Langdon HEI timing & vacuum

[martybose]

I finally finished installing my Langdon HEI today and fired it up this afternoon. The instructions suggested from 10-18 degrees initial timing without vacuum, and suggested using ported vacuum for the advance with mild cams and manifold vacuum for performance cam motors.

For no particular reason I started with 16 degrees and ported vacuum. It ran just fine until it completely warmed up, then started pinging badly at half throttle under load, so obviously I need to back down the timing. I'm also thinking that maybe I should run manifold vacuum, since I'm running about 9 to 1 compression, a mild cam, dual carbs and headers, and dropping timing as I crack the throttle seems like a good idea.

I'm curious; those of you that have done this conversion, what initial timing and vacuum source did you wind up with?

Marty

[jd52cranbrook]

I ran last summer same set up as you have now, and ran hall the time with the vacuum at the manifold, and the other half on a single carb port, (out of a dual carb set up). But I did not advance as much as Langdons said. Ran fine to me, but I have to admit I have lost a bit of hearing over the years, and can't pick up pinging too well it seems. I also had not gapped the plugs as wide as they suggest.

Now I just dropped in my rebuilt with a 230 crank, mid level cam, pistons, the works. It should be on the road by the end of this week, as I did some sound, AC, and rewiring work on the car too. I do intend to set the settings as prescribed and start from there. I have 4 weeks to get it tuned in well.

If you get any good experience with yours please share it here.

[blueskies]

I finally finished installing my Langdon HEI today and fired it up this afternoon. The instructions suggested from 10-18 degrees initial timing without vacuum, and suggested using ported vacuum for the advance with mild cams and manifold vacuum for performance cam motors.

For no particular reason I started with 16 degrees and ported vacuum. It ran just fine until it completely warmed up, then started pinging badly at half throttle under load, so obviously I need to back down the timing. I'm also thinking that maybe I should run manifold vacuum, since I'm running about 9 to 1 compression, a mild cam, dual carbs and headers, and dropping timing as I crack the throttle seems like a good idea.

I'm curious; those of you that have done this conversion, what initial timing and vacuum source did you wind up with?

Marty

Marty-

I had my timing set at 10 degrees for my trip to Tulsa, but it seemed like a bit much, so I re-set it to 8. I have the distributor on ported vacuum, attached to both carbs. I also have autolite 303 plugs gapped to Langdon's spec.

I don't see how putting the vacuum advance from the distributor on the manifold vacuum would work... The advance needs vacuum when you open the throttle, and manifold vacuum is the opposite, full vacuum at idle, and less with throttle.

Pete

[martybose]

Marty-

I had my timing set at 10 degrees for my trip to Tulsa, but it seemed like a bit much, so I re-set it to 8. I have the distributor on ported vacuum, attached to both carbs. I also have autolite 303 plugs gapped to Langdon's spec.

I don't see how putting the vacuum advance from the distributor on the manifold vacuum would work... The advance needs vacuum when you open the throttle, and manifold vacuum is the opposite, full vacuum at idle, and less with throttle.

Pete

Thanks for the info, Pete. The thing about advance is this: We have a centrifugal advance to add advance as RPM goes up, and many times the vacuum advance simply adds advance under light load conditions. If you use manifold vacuum, then the extra advance will be removed as you open the throttle to put the engine under more load. I honestly don't quite understand the use of ported vacuum for advance, as (if I understand it correctly) there is no vacuum at idle, but there is some a smaller throttle openings?

Marty

[Normspeed]

I'm curious. Don't know much about HEI distributors but why would you take a motor that basically is designed to be tuned at 5 degrees or less of initial timing, and set it to 10-18 degrees just because of a distributor change? Does Langdons provide any reasoning for that? Or, am I just waving my HEI ignorance like a flag?

[greg g]

Marty, I will chime in on this if you don't mind. Proted vacuum comes from the carburetor and is based on the vacuum created by the air flow through the venturi. More air flow equals more vacuum. So my understanding is that the vacuum created by the increased airflow throug the carb anticipates and begins to advance the spark before the weights in the centrifigal portion can react to the increase in RPM. You must have more fuel air befoe Rmps can increase. So you need advanced spark before you get higher RPM. Depending on how long you stay at WOT, the centrifigal may catch up and even pass the curve provided by the vacuum, and sustain the advance when you close the throttle to partial open, but maintain the RPMS gained during WOT operation. I remember that drag racers routinly disconected and pluged the carb vacuum stating that is wasn't necessary as they did ther starte at elevated RPM so the centrifugul was fully advanced andy way and most of the run was done at WOT, keeping the weights out and the spark flly advanced till the run was over. Street driving needs the anticipated advance that the carb vacuum provides. And high manifold vacuum at idle which decreases as RPMs increase seem counterintuitive to this.

Then of course there is this little atricle to mke things as clear as mud again::::

http://www.gofastforless.com/ignition/advance.htm

[jd52cranbrook]

After reading that link through the first time, I'm inclined to stay with the manifold advance. (Further study needed) Then run it on my G Tech for results.

