Distributor Vacuum Signal

Apologies to the Mod's if this should be in the electrics section. Kind of a nexus I guess. Anyway after diving down the PCV rabbit hole (still not fixed) I heard that older american dizzies like a full manifold vacuum signal at idle, not ported vacuum as Holley's have. Can anyone tell me if the standard Lucas likes ported or manifold vacuum? Mine has been converted to lumenition but I suspect the vacuum advance is original spec.


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IIRC the SU vac take off for the dist is below the throttle plates, I do not have a stock set to look at right now. If it is this means that the dist uses manifold vacuum. This is the opposite to your ported vacuum on the Holley.
So instead of providing a lot of vacuum advance at idle, you are getting a lot at WOT. This is maybe over advancing at the top end.
I have the same issue with the Autolite and have put a limiter in the dist so it won't over advance.

At some point I need to fit a take off in the intake and reverse the mod to try it. I work on the basis of about 35 degrees all in. I have tried with mechanical only, and tbh performance is not really affected either way, although I suspect economy might be.


Active Member
As far as I recollect the original UK spec SU set up with a Lucas distributer has vacuum created in the manifold but the take off is near the butterfly so is also influenced by the butterfly position relative to the vacuum take off port., At idle there is little or no advance and at full throttle there is no vacuum advance. The vacuum advance is mainly an economy device at part throttle openings . The centrifugal advance give the basic advance curve for most power. This all got heavily modified with emission regulations as they appeared and some have vacuum retard as well as advance, a port either side of the distributer vacuum diaphragm.


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This has been playing on my mind since you started this thread, so in the interests of keeping boredom at bay I did something about it.
I have been running a Lucas 35 DLM8 distributor and since the four barrel conversion have had to have it on ported vacuum as there was no manifold vacuum fitting.
I have now drilled the phenolic spacer and inserted a piece of copper brake pipe with a partial flare on the end in order to have a manifold vacuum take off. This is pulling 15" hg at idle. I have re timed without vacuum connected and then added the vacuum advance on top.
As soon as I have the bonnet re fitted from the windscreen job I will have a run out.
The manifold vacuum will now apply more advance at idle and part throttle cruise.
There were numbers on the vac can, but did not relate to anything.
I managed to find out that about 12" hg fully pulled the vac in, so at idle the vac can would be fully deployed.
I forgot to add the figures that you would be interested in.
Initial 10 deg
Mechanical advance built in to the 35DLM8 20 deg
Vacuum advance in the current unlimited vac can 22 deg
So I am now wondering if to peg the vac can back a bit as 52 deg at cruise may be a little too much.
Edit 2
I just realised to get all that timing in at a light cruise I would be going about 100mph !
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By my best reckoning and advice from 'British Vacuum Unit' int the USA I'll keep using the ported vacuum, but upgrade to one of their diaphragms when the budget allows. I have about 19" in neutral and about 17" at idle in the manifold (haven't measured at the port yet but should be zero looking at other carbs..) so ported is definitely the preferred option for me. Also means I need to change my powervalve to an 8.5. Jets are 57 which I think are ok. Anyway I digress - here's the info I got:
Hi Rob
I find the Holley carburetor ported vacuum usually holds up to 5 to 6 hg vacuum at idle sometimes more. If you have a vacuum gauge, check the vacuum at idle at the distributor vacuum port on the metering block. The original SU carbs had no vacuum at idle.
The Rover 3.5 should have a 5 17 8 vacuum unit.
5 hg starts to advance, 17 hg full advance and moves 8 distributor degrees = 16 crank degrees.
I manufacture special 8 18 8 units for all the different Lucas distributors when the Holley carbs are used. Make a world of difference if you have any vacuum advance at idle with the stock unit.
We have a Austin Healey 100-6 here with a new 350 aluminum block.
It had all new MSD ignition goodies, Holley carburetor and ran like crap.
I changed it to a early stock point distributor, re-curved it to full advance at 3600 rpms.
This car never ran so good.
We manufacture our own 423871 condensers and often adapt them to any distributors like the Delco's, Lucas etc.
Holley's like more advance at low RPMs
With 10 BTDC static not running timing, I set the distributor to have 5 degrees at 1000 RPMS.
That's a total of 20 degrees at 1000 RPMs. Eliminates any dead spots and bogging also helps with cooling.
This will usually give about 16 degrees at 800 RPMs. You should not have any vacuum advance at idle so this is why you want to check the vacuum at idle and use a higher starting vacuum unit it you find vacuum at idle.
Also our 8 18 8 units are made HD with a OS diaphragm folded into the outer pressed edge so they hold up without popping or leaking.



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Have already decided to re visit mine today.
Am going to keep on manifold, but limit the can to 15 degrees and set the initial to 15, this will give me 30 deg all in at 3250 purely mechanical plus the vac on top.
Did road test yesterday, car ran strong, no pinking.
Interesting read thanks.
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The result of today's fiddling is ....... I cleaned up the vac can I am using and found 3 10 8, I have another 35DLM8 and the can on that one is 5 17 8.
I will stick with the one I am using.
I limited the vac advance to 16 deg, so am seeing 30 deg advance at idle with the initial set at 14 deg, best idle I have ever had ! all deg are crank not dist.
Total advance at WOT will be 34 deg as vac will drop out and mechanical advance at 20 deg will be in.. Total advance at light cruise will be 50 deg, as both vac and mech will be in, there being 20 deg mech in the dist.
I did put a vacuum gauge on the timed port of my Autolite and there was no vacuum at idle.
Pour a drink and have a read of this, it is the path I am following.

