Boost pressure and wastegate
How the 951's boost control works, which values are quoted and which faults are typical.
In short: The 951’s boost gauge shows absolute pressure – below 1 bar means vacuum. Boost is controlled by an external wastegate, a cycling valve and the knock regulator.
How control works
A spring in the wastegate holds the valve closed; exhaust pressure and pressure from the cycling valve open it. The cycling valve under the intake manifold is driven by the knock regulator (KLR), which has a factory-set boost limit. If the cycling valve sticks closed, boost stays at about 1.2 bar.
Values
The table shows values quoted from the workshop manual; we have not yet checked them against the original. If peak boost is consistently below 1.6 bar, investigate.
Typical faults
- Low boost: leaking vacuum or boost lines, a sticking wastegate, exhaust leaks; also connectors at the throttle switch or airflow meter.
- Overboost with fuel cut: a cracked wastegate control line, KLR problems or a sticking wastegate. The fuel cut protects the engine.
- Bypass (dump) valve: a failed diaphragm causes turbo lag.
Checking the wastegate
The test uses a pressure pump on a cold engine; do not overload the diaphragm. Published guides on this partly contain psi-to-bar conversion errors – check in the workshop manual before doing the work.
Technical values
Only work with the workshop manual and the correct tools. Do not use unverified values without checking them – wrong tension values can destroy the engine.
| Value | Figure | Condition | Status | Source |
|---|---|---|---|---|
| Maximum boost (absolute gauge) | 1.75 bar from about 3,000 rpm | Quoted from the workshop manual, not checked against the original | unverified | [1] |
| M44/51: boost at 5,800 rpm | 1.52 ± 0.03 bar | – | unverified | [1] |
| M44/52: boost | 1.75 bar up to 5,800 rpm | – | unverified | [1] |
| Tolerance | ± 10 % (1.58–1.93 bar) | – | unverified | [1] |
| Limit if the cycling valve sticks closed | about 1.2 bar | – | unverified | [1] |
Sources
- Secondary source 944 Turbo (951) FAQ
Secondary source on the Turbo: charger, boost, wastegate, model years. Some figures quoted from the workshop manual, unverified here.
- Secondary source Wastegate – Checking
Wastegate and boost check; the page contains conversion errors.
- Secondary source Buyer's Guide: Porsche 944 / 944 Turbo
Enthusiast buyer's guide (1998), checks on the lift and on the test drive.