Shunting

[Muffin8372]

Hi!!!

I am new to the TVR world and have a 94 TVR Chimaera 400 HC. The car pulls like a train when it is opened up but shunts when at low speed, even for example, in 2nd @ 2000 rpm. I have already changed the plugs and CTS as seen on these forums, and the car is in exceptional condition, so I cant even say its a neglected old nag!

Any advice is gratefully received, timing etc what are your thoughts?

Cheers

[Belle427]

Have a read of this, the vacuum advance modification may be worth a try.
https://www.pistonheads.com/gassing/topic.asp?h=0&...

[Muffin8372]

Wow, I have disappeared down a worm hole!!! Thanks

[Zeb74]

I just changed mine and it was totally worn, I also had some issues at low revs and the car was running a bit too rich, now this is much better.
Don't do the same mistake I did (helped with my 2 left hands), I have removed the distributor as this sounded easier to remove the advance cap. But if the position of the distributor was marked, I forgot to mark the position of the finger. So I had to set the timing back after this.

[Belle427]

The last couple of pages on that thread are the most important regarding relocating the vacuum source.
Its always beneficial to ensure your ignition system is tip top first though such as the dreaded extender removal.

[blitzracing]

Basically shunting is due to a poor burn at low airflows with the emission control system running. This is caused by exhaust gas getting back into the plenum during the extended over lap period of the TVR cam- and why its not a problem on Range Rovers. The effect is the exhaust contaminates the inlet charge as it mixes in the plenum so it does not burn as well. The reason there are so many "fixes" is ANYTHING that improves the combustion will help get rid of it, albeit ignition or mixture control. Ideally adding a bit more fuel certainly greatly reduces it, but almost impossible on the catalyst fuel map as the ECU clamps the mixture at 14.7:1 pretty much whatever you do to the mapping. On the non catalyst fuel map you can alter the mixture, but it requires a new fuel map chip to match the TVR tune, and a under dashboard switch so you can go back to catalyst mapping at MOT time. Likewise the vacuum take off mod will also fail the emissions test, so needs to be reversible at MOT time. Whether a car shunts or not is all very marginal so you might get lucky with minor tweaks to get rid of it. Shunting becomes very pronounced where the ECU has make big fuelling changes on the fly to keep the mixture correct if there is also a fuelling fault. This will be seen as high long term fuel trims or poor cycling of short term fuel trims in RoverGauge.

[Muffin8372]

Thanks Blitz, whats the easiest way to do the vacuum take off mod?

[Belle427]

Page 8 of that thread above explains what to do.

[Muffin8372]

Thanks for that!!!!