The "Sequential" Myth
Added on 21st June, 2004

We’ve had a lot of enquiries recently beginning with the question “do you have the valve that makes that sequential noise?”. There is a common misconception that the fluttering, chirping kind of noise that people often hear, is caused by a sequential valve. This is NOT correct, in fact, there is no such thing as a blow-off valve that makes this kind of noise.

The term “sequential” is applied to HKS’ Super Sequential because it has two valves that open one after the other, the small inner valve opens first, which then pulls the second, larger valve open. By this definition, GFB’s Hybrid, Bovus Maximus and Stealth FX valves are also sequential, as there are two ports, which open one after the other.

The fluttering noise is in fact compressor surge, caused by pressurised air blowing back through the turbo when you close the throttle. Compressor surge occurs when the blades of the compressor “slip” in the air, much like an aeroplane wing stalling. This condition is most common when shutting the throttle and not opening a BOV far enough, but it can also occur with the throttle open under boost conditions if the turbo is not correctly matched to the engine. The fluttering noise is simply a result of the pressurised air trying to escape through the turbo, and the turbo attempting (under its own momentum) to cram it back in. The high velocities of the air involved can sometimes make a chirping noise in combination with the flutter.

Whilst the noise does not actually come from the blow-off valve itself, it can be caused by the blow-off valve. When the spring pre-load is set too hard, or the valve itself cannot flow enough air, compressor surge will result. Generally, if the spring is too hard, you will tend to hear the surge at low to medium RPM and boost conditions, with a normal “whoosh” at higher engine speeds and boost. If the valve is too small for the task, the surge will occur at high RPM and boost.

It is possible for the blow-off valve to open and still cause surge, as at lower RPM and boost the compressor is closer to the point of surging. Turbos can surge even with the valve half open at low RPM, and yet at high RPM even with the same valve opening, it does not surge. If you’ve ever watched the piston of a blow-off valve open when the turbo surges, you’ll usually see it flutter up and down, which is what leads people to think it is the valve causing the noise. It is in fact the pressure fluctuations in the turbo piping caused by the surge. If you hook a boost gauge near the turbo, you can see the pressure fluctuating as the compressor surges.

Ok, so how do I get that noise, then?
Quite simply, all you need to do is increase the backpressure in the turbo piping when you lift off the throttle. This can be done by removing the valve totally (not recommended – see paragraph below), or adjusting the spring on your valve harder. This will allow the valve to open at higher RPM and vent normally, and at low RPM will increase the amount of air flowing back through the turbo.

However, the noise that you get is totally dependant on your individual engine/turbo/intercooler/air filter setup. A stock intake air filter box will usually muffle the noise very significantly, pod-type filters will make the noise much more obvious. Secondly, the noise is very dependant on the type of turbo. Usually you will find that smaller turbos such as the TD04 used on WRX’s will not surge as readily, and do not sound the same at all. Nissan turbos such as those used on the 200SX and GTS-T readily surge at low RPM (accelerate moderately to about 3000 RPM, roll off the throttle, and you’ll usually hear it on a Nissan). Larger intercooler set-ups also increase the chance of surge, as there is a significantly larger volume for the valve to evacuate.

So if the noise is compressor surge, will it wreck the turbo?
This is a very grey area, dependant on too many variables to say yes or no. Generally, if the surge is only occurring at low RPM and boost, then there really isn’t going to be a detrimental effect on the turbo. You only need to compare the loads placed on the turbo at full boost near redline to the small amount of surge at low RPM to see this. If the surge is occurring when you are driving flat out at high boost (greater than stock), then the risk of turbo damage and/or a reduced turbo lifespan is greatly increased. Also bear in mind that driving style alone probably has the greatest effect on the life of your turbo!