Purple cam with low vac

[newboyracing]

Are any of you guys running the 292/509 purple cam? I have a low vac pressure of just 4 psi. There are no leaks in the manifold & I went from a 6.5 power valve to a 4.5 and it seemed to help a lot with low speed street driveing. I am thinking about going to a 2.5 power valve to see if it gets even better. Motor screems power at 3000rpm with a 4 speed but seems to lag at low speed driving.

 

[Redpolara]

The valve overlap of the 509 camshaft is enough to kill/bleed off your manifold vac.
And considering the cam is rather large, it's a foregone conclusion that the engine is going to be a bit doughy on the lower end of the RPM scale.

Tire height and rear end ratio could also be a contributing factor in regard to being doughy on the bottom end, with that particular cam.

 

[rumblefish360]

That's very true. I ran one in a 360 and would describe it the same way. Do realize that these old cams are a street strip design from 30+ years ago. They will still perform and do it well as there intended to. There timing events and quick ramp rate by compare to other street performance cams lend to that no power feeling down low.

You might want to run no power valve at all with this set up. You should also be able to pull about 5-7 on the gauge. 4 is low. (Not PSI, but inch of vaccum. or Hg.)

 

[Meep-Meep]

4" Hg is pretty low but may be expected with an auto trans idling in gear. With a 4 spd or auto in neutral you should get 10 or so - maybe 8. Do a compression check. Something tells me you might have about 115 PSI of cylinder pressure. Also is the cam installed correctly?

 

[rumblefish360]

Oh crap, your right, he has a stick. Funny, when I used tthat cam in my 360 4spd Cuda, I was able to get it about that level. I figured a few Hg. drop for an auto trans....which he has not! Duh on me.

 

[newboyracing]

292/509

Thanks guys I love the way the cam runs but will have to work around its vac issues.

 

[Histoy]

I've used that cam with a 4 speed, but it carried around 7 inches of vacuum at idle. You might try advancing the timing a little. I ran 15 to 18 degrees BTDC with no vacuum advance.

 

[Elvad]

509 cam

There are two different versions of that cam, the early one and an 'improved idle version'. They had a habit of not running at idle. Timing marks on these engines can move, thanks to the rubber insulator on the factory dampener, the outer ring can move, changing what you see. Verifying the timing marks are correct is a good idea. Or you can try timing the engine for best vacuum, forget the timing light and hook up a vacuum gauge and adjust the timing for best vacuum at idle, and see how it runs.