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Scorpio
Joined: 05 Jul 2007
Posts: 1957
Location: Brisbane, Australia
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 Posted: Fri Nov 28, 2008 5:52 pm Post subject: Ideal A/F ratio? |
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been reading up on tuning and it seems there isnt a definitive answer for the ideal A/F for my soon to be boosted NA
I know the stoichiometric A/F is 14.7 but whats a good A/F taking everything else such as power..efficiency..detonation etc into account? some say 12.7-1 some say 13.1-1
Educate me wise dudes! 
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1979 NA
MS1..EFI..
GARRETT T25 TURBO
BILSTEIN SHOCKS
GT BASED CUSTOM BODYKIT
Brisbane , Australia
Think mean think fast
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morghen
Joined: 21 Jan 2005
Posts: 9095
Location: Romania
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| Posted: Fri Nov 28, 2008 7:47 pm Post subject: |
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At WOT, when the boost kicks in...3000-3500 or whatever rev it comes in...i would even use 11.5-1..then you can lean it out a bit as it climbs up the revs.
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CBass
Joined: 03 Nov 2002
Posts: 2807
Location: Vancouver, Canada
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| Posted: Sat Nov 29, 2008 6:38 am Post subject: |
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There is no ideal AFR per se. For a turbo engine you'd want it a little richer than a NA application to keep down combustion temps. Go richer than you think you need to, start at 10.5/11:1 and conservative timing and start tuning from there.
What kind of boost levels are you expecting to see and what kind of internal build?
Access to a dyno? EGT probe?
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Raceboy
Joined: 01 Mar 2004
Posts: 2327
Location: Estonia, Europe
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| Posted: Sat Nov 29, 2008 6:53 am Post subject: |
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For NA you shouldn't go richer than 13:1 at WOT, for turbocharged car, 12-12.5 is the good point.
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'83 924 2.6 16v Turbo, 470hp
'67 911 2.4S hotrod
'90 944 S2 Cabriolet
'78 924 Carrera GT replica
'84 928 S, sold
'91 944 S2, sold
'82 924S/931 "Gulf", sold 
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CBass
Joined: 03 Nov 2002
Posts: 2807
Location: Vancouver, Canada
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| Posted: Sat Nov 29, 2008 9:21 am Post subject: |
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| Raceboy wrote: | | For NA you shouldn't go richer than 13:1 at WOT, for turbocharged car, 12-12.5 is the good point. |
That's highly dependent on the engine. Often you'll find the AFR dipping up and down on a properly dyno tuned engine. EGTs are also a huge consideration, especially if using stock pistons with narrow ring lands and a thin crown.
It's much better to start too rich and then lean out until it stops making more power while keeping an eye on your exhaust temperatures. A dyno and an EGT probe are the best way to do this. If you don't know what your EGTs are it's better to err on the side of caution and run a little rich with a little less timing.
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'81 931 in various states of assembly |
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Scorpio
Joined: 05 Jul 2007
Posts: 1957
Location: Brisbane, Australia
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| Posted: Sat Nov 29, 2008 10:35 am Post subject: |
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| Quote: |
What kind of boost levels are you expecting to see and what kind of internal build?
Access to a dyno? EGT probe? |
I want close to Raceboys level of performance (minus of course all the weight loss)..I wantthe max HP the drivetrain can happily handle
whats that ..mm 200hp at the engine?
so boost somewhere around 10 maybe maybe maybe 11 or 12 on stock compression 9:3-1
Mods are basically
1) Headstuds instead of bolts
2) newly rebuilt engine with 3 angle valve job, new rings, seals etc etc etc
2)BAE manifold
3)Garrett T25 turbocharger
4) EFI with O2 sensor...Edis4 with crank triggerwheel
5)951 intercooler
6)possible water/methanol injection
7)Thicker core aftermarket radiator, twin thermo fans
8)And a partridge in a pear tree
| Quote: | | Access to a dyno? EGT probe? |
When its all done ill get it dyno tuned...although the people ive asked dont tend to understand or wanna touch megasquirt ...might ask leadfoot and macbdog to help out
So basically start rich then move up 11.1 ---13.1
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1979 NA
MS1..EFI..
GARRETT T25 TURBO
BILSTEIN SHOCKS
GT BASED CUSTOM BODYKIT
Brisbane , Australia
Think mean think fast
all youll see is
my Porsches Arse!!! |
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huckster
Joined: 22 Dec 2006
Posts: 115
Location: Burnaby B.C.
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| Posted: Sat Nov 29, 2008 2:52 pm Post subject: |
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So you went with megasquirt and a t25 I am doing the same! Wonder who will finish first?
