Joined: Sat Sep 23 2006, 04:30AM
Location: Sharpsburg, GA
Posts: 924
Is there a way to determine in advance if I will loose too much vacuum for my bower brake booster by looking at the profile? I want to replace the cam in a mostly stock rebuild of a super commando 440, but I want to crank up the power a notch without changing the outside looks.
I read that the valve overlap and centerline are most important, but can't find any specific information. I was studying the Mopar purple shaft cams, But they provide no tech help on their website.
Joined: Sun Feb 26 2006, 08:46PM
Location: Kingston,Ontario
Posts: 5622
I know from your last thread you had yet decided on what cam you were going to use. Pick the cam you want.The power brake booster needs minimum of 10-12 inches of vacuum to operate properly.My guideline is anything over .500 lift will drop the vacuum. If the cam you go with drops the vacuum low,just install an electric vacuum pump to operate your booster. That is the easiest solution versus swapping out to manual brakes.Plus if you ever return the car to "factory correct" it is all easily reverted. The vacuum pumps are readily available from any speed shop source.
Duration has a bit to do with vacum drop,but,the overlap is the biggest factor. Overlap is a measurement of crankshaft rotation in degrees during which both the intake and exhaust valves are open at the same time. A long overlap cam brings power up at a higher rpm,but it also has its downfalls. Poor driveability due to less vacum,emissions go up,and you tend to lose low end power. A lot of cams in the 260 degree,duration category should produce vacum in the 16-19 range. Move up to the 270 degree range and you may drop a few more. When you start getting in the 280 degree range,you may be in the10-14 range and the 290 and 300 degree cams,well.....lets just say,it's time to break out that Mastercard and order up a vacum pump from Master Power Brakes pn#AC2724K. www.mpbrakes.com .As Leaburn stated you need at least 10-12 inches of vacum to operate the power booster,and there are a lot of other factors other than a long duration cam that may affect booster pressure also. Another alternative is to loose the booster altogether if you are going to a radical camshaft,although there is a big discussion on that category also.
Joined: Thu Nov 24 2005, 07:29PM
Location: So Cal.
Posts: 1111
Roller cam! !banana
I used an MP484 cam and hated it. Low vacuum, lobe center too narrow, never got it to run well. Maybe it wasn't degreed properly but I installed it "straight up." Anyway, for regular street use and an auto trans, I recommend anything with a lobe separation of 112 or higher. Matching cam profile to your intake/heads/exhaust airflow also makes a big difference. How much cam will stock heads support? I'd guess keeping lift under .500 is a safe bet.
Anybody have flow numbers for a stock big block head? Are flow rates different between a 906, 915, 542, whatever else is out there...?
Joined: Thu Nov 24 2005, 07:29PM
Location: So Cal.
Posts: 1111
Impressive! Much more flow than I would have imagined. Maybe that low .5XX lift cam isn't such a bad idea. (I'd still go with a wide LSA for a street car though)
Joined: Sat Sep 23 2006, 04:30AM
Location: Sharpsburg, GA
Posts: 924
Where could we get pictures of how/where to properly blend those heads? I have a lot of experience with a die grinder and blending/polishing but none with cylinder heads.
That MP 484 cam has a new version out with the overlap reduced from 68 to 56 deg and the centerline changed to 114 deg. I wonder what the vacuum and idle would be with it?