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Old 01-31-2008, 10:10 PM
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Ideal a/f ratio

hey everyone got a question everyone seems to have a differant answer to. Whats the ideal a/f fuel ratio ratio to be at under a boosted condition wot. Im not talkin 14.75:1 chemically ideal. im thinkin margin of safety fend of detonation good power daily driver blown motor.Any and all opinions welcome

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Old 01-31-2008, 10:27 PM
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10:1 is what most boosted applications look for at max.
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Old 01-31-2008, 10:31 PM
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Not sure this really helps, but they are the best at monitoring what's going on. Might have some usefullstuff.

http://www.innovatemotorsports.com/
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Old 01-31-2008, 10:44 PM
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A boosted motor with a fuel that has enough octane to not detonate at full power will make the most power with a AFR between low 13:1 and mid 12:1.
But if the fuel doesn;t have enough octane in itself to avoid detonation then extra fuel is needed to help cool the combustion chamber and moderate the comnbustion speed a bit to keep the motor out of detonation and preignition.
usually this is combined with some spark retard.

Injecting Water/Methanol to suppress detonation/preignition is more effective and makes more power overall, with increased safety using more normal 12.5:1 AFR and more normal spark timing. The volume of water/methanol can be up to 30% of the total fuel volume used @WOT full power. The water doesn't add to the power by itself (other than keeping things cool, allowing more normal spark timing), but the Methanol does.

The short answer is there is no ideal AFR. You give it what ever amount of fuel and take out as much timing as required to maximize the power on the available fuel octane, without burning the motor up.
Water/methanol injection and or intercooling is, more effective.

Ricardo High speed internal combustion

You can find this book and more in the Public Library. Read it, Know it, there there will be a test.

Last edited by F-BIRD'88; 01-31-2008 at 10:57 PM.
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Old 02-01-2008, 12:25 AM
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I am curious if you are using a Blower like on the Corvette in my pictures, or what I call a Super Charger, also belt driven, but much smaller. The mother board in my other laptop took a dump (hard drive is ok), or I could give you this information right now. I have emailed customers with both types asking if it shows that information.

I don't do dyno testing, but these vehicles are everyday drivers, that run crisp and clean. I change spark plugs every 2 years as regular maintenance.
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Old 02-01-2008, 12:35 AM
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weiand 142 2.17:1 drive ratio 355 sbc afr195 8.33 speed pro dish nx268h supposed to be to about 7.5 8 lbs
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Old 02-01-2008, 12:56 AM
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Not knowing your exact BARO/ALT/TEMP it should good with that combination. The only true tell is reading the spark plugs.
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Old 02-01-2008, 08:14 PM
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Lower is safer, all the ones around here that I have seen on the dyno are usually 10:1 to 11.5 to 1. Rich tend to fend off pinging and cool things down a little . You are flirting with disaster for anything over 12:1 unless it is tuned perfectly. I have the pistons to prove it. If you are not running forged pistons you need to be extra careful
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Old 02-02-2008, 08:00 PM
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So are you looking for a base line carb calibration to start your tuning precess?
What carb are you running?
Will you be tuning by eye,ear and the seat of your pants? or will you be tuning on a dyno with a wide band AFR meter?
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Old 02-02-2008, 08:30 PM
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mighty 750 blower carb aem widband and autometer vacuum/boost gauge i was having a tough time with the idle circut but got it close enough to take it out on the street for the first time tomorrow (weather permitting bc rain is crazy sometimes). not that i'm going to thrash the thing just load it up pretty good to see which way i need to go
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Old 02-02-2008, 09:13 PM
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Quote:
Originally Posted by sheldonw
mighty 750 blower carb aem widband and autometer vacuum/boost gauge i was having a tough time with the idle circut but got it close enough to take it out on the street for the first time tomorrow (weather permitting bc rain is crazy sometimes). not that i'm going to thrash the thing just load it up pretty good to see which way i need to go
not that familiar with BG carbs. They are calibrated differntly than typical holleys. If it was a holley I could tell ya what to start with and be within 3 jet sizes of optimum on your setup.
Some of the "Blower carbs" are intended for a 671 or 871 big blower not a mini blowerlike yours. The idle circuits could be way to fat, the jetting may not be right.
What is the carb model and jetting? Does it have a boost referenced power valve?
Start with 26deg of total timing. It will want lots lots of initial timing at idle. like 16 to 24deg at idle.
Don't wack the throttle untill the motor is warmed up and the blower feels warm or it may backfire on ya. the accelerator pumps will need dialing in.
Are you using a boost retard box? Use the best fuel available 92 minimum, Sunoco 94 if you can get it there.
Does this motor have cast or forged pistons?
Having Wide band is going to make the whole thing pretty easy. start over rich (WOT under boost) and work back. then work up in spark timing (26 to 32deg zone Under full boost depending on the fuel octane)

Last edited by F-BIRD'88; 02-02-2008 at 09:22 PM.
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Old 02-02-2008, 09:35 PM
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Way fat on the idle circit is a mega understatement!!

