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what effect does it actually have on driveability to block off the hole at the throttle body boot and put a breather at the valve cover? I see them on a lot of engines

I was told by a couple guys that it's just for looks- one less ugly black hose to look at. since it's only drawing air through the filter, which comes from just before the TB, it should not affect performance much. I think you go need to agjust idle speed and such to make up for the lost vacuum, which you will notice that the engine will stall if you pull that hose from the TB.

thx

I beg to disagree. I you create an opening in a closed system, which these cars have, then it's no longer closed which has to have some effect on performance. Having to adjust the idle to compensate doesn't correct the problem opening the system creates, it just allows the car to idle.

what effect does it actually have on driveability to block off the hole at the throttle body boot and put a breather at the valve cover? I see them on a lot of engines

On an FI car with the stock setup you really do not want to do that. An “open” valve cover vent (breather) allows what is called “false air” to enter the intake manifold via the valve cover, out the block vent, through the PVC tube and finally past the PVC valve and into the manifold. “False air” is air that is not being measured by the AFM (air flow meter) and as a result the mixture will become lean and also the idle speed will increase.

Notice that the factory positioned one end of the valve cover breather tube so that is between the AFM and the throttle body. As a result, the air entering the valve cover is being measured and therefore being compensated for.

However, if you have the SDS or Megasquirt controller the airflow is calculated with inputs from the MAP sensor, the TPS sensor and the rpm sensor (trigger). As a result any “false air” changes the intake manifold vacuum level which is sensed by the MAP. So, an open valve cover vent will not result in any driveability issues when using one of these aftermarket systems.

He was asking about putting the little filter on.

I agree it does affect the way the engine runs, but you can adjust things to make it run. I decided to leave the ugly hose on for those reasons. Unless somebody can think of something correct for that.

I don't remember specifically, if the engines I've seen with the rocker box vent were carberetted or F.I. (the details get blurred together after you've seen a few hundred.):paranoid:

Thanks guys, very helpfull info.

I was talking about fuel injected engines. (See 'throttle body boot' in the first post). I have a fuel sniffer that I use before I have my car smog checked and when I do tune ups. I can tell you that on both of my efi cars, when you remove the hose from the valve cover and plug it (the hose not the valve cover) the CO reading goes up. So sure, plug the throttle body and put a breahter on if you wish but even if you compensate by adjusting the idle you're still running with an over rich mixture. The will prematurely end the life of your catalytic converter (if you have one) carbon up your engine and decrease your fuel mileage. So just because you've been told somenthing by a 'couple of guys' or you 'see alot of them on engines' that doesn't mean it's a good thing.