Hi,
Im looking in the future to build a full megasquirt system, but for now im going to use megajolt as an educational tool.
I have a 72 beetle with a 1600cc engine. It will have a single 32/36 weber progressive carb. There is one manifold that splits into end castings to the left and right of the engine to the twin port heads.
What is confusing me at the moment is this talk of 'runners' and what exactly i need to tap for the MAP sensor (i want to run the MAP because eventually I want to turbo this engine with EFI). Do I have to tap it in each of the bits that split at the castings where it goes to each port (4 taps), or do I just tap either side above the castings so there is two places tapped, or could I just tap one point, say beneath where the carb sits?
Sorry for the awkwardness of the question and its wording, i am learning all this rather rapidly!!
Thanks,
newbie question
Moderators: JeffC, rdoherty, stieg, brentp
Welcome.
Typically with a single carb and manifold you would get your vacuum (and boost) reference at a point below your carburetor where all of your runners converge.
If your manifold has a vacuum port for a power brake booster, this could be a great place to tap into.
Tapping into multiple runners is only done with multiple carburetor throats feeding independent, isolated runners.
Hope this helps,
Typically with a single carb and manifold you would get your vacuum (and boost) reference at a point below your carburetor where all of your runners converge.
If your manifold has a vacuum port for a power brake booster, this could be a great place to tap into.
Tapping into multiple runners is only done with multiple carburetor throats feeding independent, isolated runners.
Hope this helps,
excellent thanks, that is what I was thinking. My manifold does not yet have have vacuum ports in it, but I will get one added. Thanks for the quick reply.
With only the one line, would I still need to run it though some sort of plenum (I read a standard fuel filter could be used)?
I will be looking to source all that I need soon, I think I will buy the kit from here or a UK supplier and build it myself
I am a Linux and Mac user, I have seen megasquirt software for Linux but not seen the 'jolt software for either Linux or OSX yet, but I may be able to port it to either if it is necessary?
With only the one line, would I still need to run it though some sort of plenum (I read a standard fuel filter could be used)?
I will be looking to source all that I need soon, I think I will buy the kit from here or a UK supplier and build it myself
I am a Linux and Mac user, I have seen megasquirt software for Linux but not seen the 'jolt software for either Linux or OSX yet, but I may be able to port it to either if it is necessary?
hi...
*no you dont need a plenum if you have a single manifold fed by a single carburettor.
*a suitable vacuum takeoff is really easy to install...
just select any accessible spot on the manifold.
i drilled a 3mm hole and used a piece of 1/8" brass tube. the tube is fractionally bigger than the hole, which allowed me to lightly tap the tube in with small hammer, for a light interference fit. no soldering or gluing required. too easy!
*my feeling is that mini fuel filter in the line is probably a good thing anyway to ensure that oil and petrol vapour dont make their way up the pipe to the MAP sensor.
regards
alexander.
*no you dont need a plenum if you have a single manifold fed by a single carburettor.
*a suitable vacuum takeoff is really easy to install...
just select any accessible spot on the manifold.
i drilled a 3mm hole and used a piece of 1/8" brass tube. the tube is fractionally bigger than the hole, which allowed me to lightly tap the tube in with small hammer, for a light interference fit. no soldering or gluing required. too easy!
*my feeling is that mini fuel filter in the line is probably a good thing anyway to ensure that oil and petrol vapour dont make their way up the pipe to the MAP sensor.
regards
alexander.
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ah.. that issue crops up again!
the problem is the vacuum advance takeoff on carburettors is placed in a way that shuts the vacuum signal off at idle. it is done by taking the vacuum from a small hole which is covered by the edge of the butterfly valve when your foot is off the pedal.
that means there is zero vacuum on the vac advance takeoff at idle, when there is, in fact, moderately high vacuum in the manifold.
ie from the vacuum advance takeoff...
idle zero vacuum
just off idle moderately high vacuum
more throttle vacuum falling again
full throttle zero vacuum again
the discontinuity between idle, and just off idle is difficult to deal with in the ignition map, so it is better to just take vacuum downwind of the throttle ie somewhere in the manifold.
fyi, the vac advance is done like that to reduce advance at idle, to make it easier to achieve a stable idle speed. you can test if your carby is like this: put you finger over the end of the vac advance hose. can you feel vacuum at idle? probably not. can you feel vacuum just off idle? probably yes.
and.. to answer the specific question, weber DCOEs as in my photo, have no vacuum advance takeoff at all, as it is just supposed that sports cars dont need the part-throttle economy that vacuum advance is supposed to achieve.
regards
alexander.
the problem is the vacuum advance takeoff on carburettors is placed in a way that shuts the vacuum signal off at idle. it is done by taking the vacuum from a small hole which is covered by the edge of the butterfly valve when your foot is off the pedal.
that means there is zero vacuum on the vac advance takeoff at idle, when there is, in fact, moderately high vacuum in the manifold.
ie from the vacuum advance takeoff...
idle zero vacuum
just off idle moderately high vacuum
more throttle vacuum falling again
full throttle zero vacuum again
the discontinuity between idle, and just off idle is difficult to deal with in the ignition map, so it is better to just take vacuum downwind of the throttle ie somewhere in the manifold.
fyi, the vac advance is done like that to reduce advance at idle, to make it easier to achieve a stable idle speed. you can test if your carby is like this: put you finger over the end of the vac advance hose. can you feel vacuum at idle? probably not. can you feel vacuum just off idle? probably yes.
and.. to answer the specific question, weber DCOEs as in my photo, have no vacuum advance takeoff at all, as it is just supposed that sports cars dont need the part-throttle economy that vacuum advance is supposed to achieve.
regards
alexander.