which port to use for timing
05-20-2007, 01:46 PM
#1
Instructor
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which port to use for timing
290hp crate engine, Edelbrock carb. all emission stuff removed. When I got the car The distributor was hooked to port that caused no vaccum at idle. Should'nt it be hooked to one that does casue vaccum at idle??
05-20-2007, 02:00 PM
#2
Drifting
05-20-2007, 02:08 PM
#3
Safety Car
If your usuing a vacuum at idle port then you are running on advance timing all the time! other words the advance wieghts are always slung open advancing timing to a point where the engine might have a hard time starting.
You have to hook the vacuum advance to a port that only pulls vacuum when you give it the gas and no vacuum at idle!
If you pull the vacuum hose off the distributer and the idle goes down then its the wrong port.
The best thing to do is not hook it up at all and set your advance at 36 at 2500 RPMs and not worry about your idle timing unless it pings when giving it gas or is hard to start.
You have to hook the vacuum advance to a port that only pulls vacuum when you give it the gas and no vacuum at idle!
If you pull the vacuum hose off the distributer and the idle goes down then its the wrong port.
The best thing to do is not hook it up at all and set your advance at 36 at 2500 RPMs and not worry about your idle timing unless it pings when giving it gas or is hard to start.
05-20-2007, 02:48 PM
#4
Team Owner
Some engine set-ups idle better with "ported" vacuum rather than "manifold" vacuum. You will set your mechanical advance curve with the vacuum advance off anyway. Just go through that process and play with the springs to bring in full mechanical advance in the 2500-2800 rpm range. Then hook up the vacuum advance and see that the car runs smoothly in the cruise range (above idle). The last step is to run the engine at idle to see whether manifold vacuum or ported vacuum is best for a smooth idle with your engine. Make your choice, then make final idle screw adjustments.
05-20-2007, 03:01 PM
#5
Race Director
If your usuing a vacuum at idle port then you are running on advance timing all the time! other words the advance wieghts are always slung open advancing timing to a point where the engine might have a hard time starting.
You have to hook the vacuum advance to a port that only pulls vacuum when you give it the gas and no vacuum at idle!
If you pull the vacuum hose off the distributer and the idle goes down then its the wrong port.
The best thing to do is not hook it up at all and set your advance at 36 at 2500 RPMs and not worry about your idle timing unless it pings when giving it gas or is hard to start.
You have to hook the vacuum advance to a port that only pulls vacuum when you give it the gas and no vacuum at idle!
If you pull the vacuum hose off the distributer and the idle goes down then its the wrong port.
The best thing to do is not hook it up at all and set your advance at 36 at 2500 RPMs and not worry about your idle timing unless it pings when giving it gas or is hard to start.
05-20-2007, 04:13 PM
#8
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Kev,
There's a lot of misinformation floating around in the forum, some of it evident in a previous post. Read BarryK's excellent write-up on ignition systems. We in the forum have already discussed most aspects of the system, and a lot of useful info was included in his report. Read it, and you will be immune to bad info in the future.
05-20-2007, 04:22 PM
#9
Le Mans Master
If your usuing a vacuum at idle port then you are running on advance timing all the time! other words the advance wieghts are always slung open advancing timing to a point where the engine might have a hard time starting.
You have to hook the vacuum advance to a port that only pulls vacuum when you give it the gas and no vacuum at idle!
If you pull the vacuum hose off the distributer and the idle goes down then its the wrong port.
The best thing to do is not hook it up at all and set your advance at 36 at 2500 RPMs and not worry about your idle timing unless it pings when giving it gas or is hard to start.
You have to hook the vacuum advance to a port that only pulls vacuum when you give it the gas and no vacuum at idle!
If you pull the vacuum hose off the distributer and the idle goes down then its the wrong port.
The best thing to do is not hook it up at all and set your advance at 36 at 2500 RPMs and not worry about your idle timing unless it pings when giving it gas or is hard to start.
I think you have a bit of a misunderstanding on some aspects of timing and how to set it up.
Please read here:
http://forums.corvetteforum.com/show....php?t=1430339
05-20-2007, 04:25 PM
#10
Drifting
If your usuing a vacuum at idle port then you are running on advance timing all the time! other words the advance wieghts are always slung open advancing timing to a point where the engine might have a hard time starting.
You have to hook the vacuum advance to a port that only pulls vacuum when you give it the gas and no vacuum at idle!
If you pull the vacuum hose off the distributer and the idle goes down then its the wrong port.
The best thing to do is not hook it up at all and set your advance at 36 at 2500 RPMs and not worry about your idle timing unless it pings when giving it gas or is hard to start.
You have to hook the vacuum advance to a port that only pulls vacuum when you give it the gas and no vacuum at idle!
If you pull the vacuum hose off the distributer and the idle goes down then its the wrong port.
The best thing to do is not hook it up at all and set your advance at 36 at 2500 RPMs and not worry about your idle timing unless it pings when giving it gas or is hard to start.
There are three kinds of advance. Initial, centrifugal (mechanical) and vacuum. Initial advance is just that, an initial amount you put into the distributer by rotating the housing (usually something like 12 to 18 degrees BTDC). Centrifugal (mechanical) advance is a speed sensing advance that only reacts to the speed of the engine. Vacuum advance is a load sensing advance that reacts to changes in load (throttle position) that the engine is seeing.
BigBlockk
Later.....
05-20-2007, 04:26 PM
#11
Le Mans Master
Since ported vacuum advance was ONLY done to help reduce emission output and your motor doesn't have any of the emission equipment on it you are probably going to be better off using full manifold vacuum advance. It should idle better, have better throttle response, get better fuel economy, and run slightly cooler operating temps.
