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The cup mod

The cup mod is simply changing the arrangement of the anciallary pipework around the carburettor and the turbo actuator.
The purpose of the cup mod is to enable the turbo to produce full boost quicker than it does normally.
The name "cup mod" is derived from the "Renault 5 elf Turbo Cup" races back in the 80's, there was a series of modications that would be done to a GTTurbo before racing. This mod was one of them.

It involves simplifying the boost control pipework to improve turbo response. Roughly speaking, it consists of connecting the front actuator port to the carb base (rather than carb inlet plenum), disconnecting the rear actuator port, and blocking the ports in the exhaust elbow and carb inlet plenum. It has the advantage of neatening the engine bay by removing a load of pipes, but may lead to premature actuator failure or poor boost performance in very cold or hot weather.
Cup Mod Diagram Cup Mod Diagram
For more information about the cup mod see this article https://www.rtoc.org/boards/showthread.php?t=12

What's a dump valve?

A dump valve or DV, also known as a Blow Off Valve or BOV, is fitted in the main boost hose from the intercooler to the carb. It's purpose is to, when the thottle is closed quickly, vent off the compressed boost air to prevent long term wear or at very high boost sudden damage to the turbo also to prevent momentary over fueling on cars that use an air flow meter. Sometimes people find it can reduce the time taken for the boost to build after a gearchange but mostly people find it increases the time. It works by having a diaphragm or piston or two that's pulled open by the vacuum formed in the inlet manifold when the throttle is shut. A spring holds it shut when the throttle is open. It also makes a nice 'ptcsssshh' noise!

Remember that Renault chose not to fit one as standard on the GT Turbo, yet most manufacturers do fit them as standard equipment (recirculating type) on turbocharged cars. The R5/R11 Turbo has quite a long boost path with reasonably wide hoses, so most pressure waves from the throttle shutting quickly are absorbed in the hoses, so you don't get the turbo damaging effect seen in cars with shorter paths. Hence a lot of people feel there's no need for a dump valve on a R5 GT Turbo or R11 Turbo.

Piston or Diaphragm dump valve?

There are two main types of DV - one uses a flexible diaphragm made from rubber or kevlar to seal the boost hose, the other uses a plastic or metal cylinder. Both work in the same way, the piston or diapragm being pulled up against spring pressure by the vacuum. Diaphragms are prone to splitting eventually, but pistons can suffer from lack of lubrication and jam. The BOVs have a second seal also held by a spring. When the vacuum pulls the main seal open the second one remains shut unless there is a boost pressure in the main boost hose of higher pressure thn the spring can hold when the second seal is then blown open and partial venting occurs. The system ramains partially pressurised. Cars with air flow meter may not idle with a DV and need the BOV with it's closed at idle feature.

Boost Pressure

There are two places to take the boost pressure measurement from. The manifold (more popular) and the carb plenum.
The standard boost pressure was around 10psi @ the manifold.
By re-jetting the carbarettor and strapping the intercooler the GT Turbo's boost pressure can safely be raised to 16psi.

What's a bleed valve and where does it go?

A bleed valve is put in the pipe that controls the actuator on the turbo to make it open the wastegate at a higher pressure than it would normally do so. This results in the boost pressure produced by the turbo being higher. It works by literally 'bleeding' off a portion of the air, so that the actuator gets a lower pressure than there is at the carb top. It goes in the pipe that goes from the carb top to the front port on the actuator. Normally it's fitted under the bonnet, but there is a version that you can mount on the dashboard (ie with long pipes), so you can increase the boost when you need it (eg for overtaking), and drop it back down to normal for normal running. However it's recommended that you fit a limiter to prevent you opening up the valve beyond a safe limit.

How do you fit a boost gauge?

A boost gauge is best connected by cutting the pipe that runs to the ignition module (ECU) and inserting a T-piece in it. Then run a pipe from here to the gauge. This will give you negative readings (ie vacuum) as well as positive boost measured at the manifold (ie what the cylinders are receiving). The standard boost gauge measures from the top of the carburettor, which will be a few psi higher than below it.

What does 1 Bar equate to in psi?

1 Bar = 14.5 psi.

What is the maximum boost on standard turbo?

Standard maximum boost pressure is about 10psi, on the standard Garrett T2. You can raise this to 15 or 16psi without shortening the life of the turbo too much, but any more than that will seriously shorten it's life, or risk engine damage.

What is the maximum boost a standard intercooler can manage?

The standard intercooler doesn't cool very effectively above 14psi. It's also prone to blowing apart at 16psi or more. An alloy version will perform well up to around 30psi. Larger intercoolers which may have to be mounted in other places can cool more effectively.

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