100 MPG Madness, Project Aprilia SR50 Part 5- More Power!

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High Gain Tuning’s prototype 26mm throttle body dwarfs the tiny stock throttle. 26mm is about the size carburetor that works well on a built engine of this displacement so we figured this would be a good starting size.

Not being satisfied with that, we decided that our induction system was the next cork in our path for more power.  The stock SR50 intake has a small 16mm throttle body and a convoluted tortuous path to the engine.  Previously we had bored our throttle body out to 20mm and ported our intake to 20mm as well.  We removed a restrictive snokus in the factory airbox and ported out the airbox intake.  We also ported out the reed cage stuffer and matched the stuffer to the intake manifold. With a Malossi high flow filter, we realized no gain in performance for all of this work.

The Stage 6 intake manifold is much bigger in diameter than the stock part and is the full diameter as the reed valve. It is also a straight shot to the reeds for maximum flow.

We figured that perhaps 20mm was too small for our level of mods so we opted to go bigger.  First we wanted to simplify our intake tract and make it more direct.  We ditched our factory intake manifold for a bigger ported direct shot part from Stage 6.  This manifold is a race intake for highly modded Minarelli  (same family of engines found on the Yamaha Zuma for instance which is the S13 of the scooter world) engined scooters.  The Minarelli shares the same intake bolt pattern as the Moroni-Aprilia engine on our scoot.  Now our intake charge can shoot straight into the engine’s cases with no restriction.

We drilled the manifold for setting the stock oil injection nozzle into the manifold with some JB weld.  We had to trash the stock rubber intake manifold to get the nozzle out so it’s no going back for us!

We also installed an experimental 26mm throttle body by High Gain Tuning.  This throttle body is in line with the carburetor size that works well on built Minarelli big bore engines and undisputedly eliminates the intake as a source of restriction.  We also installed one of High Gain Tuning’s high flow open element air cleaners to really open things up.  What was the result of all of this?  Other than sounding awesome, absolutely nothing.  We figured that like a diesel, the direct injection SR50 engine could not make more power unless more fuel was added when there was a surplus of air.  The fact that our engine ran almost normally with the throttle body completely removed reinforced that line of thought.

The intake manifold and the High Gain low restriction filter now flow more air than the engine is presently capable of swallowing.