14*, run 93 octane and work up from there. My n/a AFR165 nearly 10:1 CR combo liked 18* initial timing the most. Keep the Air-Fuel Ratio (AFR) as close to 13:1 at WOT as possible if n/a.

 

[90stang5speed]

yes, the guy that told me 38 degrees meant total timing. When I did that it cracked the plugs.