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 I think 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.

 

Have you thought about extrude honing? That'll smooth the walls while opening up the entire runner and shouldn't mess with the aero much.

 

The extrude hone may be a good approach, but a good dyno test program would be needed to be sure there was actually a gain in power. If the lower manifold is enlarged, I would think the upper should also be done the same to keep the cross sectional area relationships the same.

Extrudhoning the upper manifold looks like a near impossible task. There are some questions to answer before going down this path. Which direction do you push the putty through to avoid changing critical shapes? Do you know if plastic manifolds like the VQ37’s can be extrude honed without breaking them?

 

I haven't seen the manifolds, your description above implied there was a port size mismatch with the aluminum part.

 

Great analysis and execution!

Isn't it funny the time and money we spend for a few extra HP that we probably can't measure?

 

Since there is a tolerance range for the location of the upper manifold on the lower, the exit of the upper manifold is designed slightly smaller than the entrance to the lower manifold. This insures that the flow does not encounter an obstruction when it enters the lower manifold. Probably enlarging the lower manifold entrance out to the gasket edge would only exacerbate the cross sectional area change and cause more flow energy loss.

 

Port-matching is a time-honored method of raising power. It involves modifying the upper manifold, the lower manifold and the heads so that the ports match when all bolted together.

 

Absolutely agreed.

 

Extrude honing is not something you will likely want to do. My quote on the last manifold I did was $650 for the basic work and well over 1,000 for the more intense polishing. If you want to gain 3whp and spend $1500 then go for it. My guess is what you did by hand is more than enough. Good work

 

It probably indicates that I have too much time on my hands. LOL

 

I've been doing it for a few years and now 2-3 others are doing it. I see what you're saying but with a whole package the proofs in the pudding. Remove all bumps, smooth it, make it bigger, match to the upper, port the upper to bigger TB's, boom, power.