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Braking Performance Restoration - Existing In A Vacuum

Restoring C3 triple-ex's braking performance with a vacuum canister from Comp Cams.

Dave Young Nov 17, 2011
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We've really been enjoying project C3 Triple-Ex, especially now that we've installed our 427ci "LS7 Killer" small-block, but we've recently encountered an issue common to street engines with long-duration camshafts and power brakes. Specifically, the power brakes installed on our Stingray (and on most passenger vehicles) rely on the engine's manifold vacuum to provide the needed braking assistance, and the relatively large solid-roller cam just doesn't produce enough vacuum to adequately power the brake booster.

From the onset, we knew the Comp cam in the LS7 Killer would be marginal when it came to supporting the car's braking system, but we were willing to take that risk in order to enjoy the additional power and pulse-quickening idle. Let's face it: We built this engine with maximum power in mind, so certain compromises were a necessary evil. And while increased brake-pedal effort is certainly an inconvenience, the real issue is that with marginal manifold vacuum, the pedal effort can start off fine but quickly become stiff, resulting in inconsistent braking performance. Fortunately, Comp has a solution for this problem that's easy to install and stores manifold vacuum to assist the power brake booster.

Power brake systems in vehicles such as our '71 provide braking assistance through a booster, which is basically a large diaphragm housed between the brake pedal and master cylinder. A line is then routed from the intake manifold to the brake booster, through a one-way check valve. The engine's vacuum acts on the diaphragm, providing assistance when the brake pedal is depressed. And while manifold vacuum varies with engine rpm, it's important to have adequate vacuum at idle, when the brakes are often applied to stop and hold the vehicle.

Manifold vacuum is measured in inches of mercury. Most power brake systems operate well with 15 inches of manifold vacuum, which an engine with a stock camshaft easily supplies at idle. Engines with long-duration high-performance cams, however, keep the valves open longer, resulting in idle-vacuum readings well below 15 inches. This reduces the booster's assistance, resulting in a hard brake pedal.

The engine in C3 Triple-Ex suffered from this affliction, with only 8-9 inches of manifold vacuum at idle, and we noted increased braking effort during certain driving conditions. Not willing to sacrifice power by installing a smaller cam, we chose to correct our problem by installing a vacuum-storage canister from Comp. This canister is constructed from aluminum, installs between the intake manifold's vacuum port and the power brake booster, and stores the higher vacuum supplied by the engine during deceleration. The canister is easy to install and provides the stored vacuum to the brake booster when the brake pedal is applied, greatly reducing pedal effort.

As you can see in the accompanying photos, we bolted ours to the firewall of our Vette. The location of the canister is not so important, as long as it's mounted in an area where it doesn't interfere with the function of other underhood components. It should also be installed in a manner that makes it easy to route the vacuum lines from the intake to the canister, and then to the brake booster itself.

Once installed, we took our car for a drive and immediately noticed it was easier to apply the brakes and stop the car. Even better, the pedal effort remained consistent throughout a range of driving conditions.

To sum up, this is an easy fix in any situation where the engine can't supply adequate manifold vacuum to operate the power brakes at idle. In fact, we figure the guys over at one of our sister magazines will need one soon--since the "Anti-LS" small-block they recently built is a virtual carbon copy of the LS7 Killer. Sincerest form of flattery, indeed.


Comp Cams
Memphis, TN 38118



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