DIY Ported Lower Manifold
 

After hearing about the 3-5 hp gain by porting the VQ37VHR lower intake manifold on normally aspirated cars, I decided to blow an afternoon and port the spare one I have here.

I looked at the manifold ports to form a plan of action. The first rule of thumb for these home projects is “Do no harm”. The temptation is to go in and grind everything straight and smooth as possible. That may be the answer for forced induction, but this engine already makes 350 hp at the flywheel with a very wide torque band from just 225 cubic inches displacement without forced induction. That indicates the factory engineers have done a very good job designing the intake runners to achieve maximum and equal flow to all cylinders over the very wide rpm band. All the twists and turns in the stock lower manifold look restrictive, but those port shapes are there for a reason. Designing enlarged port shapes that beat the factory versions would require a flow bench and extensive dyno testing. That would be well beyond a DIY project. To avoid loosing power by grinding in the wrong places I chose to clean up the mismatches and roughness in the manifold casting.

The stock shape of the ports at the junction of the upper and lower manifold is smaller than the gasket window. Since the port runners should maintain a constant taper or constant cross sectional area throughout their length it is probably not a wise move to open either the upper and lower manifold runners out to the gasket shape unless the whole length of the port is enlarged. Enlarging ports to match the gasket only at the mating surface creates an undesirable sudden “bulge” in the cross sectional area of the port at the junction. These sudden changes in port area cause energy loss in the flow that can result in less cylinder filling. I chose not to enlarge this area of the ports except to insure that the upper manifold runner has an unobstructed shot into the lower manifold on all sides of the port.

There is a bump near the square entrance of the port that lies at the base of a manifold bolt hole. I removed it because it is most likely a production casting compromise for a bolt boss that was inserted into the molds after the aerodynamic design was done. Another bulge is near the port exit leading up to the injector port near the head. This may help control the injector spray pattern. I decided to play it safe and leave this one alone.

Looking into the ports from the top, some are straight from the upper intake to the head and some are curved. Also the cross section at the upper end is square and the cross section at the head is oval. There are what appear to be un streamlined shapes in the ports and they are more pronounced in the curved ports than the straight ports. Again I am betting these non intuitive shapes are there to turn the air flow and change the shape from square to round while maintaining equal flow to every cylinder. So the shapes should be left alone.


Since the air passing through the manifold ports does not have any fuel vapor to carry with it, the surface roughness can be polished down to a smooth surface for maximum dry flow.

In conclusion for the DIY’er there is not much to do on the lower manifold except to smooth the ports while maintaining their original contours and cross sectional dimensions. This can be done with 250 grit sand paper and your fingers. If you have a small hand grinder, remove the bump at the square port entrance. A lower manifold cleaned up in the way will “do no harm” and maybe get that last 3-5 normally aspirated horsepower on the dyno.

What you're saying is. You're gonna dyno test this idea? I have a spare one lying around.

I port matched my upper and lower to about an inch on each side.

If the walls of the head and the manifold match, then all you really need is the proper size gasket so that the transitions are smooth. When I was working on engines back in the '70-80s, we used RTV - run a small bead around the edges of the gasket while it is in place on the head or manifold, allow to set, and trim with an Exacto knife. Don't know if that will work on modern engines. Didn't have the too-big-gasket-hole problem on the exhaust side - they were nearly always too small and needed to be trimmed.

Back in The Good Old Days, you could get better flow with a good port-matching/relieving/polishing job. As you mentioned, the manufacturers are now doing a pretty good job of this at the factory, so gains will be small. Not worth the time and effort unless you track the car, IMNSHO.

The porting configurations I am advocating for the upper and lower intake manifold are targeted for maximum power under the curve from 4500 – 7300 rpm for a daily driver with a 7AT. Making the manifold passages larger may increase peak power, but probably sacrifice some mid range power. That is not to say that mid range power will not be increased by increased passage size, but it may not be increased as much as if the passages are cleaned up without enlarging cross sectional area.

Your on the right track for drivability and using the power at mid range. It's what I did to my 09 and it worked well even with being a NA manual and supercharged later.

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