Granatelli Wastegate Boost Spring & PSI Guide

Granatelli Wastegate Boost Springs Explained: How to Combine Springs and Calculate Base Boost

One of the most common questions we get about external wastegates is: "Which springs do I use to get the boost pressure I want?" Granatelli Motor Sports wastegates use interchangeable boost-control springs that allow the mechanical base boost pressure to be changed without replacing the entire wastegate. The basic math is simple: SPRING 1 PSI + SPRING 2 PSI = APPROXIMATE BASE BOOST

But there are a few important things to understand before simply stacking springs together.

What Does a Wastegate Spring Actually Do?

The spring inside an external wastegate pushes against the wastegate actuator and helps keep the wastegate valve closed. Boost pressure acts against the actuator. Once enough pressure is applied to overcome the spring force, the wastegate begins opening and allows exhaust gas to bypass the turbocharger turbine. That is how the wastegate helps regulate turbocharger speed and boost pressure.

The spring pressure establishes the mechanical base boost pressure of the system.

Granatelli Five-Spring Boost Kit

Granatelli's five-spring wastegate package gives the installer several different spring rates that can be used individually or combined.

Spring	Approximate Pressure	Identification
Spring #1	4.0 PSI	Outer spring, approximately 38mm diameter × 64mm tall
Spring #2	4.5 PSI	Red spring
Spring #3	5.7 PSI	Inner spring, approximately 30mm diameter × 64mm tall
Spring #4	6.5 PSI	Green spring
Spring #5	12 PSI	Yellow / Orange spring

Granatelli's current 60mm water-cooled wastegate ships with the 4.0 PSI and 5.7 PSI springs installed.

How Do Wastegate Spring Pressures Add Together?

When compatible wastegate springs are nested together, their spring forces work together. For basic boost-pressure planning, you can add the individual spring ratings together. The formula is:

Example: Factory-Installed Granatelli Springs

The current Granatelli 540259 wastegate comes with:

• 4.0 PSI spring
• 5.7 PSI spring

The math is:

So mathematically, that spring combination represents approximately 9.7 PSI of mechanical spring pressure.

Another Example: 4.5 PSI + 6.5 PSI

If a compatible installation uses the 4.5 PSI spring together with the 6.5 PSI spring:

The theoretical mechanical spring pressure would be approximately 11 PSI.

Example: 4 PSI + 12 PSI

That combination would mathematically represent approximately 16 PSI of spring pressure.

Example: 5.7 PSI + 12 PSI

The theoretical spring total is approximately 17.7 PSI.

Example: Three Springs

The same arithmetic applies when a compatible configuration uses three springs. For example:

Or:

Granatelli Boost Spring Math Examples

Spring Combination	Math	Approximate Spring Total
4.0 + 5.7	4.0 + 5.7	9.7 PSI
4.5 + 5.7	4.5 + 5.7	10.2 PSI
4.0 + 6.5	4.0 + 6.5	10.5 PSI
4.5 + 6.5	4.5 + 6.5	11 PSI
4.0 + 12	4.0 + 12	16 PSI
5.7 + 12	5.7 + 12	17.7 PSI
6.5 + 12	6.5 + 12	18.5 PSI
4.0 + 5.7 + 6.5	4.0 + 5.7 + 6.5	16.2 PSI
4.0 + 5.7 + 12	4.0 + 5.7 + 12	21.7 PSI

Important: The Math Does Not Mean Every Spring Combination Physically Fits

This is extremely important. Adding the PSI values tells you the theoretical combined spring pressure. It does not automatically mean every possible combination of springs should be physically installed together. The springs have different:

• Diameters
• Lengths
• Spring rates
• Installed heights
• Positions inside the wastegate

Only use combinations that can be correctly nested and installed in the specific Granatelli wastegate. Never force extra springs into the wastegate simply because their PSI values add up to your desired boost target.

Spring Pressure Is Base Boost — Not Necessarily Final Boost

Another common misunderstanding is assuming: "If I install 10 PSI worth of springs, the engine can only make 10 PSI." That is not necessarily true. The spring establishes the wastegate's mechanical base boost pressure. A manual or electronic boost-control system can apply additional pressure to the wastegate control system and command more boost above the spring pressure, depending on how the system is plumbed and configured.

Can a Boost Controller Lower Boost Below the Spring Pressure?

Normally, you should consider the spring combination the minimum mechanical boost level of the system. For example: If your installed springs total approximately:

you should not choose that combination expecting a normal boost controller to reduce the engine to 5 PSI. If you need a lower mechanical boost level, use a lower-pressure spring combination.

