I've been very happy with my VEA D4-engined car since getting it last year, but recently I made a simple tweak which has made me even happier with it, improving part-throttle pick up and throttle response, especially in the up-to-2,500rpm range.
Having a look under the bonnet, I noticed that the vacuum hose highlighted in the image below was curved at a rather sharp angle:
That hose delivers vacuum to the actuator that controls the bypass valve for the high-pressure (smaller) turbo. At idle, or when there is zero throttle input, the bypass valve is open as there is no vacuum in the hose. The open bypass valve allows the exhaust gas to flow equally to the high-pressure smaller) and low-pressure (larger) turbos. This minimises boost pressure at low rpm / zero throttle.
When you touch the throttle, the solenoid switches the vacuum supply to the hose, and the actuator closes the bypass valve. This diverts
all the exhaust gas to the high pressure (smaller) turbo, to quickly increase boost pressure off-idle. It's this hose and bypass valve that basically governs driveability and throttle response in everyday driving conditions.
With the hose curving at a sharp angle, I wondered if it wasn't narrowing or kinking the internal diameter of the hose, reducing the rate of vacuum increase seen at the actuator - thereby affecting the bypass valve actuation.
The factory-fit hose is an approx 15cm length of conventional 6mm internal diameter, 11mm external diameter rubber vacuum hose. It's only
just long enough to fit between the solenoid and the actuator.
I replaced it with a 20cm length of 6mm ID/11mm OD vacuum hose from my local car parts shop. The extra 5cm length is easily enough to allow a nice, smooth rounded curve in the hose with no risk of it narrowing or kinking. The factory hose simply pulls off the barbed nipples on the solenoid and actuator, and the new hose fitted simply by pushing it in place. You don't even have to remove the engine cover to do it.
Total cost: £1.50 for a new piece of hose. Total time: 5 minutes. Throttle response is now more sensitive, sharper and more positive around town and on the open road. It was good before, but this has made it even better.
Of course, this particular vacuum hose was a just a bit too short on my car. Other people's cars might have a slightly longer piece of hose fitted from the factory, in which case they probably won't experience any difference from swapping the hose. But I'd recommend any owner of a car with a D4 VEA motor should have a look under their bonnet to check if their car might benefit.