Is this a better way to go with a street engine? What kind of timing should I be running and should I disconnect the vacuum advance? First, on a street-driven engine there is no good reason to lock out your ignition timing. If we had to use a generic maximum advance that seems to work for most pump gas engines, somewhere around 34 to 36 degrees before top dead center BTDC is a good number. But with this much initial timing in the engine it will be difficult for the starter motor to crank the engine. You can get around this with an ignition cut-out so that the engine cranks and then you hit the ignition circuit, but this is unnecessarily complex for a street car. The better plan is to start with a decent initial timing of roughly 10 to 14 degrees.
It will improve idle, gas mileage by MPG, and wil not affect your top-end. Oct 10, 3. Thats where i was gonna run the dist. In the back of the intake where the brake booster will be hooked up to? Oct 10, 4. Oct 10, 5.
I don't run vacuum advance on mine. It's mainly street driven but sees road racing duties too, and the vacuum just wasn't working Oct 10, 6. Messages: 1, Likes Received: 4. Oct 10, 7. Messages: 14, Likes Received: 7.
Your Initial timing without any vacuum advance is for idle, low end power. The centrifugal timing is added as quickly as tolerable for max engine performance. Ported Vacuum advance is for cruising around at partial throttle settings where maximum economy is important as throttled operation has a less dense mixture that takes longer to burn, hence the extra timing from the vacuum advance.
That is why there is ported advance as the port is exposed when the engine approaches about 1, rpm while driving, giving the extra timing as needed. Oct 10, 8. Messages: Likes Received: 1. Even adding a few degrees at part throttle will improve mileage and responsiveness. These negative experiences fuel hobbyists' aversion toward vacuum advance. It also seemingly justifies such arguments as "my engine just doesn't like it," which then lends credence to leaving the vacuum canister unplugged.
Reality is, however, that no engine truly dislikes vacuum advance! Most original vacuum advance canisters provide more spark lead than modified engines can tolerate, which results in said operational challenges.
So how can you easily and effectively limit your GM engine's vacuum advance?
Distributor vacuum advance hookup
Highest engine vacuum occurs at sustained light-throttle cruise, which is also when carburetor fuel mixture is leanest. Since swept cylinder volume is constant, the limited number of fuel and air molecules within each cylinder during these conditions are not as tightly compacted by the compression stroke.
When spark occurs, combustion is less volatile and the flame front spreads slower, and that requires more time for a complete burn than at full throttle. Spark must then occur even earlier than what the distributor's mechanical advance is set to provide to achieve maximum performance. To accomplish this, a load-sensing device is mounted to the distributor body.
Pingback: Ask Away! Apples and Oranges. Modern computer controlled spark and fuel is not the same as old-school distributor and coil. I have a Ford with I even referred him to an earlier post by you He just refused to accept he might be wrong. Of course you would use venturi vacuum.
The Edlebrock guy was right Do you think you know more than the carb manufacturer. He keeps using venturi vacuum in reference to ported vacuum because he does not know the difference in them and think they are the same when they are not.
Venturi vacuum is situated high up in the throat of each barrel. It is a signal that is created by air rushing past it. Think of how fuel is pulled into the carb through the boosters as air flows past the booster it creates a low pressure point that pulls fuel in.
Faster air flows more fuel pulled in. Venturi vacuum functions the same way. Faster the air flows across it more vacuum is created. Ford used venturi vacuum on their Holley carbs for use with the Holley Load-O-Matic distributors that had no mechanical advance. Ported vacuum is situated just above the throttle plates and as you apply more throttle vacuum comes in off idle.
It does not increase it remains a steady vacuum signal off idle. So for you, attach your vacuum advance to manifold vacuum.
Nov 19, I have messed with the adjustment in the vacuum advance from lock to lock in 1/4 turns (about turns from lock to lock). It runs the best with if turned all the way left, I think this is least amount of vacuum advance. It still runs best without it. Anyone know what could cause this. I will take a video, upload it to youtube and paste the. Sep 04, Vacuum Advance Hookup Hello, I've noticed people have been hooking up the vacuum advance to the port on the baseplate and not on the metering block. One guy said his engine runs cooler and gets better fuel economy. I run a Mopar with a . Oct 04, See all 6 photos In a GM HEI distributor, vacuum advance is limited by the length of the advance slot. The further the rod travels, the more spark lead the unit provides. Original GM vacuum.
When your engine is under low load situations you are able to run more advance. When an engine is at idle what load is there on there.
There is no reason to run ported vacuum on distributor advance can unless you have to meet emission requirements. Outside of emission requirements it should be avoided.
The question is, what did cars run before emissions came into play? I run my hot rod with manifold vacuum. Mainly manifold vacuum and venturi vacuum, not to be confused with ported vacuum like this Matt individual keeps telling people to run which no carb has venturi vacuum today nor in the last 50 years.
1964 GMC "Crustine"- Playing with the vacuum
The one furthest from the distributer housing gets venturi vacuum to advance your timing, and the one closest to the housing gets manifold vacuum to retard the timing when you let off the pedal.
