AFR - what's the ideal number ? - Ford Forums - Mustang Forum, Ford Trucks, Ford Focus and Ford Cars
Ford Forum Ford Forum

» Auto Insurance
» Featured Product
ยป Wheel & Tire Center

Go Back   Ford Forums - Mustang Forum, Ford Trucks, Ford Focus and Ford Cars > The Garage - Tech Forums > Engine Specific Tech > Cleveland Tech
Register Home Forum Active Topics Photo Gallery Auto Loans Garage Mark Forums Read Auto Escrow

FordForums.com is the premier Ford Forum on the internet. Registered Users do not see the above ads.
Reply
 
LinkBack Thread Tools Display Modes
Old 02-05-2006, 02:47   #1 (permalink)
Registered User
 
xb351coupe's Avatar
 
Join Date: Feb 2005
Location: Sydney
Posts: 343
AFR - what's the ideal number ?

What's the ideal AFR ?

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.

thanks
__________________
1974 XB GS Coupe - 351C FMX 9". 208RWKW (280RWHP) 12.92 @ 105.9M MPH (28/6/06 WSID)
xb351coupe is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 02-05-2006, 03:04   #2 (permalink)
74 XB Fairmont
 
brenx's Avatar
 
Join Date: Sep 2001
Location: Cranbourne, Vic
Posts: 5,113
Re: AFR - what's the ideal number ?

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.
__________________
XB Fairmont (street car) 11.07@123.02mph.
brenx is offline   Reply With Quote
Old 02-05-2006, 05:03   #3 (permalink)
Registered User
 
TruBlu351's Avatar
 
Join Date: Apr 2003
Location: Newcastle
Posts: 2,398
Re: AFR - what's the ideal number ?

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 :( $$$
__________________
XE Falcon
393 Tremec TKO 9"
TruBlu351 is offline   Reply With Quote
Old 02-05-2006, 14:12   #4 (permalink)
'60 F100 Q-Code
 
davis's Avatar
 
Join Date: Mar 2004
Location: Washington, USA
Posts: 750
Re: AFR - what's the ideal number ?

Quote:
Originally Posted by TruBlu351
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.


:davis:
davis is offline   Reply With Quote
Old 02-05-2006, 16:29   #5 (permalink)
Registered User
 
xb351coupe's Avatar
 
Join Date: Feb 2005
Location: Sydney
Posts: 343
Re: AFR - what's the ideal number ?

Ok thanks guys.

One more question to be sure.

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.

TruBlue351 - He was refering to full power runs.
__________________
1974 XB GS Coupe - 351C FMX 9". 208RWKW (280RWHP) 12.92 @ 105.9M MPH (28/6/06 WSID)
xb351coupe is offline   Reply With Quote
Old 02-05-2006, 16:37   #6 (permalink)
74 XB Fairmont
 
brenx's Avatar
 
Join Date: Sep 2001
Location: Cranbourne, Vic
Posts: 5,113
Re: AFR - what's the ideal number ?

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.
__________________
XB Fairmont (street car) 11.07@123.02mph.
brenx is offline   Reply With Quote
Old 02-05-2006, 17:07   #7 (permalink)
Registered User
 
xb351coupe's Avatar
 
Join Date: Feb 2005
Location: Sydney
Posts: 343
Re: AFR - what's the ideal number ?

Quote:
Originally Posted by brenx
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.

Ok thanks,
__________________
1974 XB GS Coupe - 351C FMX 9". 208RWKW (280RWHP) 12.92 @ 105.9M MPH (28/6/06 WSID)
xb351coupe is offline   Reply With Quote
Old 02-05-2006, 23:43   #8 (permalink)
Registered User
 
TruBlu351's Avatar
 
Join Date: Apr 2003
Location: Newcastle
Posts: 2,398
Re: AFR - what's the ideal number ?

Yeah, 12.8 is AOK in my books.

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.
__________________
XE Falcon
393 Tremec TKO 9"
TruBlu351 is offline   Reply With Quote
Old 02-06-2006, 00:57   #9 (permalink)
Registered User
 
xb351coupe's Avatar
 
Join Date: Feb 2005
Location: Sydney
Posts: 343
confused Re: AFR - what's the ideal number ?

Quote:
Originally Posted by TruBlu351
Yeah, 12.8 is AOK in my books.

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.
__________________
1974 XB GS Coupe - 351C FMX 9". 208RWKW (280RWHP) 12.92 @ 105.9M MPH (28/6/06 WSID)
xb351coupe is offline   Reply With Quote
Old 02-06-2006, 01:14   #10 (permalink)
Registered User
 
TruBlu351's Avatar
 
Join Date: Apr 2003
Location: Newcastle
Posts: 2,398
Re: AFR - what's the ideal number ?

Quote:
Originally Posted by xb351coupe
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.

Please let me know what he says!!
__________________
XE Falcon
393 Tremec TKO 9"
TruBlu351 is offline   Reply With Quote
Sponsored Links
Advertisement
 
Reply

  Ford Forums - Mustang Forum, Ford Trucks, Ford Focus and Ford Cars > The Garage - Tech Forums > Engine Specific Tech > Cleveland Tech



Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Powered by vBadvanced CMPS v3.2.2

All times are GMT -7. The time now is 01:20.



Powered by vBulletin® Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Content Relevant URLs by vBSEO 3.6.0
Garage Plus vBulletin Plugins by Drive Thru Online, Inc.