05-20-2007, 05:52 PM
#13
Safety Car
You appear to be quite confused on this issue. Vacuum advance has no effect on mechanical advance. That is why they are called different names. Straight manifold vacuum is more for performance. Ported (timed) vacuum is for emissions control.
There are three kinds of advance. Initial, centrifugal (mechanical) and vacuum. Initial advance is just that, an initial amount you put into the distributer by rotating the housing (usually something like 12 to 18 degrees BTDC). Centrifugal (mechanical) advance is a speed sensing advance that only reacts to the speed of the engine. Vacuum advance is a load sensing advance that reacts to changes in load (throttle position) that the engine is seeing.
BigBlockk
Later.....
There are three kinds of advance. Initial, centrifugal (mechanical) and vacuum. Initial advance is just that, an initial amount you put into the distributer by rotating the housing (usually something like 12 to 18 degrees BTDC). Centrifugal (mechanical) advance is a speed sensing advance that only reacts to the speed of the engine. Vacuum advance is a load sensing advance that reacts to changes in load (throttle position) that the engine is seeing.
BigBlockk
Later.....
If you hook up to full vacuum at idle you are running off advance timing, so when you give the car gas you lose vacuum and the advance canister closes and the weight don't swing open, is this not backwards?
When you hook the advance canister up the car should not change idle speed up or down.
I unhook it and run idle and mechanical timing anyway.
Last edited by Tim H; 05-20-2007 at 05:56 PM.
05-20-2007, 06:57 PM
#14
Le Mans Master
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St. Jude Donor '09
Tim H Follow the above advise and do some homework before you post, vacuum advance and centrifical advance are two different deals and the vacuum canister has nothing to do with the advance weights 
05-20-2007, 07:10 PM
#15
Le Mans Master
that happens regardless of what vacuum source you use, full or ported.
When you hit the gas and accelerate you are placing the motor under load and vacuum in the motor decreases. The more you open the throttle and the more you put the motor under load the more vacuum you lose. Since vacuum is always coming from the same place, the motor, this does not change.
The only difference between full manifold vacuum and ported vacuum is at idle.
On full manifold vacuum you have full vacuum to operate the vacuum advance can at idle and as you increase the throttle under load the vacuum decreases and therefore reduces the amount of vacuum advance in the timing. On systems connected to the ported vacuum supply there is no vacuum at idle (again, this was ONLY done for emission purposes) but when the throttle plates are cracked open slightly you get full vacuum advance and than it begins to reduce the more the throttle plates are open same as if you were connected to a full manifold vacuum advance source.
Mechanical advance is controlled solely by engine RPM and vacuum advance is controlled solely by engine load. The two systems work completely independently but also together to control proper ignition timing throughout the various loads and conditions a street driven car operates under.
When you hit the gas and accelerate you are placing the motor under load and vacuum in the motor decreases. The more you open the throttle and the more you put the motor under load the more vacuum you lose. Since vacuum is always coming from the same place, the motor, this does not change.
The only difference between full manifold vacuum and ported vacuum is at idle.
On full manifold vacuum you have full vacuum to operate the vacuum advance can at idle and as you increase the throttle under load the vacuum decreases and therefore reduces the amount of vacuum advance in the timing. On systems connected to the ported vacuum supply there is no vacuum at idle (again, this was ONLY done for emission purposes) but when the throttle plates are cracked open slightly you get full vacuum advance and than it begins to reduce the more the throttle plates are open same as if you were connected to a full manifold vacuum advance source.
Mechanical advance is controlled solely by engine RPM and vacuum advance is controlled solely by engine load. The two systems work completely independently but also together to control proper ignition timing throughout the various loads and conditions a street driven car operates under.
05-20-2007, 08:50 PM
#16
05-20-2007, 08:50 PM
#17
Burning Brakes
That is not true about vacuum. Manifold vacuum goes down as you open the blades in the carb(less restriction). Vacuum from the venturi ports goes up(more air flow past the port).
05-20-2007, 08:58 PM
#18
Race Director
Its not venturi vacuum its ported.Ported is above the throttle plates and manifold is below the plates.
05-20-2007, 09:18 PM
#19
Drifting
I'll stick with my original answer.
If you hook up to full vacuum at idle you are running off advance timing, so when you give the car gas you lose vacuum and the advance canister closes and the weight don't swing open, is this not backwards?
When you hook the advance canister up the car should not change idle speed up or down.
I unhook it and run idle and mechanical timing anyway.
If you hook up to full vacuum at idle you are running off advance timing, so when you give the car gas you lose vacuum and the advance canister closes and the weight don't swing open, is this not backwards?
When you hook the advance canister up the car should not change idle speed up or down.
I unhook it and run idle and mechanical timing anyway.
When you open the throttle vacuum does decrease and the vacuum advance with it but the mechanical advance does not begin to increase until about 1300 to 1500 RPM. The mechanical advance will continue to increase with the speed of the engine until it reaches the limits of it's mechanism regardless of what vacuum is doing.
These two mechanisms are not connected.
BigBlockk
Later.....
Last edited by BigBlockk; 05-21-2007 at 12:11 AM.
05-20-2007, 09:24 PM
#20
Burning Brakes
not trying to argue but 'ported vacuum' is created by the venturi.
. The vacuum source was changed from the manifold to the carburetor venturi. This is called "spark ported vacuum." Spark ported vacuum is lowest at idle, and then increases as the throttle is opened.
from this article
http://www.corvettefever.com/techart...curving_power/
. The vacuum source was changed from the manifold to the carburetor venturi. This is called "spark ported vacuum." Spark ported vacuum is lowest at idle, and then increases as the throttle is opened.
from this article
http://www.corvettefever.com/techart...curving_power/