Stage 1 ported intake manifold Dyno results

Stage I ported A1 intake

So after much porting work with fuel injected VW A1 sytle intake manifolds and various air flow tests it was time to how these ported intake manifolds performed on the dyno.


You can find more about the porting and testing process on the A1 porting and testing page found here on the scientific rabbit. NOTE: A1 Fuel Injected VW cars at 1978 -1984 Rabbit, Jetta, Scirocco, Caddy, Cabrolet, Rabbit GTI, Jetta GLI The Dyno tests were done by a fellow vwvortex user , I offered a ported intake manifold to VW Rabbit owner in exchange for a full chassis dyno done to the car before they installed the manifold and then after the installation. Then Share the results with the community.

The rules of this exchange were simple :

  • the dyno was to be done at the same place and on the same dyno equipment,
  • at the same shop,
  • no additional changes were to be made between the dyno's other than the installation of the ported intake manifold.
  • Not even an oil change was allowed.

About the VW A1 intake manifolds: According to air flow tests the ported intake manifold flowed 10% more air than a stock non ported one. An additional benefit of the porting process is that it evens the flow between each runner. On the A1 porting page you can see that the stock A1 intake manifolds air flow between runner ports is not consistent. Consistency is important with CIS fueled cars, as the injection system can't tell how much fuel each cylinder needs - it can only tell how much all cylinders needs together.

As Noted in Greg Raven's "VW Performance Handbook" the intake manifold is the biggest single intake restriction of the engine setup.


For the test car setup and results read on.. The test car is a VW Rabbit with a 84 GTI motor. The GTI motor engine code is JH, it has a solid lifter 8 valve head, with big valves. 40 mm intake valves and 36(or 38 ?) mm exhaust valves. The car owners website is not entirely up to date, but you get the idea.

This particular motor has a few modifications already;

  • ported and polished head,
  • cam from a 1.6 gas VW motor. The 1.6 Cam is more aggressive than the stock US GTI cam(which is really the worst VW made), but the 1.6 is not as aggressive as the euro GTI / G grind cam.
  • The exhaust system is made up of the dual outlet manifold with a 2.25" cat back TT exhaust system.
  • The throttle body in use is the stock A1 small bore throttle body.

The charts shown below show the before (before installing the ported intake manifold) and after( with the intake manifold installed). What is shown in the graph is the Horsepower (HP) and Torque (Tq) of the engine that is available at the wheels, also called Wheel horsepower. This is more accurate representation of the engines power because it takes into account real sources of friction and transmission inefficiencies -ie drive train loses. All cars /trucks/cycles have these inefficiencies that take up a percent of the power the engine makes. VW cars run about 18% of this inefficiencies. Car manufactures quote crank HP that doesn't take into account this drive train loss, mostly because they sound better. So for the car and engine used in this dyno test the factory rated crank horsepower is 90, where the wheel hp is about 78. Note the before dyno shows Maximum of 84 Wheel HP with 95.1 Torque.(Pink Color on Graph) The After Dyno shows Maximum of 90. Wheel HP with 96.4 Torque(Blue Color on Graph) Thats 6 Wheel HP increase and 1.3 ft/lb of Torque by installing a ported intake manifold. Not only did the engine make more power (HP and torque) it made more power sooner in the rpm range and kept making that power longer into higher rpms. The engine is rated at a Redline of 6800 rpms. The original graph(pink lines) shows the engine running out of steam before it even gets to the high rpms.


Dyno Charts combined for easy comparison

 MK1 Rabbit intake manifold Dyno chart -compare stock vs stage I ported intake