Electronic Ignition - carb ported vacuum versus manifold vacuum?

Technical advice Q&A

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BT7
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#1 Electronic Ignition - carb ported vacuum versus manifold vacuum?

Post by BT7 » Wed Aug 02, 2017 3:11 pm

I know this is an XKE Forum but I expect that you guys have dealt with this issue too. I have a 1964 MK2 3.8 MOD 8:1 compression. I am installing a 123 ignition JAG 6-R-V unit. I have read as much as I can find about the subject of maximum advance. I plan to start my engine in the next few days. The engine was run-in briefly (not on a rolling road) with the 123 unit with no vacuum connected following its rebuild. It is currently on the number 2 clickable setting. Now the engine is back in the car. New coil, new plugs, new ignition wiring etc.

My question revolves around the subject of the source for vacuum advance. My engine rebuilder has come to the conclusion that it is manifold vacuum that should be used rather than carb vacuum, although the 123 installation instructions specifically indicate the use of carb porting.

I have done some follow-up research and I found one article that advocates for the use of manifold vacuum to be pretty convincing. I have attached the article for your review.

I am interested in thoughts and comments from the group.

Lin

http://chevellestuff.net/tech/articles/ ... nifold.htm
1959 AN5 Austin Healey Bugeye Sprite
1960 BT7 Austin Healey 3000
1964 Jaguar MK2

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bobbayers
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#2 Re: Electronic Ignition - carb ported vacuum versus manifold vacuum?

Post by bobbayers » Fri Aug 04, 2017 9:17 pm

I just had my American Series 2 E Type converted from vac retard to vac advance. British Vacuum Unit in New Hampshire USA . I sent my front carb to them to drill the vac port. It is CRITICAL to place this directly above the butterfly. I would have assumed that your Jag would already have this since it is an earlier Jag. See BVU´s website for full explanation:
https://www.britishvacuumunit.com/ported-vacuum.html
Robert Bayers

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#3 Re: Electronic Ignition - carb ported vacuum versus manifold vacuum?

Post by Woolfi » Fri Dec 21, 2018 1:35 pm

Roberts remarks about this are exatly 100% correct.
If you use manifold vacuum, you have max vacuum at idle and therefore maximum advance at idle. If you have a 16 degreee capsule, you would have 10 plus 16 = 26 degree at idle. You would have a stumblingidle and therefore a very bad smell out of the exhaust, because of unburned HC.
Why do I know ? Because I had this for three years when I bought the car. I brainless car mechanic had mounted a advance capsule and connected to manifold vacuum. After I had drilled one carb and connected the capsule to "butterfly-vacuum", the motor run very good at idle, at part-throttle and at WOT.
Regards Wolfgang Gatza

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Tbob
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#4 Re: Electronic Ignition - carb ported vacuum versus manifold vacuum?

Post by Tbob » Fri Dec 21, 2018 4:34 pm

Woolfi wrote:
Fri Dec 21, 2018 1:35 pm

If you use manifold vacuum, you have max vacuum at idle and therefore maximum advance at idle. If you have a 16 degreee capsule, you would have 10 plus 16 = 26 degree at idle. You would have a stumblingidle and therefore a very bad smell out of the exhaust, because of unburned HC.

Regards Wolfgang Gatza
This topic has been batted about on car forums for years. Wolfgang s explanation is one of the clearest and most succinct i have ever seen posted.
Well done! :thumbsup:
Bob t

LHD '69 OTS. (Former) basket case

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Tom W
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#5 Re: Electronic Ignition - carb ported vacuum versus manifold vacuum?

Post by Tom W » Sun Dec 23, 2018 9:12 am

This was discussed at length on here earlier this year.

http://forum.etypeuk.com/viewtopic.php?f=3&t=12885

Basically, if you have a standard distributor that was originally connected to a ported vacuum source, it should be connected to a ported vacuum source. The curve won’t be right if you connect it to a manifold vacuum source.

If you’re going to develop your own ignition advance curve, including the vacuum curve, then manifold vacuum is better. This is because it gives you independent control of the advance at idle/low rpm for both throttle closed and throttle open conditions.

Once the throttle is opened slightly, both systems operate the same, though the level of vacuum generated at each point might be different, so an appropriate vacuum advance canister will need selecting to suit.

Ported vacuum was developed to meet an emissions target that was measured at idle, whilst maintaining performance elsewhere in the rev range.
Tom
1970 S2 FHC

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christopher storey
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#6 Re: Electronic Ignition - carb ported vacuum versus manifold vacuum?

Post by christopher storey » Sun Dec 23, 2018 11:17 am

The front carburetter of your ( presumably ) 2 HD6 carburetted engine should have a spigot between the bellchamber and the throttle flange. This will either be a thin mushroom type for later cars with plastic vacuum tubes, or a larger screw on fitting for the earlier copper vacuum tube . I cannot recall any XK engine ( certainly prior to the later XJ6s ) which had manifold vacuum

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PeterCrespin
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#7 Re: Electronic Ignition - carb ported vacuum versus manifold vacuum?

Post by PeterCrespin » Sun Dec 23, 2018 12:52 pm

Many of the Yank S2 emissions cars had manifold vacuum AFAIK, Chris.

Given several decades of reverse osmosis* of E-Types back across the Atlantic, the chances of coming across this set-up are probably getting similar in the UK.

Pete

* The phrase should really be diffusion not osmosis in this case, but you never hear that said.
1E75339 UberLynx D-Type; 1R27190 70 FHC; 1E78478; 79 S2 XJ12L; 97 XJ6L

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#8 Re: Electronic Ignition - carb ported vacuum versus manifold vacuum?

Post by christopher storey » Sun Dec 23, 2018 7:08 pm

Peter : yes indeed, but this is a Mark 2 from 1964 , and I don't think ( wouldn't be dogmatic ) that emission controllery had crept in by then, even though California had started to see a problem . I'll have to look in my SU catalogue when I get a chance

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#9 Re: Electronic Ignition - carb ported vacuum versus manifold vacuum?

Post by PeterCrespin » Mon Dec 24, 2018 5:20 pm

Yes, sorry, I forgot it was a Mk2. I was mainly prompted by your ref to nothing before 'later' XJ6, which would mean 1974 or 1979, depending whether you mean the respective starts of Series 2 or Series 3. But with no reverse flow of saloons back from America comparable to the E-Type wave, I withdraw my comments regardless. Regards to Tony if you see him.

Pete
1E75339 UberLynx D-Type; 1R27190 70 FHC; 1E78478; 79 S2 XJ12L; 97 XJ6L

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