Valve Float During Turbocharging: How to Fix It

The Effect of Boost on Valves

Today’s Motiva Performance Engineering topic is something any performance guy should get a grip on: turbocharging and valve float.

Here’s the problem: Boost pressure into the intake side of a turbocharged engine and you boost the pressure on the exhaust side. What’s the resulting effect on a set of factory-spec valve springs manufactured to handle a mere fraction of all the pushing and pulling force?

How about a sort of oh, no, how to do I stay closed dilemma!!!? Well, welcome to the world of valve float.

Motiva Now Offers Valve Spring Upgrades

Here’s the solution: Motiva Performance Engineering now offers valve spring packages for the 1996-2004 Mustang GT, 1996-1998 Ford Mustang Cobra, 1999-2001 Mustang Cobra, 2003-2004 Mustang Cobra, 2005-2010 Mustang GT and 2007-2010 GT 500.

These packages are designed to work with the increased demand that turbocharger systems put on factory valve trains.

Many times, a customer reinforces the short block on their engine, but the the valve train and cylinder heads are not addressed. Turbocharging systems provide the BOOST in the intake manifold that allows an engine to process more air and therefore make more POWER. But this power is not free.

How to Defeat Turbocharged-Induced Back Pressure

The extra air being passed through the motor will typically “back up” between the exhaust port of the cylinder head and the turbine wheel of the turbocharger. This increase is known as “back pressure” and it pushes back against the exhaust valve. Basically, the only thing keeping the exhaust valve closed is the exhaust valve spring.

Auto manufacturers, by necessity, are always making compromises when designing and producing their power trains. They must always balance cost with reliability.

This usually means that components are designed only to handle the forces seen on a 100 percent stock engine. There is typically a fair amount of engineering margin built in to ensure reliability, but the components provided by the factory certainly aren’t intended to produce 50 to 200 percent more power than the engine left the factory with!

Everyone knows when you increase the power of a vehicle, you can expect that you’ll have to upgrade the drivetrain (clutch, transmission, driveshaft, rear end, etc.), and the short block (crank, rods, pistons, etc.) and many even go so far as to put different camshafts in. But the valve springs are rarely addressed.

On high-boost turbocharged applications, the back pressure will typically be 2:1 — meaning 2 psi of back pressure for every one pound of boost generated.

For example, on my recent engine-tuning tour in Doha, Qatar, with City Car, I came across Ibrahim Alyahia. Great guy. He liked what I did for his car. Ibrahim’s 2007 Mustang GT has a Hellion Power Systems 76mm turbocharger system installed, along with forged pistons and connecting rods.

At 20 psi of boost, this system should be making about 650 hp to the wheels at 5,500 rpm with factory camshafts, intake manifold and cylinder heads.

Well, what happens at high rpm is that Ibrahim’s valves start floating, as the exhaust valves are caught in this no-man’s land pressure cooker of opposing forces of exhaust and intake forces. The poor factory valve springs get overwhelmed. The valves “float” or, precisely, never fully close. Unclosed valves at high rpm mean lost horsepower — or worse!

Take a look at this Dyno graph of Ibrahim’s car before and after. The blue line on the graph is what happened with factory valves. The red line is what happened with Ibrahim’s same setup, but with Motiva valve springs.

Stay with me here: With the stock valve springs, the power lays over and begins falling off at 4,700 rpm, but with the extra capacity of the Motiva valves springs, the power keeps pulling until 6,000 rpm.

Now, that’s what I’m talking about!

Motiva valve-spring packages are made to “drop in” to the factory valve spring locations. The Motiva package includes the springs, titanium retainers and new valve locks, and they can even be provided with the right specialty tools to install the springs.

If you want to see how they go into your block, click here for instructions.

–Dave Rochau