Mechanical advance.

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skipvair
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Mechanical advance.

Unread post by skipvair » Mon May 15, 2017 6:30 am

I have a 140 engine with stock internals and twin Weber 3 barrels. They have no vacuum ports, so I have installed an electronic Mallory distributor with mechanical only advance. I meed to set up both the total and initial advance. If i am reading the manual correctly, then it starts with 18 degrees and gets an additional 18 degrees for a total of 36 degrees.

First question, is this correct?

Second question, what rpm should I aim for to be "all-in" with advance while avoiding pinging issues?


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bbodie52
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Re: Mechanical advance.

Unread post by bbodie52 » Mon May 15, 2017 6:59 am

The chart below should help you. The non-SMOG (no Air-Injection Reactor — AIR) 140 hp engine starts with 18° at idle, with centrifugal advance beginning at 800 rpm, with another 18° all in by 2800 rpm. Total static (idle) plus centrifugal advance = 36°.

LEFT-CLICK THE IMAGE BELOW TO ENLARGE FOR BETTER VIEWING...
Corvair Distributor Part Numbers and Specs.jpg
I always dreamed of having a set of modified heads and Weber 3-barrels on my Corvair since I was a teen in the 1960s. How do you like them, and how do they perform on a stock 140 hp engine? Are you running a stock exhaust, or headers?

:chevy:
Brad Bodie
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cnicol
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Re: Mechanical advance.

Unread post by cnicol » Mon May 15, 2017 7:29 am

Mechanical should be all-in under 3K rpm. An all mechanical approach is good for race cars where the throttle is fully open (full cylinders) or closed but not the best for the street where we're usually at part throttle. When at cruise on the street, cylinders have low mixture density (i.e. not a lot of fuel/air mix and the presence of "vacuum" is the clue). Under those conditions it takes lots of spark lead (advance) to get the fire going at the right time. VA adds 22-degrees more advance at cruise.

If it were my car, I would tap each runner and add very small vacuum lines to a central point and use that for VA. Another approach would be to employ a Megajolt EDIS and a light throttle switch to engage cruise mode. I had a 2x 40 IDA-3 car and it totally rocked! The *magic* and the reason it was so powerful was that the individual runner intakes permitted use of a "full-race" 310-degree duration cam on the street with excellent idle quality. Without that cam it wouldn't expect a Weber car to have nearly the advantage mine had compared to 4x1.
'61 140 PG Rampside
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'66 Corsa Fitch Sprint Conv. (First car 1971, recently repurchased)

skipvair
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Re: Mechanical advance.

Unread post by skipvair » Tue May 16, 2017 1:33 pm

What is megajolt and where do i find info on it. Have headers too, and overall feels a little stronger than stock 140 but a lot smoother. I dont think distributor was ever set up and am in that process right now.


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skipvair
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Re: Mechanical advance.

Unread post by skipvair » Tue May 16, 2017 1:37 pm

Running 40 IDAs with 34 venturis from Starr Cooke.


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cnicol
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Re: Mechanical advance.

Unread post by cnicol » Tue May 16, 2017 5:23 pm

Same Weber setup I had! If you were to install a monster cam as I had, you would be astounded by the power. My buddy had a 351 4-sp Mustang and we lived near the top of a long "canyon" type road. The Corvair would hang right in with the Moose. Of course he could pull the straight sections (but not by too much) and I'd catch up and try to pass in the corners. Whee!
Here's your link to Megajolt:https://wiki.autosportlabs.com/MegaJolt_Lite_Jr.

There are a couple of guys here on the forum running Megajolt EDIS. I haven't tried it but all I hear are good things.
'61 140 PG Rampside
'66 Rear Alum V8 4-dr
'60 Monza PG coupe (sold, sniff, sniff)
'66 Corsa Fitch Sprint Conv. (First car 1971, recently repurchased)

skipvair
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Re: Mechanical advance.

Unread post by skipvair » Wed May 17, 2017 1:41 pm

How wild can I get with cam and this setup and still have nice street manners. Also considering adding AC.


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skipvair
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Re: Mechanical advance.

Unread post by skipvair » Wed May 17, 2017 1:42 pm

Will put stock distributor back in with pertronix II and tap in some vacuum ports


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cnicol
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Re: Mechanical advance.

Unread post by cnicol » Wed May 17, 2017 3:04 pm

skipvair wrote:
Wed May 17, 2017 1:41 pm
How wild can I get with cam and this setup and still have nice street manners. Also considering adding AC.


If you have 2x3bbl and six separate intake runners, a 310-duration cam is perfectly streetable and will have fine (smooth) idle quality. I speak from experience. If you have some kind of shared intake system, 280-duration is what you want to stay nicely streetable.
'61 140 PG Rampside
'66 Rear Alum V8 4-dr
'60 Monza PG coupe (sold, sniff, sniff)
'66 Corsa Fitch Sprint Conv. (First car 1971, recently repurchased)

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bbodie52
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Re: Mechanical advance.

