Vacuum hoses and PCV

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bosv

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Hi

I live in Sweden so I don't have many guys to ask for advise:(. So I try here.
I have a Corvette BB LS7 that is running Accel DFI and an Edelbrock TB.
Until now I have not had a good crakcase ventilation.
Just a hose from one valve cover out in the air. I plan to go for a PCV solution.
But I am a bit confused about the vacuum connections.
Today it looks like this:
1. Brake booster goes to the big vacuum connection on the TB. I think this is wrong. It's here the PCV should be ??
2. Map sensor goes to the intake manifold 1 inch below the TB.
3. There are two smaller vacuum connections plugged.

But the car runs great and I don't want to spoil that.

I have not found any description of the connections on the TB.

Any ideas what to do ?
 
First off, Welcome to the motley crew.....


Since your car has a LS in it, I leave any comments to others more familiar....:D:crutches:
 
BBShark has it right= some pictures would be good.

I have an LS-7 BB also, I just found a picture of how I setup the vacuum.

Brake booster comes off the bottom of the carb, and the PCV and headlight vacuum comes off a secondary port on the intake.



It's changed a bit since this picture but the basic idea is there. (Different carb and fuel line.)
 
TimAT said:
BBShark has it right= some pictures would be good.

I have an LS-7 BB also, I just found a picture of how I setup the vacuum.

Brake booster comes off the bottom of the carb, and the PCV and headlight vacuum comes off a secondary port on the intake.



It's changed a bit since this picture but the basic idea is there. (Different carb and fuel line.)
Click to expand...

Tim, I make no claim that my setup is correct, but I have the two vacuum lines swapped. My reasoning is that the PCV line has a constant flow of air (and oil fumes) going through it, and by this air entering the carb base it equally dilutes all the cylinders' A/F ratio. The power brake and headlight vacuum system, if I understand it correctly, only flows air on the rare occasions when the brakes are applied or the vacuum tank needs "charging". Cylinder #8 then only goes lean on those rare occasions when the brakes or headlights are actuated, and doesn't get the lion's share of the PCV oil fumes.
As I mentioned, I don't know how the system was plumbed from the factory, but this is the only way the system makes sense to me.
 
Probably a good point, Mike. I know the brake came off the strongest vacuum signal they could find on just about everything. Most all the GM carbs have a fitting on the base of the Q-Jet that runs the booster only. The 2x4 setup I have has a T fitting in the rear carb for both the PCV and brake.
 
Hera are two pictures how it looks today.
I don't need any vacuum to the headlights since I don't have any 'pop-ups'.
thum_880516fba951d324.jpgthum_880516fba95526c0.jpg

I used the manifold connection to the Map sensor as I think this will be the most acurate point to give manifold pressure.
What do you say?
 
Last edited:
TimAT said:
BBShark has it right= some pictures would be good.

I have an LS-7 BB also, I just found a picture of how I setup the vacuum.

Brake booster comes off the bottom of the carb, and the PCV and headlight vacuum comes off a secondary port on the intake.



It's changed a bit since this picture but the basic idea is there. (Different carb and fuel line.)
Click to expand...

On the Holleys, the 3/8" port usually marked for the brake booster should be the pcv, it distributes evenly to the plenum. Booster is good in the runner. Had good pics of bottom side, but lost in the new Photobucket.

bosv said:
Hera are two pictures how it looks today.
I don't need any vacuum to the headlights since I don't have any 'pop-ups'.
thum_880516fba951d324.jpgthum_880516fba95526c0.jpg

I used the manifold connection to the Map sensor as I think this will be the most acurate point to give manifold pressure.
What do you say?
Click to expand...

Post a pic of the bottom side of that unit to be sure, but offhand I would say the pcv should be where the booster is now connected and the booster can be at any manifold vacuum, even a single runner.
 
It makes no difference if the PCV is hooked up to the big connection together with the brake booster. The brake booster has a one way valve in it. The PCV has a valve that closed at high vacuum. Engine should pull more than enough vacuum for both.

If you put in more air in the engine you need to compensate it with more fuel.
 
Belgian1979vette said:
It makes no difference if the PCV is hooked up to the big connection together with the brake booster. The brake booster has a one way valve in it. The PCV has a valve that closed at high vacuum. Engine should pull more than enough vacuum for both.

If you put in more air in the engine you need to compensate it with more fuel.
Click to expand...

Not really a question of the engine having enough vacuum, but no manufacturer that I have ever seen tee the 2 together. Must be a good reason. I assume possible contamination.
I do know that all the oem pcv ports are distributed evenly to all cylinders thru the throttle plate or with fi, in the plenum after any sensors.
BTW, no pcv valve closes at high vacuum, but they are metered though.
 
I just learned that the pcv on my car was causing all sorts of tuning issues. It's rating was to high for the vacuum produced. Causing it to fully opening at idle and a huge vacuum leak, going closed under part throttle and opening again on WOT. You can guess the rest.

I'm looking into solutions. There is a tunable pcv valve although not exactly cheap.
 
BBShark said:
Interested to see what you find. I would like to know how they "size" a PCV valve.
Click to expand...

So would I, but I have known for some time now, there are different flow rates, and, now inference about vac readings variations.....I dunno either....

All I know is that putting a PCV into a valve cover is a great no no....every damn one I have done that way, baffle or not, has sucked oil so much I can see it in the hose....

but putting it in the valley pan, with plenty of shield like on the LT1 intake I have, it's fine.....sorta the same as old Poncho engines with the separate lifter valley cover, and the PCV up front....
 
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