 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
Car Forum / Antique and Collectibles / Antique Cars / July 200562 Impala vacuum advance.
A friend and I have restored his 62 Impala. We found the original engine and carb. Long story short, the carb doesnt have a ported vacuum source for the timing advance. the manual I have shows it hooked up to manifold vacuum. If any one has this info ,it would be most appriciated. It is a rochester 4 barrel, and the engine size is 327cid. Thanx in advance. It may not make a difference for the vacuum question, but is it the 250hp or the 300hp version of the 327cid they used that year? I had one of the '62 300hp models and made some modifications but am scratching my head on the vacuum source. Both versions used vacuum advance, but the 340hp version used in the corvette that year was straight centrifugal advance if I recall correctly. >A friend and I have restored his 62 Impala. We found the original > engine and carb. Long story short, the carb doesnt have a ported > vacuum source for the timing advance. the manual I have shows it > hooked up to manifold vacuum. If any one has this info ,it would be > most appriciated. It is a rochester 4 barrel, and the engine size is > 327cid. Thanx in advance. AFAIK they were hooked to manifold vacuum on FI Corvettes as well, but that doesn't make sense since the point of vacuum advance is to advance the timing at low RPM and high manifold pressure (open throttle), not low. Dunno. Academic-I'd put on a ported carb and an electronic distributor. Points suck for driving. I,m rebuilding a car with vacuum advance but I do not rember WHY I need It, can you go through the basics...someone.? > AFAIK they were hooked to manifold vacuum on FI Corvettes as well, but > that doesn't make sense since the point of vacuum advance is to advance [quoted text clipped - 3 lines] > Dunno. Academic-I'd put on a ported carb and an electronic > distributor. Points suck for driving. > I,m rebuilding a car with vacuum advance but I do not rember WHY I need > It, can you go through the basics...someone.? Sure. The general idea is that the fuel charge in a cylinder takes a fixed amount of time to burn. You would like the burn to start just before the cylinder reached top dead center on the compression stroke, really burn well just after the piston passes TDC, and end about the time it reaches bottom dead center on the power stroke. You *don't* want it to start too early, because the pressure of the expanding gasses will push backwards against the piston if it is still rising on the compression stroke when the burn peaks. Too late, and most of the power is wasted out the exhaust system. The problem is that the burn takes a fixed amount of time, but the time it takes the piston to go from TDC to BDC depends on the rpm; the faster the engine is turning, the shorter the time span. What you want is for the spark plug to fire pretty late (close to TDC) if the engine is turning slowly and fire pretty early (perhaps 30 degrees before TDC) if the engine is turning rapidly. This will get the peak burn right after TDC regardless of the engine speed. The vacuum advance uses intake manifold vacuum to change the spark plug timing to accomplish this. It doesn't do a perfect job, but it's pretty good. George Patterson Why do men's hearts beat faster, knees get weak, throats become dry, and they think irrationally when a woman wears leather clothing? Because she smells like a new truck. God explanation, George, but its the *centrifugal* advance that adjusts for RPM. Same logic, but the vacuum advance adjusts timing for the speed at which the mixture burns. A thin mixture (high vacuum) burns slowly, so the timing must be advanced, a dense mixture (low vacuum) burns more rapidly. The combination of vacuum and centrifugal adjustments help to optimize the timing for all the varied types of engine operation. At high vacuum, high RPM operation, like decelerating down an offramp, the timing may be 36-40 degrees before top dead center (BTDC) due to the combination of advance techniques. At low vacuum, low RPM, like leaving a stop light hurriedly, the timing may only be 2 degrees BTDC. However I have never seen a satisfactory explanation as to why the vacuum signal should be ported or from the manifold. It would seem that at idle, where there is high vacuum, the timing should be advanced. But most late model vehicles (50's - 80's) seem to use ported vacuum which effectively disables the vacuum advance until the throttle is slightly above idle. If you look inside the carburetor, you can see the "port" that gives the signal its name. It sits just above the throttle plate at idle, so that the vacuum is on the other side of the plate. If the throttle is opened slightly, the edge of the throttle plate passes above the port, exposing it to the vacuum in the manifold. regards, Leon >> I,m rebuilding a car with vacuum advance but I do not rember WHY I need >> It, can you go through the basics...someone.? [quoted text clipped - 23 lines] > and they think irrationally when a woman wears leather clothing? > Because she smells like a new truck. Lots of misconceptions and half truths so far. Yes the engine wants certain amount of advance in