Curiosity really, but what dictates how quickly a gearbox drops the clutch so to speak? In the last few days I've driven 4 different P38s and have noticed a difference when pulling away from a standstill. On mine and OldShep's car when you pull away the revs go up to about 1,800rpm and the torque converter then makes the car accelerate while the revs stay constant. A bit like giving it some throttle and slipping the clutch on a manual. On the SE and one that a mate has just bought that he bought over to me to give the once over, the torque converter seems to lock up immediately and the revs go up as the car accelerates, more like dumping the clutch and booting the throttle on a manual.

I initially thought it would be down to mileage and a bit of wear but mine is on what I assume to be the original gearbox and has done 329,000, while OldShep's car only has 104,000 on the clock. The SE has 134,000 and the one my mate has just bought has 173,000 so that theory would appear to be out of the window. What dictates the speed the TC locks up at in the lower gears then? Is it the fluid, the fluid pressure, the torque converter itself or is it something more subtle like the output signal from the TPS maybe?

I have no idea and can't offer an insight.

However I can agree that same is different.

I've got 2 identical p38s. Same year, same engine. The old one goes like shit off a stick, whereas the new one I just got seems a bit more refined. Let's say one is wild, and the other is domesticated.

On mine it depends of throttle position. In citiytraffic when easy pulling away it shifts a bit like your mate's, 1-2-3 to 1800 rpm and then in lockup (a bit harsh) back to 1250-1300 rpm which is about 60 km/hr.
When entering a motorway a bit firm the lock up at 4th is at 2000 rpm, say 90 km/hr and revs drop to 1700 rpm (ish).

I found that tensioning the throttle cable exactly as per RAVE transformed the changes.

I was amazed the difference such a small input made.

Could that be the reason for the different characteristics?

That's interesting George, as far as I know the cable is adjusted correctly but I'll give it a go. As there's no kick down cable on the P38, that would suggest it has something to do with the signal from the TPS. My TPS is an aftermarket one too so may have slightly different characteristics.

lockup and stall are different things.

The 1800rpm "slipping the clutch" feeling is the TC sitting against its stall speed. However the stall speed varies a bit depending on torque input and thus throttle position. Higher torque, means a higher stall speed, simply due to the fluid mechanics inside the converter. The throttle position also determines wether the trans holds onto the gear your in, and thus lets the engine rev up beyond the stall, or upshifts.

If your below the stall speed, and apply some throttle, the engine will tend to rev up to the stall speed and sit there. This effect boosts torque output, as a) the engines spinning faster and thus able to produce more torque anyway, and b) the slip in the converter gets used to amplify the torque further. Once speed in gear matches the stall speed, the engine revs will start to rise like a manual box. At light throttle, the box will shift before or just as that happens i suspect, and thus you'll find the revs just sort of sit around 1800 while it accellerates thru the gears.

The 4.0 and 4.6's also have (i believe) different converters with different stall speeds. From memory, the 4.0 stalls higher, to extract more performance from the engine. The larger 4.6 engine stalls out sooner, making it more refined. The diesels are probably different again to due the differing power band. Given these are old cars, its possible ones had the transmission changed and a different TC fitted?

The lockup clutch usually only kicks in in higher gears, and completely locks the converter solid, like a clutch on a manual car. This improves fuel efficency by removing the losses in the TC's hydraulic coupling. Newer boxes (like the modern 7/8 speed jobs) i think can use the lockup cluch more often, but the older ones like the P38's tended to only lock up in top gear at a certain speed.

Gilbertd wrote:

That's interesting George, as far as I know the cable is adjusted correctly but I'll give it a go. As there's no kick down cable on the P38, that would suggest it has something to do with the signal from the TPS. My TPS is an aftermarket one too so may have slightly different characteristics.

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I've emailed you a copy of the SID for the Automatic Gearbox
Covers most of the controls and interactions for the box controller

Yes Mark, got that but not the sort of thing you can have a quick flick through and find the answer. I'll have another look later when I've got (a lot) more time.

Seems Aragorn is our auto box expert then and I can follow that. What doesn't make sense to me is why there are differences between cars? One thing I noticed on mine while playing with the Nano is that the calculated load value (CLV) at idle when hot shows around 16%. On the SE, admittedly when cold, it shows 25%, yet RAVE says a hot engine at idle with no load, should be around 3%. Any clues what this might be telling me?

On my Vogue using Nano data I get
Idle Load Air at 3% (from Air/ Fuel recording in Motronic)
Engine Load (at idle) 18% (From General 1 recording in Motronic)
Maybe RAVE refers to Idle Load Air as reported by Nano?

That question pricked my interest, so did a bit of digging
CLV is (crudely)an indicator of manifold vacuum and is internally calculated from inputs of air flow, air temp, air pressure
Characteristics of CLV are:
— Reaches 1.0 at WOT at any altitude, temperature or rpm for both naturally aspirated and boosted engines.
— Indicates percent of peak available torque.
— Linearly correlated with engine vacuum
— Often used to schedule power enrichment.
= [current airflow] / [(peak airflow at WOT@STP as a function of rpm) (BARO/29.92) SQRT(298/(AAT+273))]
— Where: STP = Standard Temperature and Pressure = 25 °C, 29.92 in Hg BARO, SQRT = square root,
— WOT = wide open throttle, AAT = Ambient Air Temperature and is in °C

Just for completeness an Absolute Load Value is needed. I think this is what Nano reports as Engine Load

ALV is the normalised value of air mass per intake stroke displayed as a percent
Characteristics of ALV are:
— Ranges from 0 to approximately 0.95 for naturally aspirated engines, 0 – 4 for boosted engines,
— Linearly correlated with engine indicated and brake torque,
— Often used to schedule spark and EGR rates,
— Peak value of LOAD_ABS correlates with volumetric efficiency at WOT.,
— Indicates the pumping efficiency of the engine for diagnostic purposes.
= [air mass (g / intake stroke)] / [1.184 (g / intake stroke) * cylinder displacement in litres

— Often used to schedule power enrichment.

