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Custom inlet plenum

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Does anyone have advice on this?

im looking at making a plenum for my vq35 without any cfd.

single turbo currently twin throttle body.

the factory plenum I’m sure will work fine but the orientation doesn’t really suit my plumbing.

im restricted at the front of the engine and want to go to a single throttle body so it will be either side entry or rear entry.

the lower plenum manifold has square ports so I plan to extend these with the same profile and bell mouth them.

then have a plenum volume about 4.5L.

main variables are runner length, plenum volume, plenum rough shape and TB entry location if anyone has comment?


In general shorter runners will provide more torque at higher RPMs, and longer ones will make more torque at lower RPMs. Which way would you want to go from the factory lengths?

Tell us about how you are going to construct it? Carbon Fiber molds? Combination of machined / fabricated Aluminium? 3D printing?

The design and shape will depend on the space you have available but typically plenum volume should be 1.5-2x the engine volume and aim to distribute the air equally to each runner. Generally a well designed N/A intake will work well with F/I as less intake restriction will require less boost pressure for the same given output power but with F/I you don't need to consider pulse tuning and Helmholz to recover VE like we would with an N/A engine so the design isn't as critical.

There is a lot of aftermarket support for the VQ platform with proven products available off the shelf so you could take some inspiration from those.Tthe Merlin intake looks like a good starter for you project https://merlinmachining.com/products/350z-forward-facing-intake-manifold-kit-base-kit

Hi guys.

Ill probably keep the runners about the same length as the factory ones.

I’ll be making it out of 10mm alloy plate and 3mm alloy sheet.

im not restricted on volume just front entry.

Any comments on side entry vs front/back entry.

ive had a good look at all the aftermarket plenum around.

thanks for you input.

Typically with V engines its better to have the inlet into the intake inline with the cylinders eg at the front, back or directly above. If for example your VQ had a single side entry then the further most bank would see clean airflow whereas the closest bank would be at risk of turbulent airflow due to the occurrence of eddy currents forming at high gas speed from the expansion into the plenum chamber.

Open cockpit prototypes' airboxes such as those on LMP1/LMP2 cars are a good example of how even flow distribution is achieved with a pseudo side entry, in those cases the air is directed to the rear of the airbox with a recovery cone and encouraged to scroll round the rear of the airbox for more equal distribution.

With a front/rear single entry it would be beneficial to place the inlet higher and further forward of the closest runners with a gentle short side radius to reduce eddy current formation keeping the flow laminar. Tapering down the roof height towards the back of the airbox helps maintain gas speed to the rearmost cylinders but is mainly done in N/A engines together with different height inlet runners to increase the engines volumetric efficiency using the Helmholtz effect - the Cadillac DPi airbox is a good example of this.

In short, it's much easier to achieve even distribution and flow characteristics with a front/top/rear entry than a side entry but it can be done it just requires more thinking and some form of computational analysis to be optimised. In F/I applications rather than focusing on gas speed the drivers need to be clean, non turbulent air at uniform pressure across the entry to the runners. Since there's is a relationship between air speed and pressure a well designed N/A airbox would be a good base but wouldn't require the added complexity of designing for pressure wave pulse tuning.


I have found this book pretty useful in the past if you would like dig a little bit deeper on the subject

Awesome thanks Scott.

great info.

I’ll be sure to show you what I come up with.

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