different situations and RPM. Cylinder pressure is as much a consideration as rpm. Fist starting. just enough advance to start but not enough to kick back on starter. Now the mechanical advance will advance some on start up and continue to advance till (depending on manufactures spec) 2500 to as high as 4000 but 2200 to 2500 seems a norm. The reason for ported vacuum comes from a proper explanation of what vacuum advance is meant to do. vacuum advance is mainly a gas mileage enhancer (yes back in the day gas mileage was a factor) Now the reason for ported vacuum and not manifold is that at idle and light throttle manifold vacuum is high at idle you would not want the vacuum advance pulled on full. next at part throttle and full throttle manifold vacuum drops dramatically or to 0 at WOT so if you were counting on the vacuum advance to give you advance for power when you needed it, it would not be there. Now at highway speed at light throttle a car can take much more advance because the cylinder pressures are down. This advance (46 to 52 deg total) increases gas mileage at cruise but if you were open throttle and increase cylinder pressures it would detonate (ping) like crazy so you want the advance to retard. The ported vacuum is created within the carburetor itself by the speed of the air moving through the carb and past the port inside. that speed is faster at lower throttle openings than wide open and of course shut! The ported vacuum is 0 or near 0 at idle increases at light throttle and helps the advance curve under light acceleration when Cylinder pressures are increased but still lower. At wot it is lower by design. Two ways to tune a car (almost three) 1. is to set it up as close to factory specs but that can somewhat skewed by today's gasoline's! you may get ping at light throttle thus turning some advance out of the vacuum advance with a adjustable vacuum advance may be needed with today's gas it may not like the 46 to 52 cruise advance. If the car is fine with advance disconnected but pings in some driving conditions with it on you may/will have to reduce the amount and rate of vacuum advance by adjusting it with a 3/32 allen wrench (mopar anyway) in through the advance canister port turning counter clockwise (reducing amount of vacuum advance and rate) till a result you want is attained (elimination of ping). 2. is maximum performance with vacuum plugged and forgotten as the max power and performance enhancement is the goal not the little added gas mileage that can be achieved. In this case. (most likely when mods are done and acceleration rules) you plug the vacuum advance install lighter advance springs so the manual advance comes on much faster for more performance gain. Then you set the total to your engines max total advance for performance each engine is different can be from 30 to 40 total . Total is the total mechanical and initial advance. it can be checked by increasing RPM until the timing quits advancing and that is total . Now properly applied timing take or a dial back timing light are required for this test. Now say you engine say's best results for total performance will be at 35 degree (likely in the park for a small block chev) you set total to that and drive. if it pings reduce till it's gone and be happy. if it is fine go one with your life and new found power. 3. is to tune for a combination of total performance and gas mileage enhancement. Usually not a overly successful endeavor but as with method two unhook and plug vacuum advance. tune to total performance and drive assuring no ping. Now re attach vacuum advance to ported vacuum on carb and drive. if it pings vacuum advance must be reduced till pinging disappears which may be down to 4 to 5 deg of vacuum advance down from 15 to 20 hardly worth the effort???? Now to do this if your vacuum canister is not adjustable it must be replaced with one that is. they are available for most common applications. I suggest like others. convert to a electronic ignition with a nice adjustable (vacuum that is) new distributor. You old one likely has worn bushings anyways!!!! Dodgem Took me years to understand and I'm not sure I have it all but I have a good successful grasp! > God explanation, George, but its the *centrifugal* advance that adjusts > for RPM. Same logic, but the vacuum advance adjusts timing for the [quoted text clipped - 52 lines] >> and they think irrationally when a woman wears leather clothing? >> Because she smells like a new truck. when you set the timing with vac hose plugged you are setting it in the retarded position which is where you want it to be when you are under power. when you hook up the vac hose it advances the timing . until you step on the gas then it goes back to the retarded position. >Remember, it's not, >"How high are you?" >It's "Hi, how are you?" little mike > I,m rebuilding a car with vacuum advance but I do not rember WHY I need > It, can you go through the basics...someone.? [quoted text clipped - 6 lines] >> Dunno. Academic-I'd put on a ported carb and an electronic >> distributor. Points suck for driving. |
Reply to this Message |
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2008 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.