Tuning Tips for Intake Manifold Deficiencies


Motown small block intake manifoldExcerpted From: Competition Engine Building  Written By: John Baechtel

Courtesy of: CarTech Books

Today we wanted to share an inside Tech Tip with you taken from our new CarTech book, Competition Engine Building: Advanced Engine Design and Assembly Techniques. This book has received an excellent response from our readers since it's June release. The in-depth instruction and images are designed specifically for the hardcore enthusiast interested in Competition Engine Building.

Various modifications and adjustments can be applied to intake manifolds to fine tune them for specific performance requirements.

  • Port matching the intake manifold to the cylinder head port is important to high-speed power unless it significantly degrades runner velocity.
    Changes in flow path cross section have the potential to separate fuel unless the change is minimal and does not occur abruptly.
  • A carburetor spacer usually assists fuel mixture transition into the runners. Re-jet accordingly and investigate timing reduction based on potential improvement to mixture quality.
  • Check runner and plenum surface texture and re-texture any areas that have been ground or polished (if rules permit).
  • If you identify runner discrepancies, consider using plenum floor dams or slots to redirect flow. Single-plane intakes can also utilize modified Brodix "turtles," which are plenum floor attachments that can be shaped according to flow requirements.
  • Some single-plane intakes can improve fuel distribution in problem runners (fuel separation) by laying back the base of the divider wall between the runners. If this fails to correct the problem you can reconstruct the divider with epoxy.
  • By whatever means possible check the port/runner interface for misalignment. If permitted, slot or elongate the manifold bolt holes to facilitate proper alignment.
  • If applicable, during mock-up assembly install the manifold and distributor to check distributor depth in the block. Be certain that any oil passages around the distributor housing are properly aligned and that the oil-pump drive stub meshes properly with the oil-pump driveshaft.

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  • Make certain the manifold does not bottom against the cylinder block end rails, which can affect runner seal at the port interface. You must have clearance for sealer on the end rails and to prevent misalignment of the ports and runners.
  • Check gasket fit to ensure that intake gaskets do not overhang port openings. Also check water passages to ensure the water ports do not leak. Add silicone sealer if required.
  • Use appropriate-length carb studs instead of bolts to prevent eventual stripping of the manifold bolt holes.
  • Use a silicone bead to seal the end rails. Allow it to set up prior to installation and then add a dab of sealer at each corner.
  • Use sealer on all plenum and runner accessory plugs to eliminate air leaks.
  • Apply a coating of light grease or cooking spray to both sides of the intake gasket so the manifold can be removed without damaging the gasket.
  • Block carburetor heat and/or EGR passages.
  • Investigate individual cylinder tuning via runner-length and cross-section mods and potential match-ups with individual header-pipe tuning and/or timing adjustments.
  • With boosted applications pay particular attention to intake manifold sealing issues.
  • Modifications that improve naturally aspirated performance also enhance boosted applications, sometimes to the extent of requiring less boost and less timing for a given power level.
  • If you have to use a plenum divider to improve throttle response, you probably need a different manifold, especially on a single-plane application.
  • On engines that have been run, examine the plenum and runners for signs of reversion or fuel separation. A colored stain (often reddish) indicates separation. Darker stains indicate reversion.
  •  For any given manifold configuration, think critically about the movement of air and fuel through the plenum and runners. In particular consider the effects of abrupt turns in the flow path, runner length, and area variations, and the influence of individual runners on adjacent runners. Direct paths to the valve are always more desirable.