[Don Coatney]

The prime reason for vacuum advance is fuel economy. If fuel economy is not an issue for you trash the vacuum advance and use the centrifugal advance only. If fuel economy is an issue then connect the vacuum line to the carburetor venturi as per the factory recommendation. In my opinion the manifold connection will be of no benefit period.

[grey beard]

Don is absolutely right. Matter of fact, ported venturi carb vacuum and manifold vacuum work exactly the opposite of each other. On acceleration, ported vacuum will begin to climb with rpm increase. Manifold vacuum drops in proportion to how far the throttle plates are opened. When you connect a vacuum advance to a manifold vacuum source that was made for ported vacuum, you are giving backwards signals to the combustion process. You would have high vacuum and advance at idle, and it would drop off on acceleration - not what our MoPar distributors were made for.

On this same subject, I notice lotsa' local wisdom in aid of removing the vacuum advance line to set initial timing. My own 218 timing remains unchanged at idle with the vacuum connected or disconnected. I suggest that if yours doesn't do the same, your engine may be idling fast enough to generate some vacuum on the ported side - and that's not the way to set accurate initial timing. Also, unless your idle speed is nice and low, you may get some centrifigul advance beginning to show up and affect what you see with your timing light. JMHO:)

[martybose]

I had always had the impression that ported vacuum was primarily there to enable you to have some advance while driving but not have it at idle to cut down on the NOX emissions. But maybe not.

To answer the earlier question about why the distributor change requires a different initial setting than an OEM distributor, I suspect that the Chebby distributor doesn't have anywhere near as much centrifugal advance as the stocker, so you need more initial timing to get the desired total timing at speed.

Anyway, tried the next experiment tonight. I left the initial timing (16 degrees) alone, but switched from ported to manifold vacuum, then went for a drive. My observations; (1) pinging significantly reduced, although not quite eliminated. (2) Idle speed was higher. (3) The engine hesitated while starting. The combination of the three suggest (to my way of thinking) that the harder starting was because of the combination of initial and vacuum advance while cranking, the idle speed was higher due to the same higher amount of advance, and the reduced pinging was because of the reduction of advance at partial throttle from the manifold versus the ported vacuum signal. My conclusion: Take out a few degrees of initial time (say back from 16 to 12 degrees), which should (1) reduce/eliminate pinging some more, (2) reduce idle speed, and (3) reduce/eliminate starting kickback.

I won't have time to try this Tuesday, but shoud be able to get to it Wednesday.

Marty

[martybose]

(snip) You would have high vacuum and advance at idle, and it would drop off on acceleration - not what our MoPar distributors were made for.

Okay, I'll admit ignorance on this one. My original 218 had the vacuum advance connected to the manifold, and I've always run both the 218 and my 247 (bored out 230) that way. Does anyone know for sure that the B&B carb on a 218 actually supplied a ported vacuum connection in the OEM configuration? I didn't think they had this until emissions reared its ugly head.

Marty

[martybose]

The prime reason for vacuum advance is fuel economy. If fuel economy is not an issue for you trash the vacuum advance and use the centrifugal advance only. If fuel economy is an issue then connect the vacuum line to the carburetor venturi as per the factory recommendation. In my opinion the manifold connection will be of no benefit period.

Actually, at a small throttle opening you would probably get about the same amount of vacuum-based advance from either connection; the difference would only be what happens when you step on the gas. With ported the signal might go up, with manifold it would go down. Most hotrodded motors do better with less advance at high throttle openings, compared with what they can take at reduced throttle for increased efficiency and economy.

Marty

[rearview]

I just pulled the stock carb off the manifold for my 52. The vacuum advance port is almost directly in line with the throttle plates. That should be manifold vacuum, except at idle... just like that article link above stated. It is a Stromberg BXVD-3.

[martybose]

I decided to actually find out what this Langdon HEI distributor was doing, while being fully aware of the adage about being careful about what you ask for.

First, disconnect the vacuum and confirm static timing. Warmed up the motor, slowed the idle to just over 600 RPM, checked timing. It's off by over 10 degrees! Mystified, I set the timing back to 12 degrees initial. Then I decide to check the centrifugal advance. Rev the motor up, timing barely changes. Rev it up a few more times, and notice that it isn't moving much, but it also isn't returning to the same initial setting every time. Pull the distributor cap and unbolt the rotor (standard Chebby), check the advance weights. They seem clean and move freely, so go to put the rotor back on. I'm holding the rotor as I tighten the 2 bolts, and notice as the bolts tighten it starts to sound different. Take the rotor off again, and find that the no-name original rotor has a raised pin in the very center of the underside that was bottoming out before the bolts tightened, and was jamming the advance! Shortened the pin, put it back together, sounded better. Fired the engine, reset initial timing again, then revved the motor. Found that the centrifugal advance starts at about 2000 RPM and comes in fast, and now always returns to the same initial setting. So far so good. With a 12 degree initial, got up to 35 degrees fully advanced. Was a little surprised that it was that much, might want to limit it a bit. Food for thought later.

