Moparts
Vacuum vs Duration
Posted By: GTX MATT
Vacuum vs Duration - 03/18/14 05:11 AM
What type of vacuum would you expect to get out of a stock stroke 440 with 252/260 @ .050 duration. Cam was degreed in at 104.5 ICL, its a 110 LSA Comp.
32 degrees of timing locked out on the MSD Pro Billet. I'm getting a fluctuating 6-8 inches at idle. I don't really care, I just expected closer to 10, and my desktop dyno is telling me 12 (which seems very optimistic). I have a 6.5 PV in there now so I'm going to swap it out for a 4.5, and I'm thinking that should help stabilize the reading closer to 7/8.
Just looking for some input. Throttle response is good, doesn't appear to have a vacuum leak checking with carb clean. I'm just trying to get this thing squared away before I venture out for some WOT blasts on the street.
Posted By: JD Dart
Re: Vacuum vs Duration - 03/18/14 05:25 AM
What's the idle at??
Posted By: GTX MATT
Re: Vacuum vs Duration - 03/18/14 05:43 AM
900-950
Posted By: Sport440
Re: Vacuum vs Duration - 03/18/14 06:11 AM
Your doing good IMO Id expect less.
My 3.75 stroke 451 based motor runs a 252* @ .050 cam. IIRC, it pulls about 8-10 in gear at 900rpm. I believe I'm currently running a 4.5 PV in the front of my 830 DP, with a plug in the back.
(I'm not at home, so this is just off my somewhat suspect memory)
Posted By: BSB67
Re: Vacuum vs Duration - 03/18/14 12:21 PM
What is the seat timing, and is it solid or hydraulic.
Posted By: Clanton
Re: Vacuum vs Duration - 03/18/14 02:00 PM
I was getting 14" with a .590 cam and a Domenator by leaning the fuel so I know you can do better!
Lobe separation will have a big impact on idle vacuum as well as compression ratio.
I would guess you have a fairly narrow LSA, like 108?
Keith
Posted By: DaveRS23
Re: Vacuum vs Duration - 03/18/14 04:05 PM
If the carb is adjusted okay, then changing the power valve won't have any impact on the idle. Your vacuum seems to be in the right range for the combo.
Posted By: GTX MATT
Re: Vacuum vs Duration - 03/18/14 04:15 PM
LSA is 110, advertised duration is 290/298, it is a solid cam, lashed at .014/.016 cold with aluminum heads (Hot lash supposed to be .016/.018).
Intake closing point is 69.5 degrees after bottom dead center @.015 (as given by Comp).
I will mention, the rings are not yet fully seated as I haven't gotten it on the street for a good beating. I'm just trying to quickly get the tune up close before I take it out.
The reason I want to throw in a lower PV is that it is so close to the opening point at idle, and the idle mixture screws are a little un-responsive/lazy feeling.
Posted By: DaveRS23
Re: Vacuum vs Duration - 03/18/14 04:25 PM
If the carb is adjusted right; meaning that the the throttle blades are in the correct position for idle, then the power circuit (including the power valve) does not see any vacuum signal at idle.
Now, if the throttle blades are open too far, then the power circuit could be starting to activate. But that situation needs to be fixed first.
Posted By: Crizila
Re: Vacuum vs Duration - 03/18/14 05:40 PM
A little low. What altitude do you live at?
Posted By: AndyF
Re: Vacuum vs Duration - 03/18/14 05:46 PM
PV doesn't (or shouldn't) have any effect at idle. Most important thing to do is to pull the carb off and flip it over and see where the throttle blades are in relation to the transition slots. If the blades are open too far then you need to get them adjusted correctly. If it won't idle with the blades in the correct location then you need to add initial advance and/or drill the throttle blades for idle air bypass.
PV should be set to bring the extra fuel in as the engine gets under load. Best way to set it is to hook up a vacuum gauge and watch it as you drive around. Rolling into the throttle on a long uphill stretch of road should show you when the PV needs to open up. (A wide band really helps.)
Posted By: 383man
Re: Vacuum vs Duration - 03/18/14 07:53 PM
Quote:
If the carb is adjusted right; meaning that the the throttle blades are in the correct position for idle, then the power circuit (including the power valve) does not see any vacuum signal at idle.
Now, if the throttle blades are open too far, then the power circuit could be starting to activate. But that situation needs to be fixed first.
The power valve always has intake vacum to the backside of it.
Sounds like you mean to say if the throttle valves are open to far then the main metering system may start drawing fuel into the venturi if air flow is starting to go thru the venturi. Ron
Posted By: AndyF
Re: Vacuum vs Duration - 03/18/14 08:07 PM
Right. If the carb is adjusted correctly then it shouldn't matter if the PV is open or not at idle. If fuel isn't flowing thru the main circuit then it doesn't matter if the PV is open. Of course, it is better if the PV is closed at idle since you want it closed off idle and you want it closed until the engine starts to see a heavy load.
