Cup Mod

contributed by Ian Simpson

So often asked about, here are the answers...

What is the 'cup mod'?

The 'cup mod' comes from the circuit racing series called the 'Renault 5 Turbo Cup', which ran in the late 1980s.

What does it do?

The modification causes a change in the way the way rising boost pressure and engine revs control the actuator.

I found that it varies from car to car. A lot depends how restrictive your exhaust is as the rear part of the standard system lifts the boost as the exhaust back pressure rises.

I went from a fallen off rusty residual bit of barely silenced standard system pipe back to a full standard system and got and extra 1 psi.

The backpressure exerts an effect through the one way valve I guess preventing the boost leaking away through it, so therefore presurising the rear half of the actuator instead, lifting the boost.

This does not happen if you have a large bore free flowing exhaust, or if you only rev low at low boost. So doing the rear part of the mod then would appear to have no effect. I took the rear part off one standard car with standard exhaust and the boost dropped three psi at peak.

As for the front part, well I found I got 2psi extra with the front half of the actuator fed direct from the middle of the carb than from the T from the tube from the plastic elbow to the carb middle (ie standard stsyem). This was at 6000rpm at 1 bar and with the rear, compensating, part removed. (actually I had a single port actuator by then I think)

I've have found on someone else' car than by doing the whole cup mod at once, there was much more power at higher rpm. The car accelerated faster. I guess the standard system was pegging the actuator to a pressure nearer to the carb top than the carb middle (ie before the venturi where the pressure is higher, than after it where the pressure can e several psi lower) so meaning the boost pressure in the manifold was cut down somewhat as the boost above the carb rose.

As far as spin up, well if the cup mod allows the manifold boost pressure to be 2psi higher at the same point in overall boost then the car would seem a bit faster and to have spun up sooner, which it may actually have done.

You will most likely need to turn up your boost a bit to be the same as before the mod. This is best done by adjusting the actuator rod but this can be tricky. A bleed valve in the pipe to the actuator front will do the job but may introduce other problems such as waste gate creep and fuel vapour and oil spray over your engine as you are now bleeding from after where the fuel goes in.

What have I got to change?

It involves simplifying the boost control pipework. This consists of removing the T piece and short pipes from the carb top to carburettors middle outlet and connecting the front actuator port direct to the carburettors middle outlet and blocking the carb top. Removing the pipe from the rear actuator port to carb top to turbo elbow and blocking the turbo elbow and carb top.

The carb top is easy, just loop a short length of small hose to the two disused ports. Or on some matal carb tops unscrew the two ports and fit two bolts which will be either M8 for K-Tec or 1/8 NPT for GTTuning.

Leave back actuator port open to air. Sometimes people find they get a bit more boost by blocking this. This is because the actuator spring has weakened and the actuator needs to be replaced.

For the turbo elbow Renault used to sell a plug, part number 7703075121, it's no longer avaliable and the part no has been taken off the system.

A cut down wheel bolt works but you can buy, probably for a few pence from your nearest hardware store, a 12mm diameter bolt with the standard 1.5 pitch thread, known as isometric M12.

If you or you local shop are not sure what one of those is then you can take to the shop the original fitting that held the metal pipe work into the elbow as it has the correct thread on there. Get them to offer this thread up to the bolts they sell until they find a match.

It only neds to be about 20mm in lenght with the thread going right up to the head. My local shop only had 50mm bolts with the thread going only part way.

As a temporary fix for the turbo elbow or for a quick test you can simply block the rubber pipe at the end of the metal pipe where it leaves the one way valve by the carb top.

Q - "What if I have a adjustable boost in car with a twin port actuator? which pipes are keep/removed?"
A - "Nothing, because the in-car boost controller, or bleed-valve, is T'd into the front-port actuator line, and this isn't removed with the cup mod setup.
Standard system
Modified system
1) Carburettor body,
2) Carb top elbow,
3) Turbo actuator,
4) To the exhaust elbow,
5) Actuator adjusting rod


Is it worth it?

It has the advantage of tidying up the engine bay by getting rid of a load of pipes. Most people think it's worth it just for that.

And it can make your car go faster.

So yes, it's worth it.



Some recent threads:

one

two

three


Thank's to everyone who, albeit unknowingly, contributed to this article.

17/10/2004

Ian S RTOC Profile

Showing the carb top and pipe from carb middle



Phase two standard pipe arrangement



Cup mod pipe arrangement



An early phase one with cup mod as standard



Showing early single port actuator