vacuum for distributor...where?

[Bud English]

Not to hijack Bob's thread but that Europa site link is brilliant! A lot of work was put into assembling that. Thanks for posting it Roger.

[CBUEB1771]

Vacuum advance
Begins at 5 inches Hg
All in at 17 inches Hg
advance: 22 degrees crank angle (11 degrees distributor)

Bob,
I just found a very informative web site:

https://www.britishvacuumunit.com/Lucas ... nit_ID.php

Click on the vacuum unit ID tab and find Lotus applications near the bottom of the list. These people say the S3 and S4 Elans and Elan +2 and +2S from 1969 to 1975 use the 4-6-4R unit, action starts and 4" Hg, ends at 6" Hg and produces 4 degrees of distributor retard. I can't verify this independently but I'll keep looking. It is one of the very few vacuum units that retards spark rather than advancing. I'm sure I'll be flamed by Mr. French over this.

[rgh0]

22 degrees of vacuum advance max sounds to much to me given the mechanical advance of 20 degrees and staic timing of 10 degrees also but maybe it will work. 50 degrees total advance cruising on light throttle at 3500 rpm is to much I think . You will also however need the right port connection on the carbs to not have the vacuum advance come in at idle otherwise you will have much to much advance at idel.

[dpo#4]

Hi, guys-

A few cents worth of past experience. I've also been looking at Pertronix on Ebay. New bushings, new shaft, no points are pluses. Wonder about the cap material. Vacuum advance is a real plus to me. The Elan mechanical advance curve was originally optimised for full W.O.T. acceleration, and ignored part throttle advantages because Weber dcoes had no "ported vacuum" takeoffs drilled just past the leading edges of the throttle plates. Full manifold vacuum applied at idle to the above mentioned advance unit would give 32 degrees (crank) at idle. Base idle speed would be quite high, maybe 1400-1500 rpm? Backing off the idle speed screw to get normal idle then moves the throttle plates away from the transition holes. I've seen every kind of car come in running at 30+ degrees, either through misrouting of a vacuum hose, or because (when cars had adjustable distributors) someone had timed it "by ear", meaning they had twisted the distributor to achieve the highest idle speed. This latter move did help cover a lot of poor idle issues such as flooding through the main system or big vacuum leaks, but detonation was inevitable. In the case of simple misrouting, there usually wasn't much bad effect once someone lowered the idle because 70s and 80s carbureted cars were pulling air through one 28mm primary choke, not four Weber throats.
I'm contending that any mechanical advance only engine, Lotus twincam included, could run better with an additional load compensated advance curve. Granted, Lotuses run pretty darned well as they are, and any such change requires great care - even light detonation has split the head gasket fire ring on an old Mazda truck I had - but it can be done. After all, tuning mechanical and vacuum advance values is just old style mapping. This is how we did it back in the 70s before I had a Lotus.
The car was a Datsun 510 (Bluebird to many), 1752 cc, SSS head. It was a street rod, 4.375 r&p, dual 40 dcoe. Mechanical advance was 14 degrees initial, perhaps 38-40 total (forget how much the cam slots allowed, exactly), all in at 2500 rpm. Tweaked the springs and graphed the various results with a tach and timing light. This recalibration made a huge difference! It convinced me that on-the-car calibration which includes all the backlash, wear, and harmonics is the way to go. The vacuum advance would never work with the individual runner manifold, as the severe vacuum pulses jerked the movable point plate on every intake stroke, giving each cylinder different timing. Well, it did work here as I'd previously (longer story) had the runners tapped and installed fittings to merge four vacuum hoses at a soldered 4 into 1 miniature header. Plastic tees weren't common back then. As for the ported vacuum signal, no, I didn't drill holes in the Webers! Only Keith F. could ever pull that off - I subscribe to his Yahoo group, and the guy is brilliant, if driven. The 1970 510 had a cam operated microswitch at the firewall, used to sense throttle movement away from and back to idling position. The original function was to alter timing at the dual point distributor, which had point assemblies with up to 10 degrees adjustable offset. We all ran just one set anyway, so the switch was used to control a vacuum solenoid that was from the old Hitachi carb. And it worked. Part throttle response was improved. Fuel consumption, maybe. We were young.
Our method could have been refined. A small vacuum plenum or perhaps a Ford plastic vacuum delay valve could dampen the sudden action of a solenoid. Maybe just a longer hose for volume. Additional tuning involves spring rate, preload, and total travel, which we never did. The Hitachi unit had a removable compression spring, held onto the arm by an e-clip, so more washers could be fitted for a lowered curve. Domestic U.S. Ford distributors had an allen (internal hex) adjustment screw in the vacuum advance unit. The hex key went into the suction tube and you were allowed up to 4 turns counter-clockwise(?) to mitigate part throttle detonation. Now we have $$$ knock sensors buried under V6 intake manifolds. Rant slowly subsides.
Maybe this belongs in Mods. But ported vacuum (starts sucking just off idle) could improve daily driveability at fairly low cost, since an unworn distributor comes attached to the vacuum unit. In fact, the original centrifugal advance curves have been rendered obsolete by lower octane, altered fuel burn rates, milling/boring/rod length and angularity changes, cam substitution, etc. In racing, a movable point plate is an evil thing, as timing scatter is certain at high rpm harmonics. For road use, even a hemi chamber could use a head start with less dense mixtures. The physics is there. Lowered heat rejection is the same as increased thermal efficiency. Less fuel gets burned to push the piston crown if it's lit before lots of heat sucking cylinder wall is exposed.
And no connection to Pertronix, or to the International Vacuum Advance Manufacturers Cartel. . Any comments as I duck for cover? Aloha to all - Ray

