If my engine idles @800 (with a carb) at around 14-15 inches vacc, that is in the 50 MAP neighborhood...

I didn't look at vacc at higher rpms before I took it apart to do the EFI conversion. I'm pretty sure I'll get a little higher vacc at somewhat higher rpms.

The question is what values do I put in the lower MAP cells? Should I match the cells from the idling MAP column, or taper them up slightly? Is there a post on this? I've read about other engines idling at around 70 MAP, but I didn't find what was done to the tables.

Thanks

I think I know the answer to what you are trying to ask. If you are asking about what to put in the VE cells which reside at a lower MAP than idle, that is Decel/Over-run. You will taper them down slightly as the engine needs less fuel during Decel as it pulls it off the manifold walls.

On the otherhand, it is possible for the engine to idle at 50 MAP @ 650 RPM, then turn around and cruise around town at 40 MAP @ 1,500 RPM, which will result in higher VE numbers.

I'll go for the taper down. Most of my driving (racing) is with a lot of foot on the gas, WOT and high rpm cruise (sort of) and sudden braking.

I race a Pro Stadium Tough Truck. If you've never seen it it is a little hard to describe. Drag race with jumps and tight turns. Drag race off the line, back off at the jump, back on the gass, back off a little in the turns, then do it all over again.

If I want decel to be quick, using the transmission as much as possible, do I lower the VE values more? How low can you go? Hmmmm I don't think it would be good to be below the idle values cuz that might cause a stall problem. Your thoughts?

Well too rich is more likely to cause it to load up and stall than too lean. I keep my tunes in Decel Fuel Cut until the last possible second, then fire a good shot of DFCO exit fuel to keep it from stalling. To get more decel, lower the timing advance in the cells that represent Decel in gear at your RPM range that you want it. You can get enough engine breaking to put your head into the windshield. DFCO in my G20 van doing 80 will just about put your head into the dash, yet at 55 it is nice and gentle. Just depends on where you set it to kick in. The factory setting is very RPM limited (like engage at 1,300 and drop to 1,110. Mine comes on at 2,700 and goes off at 800. It means that I am in DFCO alot more often and it is much more aggressive on high rpm throttle lifts.

GOOD!! That'll work.

How do you give it DFCO exit shot of fuel. I understand the rpm range you mentioned and DFCO... If I have a good idle at 800 and stop DFCO at say 1000, is the remaining 200 rpm range enough to prevent a stall?

I'm very familar with old-school stuff. How much less SA are we talking about to get a hard engine break. It would be a great driving benefit to let off the gas right before a tight turn rather than having to get on the brakes. Because I have to get right back on the throttle again...

What are the potential negative (damaging) impacts of greatly reduced spark while in decel at higher rpms? To the engine, not worried about the driver.

You might exit around 1100 RPM if you are concerned that it will stall at that RPM. In the TBI ECM hacks the exit shot is usually defined, you haven't mentioned which ECM you are going to work with.

As far as spark advance, the less you have,the harder it engine brakes.

The only negative effect I can think of is on the over-running clutch/sprags in automatic transmissions. It shouldn't hurt the engine. It might pull a little more oil past the rings/valve seals though due to higher manifold vacuum.

I'm not sure I'm reading this correctly, but if it's what I think you're saying, then I think you're wrong, in a little part of it.
The 2 RPM thresholds are used like this (at least on every system that i've worked on):
1 upper RPM threshold means DFCO can cut fuel at ANY RPM above this, and maintain fuel cutoff until the RPMs reach the 2nd lower RPM threshold.
It's not (or at least not supposed to be) fuel cutoff just between the numbers.
The reason it needs two thresholds is for a little hysteresis, so that if fuel comes back on, that it doesn't turn back off immediately if there's some anomoly or if say the return fuel causes the RPMs to jump up a tad. It also prevents jerk at lower RPMs, but still allows lots of fuel cutoff.

Same for the MAP thresholds. The smaller of the two allows fuel CUTOFF at all MAPs below this value, and fuel comes back on above the higher MAP threshold. This helps prevent any on off on off of cutoff, and also allows the MAP to match the RPM thresholds, such that they "fit" with the closed throttle MAP for both RPM thresholds.

Also, of course, BOTH RPM and MAP conditions must be met, and throttle must be closed in order for fuel to cutoff.

Also, if you want LOTS of engine braking (and hopefully not breaking), find a way in the calibration or software to force the IAC closed during cutoff, and use lots of IAC steps at idle to keep the throttle as closed as possible too.



Ok, Now I'm confused. If the lower MAP value allows DFCO below... then wouldn't that make the idle stuff take over? So in essence, the rpm or MAP range that is between the two values is a NO-DFCO range?

BTW I'm using a 727/730 and probably going to use 8D

Your right on that, but it still remains with ldle vacuum lower than stock DFCO is going to be very limited, if it even works at all. I know that my L82 cammed 305 TBI would never hit DFC with the stock EBL settings.

It allows DFCO ONLY IF the MAP *FIRST goes below the lower threshold (while the RPMs are above the higher RPM threshold). THEN if MAP comes above (or any other DIS-qualifier), will it return fuel.

Basically to get INTO dfco, your RPM must be GREATER than or equal to the upper RPM limit, AND the MAP must be below the lower MAP threshold, and the throttle must be closed (I think - about the throttle closed part).

To get OUT of dfco, the MAP must be greater than the higher MAP threshold, OOOOR, the RPM must get below the lower RPM threshold, OR the throttle is opened.

Idle should automatically be avoided by the lower RPM threshold.

I got so busy playing with TunerPro after reading your post I forgot to say thanks,

THANKS!!