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Lizard
Joined: 03 Nov 2002
Posts: 9364
Location: Abbotsford BC. Canada
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| Posted: Sat Nov 29, 2008 2:57 pm Post subject: |
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Well if you come out here as we discussed you could have it up and done right quick!
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Rasta Monsta
Joined: 12 Jul 2006
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Location: PacNW
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Raceboy
Joined: 01 Mar 2004
Posts: 2327
Location: Estonia, Europe
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| Posted: Sat Nov 29, 2008 5:22 pm Post subject: |
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It depends more on fuel. If you have good enough fuel (at least 98 RON), you can use AFR what's called "lean best torque" and on gasoline this point is around 13.0. Anything leaner or richer, you won't benefit (or if you will, something else is off).
On turbocharged car, one would want to go to point "rich best torque" and that is around 12.5. But, on boosted car you will be cramped by engine design and on turbocharged 924 I wouldn't go leaner past 12.2.
On properly tuned engine @ WOT, AFR's (at least on my car) are steady. I had no need to richen it up even on max torque area where knock occurs first.
_________________
'83 924 2.6 16v Turbo, 470hp
'67 911 2.4S hotrod
'90 944 S2 Cabriolet
'78 924 Carrera GT replica
'84 928 S, sold
'91 944 S2, sold
'82 924S/931 "Gulf", sold
'84 924, turbocharged, sold.
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Scorpio
Joined: 05 Jul 2007
Posts: 1957
Location: Brisbane, Australia
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| Posted: Sat Nov 29, 2008 6:48 pm Post subject: |
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| Quote: | So you went with megasquirt and a t25 I am doing the same! Wonder who will finish first?
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YOU WILL...im slow , its my first major car project , ive stripped everything on this car down to nuts and bolts (minus the gearbox) and im broke
T25 turbo hey? good choice :p
p.s when you finish first i can pick your brain and borrow your megatune tables  | Quote: |
It depends more on fuel. If you have good enough fuel (at least 98 RON), you can use AFR what's called "lean best torque" and on gasoline this point is around 13.0. Anything leaner or richer, you won't benefit (or if you will, something else is off).
On turbocharged car, one would want to go to point "rich best torque" and that is around 12.5. But, on boosted car you will be cramped by engine design and on turbocharged 924 I wouldn't go leaner past 12.2.
On properly tuned engine @ WOT, AFR's (at least on my car) are steady. I had no need to richen it up even on max torque area where knock occurs first. |
Ok thanks raceboy ...i think im getting it
just read the following article which also helped "rich best torque" and "lean best torque". (it doesnt take into account boosted applications or 924 restrictions though.
http://www.max-boost.co.uk/max-boost/internet_articles/Air%20fuel%20metering.htm
So full load at WOT were talking around 12.2
how about idle....cruising etc?
p.s will be running 98 octane
_________________
1979 NA
MS1..EFI..
GARRETT T25 TURBO
BILSTEIN SHOCKS
GT BASED CUSTOM BODYKIT
Brisbane , Australia
Think mean think fast
all youll see is
my Porsches Arse!!! |
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Min
Joined: 04 Nov 2002
Posts: 2368
Location: Vernon, British Columbia, Canada
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| Posted: Sun Nov 30, 2008 4:25 am Post subject: |
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| Scorpio wrote: | So full load at WOT were talking around 12.2
how about idle....cruising etc? |
idle around 13.5, cruise as lean as it'll go without lean surging.
Min
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1980 n/a with EDIS and Megasquirt II Injection. 7 different colors and counting. |
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tuurbo
Joined: 08 Aug 2007
Posts: 1446
Location: East Windsor, New Jersey
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| Posted: Sun Nov 30, 2008 8:08 am Post subject: |
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| Quote: | | cruise as lean as it'll go without lean surging. |
Are you trying to maximize / optimize HP or reliability or ...is that just the way it's done?
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1980 924 turbo, MSD, Meth. Inj, otherwise stock. |
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Rasta Monsta
Joined: 12 Jul 2006
Posts: 11733
Location: PacNW
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| Posted: Sun Nov 30, 2008 8:28 am Post subject: |
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I think off-load leanitude is for fuel efficiency.
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- WeiBe (1987 924S 2.5t) - 931 S3
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Min
Joined: 04 Nov 2002
Posts: 2368
Location: Vernon, British Columbia, Canada
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| Posted: Sun Nov 30, 2008 6:26 pm Post subject: |
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rasta is correct, you want it as lean as possible while cruising because that way you get the best fuel economy.
Min
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Custom means it didn't come from a box.
1980 n/a with EDIS and Megasquirt II Injection. 7 different colors and counting. |
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