[rdssdi]

rgh0 I have to agree. At cruise with light throttle therefore vacuum in the manifold certainly could give 50 degrees total advance.

This is why I asked so many people many questions. I must assume that the Director of Engineering for Pertronix can add. He approves the vacuum advance distributor for the TC engine. The variable would be where to get the vacuum so that it will not be full vacuum advance at light throttle cruise. If that is even possible.

This appears to be a question without a definitive answer. A chassis dyno would answer many of these questions but that is far beyond my ability to procure. It certainly would answer the total advance at load light throttle cruise. Maybe if I had a vacuum instrument in the cabin I could attach it to the vacuum port rather than the distributor and record vacuum readings at various load and throttle positions. That could then be extrapolated to the amount of vacuum advance once connected to the distributor.

Bob

[RogerFrench]

[ I'm sure I'll be flamed by Mr. French over this.

Why? That's absolutely correct - thanks for posting!

[RogerFrench]

Bob, If you're going to connect the vacuum advance then I agree with Rohan - direct manifold connection isn't the way to go. And, as you know by now, there isn't a vacuum advance port on the Federal Z-S carburetter.

What there is though, on each carb is a port for the evaporative loss control system. This is a throttle plate edge-drilled port, just like those found on vacuum advance carbs. You can see them on the photo of my Europa carbs - they're the brass plugs just inboard of the depression chamber covers. It would be very easy to try either one of those.

Europa Carbs

[rdssdi]

Roger. I looked at my carbs and while they have the threaded hole as you describe they are not fully machined into the body of the carb. See photo.

It appears to me that Pertronix developed a Lucas style replacement distributor for many brit engines. MG, Ford crossflow etc. On the crossflow and possibly MG and others they came with a vacuum advance. It may have been common practice with the emission engines for these makes other than Lotus. I know the 1968 crossflow engine in my TVR had a FoMoCo distributor with a vacuum advance. It very well may be that in an effort not to run afoul of the gub'ment they offer a vacuum advance unit which can be substituted for an original vacuum advance distributor and not void the emission system.

