Cam effect on vacuum
01-08-2010, 06:57 PM
#2
Mopar Lover
six-pack to go
Putting a larger cam in a motor lowers the total vacuum of the motor.
For example, lets take a totally stock 318. It has a tiny cam and, for instance, let's say that it idles at 18 inches of vacuum. (not uncommon). Now, lets change the cam to something in the 270-280 degree duration. This will lower the vacuum reading, maybe down to something like 12 to 14 inches of vacuum. Now, lets say that you have a stock 440 6 pack setup, maybe the outer two carbs are designed to open at 8 inches of vacuum. Now you put in a gigantic cam that only idles at 11 inches of vacuum. The outer two carbs will open at 8 inches so you can see what the problem is. The outer carbs will open almost as soon as you step on the throttle. This will be too much for the engine to handle and will most likely bog big time. The outer two carbs are vacuum operated. The actuating mechanisms are designed to open at a certain vacuum. If you want to change this opening point. the vacuum pods will have to be modified so that they open at the appropriate vacuum point to match the cam. I believe this involves changing a spring in the vacuum pod.
(Not 100% sure of this last statement)
For example, lets take a totally stock 318. It has a tiny cam and, for instance, let's say that it idles at 18 inches of vacuum. (not uncommon). Now, lets change the cam to something in the 270-280 degree duration. This will lower the vacuum reading, maybe down to something like 12 to 14 inches of vacuum. Now, lets say that you have a stock 440 6 pack setup, maybe the outer two carbs are designed to open at 8 inches of vacuum. Now you put in a gigantic cam that only idles at 11 inches of vacuum. The outer two carbs will open at 8 inches so you can see what the problem is. The outer carbs will open almost as soon as you step on the throttle. This will be too much for the engine to handle and will most likely bog big time. The outer two carbs are vacuum operated. The actuating mechanisms are designed to open at a certain vacuum. If you want to change this opening point. the vacuum pods will have to be modified so that they open at the appropriate vacuum point to match the cam. I believe this involves changing a spring in the vacuum pod.
(Not 100% sure of this last statement)
01-08-2010, 08:42 PM
#3
Mopar Fanatic
Putting a larger cam in a motor lowers the total vacuum of the motor.
For example, lets take a totally stock 318. It has a tiny cam and, for instance, let's say that it idles at 18 inches of vacuum. (not uncommon). Now, lets change the cam to something in the 270-280 degree duration. This will lower the vacuum reading, maybe down to something like 12 to 14 inches of vacuum. Now, lets say that you have a stock 440 6 pack setup, maybe the outer two carbs are designed to open at 8 inches of vacuum. Now you put in a gigantic cam that only idles at 11 inches of vacuum. The outer two carbs will open at 8 inches so you can see what the problem is. The outer carbs will open almost as soon as you step on the throttle. This will be too much for the engine to handle and will most likely bog big time. The outer two carbs are vacuum operated. The actuating mechanisms are designed to open at a certain vacuum. If you want to change this opening point. the vacuum pods will have to be modified so that they open at the appropriate vacuum point to match the cam. I believe this involves changing a spring in the vacuum pod.
(Not 100% sure of this last statement)
For example, lets take a totally stock 318. It has a tiny cam and, for instance, let's say that it idles at 18 inches of vacuum. (not uncommon). Now, lets change the cam to something in the 270-280 degree duration. This will lower the vacuum reading, maybe down to something like 12 to 14 inches of vacuum. Now, lets say that you have a stock 440 6 pack setup, maybe the outer two carbs are designed to open at 8 inches of vacuum. Now you put in a gigantic cam that only idles at 11 inches of vacuum. The outer two carbs will open at 8 inches so you can see what the problem is. The outer carbs will open almost as soon as you step on the throttle. This will be too much for the engine to handle and will most likely bog big time. The outer two carbs are vacuum operated. The actuating mechanisms are designed to open at a certain vacuum. If you want to change this opening point. the vacuum pods will have to be modified so that they open at the appropriate vacuum point to match the cam. I believe this involves changing a spring in the vacuum pod.
(Not 100% sure of this last statement)
01-08-2010, 08:57 PM
#4
Mopar Lover
Roger, you change springs on the outboards. You can go lighter or heavier to tune in when they kick in. Not too difficult to do, and even easier if you modify the carb lip just a bit
Thread
Thread Starter
Forum
Replies
Last Post
Rustymopar
Chassis, Suspension, and Brakes
18
01-29-2017 08:17 PM
