75 Twinspark lacks torque/driveability compared to single-spark Nord?

[Evan Bottcher]

Well that's interesting - shows a lovely wallop of torque starting as low as 2200rpm.  The chassis dyno eliminates chassis weight as a source of my concern, so I think this points to something being fishy in my 75.  Thanks for the input guys, the boys at Maranello Pur Sang are going to have a play around with the car before I take it to Tassie for the tour next month.

[Evan Bottcher]

To pick up this thread.  The car had a fairly ugly big 2.5" exhaust with an aftermarket 'high-flow' cat and it was pretty noisy, droning through the car.  Fine around town but painful on longer trips.  So I picked up a second hand middle section (cat-back) in good condition - thanks Claude - and it was fitted yesterday.  The car is a lot more pleasant, without losing the lovely note.

However - it's also relieved most of the flat spot that was affecting it down low.  It's easier to get off the line without bogging down and generally a lot more peppy down low.  No changes were made to AFM, fuel quality plug is out, variator checked, and mixtures look good on the analyser (as best can be assessed statically).  I heard from a couple of sources that putting a big exhaust on these cars makes them go backwards...

Much much more pleasant to drive in the traffic this morning.  Result!  

Pretty sure the remaining disappointment I have with the 2 litre torque is in my head - it doesn't help that the other car in the driveway is a 2.4 JTDm with 400Nm of torque at 2000rpm.  

[Paul Gulliver]

I heard from a couple of sources that putting a big exhaust on these cars makes them go backwards...

I've also heard that from a lot of guys that race with twin sparks. Not worth fiddling with the exhaust until you have opened up the engine &  spent a lot of money.

[Tristan Atkins]

Hi guys,

I originally had a 2.5 inch exhaust on my GTV TS and I found that car had no real performance below 4000RPM.  I found an exhaust calculator on the internet and after putting in all the variables it suggested that 2.0 inch diameter would be optimum at 5000 RPM. The car now starts to get going at 3200RPM and I suspect smaller diameter exhaust would reduce this number even further.

Another factor that might make the TS feel sluggish is the way that the torque is delivered. From Mat's graphs the NORD engine provide a linear torque throughout the entire RPM range while the TS levels off at around 3000RPM then picks up again.

The two advantages the TS has over the NORD would be easy starting and fuel economy. I can almost get about 1000 km of a GTV6 tank with highway driving. However, I do miss that terrific noise that the twin carburetors made at full noise.

[Joe Falcone]

Hi to all

ive sprinted in a few 75 twin sparks in the past so thought id put mw 2 bobs worth in.
the only thing that hasnt been mentioned is the main pulley. i had one that was stripped
and causing quite a bit of havoc espescially at low rpm.

just for the record im now running a 79 gtv nord ts gearbox advanced cam timing and ignition
matching any of the 75s and having more fun

cheers Joe

[tony8028]

i have owned 2 of these cars....1988 and 1991 versions.

The 88 model i had was a ripper  - around 70K on the clock when i bought and immac (still have pics if anyone is interested).

That said, the 1991 model was much more lively. I think there was a tweak to the engine around that time? It wasnt my immagination, i noticed it the second i first drove it. Still bought it because it was in showroom condition.

Overall i found the 75 a bit truck like when on its standard springs....lots of roll...At the assistance of my mechanic i did lower it at one stage and yes the handling was 150% better, but I couldnt go over speed humps and struggled to even get out of my own driveway!!

But yes, the later model 75 was a different beast when it came to engine performance.

One thing that was consistent across both model was the terrible A/C !!!!

[Evan Bottcher]

Thought I'd mentioned something that happened a couple of weeks ago.  The battery was clearly on it's way out - started to crank quite slowly and not hold charge.  Probably four years old, so no great surprise.  Funny that it happened within a couple of weeks of the battery in the 159.  I'd also noticed that the alternator light had been coming on in the ARC from time to time, but would go out if I re-started the car.  Thought nothing of it.

Put a new battery in it - a supercharge supplied by Maranello Pur Sang - and I was surprised to find the car has a new lease of life.  Sharper throttle response, smoother idle, and better mid range torque.  It's not a rocket ship by any means, but a real nice improvement.  Bruno says he has seen it before - a faulty battery won't supply sufficient current for the EFI computer to work properly (or something like that - I missed exactly what he was saying).  Mostly I notice that the engine just runs a lot smoother right through the rev range.  Alternator is supplying 13.3ish volts so that's fine, and no alternator light at all in a couple of weeks regular driving.

Anyone else heard of this result?

[festy]

The ECU itself will happily run on as low as 8 volts, but I can think of a couple of explanations for why it's working better with a new battery:

1) If a fault was logged by the ECU at some point (i.e. transient sensor malfunction etc) then that fault won't be automatically cleared until the engine has been restarted 50 times.
Some fault codes will result in reduced power because they disable peripheral functions like VVT, or invoke various levels of "limp home" modes.
Replacing the battery (or just disconnecting the battery) will clear the ECU's memory, and the fault codes all disappear.

2) Although the ECU will run on low voltages, it tries to compensate for the lower voltage when calculating the injector pulse times and ignition dwell.
Lower voltage means the injectors take longer to open, so need to be open longer.
Lower voltage affects dwell because the coil needs longer to charge between fires.
As both the injectors and coil draw a reasonable amount of current, if your battery was weak then the voltage might be dropping even further as they activate due to the load, resulting in insufficient fuel and/or weak spark.

Either way, it sounds like you got a good result by replacing the battery so it doesn't really matter
Here's the twinspark's injector battery bias and dwell maps, just in case you're interested...
   

[Evan Bottcher]

Very interesting stuff Mr Festy.  Thanks for sharing the info.  Both those scenarios make good sense!  Cheers.

[Mat Francis]

Probably irrelevant now that Festy has proven the point, but I would fully support your theory. I replaced the battery in the work commodore yesterday and  it made a noticeable difference. Well at least for this morning, before a coil or something died again. Most unreliable POS I've ever had the pleasure of driving!