Manifold Vacuum & Carb Idle Circuit

[Fly2xs]

Hello,

Just built a standard 289 to the HiPo standard of the day (65/66) with a comp cam with the specs below.  I have an Autolite 1.08 4100, Wieand Stealth intake, repo hip exhaust, single piont distributor recurved to the boss 302 specs, engine bored twenty over.

My question, what can I expect in manifold vacuum?  What the the origional 65/66 HiPo's for manifold vacuum.

With the Comp Cam installed, I cannot stay on the idle circuit on the Autolite.  I have to open the primary idle a bit.  Can this be expected with the cam specs below?  Did the original 65/66 HiPo engines stay on the idle circuit?

Thanks for any help in advance.


RPM Range 1500-5500
Lifter TypeHydraulic Flat Tappet
Int. Duration Advertised (Deg) 268
Exh. Duration @ .050 in. (Deg) 218
Exh. Duration Advertised (Deg) 268
Int. Duration @ .050 in. (Deg) 218
Lobe Separation (Deg) 110
Exhaust Valve Lift with Factory Rocker Arm Ratio (0.456
Intake Valve Lift with Factory Rocker Arm Ratio (i0.456
Valve Lift with Factory Rocker Arm Ratio 0.456 int./0.456 exh.

[Royce Peterson]

The problem with camshafts is the descriptions found in catalogs or online assume that you are building a 350 Chevy. A 289 is much smaller so it is affected a lot more by camshaft duration and lobe separation than a 350.

I built a real 289 HP and used the same camshaft in a '66 Shelby. It had the stock Shelby intake manifold, tri - Y headers and 4 speed.  Rear gears were the standard 3.89:1. In that car with a stock 715 CFM Holley we had no problem keeping the carburetor in the range where the idle mixture screws were still effective. The throttle response was a little soggy off idle and the effective RPM range was more like 2000 - 6200 RPM. It idled around 700 RPM.

In my case the owner was unhappy with the 268H especially with the (non - stock) Fomoco under dash factory style AC running. So we swapped out to a 260H cam and he was very happy with the performance after that. It idled around 600 RPM with the AC on.

At no time did we ever put a vacuum gage on the car but I expect it would have been 12 - 14 inches of mercury at idle. Swapping to the 260H no doubt bumped up vacuum considerably but in a car with manual brakes and no vacuum accessories and a standard transmission it was unimportant to us.

[gt350hr]

   Fly2xs,
      The cam you have chosen is smaller than an original HiPo 289 , same for the carb. What is hurting you is the Weiand Stealth intake. It is larger than the original Cobra intakes which combined with a smaller cam , hurts vacuum signal to the carb. To get the primary "blades" closed enough to make the idles screws effective , you could drill some 1/8th holes in the blades to let air through  with less blade opening. It is unusual to have to do this with such a small cam but the combination you have seems to need the modification. You could also "crack open" the secondaries and do the same thing.
   Randy