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MG MGB Technical - Manifold vac questions

I hooked up a vacuum gage to the manifold, and did a few quick errands, noting when there was vacuum and how much. I was looking for the info for tweaking my timing, but it also relates to the PCV discussion.

On a 18V672 stock LC engine, 2 HIF4's with functioning poppet valves, I get the following vacs (in of Hg)

Idle - 14
Accelerating at all - 0
Decelerating (engine braking) - 20
Flat Cruise @ 60mph - 4
Flat Cruise @ 70 - 0

In other words, if everything is working correctly I only get vac advance at idle, overrun, and something just slightly shy of overrun.

But does that also mean the PCV is really only drawing at idle/light throttle? Wouldn't this be the time it's least necessary?

Steve
Steve Aichele

Hi Steve.

I have only recently connected my vac gauge (to a 'T' on the distributor vac feed), but I seem to get a lot higher vac figures than the ones you list.
What do other members usually see ?.

Don
Don

It depends on exactly where in the system it is "seeing" the vaccum signal from. PCV connection to HIF? Manifold in balance tube? Advance tap on carb? All will be different to a greater or lesser degree (and engine condition/state of tune will change that too!).

Mike!
mike!

This was a connection to the manifold, right next to the vac advance take off.

So how much vacuum should I be getting? The car seems to run pretty well, if maybe a little on the warm side.

Steve
Steve Aichele

Steve,
Vacuum is at it's highest at idle and de-acceleration. If you are trying to set timing via vacuum,,,,, advance your distributor to the highest vacuum reading and then retard it by three degrees that should be the hot set-up. It's worked for more years than I'll admit to. Hence if you are shooting for fuel economy hook a vacuum gauge and strive for the highest readings all the time.

Roy
Roy Dougherty

Steve,
I said retard your distributor by three degrees....What I meant to say was retad it by three inch pounds on the gauge.
Roy
Roy Dougherty

Steve,

It is a popular misconception that the PCV provides vacuum to ventilate the crankcase.

It actually LIMITS the vacuum so that you only get a low level of vacuum in the crankcase - only an inch or so.

When there is less vacuum than this available in the manifold then crankcase pressure will still cause the fumes to be carried into the manifold.

The later type of ventilation, where the chest cover connects directly to the carbs via a pipe entering the carb in front of the throttle plate, never sees more than 1 or 2 inches of vacuum.

Absolute levels of vacuum in the manifold depends on the carb setting and type, the camshaft being used and the condition of the valve seats - on engine overrun the figure should be around 20 inches - carbs with overrun valves will show less than this.
Chris at Octarine Services

The poppet valves limit max vac to about what you say. The crankcase ventilation system is connected to the constant depression area of the HIFs, which is as the name indicates - constant depression. That means vacuum, not your state of mind when you're trying to make your car work! "Constant depression" is how SU and ZS carbs work, and is achieved by airflow balancing against the vacuum chamber piston/spring. It's also where the "CD" in the ZS model number comes from.
Early SUs don't have the CD pipes. The CD connection was instituted precisely because, as you thought, manifold vacuum goes away when the throttle is open, leading to nasties escaping. Formerly the fumes were just sent to the general area of air intake, whence they went into the engine and were burned if they felt so inclined.
FRM
http://www.usachoice.net/gofanu
FR Millmore

Thanks. The PCV setup makes more sense. I'm curious about this idea of using the vacuum to set the timing. As you advance you'll get somewhat higher RPMs, which should result in higher vac (up to either stall or the poppets open). And would you do that with the vac advance hooked up or not?

Thanks
Steve
Steve Aichele

A vacuum gauge is a very interesting and usefil device for tuning, fault finding and driving and one not heard of too much these days. Normal idle vacuum is 21hg but it is normally accepted that 17 - 21 is acceptable there are all sorts of things to look out for. Have a look at http://www.centuryperformance.com/vacuum.asp which will provide you with interesting.

Now only a few weeks ago someone on this site said that a vacuum gauge could not be used with certain tuned engines. As far as I am concerned the jury is out on this one and I'd be very interested to hear all your comments. The vac gauge in conjunction with a carb balancer is ideal for adjusting carbs. I'm not so happy with it when adjusting ignition. The method is to set the engine to idle and then gradually advance the engine until the gauge needle just starts to kick. Then retard by one and a half hg. This is not precise enough and usually results in too much advancement necessitating the need to retard to avoid heavy pinking.
As a guide both my Bs are standard, new condition and idle at 18hg on a steady needle a figure I'm not entirely happy with but this may be a good baseline to aim for.
Iain MacKintosh

This was VERY informative.....I have a vac guage fitted on my 78 B (with weber 38/38/Peco/new cam/valves/head job/euro dizzy etc etc..."ignition miss" at idle is what I appear to suffer from with a bouncing needle at idle just like in this article.....SO....why am I suffering a miss...it runs great off the line...but any electrical load seems to stress the ignition....battery reads 13.5 volts with engine running.....something in the ignition circuitry??????????????????? anyone?
P J KELLY

I don't have much bounce in the needle. Each of those readings is pretty solid, maybe fluctuating within a range of 1 inch or so. If you're getting a lot of fluctuation at idle, it seems like maybe something in the valve timing or lift. If it's not a normal vacuum leak, it could be a drop in the draw into the engine. If you had a relatively large lag between the time one intake closed and the next opened, your vac would drop dramatically. Don't know exactly how that plays out in terms of driving...

Steve
Steve Aichele

Can't see why a vacuum gauge *can't* be used with tuned engines, it is just a closed off, narrow bore, length of tubing after all. It may not give the same readings as an untuned engine, but that is something different.

Factory timing errs on the conservative side to take account of manufacturing tolerances. This means that whilst some engines cannot take additional advance without pinking, most will be able to. I have had a Redex gauge since the 70s and used that for setting-up a BL A-series engine, much as Iain says although only backing off 3/4" Hg, which gave noticeably improved acceleration and economy over the factory figures. While the car was under manufacturers warranty I had it dealer serviced, and it always came back running like crap until I reset the timing. I tried it once on the MGB but it resulted in the same timing, so I haven't bothered since. I have found it no different to using lifting pins and the MkI ear when setting up carbs, so never bothered with that. Steady speed cruising should be giving you a vacuum almost as high as idling, sometimes higher, that is economy driving.

You should be getting about 14.5v at the fuse box at a fast idle with minimal load, dropping only to about 13v with maximum factory loads. A low starting voltage, coupled with ignition problems as electrical laod increases, both indicate to me that there are bad connections in a common point feeding the ignition and the loads. If headlights are causing the problem this is almost certinly at the solenoid. If headlights are OK but things like brake lights are the problem, then the brown feed to the ignition switch, the switch, or the white feed from it.

There are many different 'bouncing needles' shown on my gauges instructions, all with different causes. But if you have a miss as well, then you need look no further with the gauge.
Paul Hunt

Think you may be right Paul as I can never remember exactly what to back off as 4cyl and 6cyl are different. I'm not sure where the comment came from re the use with tuned engines but it was on this BBS three or four weeks ago. I know that 17-21 is the figure to aim for buit both mine read a steady 18 on idle which I'm not 100% happy with as I've always achieved on other cars. I felt that it may be something to do with the valve overlap but maybe if I timed the cams accurately I would get the higher figure. Its hardly worth the bother though of hauling the rad and everything out when the cars both run very well.
Iain <acKintosh

This thread was discussed between 07/09/2004 and 11/09/2004

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