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Can someone explain the MAP vacuum re. MJ


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I've got my 4.3 V8 (carbed on Eidelbrocks) running on MJ. Drived fairly ok, a little missfire under heavy load at times but mostly ok (there IS a rythimic pulsing of the engine in the mid rev range but that must be something else...)

I'm analysing my spark table which is basically a safe map from this site, and Im a touch confused as I don't totally inderstand how the carb vacuum relates to engine load.

My Map is: (that was taken with the engine at tickover)


Now, on the MJ instructions PDF it shows an example map with, on the left, an arrow going up with "Vacuum increases or throttle closes), then an arrow down with "Pressure increases or throttle opens". Advance increases with rpm and increases as Vacuum increases or throttle closes (their words).

So that tutorial seems to tell me that the higher KPA readings - at the bottom of my map, represent.the engine "under load", and the lower KPA readings higher up in the map represent the engine "coasting" ("Vacuum increases or throttle closes"). This tallies with what I've read about engines needing less advance under load and more when coasting.

So from this I assume that the KPA figure is high when the "pressure increases or throttle opens" and drops off when you back off the throttle.

This is where I get confused. On my car, at tickover, the green square floats around the bottom left of the map. Should it not be the top left as that is where it should be for "closed throttle/vacuum increase". Or is it because, revving the engine at standstill is not a "load" and you have to actually strain the engine for teh figures to be right?

When I blip the throttle (standstill) the green square starts bottom left and then , naturally goes right as the revs increase, and also it climbs up the screen (to lower KPA figues)

So it's doing kind of the opposite of what I'd expect from the MJ tutorial diagram. It's also confusing as the MJ diagram talks about both vacuum increasing and pressur eincreasing. Which is it? Do you start with a vacuum that turns to positive pressure under load?

(p.s. I'm using the non timed vac port on my carb - I've tested it, and the timed port to be safe)

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For me yout table is back to front !

I find it seems to work better with 100kpa at the top = foot hard down....and lowest number st the bottom = foot off / hard overrun.

Whilst it will work either way around seeing box going UP as foot goes down seems to keep my ocd more gappy lol

Tick over on a v8 should be around 35-60 mark...


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It's MJ, I don't think you can change the way up the table is.

But.... You have a very odd table, it only goes up to 78, should be 100. Should be around 45 at idle depending on spec of engine.... Seems to me your map house is dead, blocked or the sensor is duff.

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It depends which side of the throttle your MAP sensor is hooked up to. Is it on the engine side of the throttle butterfly or the air filter side ? You can get that kind of reversal if you've connected the sensor to the air filter side of the throttle, using the old distributor vacuum connection, for example.

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Thanks guys - I've changed the map to go up to 100 now. Idle, once the engine has wamed, settles around 65. (might be something to do with the cam I have etc - not sur ewhat I have, I know the engine is tuned to some extent.

Now when you floor it, the green box runs along the bottom of the screen, then returns in a big circle (anti clockwise) as you would expect it too. I just expected idle to be further up the screen (lower KPA) but some quick googling suggest certain types of cams can change this.

I run the vac of the non timed port on my eddy carb - i've tested the other port and it's timed (100kpa at idle, then jumps back down when you touch the pedal) - which I don't want.

I undertsand the idea of it all now. The MJ manual confused me as I though the legends on their engine map were descriping the axis, but they are not, they are explaining what causes the green bar to go in each direction (confusing, but you might know what I mean!).

Incidentally, I set the map to max at 78 as at the time, that was my idle tickover and I assumed the map went in the other direction when you floored it. My mistake!

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The way i always think about the pressure after the butterfly is that the higher the engine rpm the more air it's pulling through (related to the size of the cylinders and the number of times they go up and down right?). The more closed the butterfly is the higher it resists that air flow. So at idle with the throttle 10% there is a certain negative pressure. As you floor it and fully open the butterfly you have removed that restriction bit the rpm hasn't increased yet so the negative pressure will drop off to be close to the atmosphere pressure. Assuming the air intake is big enough as the rpm builds that negative pressure will stay low. Then you let your foot off the butterfly closes, the rpm is still high so there's still a huge demand for air but you've put a restriction in the way so the negative pressure will shoot right up. Eventually the rpm comes down and you end up back where you started.

I find if you think of it like that you can work out what it should be doing in between.

Does that make sense?

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