Example: You Want 15 PSI of Boost

Suppose your target boost is approximately 15 PSI. You have two basic strategies.

Option 1: Spring Pressure Close to Target Boost

You could select a compatible spring combination with a mechanical base pressure reasonably close to your intended target. For example: 4.0 + 4.5 + 5.7 = 14.2 PSI If that exact spring arrangement is approved and physically compatible with the wastegate, the theoretical base would be approximately 14.2 PSI.

Option 2: Lower Spring Pressure + Boost Controller

Another approach is using a lower mechanical spring pressure and allowing an electronic or manual boost controller to raise the final boost level. For example:

• 9.7 PSI Base Spring Pressure → Boost Controller → 15 PSI Target
• This can give the boost-control system additional flexibility.

Example: You Want 20 PSI

If the engine's final target is around 20 PSI, that does not automatically mean you need exactly 20 PSI of mechanical spring pressure. A setup could use approximately: 10 PSI Mechanical Base + Boost Control = 20 PSI Final Target The exact strategy depends on the boost controller, turbo system, wastegate plumbing, engine combination and tuner.

Why Not Put the Stiffest Springs in the Wastegate?

More spring pressure is not automatically better. If the mechanical spring pressure is too high, the wastegate may not open soon enough to maintain the lower boost level you actually want. That can create an overboost condition. Your spring combination should be selected around the lowest boost pressure you need the system to run safely.

How Spring Pressure and a Boost Controller Work Together

Think about the system in two layers:

Simple Granatelli Wastegate Boost Formula

For spring selection:

Example: 4 PSI + 5.7 PSI = approximately 9.7 PSI base spring pressure

Why Actual Boost May Be Different From the Spring Math

Spring calculations give you a starting point. Actual boost pressure can vary because the complete turbo system affects how the wastegate behaves. Variables include:

• Wastegate size
• Wastegate location
• Turbocharger size
• Turbine housing
• Engine displacement
• Exhaust manifold design
• Exhaust backpressure
• Boost-reference source
• Boost controller
• Pressure-line routing
• Single vs. twin wastegates

That means a theoretical 9.7 PSI spring combination may not result in exactly 9.7 PSI on every vehicle.

What If the Car Makes More Boost Than the Spring Rating?

If actual boost is substantially higher than expected, do not automatically assume the spring is wrong. Check:

• Wastegate reference lines
• Boost-controller plumbing
• Wastegate valve operation
• Wastegate size
• Wastegate placement
• Exhaust manifold flow into the wastegate
• Signs of boost creep

An undersized or poorly positioned wastegate may be unable to bypass enough exhaust even when fully open.

What If the Car Makes Less Boost Than Expected?

Possible causes include:

• Incorrect spring installation
• Boost leaks
• Wastegate valve not sealing
• Incorrect reference-line routing
• Exhaust leaks
• Turbocharger sizing
• Boost-controller configuration

Granatelli 44mm, 50mm and 60mm Wastegates

Granatelli Motor Sports offers:

• 44mm piston-actuated wastegates
• 50mm piston-actuated wastegates
• 60mm piston-actuated wastegates
• 44mm water-cooled diaphragm wastegates
• 50mm water-cooled diaphragm wastegates
• 60mm water-cooled diaphragm wastegates

Granatelli wastegates include adjustable spring packages so builders can match the mechanical boost setting to the requirements of the turbo system.

The Bottom Line

Granatelli wastegate springs make mechanical boost adjustment straightforward once you understand the math. For compatible stacked springs:

• Add the spring PSI ratings together to estimate mechanical base boost.
• For example:

• 4.0 + 5.7 = approximately 9.7 PSI
• 4.5 + 6.5 = approximately 11 PSI
• 4.0 + 12 = approximately 16 PSI
• 5.7 + 12 = approximately 17.7 PSI
• 4.0 + 5.7 + 6.5 = approximately 16.2 PSI

But remember: The spring total is your mechanical starting point—not a guarantee of the exact boost pressure the engine will make. Actual boost depends on the complete turbo system, and not every mathematically possible spring combination should automatically be installed together. Always choose a mechanical spring combination appropriate for the lowest boost level you want to run, then use the boost-control system to increase pressure from there when appropriate.

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Written by JoJo Granatelli — third generation at Granatelli Motor Sports in Oxnard, California, and the one who drives what the company builds. He set a quarter-mile record of 7.06 seconds at 157 mph at Famoso Dragstrip in August 2026. More about JoJo.