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08/03/2005

  Engine flatspot?

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Scorpio Power Delivery

'Cammy' Cosworth - or a Flat Spot?

In messages on the Confidential List some owners have complained of a 'flat spot' in power delivery between 3000 to 3500 rpm and have been asking if their cars have a fault which needs repairing. It is not that the engines have been running badly, it is just that the engine does not seem to deliver power when it is expected. The engine picks up, but after a moment of delay.

The answer lies with the Variable Resonance Inlet System (VIS) which enhances torque at higher engine speeds.

V6 Cologne (2.9 12V)

Below is the 12V torque/power graph for the 12V V6 Cologne engine. This has 2 hydraulically-operated valves per cylinder and a standard inlet manifold. Of the two plots, the Torque line is of more interest because the automatic gearbox uses the torque of the engine to transmit power to the wheels, not horsepower. Maximum power of the engine is delivered at 5,600rpm, but these revolutions will never be normally be reached: instead the engine rises to the revs at which it delivers maximum torque, at about 3,600rpm and stays there while power is required, before falling away as the accelerator is lifted and the torque converter clutch engages. At cruising speed the engine will be spinning at or slightly under 3,000 rpm and still developing 225 Nm of torque. The engine is still on an upward power curve and it will easily meet further torque demand from the gearbox. This is what gives the engine that wonderful 'grunt' which I so enjoyed in my previous three 12V Granadas.
 


24V Cosworth

With the Cosworth 24V engine in the Scorpio the engine benefits from four cams and 24 hydraulic valves. Because of their breathing characteristics, multi-valve engines tend to develop more power higher in the rev band, which is bad for economy, and to counteract this Cosworth developed a variable inlet system. See VIS. Restricting one of the inlet valves at lower revs (below 3,200 rpm) and using resonance techniques in the inlet, as well as better porting, Cosworth have produced a much steeper power curve from the same capacity. See the diagram below.
 

Already, below 3,200 rpm the 24V has pushed up the Nm from 225 on the 12V to over 260 Nm and this is developed to coincide with a cruising speed of up to 90 mph. There is ample power to breast hills, however steep, even with a full load of passengers and fuel - this is cruising at its most relaxed.

Beyond 3200 rpm, however, there is a drop in torque which is caused by changes in the inlet manifold as the VIS valve opens and the secondary resonance is established. This lasts for only about 500 rpm, and then the engine is once again at the bottom of a very steep power curve in which the Nm soars from 253 Nm to 281 Nm in only 700rpm. This is an impressive achievement - only 12 Nm less than the 2.5 litre turbo diesel - indeed, at the time of its introduction this engine was the most powerful normally aspirated sub-3 litre petrol engine in the world. This is the reason for the exhilarating surge in acceleration which can leave so many cars behind, floundering in its wake.

However, it needs a slight change in driving style to accommodate this characteristic. While cruising at 80 mph, for example, the driver faces a delay of 700 rpm between single inlet operation and the onset of the double. If he just depresses the accelerator the car will only slowly gain speed - the torque of the engine is on a downward curve and it feels almost sluggish.

Instead, the driver should flick the car out of overdrive 4th. Immediately, the revs rise by 500 rpm, and if the driver now smoothly matches the revs with the road speed the engine is poised at 3,600 rpm, exactly at the bottom of the next power curve, ready to unleash that incredible surge of Cosworth quad-cam power. Gently depressing the accelerator now delivers a immense surge of torque which is such an incredible experience for the new owner - and remains available to 140 mph.

This is not even using kickdown! From rest, if the driver buries his right foot into the carpet, the EECV Powertrain Control Module detects a Wide Open Throttle (WOT). It automatically times the injectors to a rich mixture and switches off the air conditioning compressor clutch to reduce engine load, while the EGR valve closes to allow a full air/charge mixture and the torque converter clutch unlocks to allow the torque converter to spin up to maximum torque delivery. The engine pours its torque into the converter and the variable inlet opens as soon as the rev band is reached. At the rate the revs are increasing the VIS changeover is almost unnoticeable: now the engine note rises to a formula one howl, the back axle has settled on its springs and the bonnet has lifted slightly and the speedo needle is sweeping through to the limit - your passenger's eyes are agape with astonishment and his head is jammed back against his headrest. His words are normally unprintable ...

DOHC Power Delivery

The 24V is not the only recipient of VIS - the DOHC2000 16V engine also uses variable inlets to enhance torque delivery. Below is the power chart of the DOHC 8 valve. Here, the torque is a relatively flat curve peaking at 166 Nm at only 2,500 rpm. This torque remains above 160 Nm until 4,600 rpm, when it tails off fairly quickly. The 8 valve Scorpio was only available with manual transmission, and the power (KW) curve shows a healthy line from 30 Kw at 1500 rpm to 84 Kw (115 bhp) at 5,600 rpm.


Now, below, the result of four valves and VIS on the DOHC engine. For the manual engine (although timings can be programmed differently into the EECV depending on the transmission) the power graph shows an almost straight progression up to 100 Kw (136 bhp) at 6,400 rpm. In any manual gear the driver will have a smooth progression of power available as the revs rise.


Here, to a less marked degree, the influence of the VIS has a similar effect. At 3,600 rpm the single inlet had done its work and has given its best. The torque produced by the engine would continue to fall now through the remaining revs on the engine, but at 3,600rpm the second inlet opens, both the inlet valves are now admitting resonating air and the torque rises swiftly to 175 Nm at only 4250 rpm. Again, this is a useful boost in torque available to the driver of the automatic - it is welcome extra 'grunt' that he will be able to use on overtaking and breasting hills.

2.3 DOHC

Regrettably, the publicity brochure from which I obtained these power graphs does not show the 2.3 engine - it was introduced to the range a full year later. I have another brochure which covers the 2.3, but in this publication the power graphs have been omitted altogether. On this engine, Ford claimed that the new composite material inlet manifold was so efficient that it did not need the VIS system and it would have been nice to see if this was the case. The maximum power of the 2.3 is 147 bhp. @ 5,600 rpm.

If you have a brochure which does show a 2.3 power graph, please let me know.

2.5 VM TDi

The Vittori 2.5 turbo diesel was only available with a manual transmission until the last 8 months of production in 1998, when an automatic was introduced. The graph above appears to be a little generous to the earlier diesel - an uprated VM engine was not introduced until September 1996.


As expected, being a compression-ignition engine, the torque curve rises steeply to a maximum of 293 Nm at only 2200 rpm. Thereafter torque trails off sharply. A driver would normally choose to keep the engine between 1750 and 2,500 rpm to make maximum use of this torque. The power curve rises smoothly until 4,200 rpm, where it is developing 82 Kw (125 bhp)


Graphs reproduced from December 1994 Scorpio brochure - Ford Copyright.
 

EricR

 

 

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