As the Lotus TC uses the same distributor it was a simple matter of adding a side entry cap and cataloging it for the TC engine without any knowledge that Lotus did not use a vacuum advance on the TC engine. This is in line with what the tech person at Pertronix said. He told me that it is a generic distributor for Brit cars using the Lucas distributor. My assumption that as the Lotus TC "street" vacuum advance distributor by virtue of a unique part number would translate to a unit tailored for this application is false. Therefore it appears I am wasting my time. While it is true that a vacuum advance distributor with a properly tailored advance curve and correct vacuum source would deliver a better running cooler engine, as Lotus did not offer it I am left with engineering this rather than installing it and go.

I do love a challenge but this may be beyond my capabilities as the curve and advance provided may be wrong for the TC as well as the issue of finding the correct vacuum source. Pertronix considers this distributor correct for the TC engine based upon the fact that they have not received complaints rather than a proper chassis dyno testing.

That is the way I presently see it. It appears that I should run the distributor with the vacuum not attached and consider a better running cooler engine a fantasy. I have little doubt that Lotus used a centrifugal advance only due to its simplicity, performance over drivability and probably lower cost.

Bob

[rdssdi]

I looked at the Lucas vacuum Advance page posted by CBUEB1771. It shows for 1969 Lotus Elan +2 41225 (distributor number) 54415328 (vacuum capsule number) 2-6-3 ( advance parameters). My original distributor says 2-4-3 (advance parameters) 54420293 (capsule number) with a distributor number of 41225A. That would translate for listed distributor- start advance at 2 inches of Hg finish advance at 6 inches of Hg and have a total vacuum advance of 3 degrees at the distributor or 6 degrees at the crank.

My original distributor markings translate to start advance at 2 inches of Hg finish at 4 inches of Hg with a total vacuum advance of 6 degrees at the crank. Very close.

There goes the theory that Lotus only used a vacuum retard capsule. According to the page aforementioned a retard capsule would have an R suffix in the parameter code. So another bit of information. It certainly looks like a 22 degree at the crank total vacuum advance in at 5 inches Hg and finish at 17 inches of Hg is a long way from what Lotus used.

What also confuses me is the Lotus workshop manual drawing of the emissions system. I cannot determine where the one hose that starts at the idle retard valve goes. there also appears to be a plastic tee with a clamp at the right of the drawing that I cannot determine where and what attaches to it.

Bob
Now thoroughly confused.

[rdssdi]

Just noticed label on air box. It states: Engine idling speed 950 rpm, timing static 12 degrees BTDC Idling (with suction RETARD operating 7 degrees BTDC. So the capsule , while not having an R suffix, is indeed a vacuum retard. So it now appears that Lotus did only use a vacuum retard on the TC.

So I will use the distributor with no vacuum and a 10 degree idling timing. I wish Pertronix knew what they were doing.

Bob

[CBUEB1771]

So the capsule , while not having an R suffix, is indeed a vacuum retard.

Bob,
We all should have turned a couple more pages in the workshop manual. The function of the idle retard valve and its interaction with spark timing is present on page 50 for Section L. Here it states that a vacuum retard capsule is fitted to the distributor. The vacuum retard valve exposes the diaphragm in the retard capsule to inlet manifold depression only when the throttles close. Under other conditions the retard capsule is vented to atmosphere and therefore has no effect on spark timing. It seems the intended effect is to retard the spark to reduced emissions on the overrun. Given this description you would probably be best off not connecting the capsule on your Pertronix distributor to anything. The link I posted with the vacuum advance/retard applications is interesting in that there is a very strong correlation between applications of Stromberg CD carburetors, manufacturing dates between 1969 and the late 70s and specification of a vacuum retard. The earlier applications and without Strombergs are almost exclusively with vacuum advance.
Russ

[British Vacuum Unit]

this may help,
Lotus distributor number 41225 OE vacuum unit.
After a little research and digging, the OE unit for the 41225 is 54420293 with retard of 2-4-3.
Lucas superseded it to 54415328 2-6-3 when they ran out of the OE units.
We do have OE 54420293 2-4-3 units in stock. If you need any unit, we usually have all OE units in stock.
Thanks, Rob
British Vacuum unit
http://britishvacuumunit.com
[email protected]
603-783-0566