Double throttle system - duh...

[trotter]

Can anyone explain to me how / why this thing works? I\'ve been trying to figure it out for a few weeks now and am just going in circles!!

What I have so far....

1) Only works on cold startup
2) Double throttle system keeps the upper (secondary?) throttle plates closed so that you cannot rag the t!ts off when cold
3) After coolant reaches a certain temperature, the wax pellet thingie melts, disconnects the double throttle control and she is good to go.

Correct?

graeme

[Jono FD3]

yep, that is it in a nut shell.... stops you WOT untill it\'s fully warm!

Jono

[philiwily]

The double throttle is two separate throttle plates which close off the secondary runners to stop it being thrashed when cold.
It\'s opened and closed by a vacuum actuator which is switched by the ECU via a solenoid.
The wax pellet is for the fast idle cam.

Phil

[RXSpeedy]

Correct !!!

[m1tch]

This will of course only work if you still have your secondary butterflys

[trotter]

Right, I see!! So I assume then that there is a secondary set of butterflies behind the ones you can see. i.e. throttle still activates the \'front\' ones but the actuator holds the \'rear\' ones shut?

And what is the fast idle cam for? Do I assume that it\'s used when starting from cold(hence the higher idle speed) before it begins to drop off?

graeme

[philiwily]

Right, I see!! So I assume then that there is a secondary set of butterflies behind the ones you can see. i.e. throttle still activates the \'front\' ones but the actuator holds the \'rear\' ones shut?

Spot on.

And what is the fast idle cam for? Do I assume that it\'s used when starting from cold(hence the higher idle speed) before it begins to drop off?

graeme

Spot on again. It\'s a cam that stops the throttle linkage fully closing. It\'s controlled by a wax pellet which is heated by the two coolant hoses going to the throttle body. Because of this when setting your TPS when cold you need to disengage the cam.

Phil

[trotter]

last question, i promise!

Whats the reason for the higher initial engine speed? is it to get it to running temp quicker?

[trotter]

Sorry, ignore previous post. Let me see if I have it sussed:

1) Double throttle system:

Purpose is to prevent driving with WOT on a cold engine. Achieves this by holding the number 2 set of secondary butterfly valves closed. i.e. although the number 1 set are open, no more air can get into the engine as the pathway is blocked by the number 2 set. At a certain point (engine temperature) the ECU actuates the solenoid, releases the vacuum and the 2nd set fully opens and remains so, thus allowing more air into engine = WOT.

2) Fast Idle Cam:

Purpose is to get the engine to operating temperature quickly. Does this by a plug of cold thermo wax holding the primary butterfly valve open (hence high initial idle ~1500rpm). Once coolant reaches a certain temperature, the wax melts and allows the primary valve to close completely (idle drops to ~1000rpm). Note that this is NOT the Accelerated Warm-up System that was installed on US cars which had the purpose of also increasing the temperature in the O2 sensor and cat. This system will hold revs at 3000rpm.

graeme

[philiwily]

Sorry, ignore previous post. Let me see if I have it sussed:

1) Double throttle system:

Purpose is to prevent driving with WOT on a cold engine. Achieves this by holding the number 2 set of secondary butterfly valves closed. i.e. although the number 1 set are open, no more air can get into the engine as the pathway is blocked by the number 2 set. At a certain point (engine temperature) the ECU actuates the solenoid, releases the vacuum and the 2nd set fully opens and remains so, thus allowing more air into engine = WOT.

2) Fast Idle Cam:

Purpose is to get the engine to operating temperature quickly. Does this by a plug of cold thermo wax holding the primary butterfly valve open (hence high initial idle ~1500rpm). Once coolant reaches a certain temperature, the wax melts and allows the primary valve to close completely (idle drops to ~1000rpm). Note that this is NOT the Accelerated Warm-up System that was installed on US cars which had the purpose of also increasing the temperature in the O2 sensor and cat. This system will hold revs at 3000rpm.

graeme

Couldn\'t have said it better myself. I have done away with both systems with no ill effect.

Phil

[trotter]

Excellent, another mystical being gets found out!!

Cheers Phil, (and others)

graeme

[bumpstart]

Sorry, ignore previous post. Let me see if I have it sussed:

1) Double throttle system:

Purpose is to prevent driving with WOT on a cold engine. Achieves this by holding the number 2 set of secondary butterfly valves closed. i.e. although the number 1 set are open, no more air can get into the engine as the pathway is blocked by the number 2 set. At a certain point (engine temperature) the ECU actuates the solenoid, releases the vacuum and the 2nd set fully opens and remains so, thus allowing more air into engine = WOT.

graeme

i will clear this up a little further
there is several variations of double throttle system
,, the one described almost correctly above
 is the one with the thermopneumatic switch that holds off secondary operation when cold
 s5\'s lack the thermopneumatic valve and also the diaphragm
these work purely on dashpot drag tension on a lever
there is also an interim later s4 TB with features of the early combined with the plastic drag bob weight mechanism


in ALL systems the forward most pair of throttles is the delay set,,,
and these will only ever open to the same opening as the other throttles
they will only open 100% if the main throttles are 100% open

 the rear set work directly with mechanical action of the primary throttle linkage once the primary throttle is about 15-20 % open


as demonstrated above,, the later system has NOTHING to do with denying the secondaries on warmup
the delay secondaries will open at the same rate hot and cold

in all examples the delay secondaries are added because the ECU has limited clockspeed to deal with such large fluctuations in airflow if all throttles snap as one

the throttle delay system allows simple throttle pump strategy to work without leanout pop and snap bad manners

those that remove the systems often have aftermarket ECU,,
and have brought changes in ECU clock-rate,,and staging and throttle pump enrichment strategies


NB
i search for these ealier TB\'s with the pneumatic delay
- a few tricks allows you to use them on very large ported engines and use the delay to deny the engine secondaries until boost or load occurs in the primary runners
- this brings a TQ and spool bonus on said engines by restoring throttle staging to load demand

[philiwily]

My mistake for not making it clear that we were talking about FD\'s, which work exactly as described in post #9. I will put my hands up and say I know nothing about FC\'s.
Phil

[bumpstart]

My mistake for not making it clear that we were talking about FD\'s, which work exactly as described in post #9. I will put my hands up and say I know nothing about FC\'s.
Phil

i suppose that clears that up
,, but does make the interesting observation that the FD ECU is quick enough to catch the throttle change without need for a mechanical drag delay

in review

s4 thermo-pneumatic switched with mechanical drag dash pot ( cold thermo hold out function and drag delay )
s5- mechanical drag dash pot,, no cold thermo function ( drag delay only )
FD- electric-pneumatic solenoid control ( cold thermo hold out function only )

[philiwily]

but does make the interesting observation that the FD ECU is quick enough to catch the throttle change without need for a mechanical drag delay

I did actually find that very interesting, when I was writing my last post I wrote almost the exact same as the above quote but deleted it as it read like I was being sarcastic (which I wasn\'t).

Phil