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#1
11-30-2009, 07:15 PM
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Hydroboost and vac line?
Hi, If you have a Hydroboost system can you use the carb vacuum line for a PCV line from the valve cove in addition to the stock one in the valley pan to the front of the carb?
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#2
11-30-2009, 08:20 PM
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Quote:
Stewart
__________________
1976 TA, nose converted to 1970 style, 406, ported #13 Heads, '70 iron intake without crossovers, Q-Jet - Cliff style, RARE OS manifolds, Pypes duals w/crossflow, UD 230/238 custom HR 4/7 swap cam with solid roller lifters  , Hydro-Boost 4-wheel discs, 4 Speed, 3.23 posi.“Nothing in life is so exhilarating as to be shot at without result.” - Winston Churchill |
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#3
11-30-2009, 08:27 PM
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Yes, but:
1) A carburetor has a certain "sweet spot" in the throttle blade position where the carb designer wanted the blades to be. That blade position will allow a certain amount of idle air to the engine. 2) PCV valves have given flow rates through them. The highest flow valves flow about 4.5 cfm. 3) An engine about 400 cid needs about 17 cfm to idle properly at about 750 rpm. Of the 17 cfm, 3 cfm might come from the factory pcv valve. A larger engine like a 455 engine might need 19 cfm. To get 19 cfm the engine might have a larger carb with bigger throttle bores (850 carb vs 750 carb). Only 1/16" inch difference in each blade diameter but enough to get 2 more cfm. 4) So now you plan on adding a second PCV valve flowing 3 more cfm (like the factory valve) and you now have 6 cfm worth of PCV flow so the total flow is 21 cfm. 21 cfm might cause the idle to jump to 1000 rpm. You want the stay at 750 cfm so you close off some of the idle air flow from the throttle blades. 5) The reduced idle airflow makes the carb idle at 750 rpm but now you have a off idle hesitation in the engine because the blades are in the wrong spot in the bores. EVERYONE ASSUME THAT PCV VALVE CALIBRATIONS ARE SO SIMPLE THAT ANY DUMMY CAN INSTALL A PROPER SYSTEM, WRONG! PCV valves are designed for specific engines and therefore will sometimes take a bit of trial and error to make things right. Unless you have a Boosted engine where you are creating a lot of crankcase pressure, a simple single PCV valve will typically do the job. Like I said, you can easily add a second PCV valve in the location you described BUT be prepared for some serious tuning to make everything work right. Course some people hit the right combination the first time and say Tom's post is BS. Ok, if you say so. Ford has a Technical Specialist who does ONLY PCV systems as a full time job. That should tell you something. Tom Vaught
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"Engineers do stuff for reasons" Tom Vaught Despite small distractions, there are those who will go Forward, Learning, Sharing Knowledge, Doing what they can to help others move forward. |
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#4
11-30-2009, 08:42 PM
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Thanx for the lesson Tom. I was way too simplistic. Now I'm wiser.
 Stewart
__________________
1976 TA, nose converted to 1970 style, 406, ported #13 Heads, '70 iron intake without crossovers, Q-Jet - Cliff style, RARE OS manifolds, Pypes duals w/crossflow, UD 230/238 custom HR 4/7 swap cam with solid roller lifters , Hydro-Boost 4-wheel discs, 4 Speed, 3.23 posi.“Nothing in life is so exhilarating as to be shot at without result.” - Winston Churchill |
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#5
12-01-2009, 12:49 AM
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And yet, just TRY to find flow rates, spring tension, or any other specifics related to PCV valves...
If there's a source, I don't know about it. |
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#6
12-01-2009, 12:57 AM
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Quote:
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 John J.  
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#7
12-01-2009, 08:08 AM
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I was just thinking the more the better. It would be cheaper than getting a vacuum pump. I am not even sure I need more vacuum I just figured it would be cheap insurance. This would all be for a 550hp (occasional 150NOS) street car. I didn't realize it was so complicated.
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#8
12-01-2009, 10:08 AM
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it's not complicated: simply run one pcv valve in the valley pan, cap the other carb fitting that would have run your brake booster, and you're good to go.
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John J.
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#9
12-01-2009, 10:24 AM
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What John said.
On my conversion to h-b I completely removed the vacuum tree from the back of the carb and plugged the hole with a pipe plug. No muss, no fuss and a lot cleaner looking. FWIW, Stewart
__________________
1976 TA, nose converted to 1970 style, 406, ported #13 Heads, '70 iron intake without crossovers, Q-Jet - Cliff style, RARE OS manifolds, Pypes duals w/crossflow, UD 230/238 custom HR 4/7 swap cam with solid roller lifters , Hydro-Boost 4-wheel discs, 4 Speed, 3.23 posi.“Nothing in life is so exhilarating as to be shot at without result.” - Winston Churchill |
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