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Old 05-26-2005, 01:34 AM
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Another Carb Tuning ?...TECH @ BG Help

Well I ve built a few short track stockcar engines lately. Since I primarily build street/strip and drag engines I find myself at abit of a disadvantage when it comes to tuning these engines. The rules for these engines call for a 390 cfm 2bbl carb (most flow more like 420 to 450 cfm) . This makes for some interesting tuning challenges on a 362 CID SBC turning 7200 rpm's. I have noticed that when you hammer the pedal to wide open throttle the vaccum doesnt drop to or even close to zero and it recovers to a higher vaccum much quicker. I realize this is becuz the carb is so small that it maintains a better vaccum signal. My question is would this type of setup benefit from a slightly larger high speed air bleed than what holley uses stock in these carbs. My thought is that the since the carb is so overly responsive due to its small size that the higher vaccum signal it maintains going to WOT is causing it to draw more fuel than the engine needs at that moment. This in turn causes a rich condition during the transition to WOT that makes proper tuning difficult. I would think that increasing the size of the high speed air bleed would weaken the vaccum signal at that point and cause it to drop closer if not to zero thus eliminating the overrich condition and make it easier to tune. I'm not sure Im on the right track so any input would be good. Just tryin to get the most outta this combination.

Doc

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Old 05-26-2005, 07:44 AM
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The short answer is yes. Restricted engines have a tendency to richen the mixtures at speed. The problem get's worse as CI goes up. You can e-mail me for the longer version.
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Old 05-26-2005, 11:30 AM
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Quote:
Originally Posted by Docs91RS
I would think that increasing the size of the high speed air bleed would weaken the vaccum signal at that point and cause it to drop closer if not to zero thus eliminating the overrich condition and make it easier to tune. I'm not sure Im on the right track so any input would be good. Just tryin to get the most outta this combination.

Doc
We do TONS of these restricted 2 bbl motors on the dyno. We typically see 5" vac and the last one pulled 7". We have played with all kinds of trick carbs and mainly use Braswell units. HP gain from a super trick versus standard is from zero to 10 HP, depending on the customers base carb and condition.

Changing the air bleeds will not change the vac restriction you see in the manifold. To verify that you are getting a rich condition you need to run it on the dyno with sensors hooked up, A/F meter, airflow meter and brake specifics. What your chasing though with the airbleeds are small gains at best, unless jetting is WAY off. Still though worth doing as johnsongrass1 pointed out.

Power on these motors is in the camshaft (lots of duration is needed) and futzin with the intake (in a legal not able to notice type of way) and in spacers. Spacers shaping can get you 10 horse. Play with a standard Brezinski 2 hole. cut out half the thickness inside where the 2 hole is inside the plenum (cut out the tickness on the bottom) and open up the 2 hole to an open design. These are a few of the modified spacers we test out on the dyno.

Although it is illegal in these types of classes (typically) most shops that build these motors have an affinity for acid. Not the type you pop to get high but the type that cleans pools (re: industrial grade muratic). Brezinski had a nice webpage showing what your not supposed to do.

For a general feel for what we have found a fully prepped restricted motor that will/should/usually/hopefully pass tech will dyno 30 to 40 HP more than a motor built to the strict letter of the law per the rules.

Lightweight crank, rods, pistons, (1500 or less bobweight) coatings on everything inside are some of the other basics that are common on a lot of the top motors.
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