Copied from Chevelle stuff.

As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance.


The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whiz-bang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. 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-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, drivability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.

Courtesy John Hinckley
Retired GM/Chrysler Engineer
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Thanks @cobraboy that has changed my opinion entirely about vacuum advance, and has really cleared up some fog for me regarding it and timing. I'm really enjoying learning all this new stuff. I've driven old cars for ages but I have a new invigoration for tuning (I used to just run all factory stuff at factory settings) now. Looking forwards to when I can get under the bonnet again!

Having re-read this thread it seem that the GM engineer and the British Vacuum Unit people are skinning the cat two different ways. One is saying no vacuum advance at idle and one is saying manifold vacuum (in my case about 19/20") at idle. This would have to be using two different vacuum cans, surely?? I wonder what can the GM guy would recommend?

Further down the rabbit hole I come across this thread Ported vs Manifold vacuum advance. which again purports the use of ported not manifold, and goes so far as to say the quote above from the Corvette Forum is NOT by a former GM Engineer - scandall!!!
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Well, its a bit like a ZDDP conversation, folks are never going to agree, you just pick a side.

As an ex Mustang owner I can tell you that one of the posters on that thread you highlighted barnett468 has been banned from almost every Mustang forum on the internet. He is a serial troll and takes great delight in posting misinformation and lies to turn everyone against each other.

I am not going to recommend either of the vac take off options. As stated by both camps, once you are 'on it' neither set up is providing a vac signal so you are really tuning for economy, smoothness and cooling. So try both and see how it works out.

As stated by the GM guy, if you have a lot of cam then you would need a vac can that would deploy early to get a steady idle. Your 19/20" would work fine for a stock Rover can.

It is interesting to note that your code that you listed for the Rover can 5 17 8 and my 3 10 8 both have the same 8, meaning the advance is 8 dist degrees and 16 crank degrees, BUT my can put on 22 degrees of crank advance !

You may also find that your idle mixture may need re setting. After setting my dist to manifold vac and setting the idle speed I then had to lean out the mixture, I can only assume that as the GM guy has said, the charge is being lit earlier and burning more efficiently, therefore you need less fuel.

With all that in mind just set your timing so you get the magic 34 degrees at 3500 with a disconnected/ plugged vac line and see what that gives you at idle with no vac, then plug the vac in and check, if you are on ported then you will not see much change. Drive it and listen for pinking at high load in high gear. With ported vac you might get pinking as it will add timing on throttle opening.

Did you know that you can use your vacuum gauge connected to MANIFOLD vacuum to set your idle mixture ?
You connect the gauge and then tune for maximum vacuum using the mixture screw. If you have an undamped gauge and the needle is flickering wildly then get a brake line clamp and fit to the pipe, turn the clamp until the needle steadies.

Maybe get a big gun and start on those rabbits ?
lol I do tune my idle to manifold vacuum & ear normally. I'll have to get the appropriate plumbing to give manifold vacuum a try with the dizzy. I guess I should also get a tach so I know when I'm at 3500..... :p
As I'm now having an issue with lumpy tick over I just dropped on this thread, looking for any guidance on this and any other thread. I removed the vacuum pipe from the inlet manifold to suck it with my vacuum gun to see if the advance on the distributor was working properly and was surprised to find that the pipe on the inlet manifold is not sucking air. I'm almost sure this should suck air. Am I correct in thinking the should be a change in tick over when this pipe is removed. Sorry for hijacking this thread bye the way.
Regards Terry H


Well-Known Member
If the vacuum pipe is connected to manifold vacuum there will be suction at idle, if the pipe is connected to ported vacuum there will not be suction at idle , but there will be when the throttle is opened.
Manifold vacuum take of point will be below the throttle plate, ported vacuum take off will be above the throttle plate.
On the 2000 TC the vacuum pipe is before the throttle plate and only on one carburettor. Thanks for your advice. So air flow runs through filter then suction chamber, piston and needle jet, then past the vacuum pipe and then throttle plate into inlet manifold.
Regards Terry H
You have ported vacuum. If you open the throttle slightly, the vacuum port should be exposed and the engine will draw air through the port. You can still test the vacuum canister with your vacuum pump though. Just look to see if the distributor base plate moves when the vacuum is applied.

Whilst that GM article is very good, ported vacuum is not the disaster it makes out. That’s specifically referring to ported vacuum to meet idle emissions targets, and in that example, the advance settings are a compromise.

There is another reason to use ported vacuum though. That being getting enough airflow at idle speed. Carburettors are very dependent on sufficient airflow. No airflow, no fuelling. By reducing the advance at idle slightly from optimum, it’s necessary to increase the airflow slightly to compensate and maintain idle speed. This increased airflow makes the fuelling far more stable, and the idle less susceptible to change due to external influences.

Once the throttle is slightly open, both systems perform the same, providing you have an appropriate vacuum canister for both the engine, and the point you’re measuring the vacuum signal.
Thanks both, will have to turn the throttle up a little to get the tick over right. The base plate moves when I suck with the vacuum pump but it's not moving very quickly, either that or my vacuum pump isn't sucking very well.
Regards Terry H