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Any Big Blocks With Purpleshaft .509 x 292 deg Cams
Posted: Thu Jun 13, 2013 12:25 pm
by 71_bronze_gtx
If you have this cam or something fairly similar, what kind of vacuum are you getting at idle?
I'm getting ~7.5 inches at 1,000 rpm. I have this same profile cam in a 340 car and it's only making 5 inches at 1,000 rpm. I don't know how either cam was degreed. Seems like the 340 one might be a little late.
Anyone?
Re: Any Big Blocks With Purpleshaft .509 x 292 deg Cams
Posted: Tue Aug 20, 2013 11:50 pm
by 71Satellite98661
I wouldn't be too surprised that the big block pulls more vacuum than the small block with the same cam specs. The volume of the cylinder at BDC is the key. I could be all wet but that's my thoughts. On another note, I ran this cam in my 71 Satellite with 440 and AT and loved it. To run the best on the street I had to select the proper converter and ran 3.91 rear with 275/60/15 tires. Prior to the converter I had a heck of a time 2-peddling at every stop light. I know these new big hydraulics are the bees knees but that cam was designed well and made huge power. Not to mention that it turned heads everywhere I went! I miss that cam but unfortunately 10 years ago I wasn't aware of the effects of the lack of Zinc dialkyldithiophosphates (often referred to as ZDDP) and wiped two lobes which took out rod bearings before it was noticed.
Re: Any Big Blocks With Purpleshaft .509 x 292 deg Cams
Posted: Wed Aug 21, 2013 9:17 am
by 71_bronze_gtx
I've since had a few experienced motorheads say that it's typical for a big block to pull a little more vacuum. If so, there is some science behind it.
The intake manifold is where the measurment is taken, and that is the chamber being "evacuated" by the motor, which is acting as an air pump. Vacuum level is set by how quickly air is "trying" to be evacuated versus the restriction letting air into the chamber, which is the carburetor in idle position.
It could actually be that the carburetor opening is what's making the difference - the carb for the 340 is opened proportionally more to get the same 1,000 rpm idle. But if the two cams are "degreed" differently that would also affect the vacuum, as well as performance.
That said, both the engines were built to make power at high-rpm. Not the ideal for red-light or "from a roll" street racing. But they sure sound good. And who needs brakes?