Assuming, in general, that running a (what's considered a large type cam) that the loss of vacuum associated with the cam results in poor idle----does this mean that that is the reason the larger cams idle lousy? And if this is the case is there not a device that one installs that replaces the vacuum thus eliminating the lousy idle of the larger cam?

And...is the drivabilty of larger cam affected by the loss in vacuum or is that strictly a matter of the cam's profile which requires a higher RPM to get the peak HP/TQ?

The reason I ask is I am just curious if you can make a larger cam behave as a DD if you replace the vacuum with the device ( which I cannot recall is used).

Thanks much.

 

The lack of vacuum is only part of it. There are vacuum pumps that can be had, but those only replace the lost vacuum to other devices in the vehicle, such as power brakes or vacuum operated wipers. The only way you're going to use such a device to "replace" the lost vacuum is to install it internally in the engine, and create vacuum in the cylinders.

Another part causing lousy idle is the large overlap.

 

A large cam doesn't idle poorly because there's low vacuum. There's low vacuum because of the overlap on the large cam. You can't add a vacuum pump to the intake manifold, if that's what you are asking. Vacuum is developed by the falling piston, and it can't pull a high vacuum if both the intake and exhaust valve are open for an appreciable amount of time.

 

An ITB intake manifold would make a car with a cam idle alot better.

 

Yup, that's why there is always some trade off of low end for high end with a single cam engine. This is the reason I like superchargers, if the engine can handle the boost.

Dan