I think this symptom is a lot more complex than just accel enrichment needing tweaking.
These cars typically cruise/maintain speed around 40-50kpa, but often run a decel fuel cut at around 20-25kpa (or lower). With no ISV, drop-throttle deceleration from 3000rpm+ can result in a manifold pressure of 15-20kpa depending on cams and throttle body settings.
On big injectors, it is typically difficult with standalone to maintain proper fuelling below about 35kpa due to fall-off in injector flow rate, crude injector models, and limited resolution in the fueling maps. Injector Dynamics has an excellent article on the injector behavior here:
http://injectordynamics.com/articles/low-pulse-tech/.
Imagine you're driving along at 2500rpm and you let off the throttle. If you're not running a dashpot on the throttle blade (the thing bolted to the throttle body that makes it slowly close over the last few degrees), the throttle is going to slam shut, giving you a fuel cut and immediately put you into engine braking. Any loose drivetrain or engine mounts will generate a thud on the load reversal. Now you're decelerating at 0% throttle and say, 20kpa manifold pressure, while sucking the remaining fuel film off the cylinder head port walls.
You go to pick up the throttle. For the ECU to leave decel fuel cut, you need to exceed a certain throttle %, or a minimum kpa. The first 3% of your pedal travel goes towards getting the ECU out of decel. Now you're at say, 28kpa and the ECU is injecting fuel based on your fuel map and injector model. Additional pulsewidth is temporarily added depending on your acceleration enrichment settings. The purpose of this is to restore/maintain the film of fuel being deposited and then removed from the cylinder port walls during the influx of air charge. This film is pretty important, and it can be tricky to tune the enrichment correctly at varying engine speeds, throttle deltas, etc.--it's easy to go too rich or too lean, especially with big injectors. Regardless, this effect is transient and temporary--after a short period of time or a number of engine cycles, that added pulsewidth is no longer there and the car is being fuelled solely off the 25-35kpa cells on the fuel map.
The problem is that due to the limited usage of these cells, and the aforementioned injector behaviors and fuel model issues, these cells are not tuned very well--and sometimes they can't be on VEMS and 034's systems. So you often run very lean from 25-35kpa, causing the engine to stumble, buck, or at a minimum not make enough power to maintain vehicle speed. This is easily felt as a dead spot in the pedal travel while the engine is running like that. To get around it you just apply another 10-15% throttle until you get into the 40-50kpa cells, which are typically well tuned as the car cruises there. Suddenly the engine picks up and becomes responsive to pedal inputs, delivering consistent changes in power with small changes in pedal movement, which it did not do for the first 15-20% of the pedal.
I spent a ton of time working on this, as when I bought my GTI it was immediately obvious how much better the driveability was at low loads--there is a LOT of things going on in its modern DBW ECU to make sure that thing is smooth and does not jar the passengers or the drivetrain under any circumstances--for example, being rowdy getting on and off the throttle in first gear is totally civilized, whereas in the Coupe you would get a concussion. Here's how I approached trying to attain that with my old car running standalone:
1) I ran an ISV, and configured it so that it would progressively open more and more at high RPMs. I have it set so the lowest manifold pressure I ever see is around 20-25kpa. This means I don't have to open the throttle a bunch just to reach the decel fuel cut kpa, and I don't have to worry about trying to tune 15kpa map cells. My decel fuel cut kpa value was based on my injector opening time was (so I quit trying to inject fuel when pw gets low enough that the injectors can barely open), and where the car was running under deceleration.
2) I did a bunch of work to figure out what it took to make my injectors work at low loads. For 034-specific information, see my Motorgeek post here:
http://motorgeek.com/viewtopic.php?f=2&t=546393) I spent a bunch of time dialling in the 30-50kpa region of my fuel map, from 1000-6000rpm. This was often done just sitting in a parking lot revving in neutral, which puts next-to-no load on the engine. Getting this tuned so that the engine would run lambda 1 made a huge difference. When driving hard or shifting at high RPM, throttle response is sooooo muuuuchhhhh bettttterrrrrr. It's worth noting that as a result of this, the bottom cells of my fuel map are weird--it goes from lowish values to very high values, again to make up for the lowered flow rate at low pulsewidth (meaning you need disproportionately higher pulsewidth to deliver a given amount of fuel compared to the rest of the map).
4) Once all of that was in place, I then played around with accel enrichment and got that dialed in.
It was a ton of work, and required a bunch of reading/learning to figure out what was going on and how to fix it, but the car now drives really, really nicely.
Sam
) But I didn't experience any different.