Venturi vacuum was used before distributors had mechanical advance as it was a increasing vacuum signal as the engine ran faster and faster. Ported vacuum was the same amount of vacuum at 1, rpm as it was at 2, rpm. Mine how ever is manifold vacuum on the outter can and the inner can is venturi vacuum. Mine is done this way so when you apply hard throttle and loose manifold vacuum the outter advance is slammed back via spring pressure creating a instant retard to prevent ping under load.
Yours will be the same, only difference is one will be manifold vacuum and the other will be ported vacuum so when you apply heavy throttle the manifold vacuum going away will result in a instant retard of your ignition timing to prevent ping under load. Jefftrying to replace distributor pick up in ford 5. Its very obvious no one monitors this considering your complete lack of understanding on this matter spreading disinformation as fact when you sir are in fact incorrect and wrong.
We only have two forms of vacuum, manifold and ported. Ported vacuum was used for emission reasons to prevent too much advance at idle as it was believed to increase emissions. In fact it created a worse situation as it actually increased emissions slightly over manifold vacuum. You cited Ford numerous times of running ported vacuum as the cause for why its right.
You fail to grasp that Ford used venturi vacuum first, then manifold vacuum, then they went to ported vacuum during the emission era. If you are running your Ford on ported vacuum I feel sorry for your engine and how you are gimping it by using the emission era form of vacuum advance over the proper non emission era manifold vacuum.
Hi Jeff, I have a question that no one can seem to answer. I get about 25 degrees advance from the centrifugal mechanism, and about 45 degrees on top of that from the vacuum.
Hei vacuum advance hookup
I appreciate any advice! But under the assumption that this is accurate, best thing to do is to remove the vacuum advance unit and braze or weld or use JB Weld to limit the amount of vaccum advance. There is normally a rod that travels in a slot. Limit the length of the slot and this will limit the vacuum advance. But before that - check to make sure you ahve the correct advance unit - It could be the one you have is incorrect. Limit the advance to around 10 to 15 crankshaft degrees.
Timing is Everything
I have another distributor that appears the same as the one installed. These are both stock Ford duraspark era units. I really appreciate the help on this!
Voes hookup pangulong marcos is no fitting at the air cleaner if you if there are the centrifugal-advance distributor - one wire hookup. All gears except , mallory. Do stock coil hei distributor should be too much contradicting info on the vacuum at 6btc with the provided. Jan 04, In certain instances, there can be a slight vacuum between the carb and the top of the blower and the last thing you want is for the distributor to get a false vacuum signal and advance timing.
I have a mach 1 Q code, looking for advice to adjust single port vacuum advance on distributor, bought a new vac advance and wondering if it needs to adjusted before I install it, I,m more of a backyard mechanic. How many turns ccw to do that? I would just start with how it came out of the box rex.
Start off by checking and confirming your total advance with the vac canister disconnected and the line to the carb plugged. Take that number, and subtract the total mechanical from earlier from it to get your vaccum advance number. As a general rule of thumb, we usually look for a vaccum advance to take the total timing to between 45 and 50 ish degrees when active.
At the end of the day, give your engine what it wants. Set a certain degree of timing to begin with, then take the vehicle and test it.
If you notice any Detonation loud pinging then you have too much for your engine. Your email address will not be published. Your Website.
Mar 27, The effect on the vacuum advance is consequently the same. The important thing is to use vacuum advance, and adjust it correctly. Most V8 will happily cruise in high gear on level ground at 50° to 55° BTDC ignition timing. Vacuum advance was the only way to achieve this, centrifugal weights and springs alone can't do it. Sep 16, Modified engines with more cam that generate less than 15" Hg. of vacuum at idle need a vacuum advance can that's fully-deployed at least 1", preferably 2" of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC advance can (about $10 at NAPA) provides the same amount of advance as the stock can. Oct 11, Using manifold vacuum advance will add ? of extra advance at idle and the engine speeds up a lot. Nail the throttle and that extra 20? timing drops away and a bog will probably occur. That is why there is ported advance as the port is exposed when the engine approaches about 1, rpm while driving, giving the extra timing as needed.
Save my name, email, and website in this browser for the next time I comment. This site uses Akismet to reduce spam. Learn how your comment data is processed. Author: Jeff Smith Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age After graduating from Iowa State University with a journalism degree inhe combined his two passions: cars and writing.
Smith began writing for Car Craft magazine in and became editor in Inhe assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. SinceJeff has held various positions at Car Craft including editorhas written books on small block Chevy performance, and even cultivated an impressive collection of and Chevelles. Now he serves as a regular contributor to OnAllCylinders. Great article! Many people do not understand this and you explain it well!
Jeff says: March 28, at pm. Richard Petitpas says: March 29, at am. Ken Chorney says: March 29, at pm. Scott says: March 30, at pm. Ted says: August 25, at pm. Robert Fortier says: September 5, at am.
Ross says: September 8, at am.
Thanks Ross. Steve Koen says: January 31, at pm. Jeff Smith says: February 1, at pm.