Unread post by bbodie52 » Wed May 17, 2017 3:05 pm

skipvair wrote:Will put stock distributor back in with Pertronix II and tap in some vacuum ports
I believe this approach will not necessarily solve your problem. On the stock Corvair carburetor, the vacuum port (spark port) that is intended to be connected to the vacuum advance on the distributor is drilled in the carburetor body to tap from vacuum in the Primary carburetor throat that is physically ABOVE the throttle butterfly valve. That means that when the engine is at idle (throttle butterfly closed) there is essentially no vacuum applied to the spark port. At idle speed (approximately 600 RPM) the vacuum advance is inactive.

The vacuum advance begins to take effect as the throttle begins to be opened, at low speeds. It is intended to add ignition timing advance at the point in engine operation when the engine RPM is increasing, but has not yet reached a speed that is sufficient to begin engaging the centrifugal advance. It is, in effect, not unlike a computer system that is linked with sensors, electronic ignition control, and fuel injection that would likely be programmed into a modern car — except that computer control based on electronic sensors was not available in the 1960s. Instead, engineers took advantage of carburetor airflow characteristics and centrifugal advance characteristics and compensated by using the spark port–actuated vacuum advance to serve as an interim timing control that would automatically phase-in as throttle was initially applied and would automatically be reduced as engine RPM increased and manifold vacuum decreased as the engine approached wide-open throttle settings.

It is not desirable to have the vacuum advance engage while the engine is idling — which would be the case if you were to tap into intake manifold vacuum below the carburetor throttle butterfly valve. At idle speeds, with the throttle butterfly essentially closed, intake manifold vacuum is high. Connecting the vacuum advance to the intake manifold (which is what would happen if you were to connect the vacuum advance to the horizontal vacuum port on the Rochester carburetor), is not desirable. Doing so would fully engage the vacuum advance and would apply maximum timing advance when the engine is only idling. Timing advance is needed at low speeds, but not at idle speeds. You would either need to create a spark port on the Weber carburetor (a single selected carburetor barrel only), or you would need some sort of electronic programmable ignition control over ignition timing advance that could be controlled via a computer program to match the needs of the engine. Such a system is found on the GM electronic fuel injection and distributorless ignition system that has been adapted for use on the Corvair http://www.corvairefi.com/. The system uses EFI throttle bodies and six individual coil outputs (one per cylinder), and a number of sensors that allows the programmable computer system to match fuel injection and ignition performance and timing advance to the needs of the engine via a tunable computer algorithm.

Most carburetors that are intended for street use — including a number of aftermarket Holley performance carburetors — included a provision for a vacuum spark port in their design. High-performance racing carburetors, such as the Weber and Dell’Orto carburetors, had no such provision in their design.

The Megajolt system appears to be a tunable electronic ignition control. As recommended earlier this system may permit you to sort out a timing curve for your engine that would be optimum for the desired street based performance coupled with the extensively modified intake and exhaust systems. If you were ever to install a camshaft that was optimum for this engine, the Megajolt ignition control system would allow you to retune the ignition set up to meet the new engine breathing parameters. It is unlikely that you can successfully optimize a stock Corvair carburetor and vacuum advance to work well for a street based engine with the carburetor and exhaust modifications you have added. Even the tunable Stinger distributor that was recently introduced by Seth Emerson at Performance Corvairs https://www.perfvair.com/stinger-ignition-distributors/ may not meet your needs — although it may come a lot closer than attempting to adapt a stock distributor. You might possibly try contacting Seth Emerson to see if he has any experience with customers fitting the Stinger distributor to a Corvair engine using Weber three barrel carburetors and custom headers.
Image
:link: https://wiki.autosportlabs.com/MegaJolt_Lite_Jr.
Brad Bodie
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Image 1966 Corvair Corsa Convertible

notched
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Re: Mechanical advance.

Unread post by notched » Wed May 17, 2017 4:09 pm

MSD 6AL2 digital part# 6530 also is programmable and lets you keep the distributor. You still need a vacuum signal for the map sensor. Even the Megajolt is more accurate with a map sensor and that is speaking from experience as I have Megajolt on my Corsa.
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skipvair
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Re: Mechanical advance.

Unread post by skipvair » Thu May 18, 2017 8:20 pm

I found the total advance on the Mallory was never adjusted. I bought the kit and adjusted it and have all in at 2800 for a total of 36 degrees. It runs substantially better and seemd s to have eliminated the ping I was getting at cruising speed ( 75) had too much total advance. runs smooth as hell with these webers.

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