Now back to vacuum advance. With engine running, try all three of the connections on my Carter-Webers. I find that the ported vacuum worked as advertised; no advance at idle, then more advance as the throttle is opened up. Free revving at partial throttle and about 3000 RPM gave about 45 degrees of advance; Oh Boy! Then tried the other 2 connections; both gave me no advance under any condition! Conclusion: I have one ported vacuum connection only, maybe the other two are emissions related somehow. If I want to try manifold vacuum, I'll have to add my own connection in the manifold or the carb adapters. Bummer, because I would have to take the carb off to drill and tap a hole in the adapters. I can't even imagine drilling a hole in my Edmunds manifold, thank you!

So to anyone with a Langdon HEI; you might want to confirm that your centrifugal advance actually works; if it doesn't check the underside of the rotor. Further testing will wait until I feel like adding the manifold vacuum connection; for now I'll run with centrifugal only, but might back it down a few more degrees to something like 8 degrees.

Marty

[Normspeed]

Marty, why don't you want to use the ported vacuum hookup if that seemed to be functioning?

Now back to vacuum advance. With engine running, try all three of the connections on my Carter-Webers. I find that the ported vacuum worked as advertised; no advance at idle, then more advance as the throttle is opened up. Free revving at partial throttle and about 3000 RPM gave about 45 degrees of advance; Oh Boy! Then tried the other 2 connections; both gave me no advance under any condition! Conclusion: I have one ported vacuum connection only, maybe the other two are emissions related somehow.

Further testing will wait until I feel like adding the manifold vacuum connection; for now I'll run with centrifugal only, but might back it down a few more degrees to something like 8 degrees.

Marty

[jd52cranbrook]

Good info Marty, thanks for posting this.

I have Webber/carter carbs, as far as I know there is only one ported outlet on it. The Offy intake has two holes to use for vacuum if one wants to.

I will be out tomorrow to check. I only have a few more items on my check list to complete before the car goes on the road., like putting in the front seat again some other minor stuff,,

[martybose]

Marty, why don't you want to use the ported vacuum hookup if that seemed to be functioning?

Mostly because with no vacuum advance at all I'm still getting the slightest audible pinging at full throttle in second gear; adding another 15 or 20 degrees of advance would probably blow the head gasket out of it!

Marty

[greg g]

Marty, The manual calls fo a maximum of 10 degrees of mechanical and 9.5 degrees of vacuum adcance at 16 inches of vacuum, so 20 degrees of total advance at the distibutor, it could be double that at the crankshaft. The crank specs are 18 to 22 degrees at 2850 Rpm, and 14 to 18 degrees at 14 inches of vacuum. So the crank could show 40 degrees of advance at 2800 to 3000 rpm. I wonder why Tom specs such a lot of initial advance. Seems to me that if you had 6 to 8 of initial, and 30+ at 3000 you would be ok.

[Merle Coggins]

, so 20 degrees of total advance at the distibutor, it could be double that at the crankshaft.

WHat??? Why would you double it at the crank? You measure the distributor at the crank anyway. 20 degrees of total advance would be crankshaft degrees.

All this distributor advance talk has prompted me to get out my manual and read up. Found interesting things on the vaccume advance, purpose and function, advance timing and etc. Although my manual is for a truck, the theories here should be the same.

Links to a few pages scanned from my manual on this subject;

http://i66.photobucket.com/albums/h254/mac2026/Manual%20Pages/Distributor.jpg

http://i66.photobucket.com/albums/h254/mac2026/Manual%20Pages/Distributor2.jpg

http://i66.photobucket.com/albums/h254/mac2026/Manual%20Pages/Distributor3.jpg

[Plymouthy Adams]

I think Greg made reference to the timing at the distributor if you were curving this distributor on a machine and not on the engine itself. Yes total advance is set by the average man as indicated on the crank and what you see is what you get. (indicated by the light) Mechanical advance is necessary for that WOT take off..and vacuum advance will come in when the engine has obtained the speed you are running...this is what gives you the economy in driving. Lot of vacuum advance mechanisms are adjustable with an allen wrench to set total max advance to prevent pinging...this is indicated by what is called "light throttle ping" and is most noticible if you were crusing along and start up a slight incline and under the added load your engine pings..

[greg g]

That is what it says in the manual.......

[Don Coatney]

I wonder why Tom specs such a lot of initial advance. Seems to me that if you had 6 to 8 of initial, and 30+ at 3000 you would be ok.

Greg;

I agree. The benefit of installing an HEI distributor should be increased spark energy through all RPM speeds, not altering the distributor advance curve. Although altering the distributor curve can be bennifical in known specific applications such as full out racing.

[40desoto]

When using a triple carter ball n ball setup.  Is it beneficial to run the vaccum advace from all three carbs?

 

[Don Coatney]

On my dual carburetor setup I inly used 1.

 

[40desoto]

Thank you Don.