I was under the impression that mid 12's was close to the mark. The reason I ask is I had a guy tell me I needed to be around 14, now he's got me second guessing what I believed to be correct.
My AFR's vary throughout the rpm range but pretty much stay between 12.8 and 12.0. My tuning was done on an engine dyno. I was told that's pretty much spot on.
When I had the 351 it was also around the mid 12's.
Ask him whether he was referring to cruise, economy AFR or max power.
Stoichiometric (on the $$ for economy cruise) is 14.7:1.........this would melt your pistons @ WOT, if it didn't ping to bits before it got there.
Full power requires a richer AFR to keep combustion temps low to avoid detonation. 13.2:1 is "supposed" to be the magic number, but this doesn't always seem true on dyno runs WRT various n/a engine combos......ie: little richer OK. Definitely don't go leaner than this.
FYI, late model EFI engines lean out to about 18-19:1 when cruising with the use of knock sensors. Wish mine did that :( $$$
Ask him whether he was referring to cruise, economy AFR or max power.
Stoichiometric (on the $$ for economy cruise) is 14.7:1.........this would melt your pistons @ WOT, if it didn't ping to bits before it got there.
Full power requires a richer AFR to keep combustion temps low to avoid detonation. 13.2:1 is "supposed" to be the magic number, but this doesn't always seem true on dyno runs WRT various n/a engine combos......ie: little richer OK. Definitely don't go leaner than this.
FYI, late model EFI engines lean out to about 18-19:1 when cruising with the use of knock sensors. Wish mine did that :( $$$
13.2 to 13.4 is a good number for good power and reasonable economy. With a carburetor, the "ideal" is to usually jet the fronts so that you maintain about 13.2 on the primaries and jet the backs to that you maintain about 12.8-12.5 on the secondaries. This will give you "best" cruise and WOT so that you can enjoy the both worlds without nuking your engine. If you live in a fairly humid climate, you can usually get by a bit leaner than in a drier climate.
So if I'm 'running at 12.5 and want to increase to say 13 odd, All I do is fit larger jets ? or working the other way if I was running at 14+ and needed to get down to 12's I change to smaller jets ? Or am I back to front.
From memory I was at around 12.5 to 12.8 so I'm in the ball park. Should I bother changing jet sizes, or is all ok and I'm safe ? I don't have any pinging issues, actually I've never had a problem even when I run 92 to 95ron fuel. I guess my 10 to 1 compression helps.
If your in the 12's? Don't change anything unless your trying to make the mixtures more consistant than they are throught the rpm range. Your compression would assist with detonation.
If your in the 12's? Don't change anything unless your trying to make the mixtures more consistant than they are throught the rpm range. Your compression would assist with detonation.
But, in answer to your question, yeah, you got it backwards. If you want to go leaner, ie: 12.5 going to 13.2 you need smaller jets. LEAN is a high AFR, RICH is a lower number. ie: 12.8 parts air to 1 part fuel. Just like mixing cement, or was that 3 parts sand and 1 shovel of cement? So if you halved your jet size you'd double the AFR so to speak.
But, in answer to your question, yeah, you got it backwards. If you want to go leaner, ie: 12.5 going to 13.2 you need smaller jets. LEAN is a high AFR, RICH is a lower number. ie: 12.8 parts air to 1 part fuel. Just like mixing cement, or was that 3 parts sand and 1 shovel of cement? So if you halved your jet size you'd double the AFR so to speak.
Well now that bloke has got me completely worried, He was telling me I was running to lean at 12.5 to 12.8 and said he would run richer and quoted 14 as a better number.
Well now that bloke has got me completely worried, He was telling me I was running to lean at 12.5 to 12.8 and said he would run richer and quoted 14 as a better number.
Tell him its like making raspberry cordial.
AFR = Air to fuel ratio, ie: a comparison of air molecules to fuel ones. So the first number is the AIR and the second is the FUEL. ie 13:1. It's not Fuel to Air ratio which would be 1:13. And, if it was backwards and actually 13 parts fuel to 1 part air, you may aswell block up 3 holes in your 4 barrel carby and put the fuel feed straight into the venturi!!
So back to the cordial (cordial = fuel and water = air). Tell me that a 2:1 cordial mix wouldn't make you pucker up!....its a rich mix (low first number)And the skimpy 20:1 diluted crap is awfull because there's too much water too lean on the cordial.
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So if you halved your jet size you'd double the AFR so to speak.
Re: AFR - what's the ideal number ?
(low first number)And the skimpy 20:1 diluted crap is awfull because there's too much water 
too lean on the cordial.
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