ZL-1 and Power Brakes

[wvhylander]

I had Comp Cams grind a roller for my BBC specifically with power brakes. They said for good vacuum numbers I needed to stay under 250 degrees duration @ 50. The cam came in as 236/242 547/567 lift. I've got vacuum but not the hp numbers I was looking for. So my question with the duration and lift numbers I've seen on ZL-1 cams how did GM get vacuum to operate power brakes? I've seen duration numbers listed @ 260/270 @ 50. I know it's is not as simple as looking at duration, but Comp Cams didn't grind a cam with numbers close to the GM cam. They offered to regrind closer to 250 duration, but no guarantees on vacuum numbers.

[napa68]

I had an iron headed L88 with a ZL1 cam in it. Didn't take much to run the booster out at an idle.

Tim

[x33rs]

When comparing duration numbers on the solid cams, about 10-15 degrees should be subtracted from the .050 numbers because you loose at least that much with lash, especially those old profiles.

It gets even more confusing with modern grind cams, comparing with 50 year old grinds becomes difficult because the lobe profiles used will affect the duration lost with lash. Some modern lobes might be closer to 8 or 10 degrees lost while old lobes can be as much as 15 or more.

The size of your BBC and the amount of compression it has will play a roll in how much vacuum it'll produce. A big player is also where they put the LSA and the ICL position of the cam when installed.

You could always request to have the LSA spread out a couple degrees to help with vacuum production (little smoother idle) Comp grinds most of their cams on a 110 LSA. Then maybe move the install position forward a degree or 2.

As an example I run a 242/248 @ .050 hydraulic roller in my 10:1 454. It's on a 110 and installed at 106 ICL. Personally I think I would have liked to install it at 104 but never got ambitious enough to dig back into it. It's not an overly large camshaft, but it's not a big vacuum producer either, idles about 5 inches in gear up here at 5,000 feet which is about 8 inches at sea level. I run an extra hidden vacuum can for the booster and that's worked "okay" for many years but it's only good for a couple stops before it needs time to replenish, which just takes a few seconds. Wouldn't want to daily drive it this way but it's been okay for a weekend fun car.

For comparison I built a 455 Pontiac for a customer, and did a custom 239/243 @ .050 hydraulic roller. 9.98:1 compression. That cam was on a 112 LSA and I installed it at 106.25 ICL. That engine made great vacuum, at our elevation it idled with 12 inches, and that number increased at sea level. It worked the power brakes perfectly.

[luzl78]

Originally Posted by x33rs

When comparing duration numbers on the solid cams, about 10-15 degrees should be subtracted from the .050 numbers because you loose at least that much with lash, especially those old profiles.

It gets even more confusing with modern grind cams, comparing with 50 year old grinds becomes difficult because the lobe profiles used will affect the duration lost with lash. Some modern lobes might be closer to 8 or 10 degrees lost while old lobes can be as much as 15 or more.

The size of your BBC and the amount of compression it has will play a roll in how much vacuum it'll produce. A big player is also where they put the LSA and the ICL position of the cam when installed.

You could always request to have the LSA spread out a couple degrees to help with vacuum production (little smoother idle) Comp grinds most of their cams on a 110 LSA. Then maybe move the install position forward a degree or 2.

As an example I run a 242/248 @ .050 hydraulic roller in my 10:1 454. It's on a 110 and installed at 106 ICL. Personally I think I would have liked to install it at 104 but never got ambitious enough to dig back into it. It's not an overly large camshaft, but it's not a big vacuum producer either, idles about 5 inches in gear up here at 5,000 feet which is about 8 inches at sea level. I run an extra hidden vacuum can for the booster and that's worked "okay" for many years but it's only good for a couple stops before it needs time to replenish, which just takes a few seconds. Wouldn't want to daily drive it this way but it's been okay for a weekend fun car.

For comparison I built a 455 Pontiac for a customer, and did a custom 239/243 @ .050 hydraulic roller. 9.98:1 compression. That cam was on a 112 LSA and I installed it at 106.25 ICL. That engine made great vacuum, at our elevation it idled with 12 inches, and that number increased at sea level. It worked the power brakes perfectly.

It’s nice to hear when someone knows what he’s talking about

[big gear head]

I took the booster off of mine because I knew that the cam I wanted to run wouldn't produce enough vacuum to operate it. I know that this isn't what you want to do because that isn't how these cars were built, but I didn't want any surprises when I tried to stop the car.