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what is a good air fuel ratio for a fuel injected turbo motor at full boost? the engine is a sbc with sb2 heads and is water to air intercooled. right now it is running 18 pounds of boost and the air fuel ratio is about 9 to 1 using c16 fuel. is 12 to 1 to lean for a turbo motor?
 

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yea, the car is running pretty good (5.72 in the eighth), just trying to creep up on a good air fuel ratio without hurting anything. thanks
 

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What compression? With C16 you could run as lean as 12.2 to 12.5 to 1 if the compression isn't stupid at that boost level..
 

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From what I have read, SB2 stuff likes lower timing and fatter a/f ratio, maybe Dustin will chime in.

I like around 11.0-11.5 on my stuff, but it is non intercooled!
 

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From what I have read, SB2 stuff likes lower timing and fatter a/f ratio, maybe Dustin will chime in.

I like around 11.0-11.5 on my stuff, but it is non intercooled!
It might but that is well below the generally accepted levles of beneficial cooling effects and is moving into a auto ignition/ preignition zone of danger. To much fuel can be just as bad ass to little but in a very different way.
 

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It might but that is well below the generally accepted levles of beneficial cooling effects and is moving into a auto ignition/ preignition zone of danger. To much fuel can be just as bad ass to little but in a very different way.
Yeah, I have seen some of my nitrous buddies tear up some ring lands from being too fat! It's a fine line for sure.
 

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Nitrous and boosted applications are night and day different when it comes to excess fuel..........

I don't think you want to go above 12.0 A/F at full-load, it probably isn't worth the risk. Make a pass on a new set of plugs and see what they look like after a run, especially the heat in the straps.
 

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Nitrous and boosted applications are night and day different when it comes to excess fuel..........

I don't think you want to go above 12.0 A/F at full-load, it probably isn't worth the risk. Make a pass on a new set of plugs and see what they look like after a run, especially the heat in the straps.
I don't disagree here. But if you keep adding fuel you drive up the compression ratio.
 

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You are right, the extra fuel in the chamber causes a higher compression ratio which a lot of people never think about. The same thing happens with the Water/Methanol injection stuff too. While to a point it will help cool-down the chamber, it is also raising the compression ratio at the same time. It's all a balancing-act.

I don't like to run anything boosted below 11.5 A/F, but that is just me and I have never tuned anything making more than 1400hp. I would NEVER run a N2O engine below 12.0 A/F either, but I have never hit it with more than 300hp so far.........
 

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we're at 12.0 currently running 11#s of boost on turbo blue fuel. It traps at 146. My target AFR is 11.4 at 20#'s but we haven't put any real boost to it yet.
 

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You are right, the extra fuel in the chamber causes a higher compression ratio which a lot of people never think about. The same thing happens with the Water/Methanol injection stuff too. While to a point it will help cool-down the chamber, it is also raising the compression ratio at the same time. It's all a balancing-act.

I don't like to run anything boosted below 11.5 A/F, but that is just me and I have never tuned anything making more than 1400hp. I would NEVER run a N2O engine below 12.0 A/F either, but I have never hit it with more than 300hp so far.........
I typically try to keep the plug reasonably clean and cool while trying to keep the big HP motors in the mid 11's myself. I have a few motors a bit over 1400hp and nothing really changes all that much. The small CI engine with lots of boost are different creatures however. To much fuel at a certain HP per CI point starts to hurt parts but in a different way. You get into a hydrolock situation if you aren't careful.
 

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You bring-up an interesting point there Collinsautomotive. I don't do EFI for a living but have worked on this stuff for 20+ years now. I never really thought about this until you mentioned the small CI stuff above: If you have a large cubic inch engine producing 1500hp and a small cubic inch engine producing the same amount of power, the amount of fuel in each cylinder will certainly change things won't it? I guess that would be true provided each engine was using the same amount of fuel to produce that power........... Typically a larger cubic inch engine will consume more fuel that is being burned for the heavier rotating assembly, higher frictional losses, etc.............. What are your thoughts? I am also saying that each engine is making the power the same way, whether they are N/A, turbo, blower, N2O........... Can't really compare one method of producing power to another.
 

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I say 10.5 to 11.2 to one is what most mid compression with a decent combustion chamber with c16 and c16 is all I will recomend for boosted aplications. As for cubic inches and fuel consumption thats splitting hairs. i say 300 to 500 cid, theses ratios will get you close enough to start reading plugs. I would start with NGK 10s and may go a little hotter if it needs it. If a guy says he can tell you what ratio or what to set your time at run as fast as you can away from them. Read the plugs and give the engine what it wants and dont get hung up on numbers.
 

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The perfect A/F??? There really isnt one that applies to every engine in all the different environments they are running in - I dont think anyway. The "optimum" A/F @ 10-15lbs boost is different to 20, 25, and 30+.

But i think most of you have said this as well as the safest way is to just creep up on the set up and monitor everything closely....

Ive been running an SB2.2 headed SBC for some years now originally nitrous and turbo for the last year and half and these things respond to minor changes in a big way. The chambers and port shape/design is very efficient and tends to be a little sensitive - so dont lean on it till you have an idea of how it responds to boost, spark and fuel is my best advise.

Having said that, your 9.0 AF is way too fat and im sure youll be "flat" in top gear... Whats your rpm line like on your log??? I bet it looks like a lumberjacks saw!!!

As already mentioned - read your plugs cause they dont lie as they are in the chamber!!! not transfering electronic data from a sensor in the form of an AF ratio....

Once you get the correct timing, and fuel burn going by the plug - replicate it every time via your AF (once you find that out).

Pete
 

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I'm suprised the motor even runs at 9.0 a/f with only 18# of boost. 12.0 is safe at that boost level.

The Sb2 heads don't need quite the timing of a conventional head and they don't need as much fuel either.
 

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what is a good air fuel ratio for a fuel injected turbo motor at full boost? the engine is a sbc with sb2 heads and is water to air intercooled. right now it is running 18 pounds of boost and the air fuel ratio is about 9 to 1 using c16 fuel. is 12 to 1 to lean for a turbo motor?

well are you gonna be tuning on a track or on a dyno? tune for the most hp and when you get to the most hp, make sure there is a safe margin of knock threshold. the more efficiant your heads are on the aspect of quench and air/fuel mixing, the leaner you can go on the air fuel ratio.... my stock headed buick motor makes the most power onthe dyno at around a 10.9-1 afr which is really shitty from a "combustion chamber" aspect.

if i were to change the heads out with a set of GN1 al heads and a set of flat tops the most power could prolly be made at a leaner AFR. hope this helps
 
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