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cams and vacuum
Can anyone out there educate me on what a cam has to do with manifold vacuum? I am installing an assembled 428 with a cam that has an intake of 275 lift x .515 duration and an exaust of 285 lift x .541 duration. I understand I will have to install a vacuum storage canister because I have an automatic trans and power brakes.
Question #1 Is this too much cam for a street engine? Question #2 Do I need a storage canister and why? HELP:confused: |
Check out this link from Comp Cams. On the second page, it has a short discussion of overlap and its relation to manifold pressure.
http://www.compcams.com/Information/Tutorials/ Hope this helps. |
275 intake duration is not that much (if that is TOTAL duration). The lift makes no difference. What is your lobe separation? That plays a big role in idle quality, vacume, and low end torque. The range is usually between 106 degrees and 114 degrees. A lower number has lower vacume and rough idle, but a lot more *peak* torque and hp.
Unless you have really low lobe separation, I would think you would be fine for a street car. Ed |
Chopper:
Excellent site on the camshaft. Thanks. I will put that in my notebook. |
Sorry for the delay. I had to obtain the cam data. The lobe seperation is 110. Apparently, according to the Comp Can tutorial, both valves are open together at one point and that creats a vavuum. The lobe seperation determines how long they are open together. Fascinating stuff, I have a lot to learn. Thanks all.
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Um, if I may, manifold vacuum is caused by the pistons pulling air into the engine past the restriction of the throttle plates. Most passenger car engines (ie, non-high performance with little overlap) at idle will pull approximately 18-20 inches of manifold vacuum. Higher performance engines with high-overlap camshafts pull less manifold vacuum at idle due to the overlap. That is, with both valves open while the piston is moving downward in the bore, the cylinder is pulling less air through the intake than it would if the exhaust valve were closed; hence less manifold vacuum. My car, for example, idles at about 10.5 inches of manifold vacuum at 900 rpm.
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If you're running a warm cam that lacks in idle and bottom end due to low vacuum, and it's a flat tappet hydraulic cam, you can get your vacuum up, and idle and bottom end improved by replacing your lifters with Rhoads variable duration lifters. They bleed down at low RPM and sound like you're running solid lifters.
Check them out at rhoadslifters.com. They've been around awhile, I've used them on big block Pontiacs, and they work great. If they made hydraulic rollers, I'd be using a set now |
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