Aftermarket camshafts can add significant power to an engine when properly matched with other parts. But remember, the camshaft grind doesn’t just determine engine rpm operating range, it affects peak manifold vacuum as well.
While displacement, intake manifold design, valve size, and piston ring seal also play a role in vacuum, valve overlap is the most significant factor.
As valve overlap increases, idle quality and vacuum degrade. An engine must maintain a minimum of 16-17 in.Hg of vacuum at idle to retain vacuum boosted power brakes without requiring alternative solutions.
A typical 350-based street engine can usually handle a cam grind with up to 60 to 70 degrees of valve overlap and still have enough manifold vacuum to operate power brakes. But even then, matching engine components to the camshaft is critical for it to perform efficiently.
Dual-plane intakes are best for street applications because they separate the manifold into two plenum sections. Each plenum and set of runners connects to every other cylinder in the firing order (180 degrees of crankshaft rotation between pulses), unlike a single-plane manifold with pulses every 90 degrees.
Exhaust reversion is reduced with a dual-plane design, increasing manifold vacuum. Single-plane intakes should only be used for higher rpm or racing—rarely will they work with power brakes.
3 Power Brake Solutions for Vacuum Loss from an Aggressive Camshaft
Alternative brake-assist solutions include a vacuum canister, an electric vacuum pump, or a hydroboost system.
- A vacuum canister (aka a vacuum reservoir) stores additional peak vacuum and usually only works for situations with borderline vacuum issues.
- An electric vacuum pump is a common alternative—most units are self-contained and install easily with just two wires.
- Hydroboost systems work off power steering pump pressure and are an excellent choice in applications with repetitive brake use where a vacuum pump might not keep up. They are common in road racing, off-road trucks with larger tires, and aggressive pro street cars.
If you need help with camshaft selection or a brake assist solution, contact the Summit Racing tech folks. They’ll help determine what’s best for your application
I have seen that David Vizard, Overlap chart before in articles and his books. When you “Really” look at it and “Try” to Apply it to Selecting a Cam for an egine, It’s Way Too vague to be Any use at ALL. The more I talk to Engine Builders about Cams, The more I realize they don’t Really know either. Same goes for Cam Companies, which I bet why they Never reply to questions Or thier Cam recommendation forms.
Only way to find out, ” What Works and What doesn’t” Is to try Multiple different cams on the Street And Strip or with Dyno sessions.
I bought a C4 with a “built” engine per its sign in the yard….Chevy dealers Corvette Certified mechanic would not touch the car and stated he had no clue what was in it. The TPI Specialty injection system, which I think to be a Ram or mini ram probably scared him away. I can hear the cam but am unsure of the size. It also has an electrically driven vacuum booster pump.. From the Precision gear 354 rear end and Stainless Steel Brakes Iam assuming this was a street racer… Less than 50,000 miles…. The little old lady who owned it left it with an unused, unopened bra ( for the car perverts).
My question is what size range cam begins to need the vacuum boost pump for normal street and highway driving….and can the vacuum pump switches be adjusted…if s9 how.
Are you having issues with any of the vacuum power-assist features? How much Vacuum do you have at idle now?
Not functioning brakes without the aux vacuum boost pump working which fails about once a year when the relay goes tits up ..
Good point though. I will check that
Idle vacuum jumps between 5 and 15 at idle.. Also found they put Dart 200 heads on but question if it were enhanced to a 383. The builder put a Mallory Hyfyre 6 on it indicating to me the goal was very high RPM and racing not street.