Mine sits at around 25% all the time, sometimes more, when the engine is idling. I wonder if that's why it's so bloody gutless between 2000 and 3000 revs, it's not sitting in the right parts of its fuelling and timing curves?

gordonjcp wrote:

Mine sits at around 25% all the time, sometimes more, when the engine is idling. I wonder if that's why it's so bloody gutless between 2000 and 3000 revs, it's not sitting in the right parts of its fuelling and timing curves?

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On a GEMS (and to a lesser extent the Thor) the mid-range gutlessness is due to fuelling/ timing designed to "cheat" the emissions test. The famous Tornado chipset is designed to address that
"Normally these engines are not very interested in revving much further than 4500RPM, and they also feature a flat spot on full load between 1750 and 3250RPM. Most of the available response is seen on the first and last 25% of the throttle pedal travel, whilst the 50% in the middle doesn’t make much difference."

The Nano items are labelled differently between Thor and GEMS, GEMS doesn't have an Idle Load Air reading but has the CLV which the Nano manual describes as "Calculated load value (%): This is an internal value where the GEMS keeps a track on the load that the engine is under it is used in internal fuelling calculations." You've got an Engine Load figure of 18%, so, assuming that is the same thing, yours is reading roughly the same as mine.

RAVE quotes a CLV figure of between 2.8 and 3.8% as being correct at idle with no noted differences between 4.0 litre or 4.6, GEMS or Thor. The only thing I did notice is that RAVE quotes mass airflow as 20 kg/hr +/- 3 while I got 17 or 18 ish, so within spec but only just.

I'd never head of the Tornado chip (being a newbie to Rover V8s). Some pretty startling claims are made - but £385 plus VAT and shipping is a little harsh!

You've never heard of Mark Adams? The chip does make a hell of a difference but as you say, it's not cheap......

Morat wrote:

I'd never head of the Tornado chip (being a newbie to Rover V8s). Some pretty startling claims are made - but £385 plus VAT and shipping is a little harsh!

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Mark is pretty much the global authority on the GEMS ECU. All of his numbers are backed up with dyno figures. He's extended that to the Thor as well, although, as he says, you don't get the same overall power increase as you do with the GEMS.
My GEMS has the Tornado chipset. I've only driven 1 other GEMS and mine certainly feels like it has much more poke in the mid-range than that one did.
He does also supply ECUs for GEMS and Thor with the Tornado upgrade that have the security stuff programmed out- designed for using the engine in other cars. No danger of the BECM and engine ECU conspiring to immobilise the vehicle.

Agreed with Aragorn on autoboxes / TC's. The TC in my car (Grand Voyager) locks up in top (4th), will lock in 3rd if it's been sat in 3rd for a long while or if the shifter is set to 3. It will lock up in 2nd if the gearbox temp gets very high... When TC's slip they generate heat, too much heat is very bad for autoboxes. I thought about fooling the box temp sensor to make the box lock in 2nd at the press of a switch, would probably help prevent box getting hot when pulling the caravan up that steep incline on the A30... But I'll probably use the ML for towing instead lol, finally reserved a bit of time to work on it and it passed it's MOT the other day.

Engine load figures are fairly abstract - Is 100% load at the red line with full throttle, or is it at full throttle at rpm where the engine makes most power (which could be below red line rpm), or is it at torque peak rpm (which is well below red line) with full throttle? Then there's part loads e.g. 50%, is this at half throttle at the red line, or at full throttle at half red line rpm, or both? Because they place very different demands on the engine, fuelling and ignition timing won't be the same between them... The load figure cannot be used directly for ignition timing or for fuelling. Some LPG ECU's calculate engine load but there is always the disclaimer about it's abstract nature. Some allow you to set a change back to petrol after a certain engine load, but you'd be hard pressed to guess what combinations of rpm and throttle position would set the switchback condition.

A lot of OBD2 compatible cars with MAF sensors show g/s in live data, I take this to be grams per second...Can confirm if grams / seconds interpretations are near correct... An engine of around 1.6litres generally reads around 3g/s at idle when warm, which would equate to 10800 grams per hour, 10.8Kg. Given stochiometric ratio of 14.7:1 for petrol this would equate to 0.74Kg of petrol burned per hour and since petrol has a density of 0.77Kg per litre this equates to around a litre per hour (0.22 gallons per hour) which won't be far off for warm idle for a 1.6. A car with this engine might do 40mpg at 60mph which is 1.5 gallons per hour, thus using 6.8 times as much air (20.4g/s) and fuel to cruise at 60 than sat at idle, which also makes sense.

Simon

If I remember correctly, RAVE only mentions the accelerator cable adjustment for GEMS, not for Thor.

That said, I can't see how it makes any difference on either - given the TPS is driven on the same shaft as the throttle butterfly, which is driven by the cable. With no kickdown cable, I can't see how it would make any difference on either management system...

Thoughts?

There is cable setting info in RAVE (Section 19 Page 2) for both types, but for GEMS throttle cable adjustment RAVE does say
NOTE: Accurate setting of this cable is
critical to correct operation of automatic
transmission.

Yes, but why? There is no potentiometer on the pedal itself to match up to, or anything between it and the throttle body... and the cable just pulls the butterfly shaft which directly operates the TPS on the other side. So I can't see how its critical at all.

Unless it was something accidentally left in from the Classic/whatever else actually had a kickdown cable perhaps, that might need some more careful attention.