Holley messed this up years and years ago by using vacuum idle as a measurement point. Then the hot rod magazines made a mess of things by talking about engines loading up at idle due to open power valves.
A power valve with a hole in it will make a mess at idle since fuel will go into the intake manifold. But an open PV shouldn't change the AF ratio at idle by much if any.
Posted By: GTX MATT
Re: Vacuum vs Duration - 03/18/14 08:27 PM
Quote:
Right. If the carb is adjusted correctly then it shouldn't matter if the PV is open or not at idle. If fuel isn't flowing thru the main circuit then it doesn't matter if the PV is open. Of course, it is better if the PV is closed at idle since you want it closed off idle and you want it closed until the engine starts to see a heavy load.
Holley messed this up years and years ago by using vacuum idle as a measurement point. Then the hot rod magazines made a mess of things by talking about engines loading up at idle due to open power valves.
A power valve with a hole in it will make a mess at idle since fuel will go into the intake manifold. But an open PV shouldn't change the AF ratio at idle by much if any.
Gotcha, I knew this at some point now that I think about it and forgot it. 
I will check to make sure that the throttle blades are not open too far.
Posted By: polyspheric
Re: Vacuum vs Duration - 03/18/14 09:17 PM
Dropping your initial spark to 20° may help raise the idle vacuum, but it's a PITA.
Posted By: AndyF
Re: Vacuum vs Duration - 03/19/14 01:31 AM
It is easy to test though. At idle you can just turn the distributor in order to try different initial advance points. If he swings it from 20 to 32 degrees and sees a big difference in idle quality and/or engine vacuum then that is a clue.
If the vacuum is a lot better at 20 degrees then the decision has to be made to put an advance curve back in the distributor or just live with the 32 locked for best high speed performance.
Posted By: BSB67
Re: Vacuum vs Duration - 03/19/14 02:15 AM
So I guess it is an xtreme energy solid. Its specs are similar to the MP .577 solid. Maybe ask if anyone has/had that cam for comparison purposes. I think Ron (383Man) used it in his 440.
If you are just using a pan gasket for the intake, don't rule out a vacuum leak. The aerosol can spray method is not always reliable.
Posted By: GTX MATT
Re: Vacuum vs Duration - 03/19/14 03:48 AM
Yup this is the extreme solid line.
The engine idles OK with less initial, but idled better and better the more initial I gave it, thus why I locked it. Although I did not verify with a vacuum gauge I will double check this weekend, RPMs came up and I reset curb idle to get them back down. Once warmed up the engine fires right off at the touch of the key, doesn't even get a full revolution in really before it comes to life.
I am running a valley tin with paper gaskets on either side. The gaskets seemed to crush well, but the reason I am worried about a potential vacuum leak is because I had a little trouble getting the intake on. Port alignment was good, and even with no gasket/tin the intake face to head alignment looked flush, but I did have to touch up the bolt holes to get the bolts started easily.
The only other way I can think to more accurately check for a vacuum leak is with propane and a long rubber hose. Someone on here had suggested something good a few months back for checking for vacuum leaks, and now I can't find the thread. I thought I favorited it.
I've had a cold since Monday AM so I haven't gotten out to play with the car after work.
Posted By: cudaman1969
Re: Vacuum vs Duration - 03/20/14 05:00 AM
Quote:
Yup this is the extreme solid line.
The engine idles OK with less initial, but idled better and better the more initial I gave it, thus why I locked it. Although I did not verify with a vacuum gauge I will double check this weekend, RPMs came up and I reset curb idle to get them back down. Once warmed up the engine fires right off at the touch of the key, doesn't even get a full revolution in really before it comes to life.
I am running a valley tin with paper gaskets on either side. The gaskets seemed to crush well, but the reason I am worried about a potential vacuum leak is because I had a little trouble getting the intake on. Port alignment was good, and even with no gasket/tin the intake face to head alignment looked flush, but I did have to touch up the bolt holes to get the bolts started easily.
The only other way I can think to more accurately check for a vacuum leak is with propane and a long rubber hose. Someone on here had suggested something good a few months back for checking for vacuum leaks, and now I can't find the thread. I thought I favorited it.
I've had a cold since Monday AM so I haven't gotten out to play with the car after work.
carb cleaner, spray around the gaskets and bottom of carb, if engine idles up or changes there's your leak. The more advance the better it will run but harder to start. If carb is right on it will idle at 4 to 5 hundred rpm. Get your air bleeds and idle fuel restrictions right and it won't matter what the vacuum is.