The Intake Tube Modification

This page describes a modifcation of the stock intake tube to increase airflow to the motor.

After many track sessions I have determined that this modification is good for both torque and HP over the stock tube and on a car with a stock MAF sensor it outperforms a 3" rubber hose inlet tube. On a car with a higher flow MAF, the modified tube shown here may still outperform a 3" rubber hose but the margin is much smaller. Using a recalibrated ported 70mm MAF the following results are given. Over the stock airtube, the modified one gave a consistent .12 seconds and 1.3 mph improvement. Over the 3" rubber hose it gave a consistent .10 seconds but only about .3 mph.

Summary, on a stock intake using a K&N panel filter on the stock airbox with the silencer removed, the modifications shown here were valuable in improving performance HP and Torque over the stock tube with a stock MAF. For cars with highflow MAFs, this modification is probably equivalent to a 3" intake tube in most respects. It is possible that a truly smooth bore 3" mandrel bent inlet tube would outperform this modification, but a tube of this nature will most likely require a modification to the hood crossbrace for clearance and to date there are no tubes of this type available.

The Tube is the large embossed plastic piece, the bellows hose from the MAF to the plastic casting is restricted at the clamping point of the MAF. Look through the MAF with this piece installed and you will see the smaller diameter.

The real restriction is where the plastic inlet crosses the fuel rail right near the PCV inlet shown just to the left of the alternator.

Bottom of tube showing restriction. The smaller tube and elbow is the PCV inlet. Flow is from right to left in this picture.

The restriction viewed from the intake side looking downstream relative to airflow.

The tube after cutting and patching with a glass reinforced silicone sheet. The plastic must be primed and a high temp silicone adhesive was applied. The assembly was clamped with padding to apply conformal pressure to the area.

Looking where the restriction was. The grey material is the silicone adhesive. Compare this to the picture of before.

A variety of tubes, left is the stock restricted tube, center is an 80mm smoother bore currently in use, right is a smooth bore 3" neoprene. The MAF and the inlet to the cast plastic piece is 80mm and the center tube is the best available for this. Notice the stock bellows tube has a dark shaded ring about 1 inch from the lip. This ring sits right up against the exit of the MAF and is smaller bore than even the MAF bore. Also, this tube has very large corrugations which will cause turbulence in the airstream.

Bevelling the edge of the tube where the rubber hose attaches. The air enters the cast plastic piece here and the material is about 3/16 of an inch thick.

This modification looks stock when installed and intake noise is increased. I would estimate the gains on a stock intake with a K&N filter to be about 5-7 HP and 4-8 ft-lbs of torque.