The V8 Owners Forum -

I'm going megajolt with a MAP sensor. I've gto two choices as I see it.

a)

The red bolt that's blanking some kind of hole




b)
Then at the other end:



I've got that little "blanked off with a bolt" 10mm pipe.

Are they both vacuum ports? is there a better one to use?

I notice in triggerwheels they offer 1/8th npt threaded barbs which look remarkably like the same thread size as the big port with the red bolt in it.

I would not use the ports on the manifold, I'd use one of the small ports on the front of the carb. If you look very closely at the two ports you will see that one is slightly lower than the other. The low one is open to manifold vacuum all the time, the other port is 'timed', basically this means that it is not subject to vacuum at tickover but as the throttle starts to open it then 'see's' the manifold vacuum. I won't know anything about Megajolt system so I can't tell which port to use. I use a programable MSD system with a MAP sensor, my manifold vacuum at tickover is just about the same as the vacuum at cruise RPM so I use the timed port as this prevents the MSD from adding a load of ignition advance at tickover. It's worth getting this sort of stuff spot on along with getting the Eddy carb setup correctly, my mate runs a stage III 4.6 in his SD1, it runs the same carb and MSD setup that I run, on a recent run at 50-100 MPH he got 33 MPG!!! We reckon that we could get 37 MPG if he could keep his lead foot off the throttle! We both run alot of advance at idle, around 18 degrees, the advance curve is quite agressive, we run around 30 degrees all in at 2750 RPM. If you are running a 3.5 engine the figures that you want will differ.

One other thing, the rubber blanking plugs that cover the ports have a habbit of cracking up so its worth checking them!

as sidecar says lower one on the front of the carb is the one to go for, run my MJ there no problem at all

Do you mean the rubber blanks on the left and right of this shot?

Sidecar uses the timed upper one, and JSF55 uses the lower one.

Most vacuum advance takeoff ports are designed in such a way that vacuum is measured through a hole, which is actually covered by the edge of the throttle plate at idle. That is called Ported Vacuum. That is done to cut off all vacuum advance at idle, which in turn slows the engine and makes it easier to achieve stable idle with the tiny throttle plate opening used at idle. Vacuum measured on such a port, therefore, has no vacuum at idle, moves to maximum vacuum just off idle, and then decreasing vacuum as the throttle is opened further. The discontinuity just off idle makes such a port awkward to deal with on the load map.

You can test whether you have ported vacuum or not: pull the vac hose off the vacuum advance unit. With your finger over the end of the hose, you should feel vacuum with the engine revved above idle. Assuming yes, let the engine speed drop back to idle. Is there still vacuum? If not (most likely), that means you have ported vacuum, and should find another source of vacuum, somewhere on the manfold.
From here
http://www.autosportlabs.net/MJLJ_V4_ve ... tion_guide

Nice info. I understand the difference now.

To confirm, the rubber capped little pipes either side on my photo are vac ports?

And Sidecar uses the top left timed port with his system to avoid too much advance at idle and and JSF uses the bottom right port on your / his Megajolt system ?

Just want to clarify as you both seem to use different vac sources.

I'm confused because JSF says above "as sidecar says lower one on the front of the carb is the one to go for, run my MJ there no problem at all" ... But sidecar said he uses the upper one.... That's what is really confusing me.

With your MJ which one are you using?

For MJ you should use the lower one.

Sorry for the confusion ! Sidecar did say he used it for his MSD tho, not a MJ . Lower right is the one and cap off all the others

Thanks - was a touch confusing! - but I understand the reasons now too - which is always preferable. I'll look forward to hooking up the MJ to the vac BEFORE i've installed anything and look at my vac readings (a complicated way to see them without a vac gauge!)

Personally I'd say that you will have to do some tests to determine which is the best port to use. If the my system knows not to apply vac advance at idle then you can use the non timed port but if it does not have this function then the timed port maybe the best one to use. Basically I would not just follow everyone else, you need to understand how your system works then decide how best to use it on your engine, use a vac gauge and take some readings from both ports at idle and whilst cruising on a light throttle at 40 - 60 mph. The vac reading of the timed port should be zero at idle. You could also take readings as you start to crack the throttle open when accelerating, this is where the standard Lucas vac canister does not work well with an eddy carb. The vac system adds advance and the engine can kick back if you are running a decent amount of static and bob weight advance.

Finally you need to decide what sort of timing figures you want to run, I would not just set your new system up with the standard Rover factory figures as they are not very good in my experiance! People on the forum can give you some starting point figures but we will need to know a bit about you engine and car. Things like the size of the engine, the compression ratio and the spec of the camshaft are important. The size and weight of the car is also usual to know.

The MJ needs full manifold vacuum to work properly. If you use ported vacuum the unit will think you have floored the throttle at idle and will pull the timing back and will be totally confused. The MJ has all the facilities to increase timing to suit the engine load, revs and timing at cruise for economy also, when using full manifold vacuum. This was one of the reasons I changed to MJ from my MSD programmable unit as my particular MSD did not adjust timing for load and for light load cruising.

If you are wanting to run carbs I believe the MJ unit is the way to go but That's just my opinion from personal experience.

