103MM Throttle Body on LS3 Intake Manifold. How Much Power?

One of the questions that we’ve often been asked by our customers is “Can I add a 103mm throttle body to my OEM LS3 intake manifold?” We recently had the opportunity to put this question to the test on our chassis dyno, and come up with a definitive answer.

The simple answer is yes, you can run a 103mm throttle body on your LS3 intake manifold, but should you?  Read on as we take you through our research and results.

Can I add a 103mm throttle body to my OEM LS3 intake manifold?

Our test vehicle was the author’s 2011 Camaro SS, equipped with a manual transmission, and an LS3 with our SS3 camshaft, long tube headers with cutouts, 25% underdrive pulley, ported rod-mod intake manifold, ported 90mm throttle body, and Cold Air Inductions cold air intake. Before making any changes, we conducted some baseline dyno pulls to nail down our starting point. After validating that spark timing and air fuel ratio were optimized we observed a consistent 487whp at 6500rpm. Wide open throttle spark timing and target air fuel ratio remained consistent for all tests to eliminate tuning as a variable in the results. Notably, we observed engine vacuum rose approximately 4kpa from 3000rpm to 7000rpm. Rising engine vacuum as engine speed increases at wide open throttle is indicative of a restriction in the induction system.

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Our first change was to remove the cold air intake as a variable from the system, so that any changes we made to the throttle body would not be influenced by potential induction system restrictions. We quickly fabricated a “Vara-stack” velocity stack from pieces of a Vararam OTR intake. This allowed us to pull unrestricted airflow into the throttle body, and we gave it an upward angle to ensure that we drew room air from the dyno cell, rather than hot air right off the top of the radiator.

We re-tuned the car in speed density mode, and again ensured we had an optimal air fuel ratio for maximum power. Unsurprisingly, with no air filter in place, we observed a consistent 501whp at 6400rpm. Again, we observed rising engine vacuum, but we improved to a 2kpa gain in vacuum. This tells us that the cold air intake represents a small, but observable intake restriction compared to our “vara-stack”.

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Next, we snatched the intake off the motor and headed to the porting room. This intake manifold was already equipped with our rod-mod and standard porting. The rod-mod is a modification where we insert 1/2” dowel rods down the length of the manifold to provide a smooth radius for air to follow into the runners. From the factory the intake runner entry is extremely sharp, which disrupts airflow and hurts power. Then our standard porting involves removing the stands inside the manifold to increase plenum volume, touch up the intake throat, and cleaning up each intake runner injector boss area. We’ve documented that the rod mod and porting is typically worth 10-15whp for cammed cars.

For our test, we went to work on opening up the inlet flange to accept a 103mm Nick Williams throttle body. Porting the opening to that size requires completely milling away the factory o-ring groove.

When we finished our work we observed that we had certainly increased the overall size of the opening, but the throat of the intake at the roof still pinches down significantly from the initial opening diameter. We utilized a gasket sealing compound to seal the throttle body to the intake and prevent any air leaks, and we confirmed that there was no throttle blade interference.

With the intake back on the car and the “vara-stack” in place we fired it up and reconfirmed our air fuel ratio was optimized. With our coolant and oil temperatures within our test parameters we made a few dyno pulls and observed no appreciable change in power. We observed the same gain in vacuum as we had with the 90mm throttle body. What we’re seeing here is that there is an intake restriction that lies within either the cylinder heads, or most likely within the plenum and intake runners themselves.

In conclusion, we have empirically validated our long held position that a 103mm throttle body on a OEM LS3 intake manifold does not offer any performance benefits. Milling the intake flange to accept the larger throttle body is a fairly tedious process, and makes a quality seal between throttle body and manifold a bit more difficult with the need for a gasket sealer and a keen eye towards fitment. Our previous intake tests have demonstrated that a stock throttle body and ported rod mod will match a F.A.S.T. LSXR manifold with a 103mm throttle body for power on a LS3 with a healthy cam, and that the stock throttle body and ported rod-mod intake remain one of the best “bang for your buck” mods you can do to your LS3.

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One thought on “103MM Throttle Body on LS3 Intake Manifold. How Much Power?

  1. James Stevenson says:

    Love your guys’ information. It makes understanding the science of performance LS engines much more understandable. Thank you

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