dan_t

I have a 383 LT1 Intaked SBC behind a 6spd Manual in a C4 Corvette and I would like to try and play with lift-off throttle settings.

At the moment I feel like there is too much engine braking when I initially come off of the throttle.

In situations where I no longer want to accelerate but want to hold speed I find it hard to modulate the correct throttle opening. I feel like I have to "Roll" off of the throttle not to un-settle the car.

In situations where I am ON OFF ON the throttle I think that having less engine braking would make the car feel more stable at high speed.

Maybe I'm trying to tune around problems with my driving but I thought I would ask!

Orr89RocZ

iTrader: (20)

Whats your timing tables look like in the higher rpm, low map regions? Playing with those values will help some.

dan_t

High 40's for most of it... does timing really matter if the injectors are being cut though?

Orr89RocZ

iTrader: (20)

Probably not, but I'm not sure how quickly dfco enables after letting off the throttle. If you were to disable dfco, and keep injecting fuel, timing would make a difference how it acts.

RBob

iTrader: (1)

Look at the throttle follower tables for the IAC. I know there is one vs. MPH.

Sometimes DFCO will also open the IAC which helps the vehicle to roll-on. But with a quick look at the calibration I don't see one. There may still be one.

As the IAC is opened, or held open, there is less engine braking.

RBob.

dan_t

Which way would you adjust the timing to create less engine braking?

Low RPM's is fine is around 4000 RPM and upwards

Orr89RocZ

iTrader: (20)

I need to look at settings for a manual MAF bin but i know in ARAP DFCO enables above 1200 rpm and when LV8 is less than 33 LV8 but there are other things in there that may keep it on. So there shouldnt be any fuel

If you pull timing in the upper rpm lower load areas, it will engine brake harder. More timing will lessen the decel effect. So you probably want to run as much advance as it will take.

Also i think if its richer it wont brake as much.

bl85c

iTrader: (2)

I've gone the opposite route and increased braking by raising the load qualifiers, dropping the min time to enable and closing the IAC. Makes it easy to coast downhill without picking up speed. I imagine the opposite would work well for you.

dan_t

DFCO , Enable % TPS Vs. RPM

Which way does this table work???

bl85c

iTrader: (2)

I'm using 6E so I can't give you specifics, but in 6E there's one tps enable point. If the throttle drops below that point it's enabled, but only after a decrease in throttle.

RednGold86Z

iTrader: (1)

This is one thing that I've spent years perfecting (trying to) for our OEM applications. I think I'm getting close to pretty darn good, but I think not quite as good as the newest Delphi/Bosch/Others. We're not using true torque model based software, more ad-hoc with brute force.

Getting the tip-out to cut-off to feel good needs 1) IAC decaying from "more open" to "closed enough" to control the deceleration MAP to a point that is in the negative torque region, but not too far below zero-torque 2) Fuel not cut off immediately (very important - over 1 second delay is pretty comfortable) 3) Spark to slowly decay during this pre-cut fuel-on delay period to very low values (even 0). All this combines to go from positive torque to negative torque to cutoff negative torque in the best way.

Closed throttle Tip-in is a real pain (especially if fuel is cut) - here's how I do it 1) Start with super retarded spark (see above) 2) Don't let the MAP get too low during the closed throttle portion (see above) 3) Filter the spark so that it doesn't increase quickly 4) Begin the fuel as early as possible (use lower MAP for returning fuel - start fuel in the negative torque region) (minding that you must go rich at nearly the first combustion in order to keep NOx emissions from going directly through the O2 filled cat) 5) Use algorithms to retard spark based on dMAP transients and dRPM surges.

Also, you don't HAVE to cut fuel at your normal driving RPMs, although you might get some exhaust pops if you don't. There is very minimal fuel coming out of the injectors at low loads, so the savings are minimal - but, if you're losing control of the flow, then I'd suggest using cutoff. Also if using cats, you don't really want to be having too many pops and misfires when fuel is not cut.

dimented24x7

iTrader: (2)

Im using a newer GM PCM. Doesnt have the torque model like the '411 Im working on now, but it does have a reasonable ammount of control in DFCO.

You can try disabling DFCO, but my experience is that the engine will enter "DFCO" automatically with larger injectors as the PW gets so short that the pintle doesnt come off the seat, effectively forcing fuel cut-off, with LOTS of backfiring when the injectors do finally open at lower RPMs.

Keeping the IAC open and having some min air helps the engine braking. Also not decaying the fuel out right away helps to lessen the sudden engine braking, if you have that much control. Depends on what ECM/PCM your using. If your using a $6E (a dinosaur by todays standards), theres probably not much you can do outside of the throttle follower settings. RnG's suggestions work well in my experience, but some of those are not options available to you with the older ECMs, especially when exiting DFCO, which requires fuel logic running syncronously with the crank reference pulses. The newer ones even have a model that accounts for residual air scavanged out of the exhaust after DFCO to ensure accurate fueling transitions.