Regards Denis

DEVONMAN wrote:The MJ needs full manifold vacuum to work properly. If you use ported vacuum the unit will think you have floored the throttle at idle and will pull the timing back and will be totally confused. The MJ has all the facilities to increase timing to suit the engine load, revs and timing at cruise for economy also, when using full manifold vacuum. This was one of the reasons I changed to MJ from my MSD programmable unit as my particular MSD did not adjust timing for load and for light load cruising.

If you are wanting to run carbs I believe the MJ unit is the way to go but That's just my opinion from personal experience.

Regards Denis

MJ is not my thing so Im just interested here.....

If the total timing at tickover is a combination of the static timing and the timing that the vac system has added then if you floor the throttle at tickover won't the MJ pull out a load of ignition timing which will leave the engine running with hardly any ignition advance?

Just for info the MSD that I run is the fully programable 6530 system which has many features including being able to program a MAP system. The total advance at tickover is set to whatever I set it to...around 18 degrees, the RPM advance system will start to add timing from 1000 RPM, the MAP system will add up to 14 degrees on top of what all the other systems are adding but only when the timed port is pulling around 17 inches of mercury.

I know that drag racing is a big feature on this forum, the MSD system also has a burn out rev limiter, launch control and a retard system for when the NOS system is running. (I use this last feature on the road).

http://www.msdignition.com/Products/Ign ... 6AL-2.aspx

sidecar wrote:

DEVONMAN wrote:The MJ needs full manifold vacuum to work properly. If you use ported vacuum the unit will think you have floored the throttle at idle and will pull the timing back and will be totally confused. The MJ has all the facilities to increase timing to suit the engine load, revs and timing at cruise for economy also, when using full manifold vacuum. This was one of the reasons I changed to MJ from my MSD programmable unit as my particular MSD did not adjust timing for load and for light load cruising.

If you are wanting to run carbs I believe the MJ unit is the way to go but That's just my opinion from personal experience.

Regards Denis

MJ is not my thing so Im just interested here.....

If the total timing at tickover is a combination of the static timing and the timing that the vac system has added then if you floor the throttle at tickover won't the MJ pull out a load of ignition timing which will leave the engine running with hardly any ignition advance?

Just for info the MSD that I run is the fully programable 6530 system which has many features including being able to program a MAP system. The total advance at tickover is set to whatever I set it to...around 18 degrees, the RPM advance system will start to add timing from 1000 RPM, the MAP system will add up to 14 degrees on top of what all the other systems are adding but only when the timed port is pulling around 17 inches of mercury.

I know that drag racing is a big feature on this forum, the MSD system also has a burn out rev limiter, launch control and a retard system for when the NOS system is running. (I use this last feature on the road).

http://www.msdignition.com/Products/Ign ... 6AL-2.aspx

Unlike the MSD you are using, my MSD only had the capability off setting the ignition curve against revs and retard for boost. Sound like yours is a big improvement on what I was using.

The MJ won't pull the timing when you floor the engine as you can simply set any advance for any rev point/load.
With a lot of time and effort the timed port could possibly be used but because it's effect varies wildly compared to manifold vacuum it would be very complicated to set up given it's effective window is mainly at light throttle. Using the timed port, the MJ would see no significant vacuum at idle which would further complicate the set up.

Is there a reason why you don't use full manifold vacuum with your MSD and set things accordingly?
Regards Denis

Hi Denis,

I can not use the non-timed port because the MSD will add advance whenever the MAP sensor 'see's' vacuum, the level of vacuum and the amount of advance can of course be programmed in via a laptop with some nice, easy to use software. The problem that I had was that the idle vacuum was around 17" and the cruise vacuum was also the same figure, this means that when the engine is idling the MSD if connected to the non-timed port would think that it needs to add a load of ignition advance because the engine was in a 'cruise' state. (The RPM curve and the load curve being independent 'things' within the MSD).

In fact when I say 'add timing' the MSD works exactly the opposite way round, the dizzy is statically set with as much timing as you would ever want, it is also locked out so that it does not have any bob-weight movement at all (46 degrees being the all-in plus vac in my case). The MSD then 'adds' retard. So for example if I want 18 degrees at tickover I would program in to add 28 degrees of retard, 46-28 =18. The MSD has a very fine scale to operate in so you can do things like add even more retard from 0 to 500 RPM which gives the starter motor an easier time. You can then ramp up the timing over the next 100 RPM and hold that figure (600 -1000 RPM) so that this is the idle timing. You can then program a curve with as much detail as you want after that, for example I Actually run 28 degrees at 4800 RPM but then ramp in another 4 degrees over the next 200 RPM. (I do this because I 'believe that the VE must be dropping like a stone above 5000 RPM, this means that the cylinder pressure will be lower and therefore the charge needs to be ignited earlier in the compression stroke)....This is all just theory by the way! Independent of all of that the MSD can 'add' another 14 degrees of ignition whenever the timed vacuum ports is under high vacuum.

Cheers,

Pete

Hi Pete.
Thanks for the explanation on how you use the MSD unit. It's pretty much as I had my MSD set up previously except my unit didn't have the facility to add the 14degree advance at cruise. Otherwise the locked dizzy etc was the same with the MSD pulling the advance back for idle and lower revs.

I chipped in with my comments as the OP said he was using the map sensor version of the MJ and I felt that anything other than full manifold vacuum would complicate the situation and contradict/confuse all the user guide installation instructions.

Regards Denis