General Category > Quadrajet Carb Talk and Tips

vacuum at WOT

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Miles:
Good morning,
on a dyno, is a vacuum of 1.4" an indication that the carb is too small or is that a normal reading.
Thank you

70GS455:
Yes, specially if it's near max rpm. Should be 0.5" or less, ideally. Actually it means your engine is too big or you're making too much hp =D

Cliff Ruggles:
Don't be too quick to blame the carburetor, could be other things including the intake port capacity, intake manifold, etc.

What are the engine specs and dyno numbers?

Miles:
463 cid, 10.49 static compression ratio, 230/236 at 0.50, 112 LSA, gross lift .575/.585, 108 ICL, full roller, CNC ported fast burn aluminium heads 204 cc ports  flow 305/220 at .55 lift. EPS 800 95/110 jets 65/47 rods and 750 HP 73/73 jets, HEI 16 initial 29 total no vacuum can, port matched high rise dual plane no heat
490HP at 5200 and 552 lbft at 4200 0.6" vacuum at 3000 rpm 1.35" vacuum at 5200 rpm with the EPS 800, 500 HP at 5300 rpm and 561 lbft at 4000 rpm no vacuum numbers with the Holley 750.
I'm just wondering what the ball park vacuum number at WOT max rpm is for a street engine. I've heard anything over 1.5 is a restriction in a street set up and racers like 0.5-0.75".  Don't know it that is true or BS.
Thank you.

Cliff Ruggles:
Most of the ones we've done have been around .4 to .9" at WOT.

I don't pay a lot of attention to such things to be real honest.  In a street driven vehicle the engine will spend very little time right up at the shift point anyhow. 

Far more important is average power and power in the loaded rpm range where the dyno can measure it.  The broader and flatter the power curve (torque) is the better.

Great torque numbers, HP seem a bit "light" for the CID and head flow and it's quitting pretty early in the rpm range, probably needs a bit more cam, or just could be the dyno as they vary some place to place.

I'd also add that making peak torque at 4200rpms should put peak HP up near 5600-5800rpm's based on the builds we've done here with similar CID, head flow, cam specs, etc.

I would run vacuum advance if this is street driven, about 10-14 degrees would be enough.  There I no reason not to use it, and you simply loose power and efficiency in the "normal" driving range running w/o it......Cliff

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