Forged pistons. Its at 17 inital and a total of 32 feels like it needs more though. Put brown cams in front and back with 35 squirts could probably pull back a size or two on the primary squirter but good to go around the block.

E Curve dizzy till i get things dialed then i'll add a 685 mallory box with boost retard. Carb # is 5402010bc dropped primary jets to 72 and 2.5 power valve for tuning idle circut . probably put 75 jets back in and 6.5 power valve it has 83 jets in the back and yes power valve is boost referanced

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Old 02-02-2008, 10:23 PM
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i would start with 75 and 88 jets. (fat jetting is safe) 6.5 power valve is good as long as you have more than 6.5" manifold vacuum at idle.
More initial (20ish+) but 26-28deg total spark. Start conservative on total spark lead. Lock out the timing and set at 26 BTDC full time if nessessary.
You can work up from 26deg from there. More intial = better throttle response.... to much total = broken pistons
Remove the carb and look at the throttle (all 4 transfer slots exposure .02-.03") reset as nesssessary.

Is there a 3/8" vacuum fitting on the base of this carb.?
Hook up the PCV on the back of the carb if there is a spot there.
Will lean and tame the idle fuel calibration. If you don't have proper valve cover PCV vent baffling just let the pcv hose and valve hang free for now.
reinstall the carb and reset the idle mixtures. Try not to touch the idle speed ( throttle opening at idle)
Hook up the vacuum advance to the intake manifold below the blower NOT ON THE CARB or leave it plugged off for now. Vacuum advance will need carefull dialing in. Hook up the power valve reference hose to the intake manifold below the blower.
What is the manifold vacuum and idle speed at idle while idleing in gear?

Make sure the fuel in the tank is 100% 92 octane or better. Low octane fuel will destroy this supercharged motor.

Check out the Snow water injection systems. They work very well.
You can really crank up the boost (12lbs+) and keep all or most of the timing under boost without detonation. ( way, way more power with reliability)

Did you need to use the IDLE EZE to get the idle right?

Last edited by F-BIRD'88; 02-02-2008 at 10:50 PM.
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Old 02-02-2008, 10:50 PM
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picked up a 1"spacer with pcv port "td2103" couple days ago i was gonna throw on tomorrow still gotta make a baffle in the valve cover for the pcv.

best i could get it to idle was about 1000 rpm 12" of vac (I think it should be higher) and 13.8 afr with the idle eze wide open and about a 1/4 turn on the front and back idle speed screws idle doesn't change much in gear pretty loose 2400 holeshot converter vacuum advance is still unhooked for now but it's pretty adjustable with the e-curve.

Good call on the pcv I was wondering about that. The power valve is run ref appropriately. I run chevron 94.
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Old 02-02-2008, 11:10 PM
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You'll probabily want a looser higher stall converter. Remember its just a low compression mutt until the boost hits. A 3000-3500 stall is best. You launch at full boost. but thats up to you...

With the NX268cam it should idle "like a stocker". (750rpm in gear)
Remove the carb and look at the throttles. If they are too far open at idle or uneven at idle the idle quality will be off. Reset them all and reinstall the carb with the PCV hooked up. Close the idle EZE and lock out the timing curve (26deg full time to start tuning)
Let it warm up and then re-adjust the idle mix screws . If you need more idle rpm open the idle eze a bit. If the idle speed is too high, remove the carb and close the throttles a little bit, but evenly. Do not open the throttles once pre-set.
IDLE transfer slot exposure). With the combined PCV and idle EZE you should be able to get a pretty good idle without excessive throttle blade opening. The idle AFR on the meter is not that critical and can even give false reading from raw oxygen getting pumped thru the motor (cam overlap at idle). A clean steady idle at around 750rpm in gear with maximum manifold vacuum is what you're after. (warmed up engine) 12" in neutral is about right. What is it in gear after you do the readjustment? as long as it more than 6.5" in gear, your fine. the idle AFR's are good. Don;t try to get a 14.7:1 ratio at idle. its not going to happen on a carbed motor. all you want is the best idle quality at 750rpm in gear. If the AFR meter says 12.5:1 at idle so be it. (warm)
Do not test at WOT with leaner than stock primary jetting (yet) once the idle is sorted out the 75's won't be too fat.
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