Blazeracer

iTrader: (2)

Hey all, I'm hoping to tap into the wealth of knowledge here. I think I'm not completely understanding how the MAP table works on the MSD. I like the idea of the computer pulling timing for part throttle cruise and starting. For those who don't know, a jet boat is a completely different animal. Cruising speed is about 4,000 rpm and the tach aproaches 7 on the hit. My start up and part throttle seem to be just fine. Plugs look a little rich, but I'm OK with that for now how hard this motor runs.

Yesterday I made a new MAP table. The boat launches hard and pulls very well IF I lift the pedal just slightly. It's pulling about 5,300 rpm at WOT and picks up a couple of hundred more if I lift the pedal to about 5,500. It didn't do this before I made the change. I'm wondering if my MAP table is backwards and taking out timing at WOT and putting it back in when I lift the pedal.

Oh, I took 2 degrees total timin out on this curve also to 34, and it seems to like it. We can run a little extra timing in a boat cause lake water keeps them heads nice and cool.

EDIT for motor specs. Stock bottom 5.7 with ARP everything. AFR 210 Mongoose heads and LG Motorsports G5X3 cam thru a Marine Demon 850 jetted at 74/80.

Blazeracer

iTrader: (2)

I guess my real question is, and I can't find it anywhere, what do the values across the top and down the left represent on the MAP chart? The timing chart is easy, RPM vs. timing in degrees.

Vacuum vs. timing removed in degrees would be too easy.

Blazeracer

iTrader: (2)

Maybe the table is measured vacuum vs. timing added? That would explain why it adds no timing when the hammer is down and adds a little if I lift the throttle a touch.

Any thoughts??

ZONES89RS

iTrader: (25)

You should be able to watch the lap top while it is running. When at idle, watch the MAP, then go open throttle, it will show you where the pressure drop direction is from losing vacuum. I cannot remember which way it goes but it is reverse from what you would think if I remember right.

Blazeracer

iTrader: (2)

Yeah, and pickle fork jet boat and a lap top.... LOL Been thinking about trying that. Don't want to wreck a lap top though. Need a way to strap it down. **** gets thrown to the back on the hit and fly's back forward when you lift. Water has a lot of drag, and it takes a ton of thrust to get em movin. Sure it fun though, no speed limit on a lake either.

89gmcs15

iTrader: (7)

Ive got a 340 hp sbc in a glastron with a berkley jet. Its fun, but would be way more fun with about 500 hp. The map sensor would help fuel economy tho!

Pop N Wood

iTrader: (1)

What you are seeing makes sense. The MAP curve measures absolute pressure, not vacuum, across the top of the graph. More throttle means vacuum goes down (i.e. higher manifold pressure) hence the box pulls timing.

You can either put some more static timing in or move the MAP curve to the right more. Carb'd LS motors like a bit more timing than EFI. 34° isn't uncommon for many of our street engines.

Blazeracer

iTrader: (2)

Before this boat I had a 78 Glastron CVX 20 with an EFI 5.3 that SCREAMED!! That boat was a blast, but still slow in comparison to this one. Sold the daycruiser to get a picklefork.

This boat was cherry. Had the best of everything in it.

Blazeracer

iTrader: (2)

Engine running, throttle blades closed, absolute pressure is negative, meaning vacuum. In fact, without a blower or turbo manifold pressure will never actually be pressure.

Numbers across the top read 0.0 to 15.0. What do they represent?

Numbers on the left read 0.0 to 65.0. It's probably safe to assume they represent timing in degrees, but is it timing added or timing taken away from the base timing?

This is probably why everyone zero's their MAP out cause they don't know what it all means. I wanna figure this thing out and understand what it all means, not just move dots around till it works.

Prolly could call MSD and just ask.. haha

89gmcs15

iTrader: (7)

The timing is added and the map reads higher numbers with less vac. Start higher on the left for adding withvac and taper down to the right with lower vac

89gmcs15

iTrader: (7)

Heres a quick vid I threw together for ya to see map readings vs vac and load. Any questions just ask

Pop N Wood

iTrader: (1)

The numbers are absolute pressure. 0 psi is a perfect vacuum, no air. It can never go negative.

Here is a link to a table converting absolute pressure to vacuum gauge reading.

http://www.engineeringtoolbox.com/va...ter-d_460.html

Just look at the psia column and the "Inches Mercury Gauge" column. Note that 14.7 psia is zero inches of vacuum. As absolute pressure drops, the vac gauge reading goes up. Closed throttle = max vac = minimum absolute pressure

Confusing but that is what happens with MAP sensors, they measure absolute pressure, not vacuum. That is what EFI needs to know to figure out how much air is being ingested. We carb guys just get to inherit their technology.

Most people zero out their MAP tables because they don't have a MAP sensor installed.

Blazeracer

iTrader: (2)

Got it, thanks. That video helped tons.

89gmcs15

iTrader: (7)

Glad it helped!