Low speed shunting...

[Jasandjules]

Anyone got any sure fire tips to resolve this? (5.0 engine)

[bobfather]

There are several stories relating to low engine speed shunting. The current front runner is that the trumpet length encroaches too close to the top of the plenum cover. This causes the trumpets on induction cycle to create a negative pressure in the vicinity of the top of those trumpet which causes air starvation to adjacent trumpets. People who have installed plenum spacers to lift the plenum and thereby open up the tight air space report complete elimination of low engine speed shunting.

[Paulprior]

But mine is standard as far as I know and drives at low speed in town just as you would expect any car to, so surely it's not a problem on all cars, the only time I had a kangarooing sensation at low speed was after fitting a dodgy ignition amplifier.

[bobfather]

Low engine speed shunting happens on all of them as far as I know, you may be someone who doesn't normally let the revs go low enough.

[carsy]

A little more fuel in that rpm area usually helps.

Go get booked in with Jools and let him wave his wand over it.

[ClassiChimi]

^^^^^^ What he said
After market Ecu will sort it!
Couldn't resist,,,
Failing that a trip to someone who can adjust the map after making sure you've current tested everything and ensured all the other associated parts are in fully working order when hot.
Or just buy Rover gauge and start from there.

Edited by ClassiChimi on Tuesday 9th August 17:36

[Paulprior]

Surely I'm not the only one to have smooth low rpm? I spent about 1.5 hrs in town traffic jams this weekend without any problems, and when I am moving steadily but slowly I'm rather lazy and sometimes find myself in 5th gear st 1000rpm, that's still ok, but I'm sure I have heard others say the same?

[bobfather]

Paulprior said:

Surely I'm not the only one to have smooth low rpm? I spent about 1.5 hrs in town traffic jams this weekend without any problems, and when I am moving steadily but slowly I'm rather lazy and sometimes find myself in 5th gear st 1000rpm, that's still ok, but I'm sure I have heard others say the same?

Could this be the result of an overly retarded ignition? In my youth I used to mess with advance & retard on old carb Fords and rough tickover was an indication of being close to max advance

[blitzracing]

bobfather said:

There are several stories relating to low engine speed shunting. The current front runner is that the trumpet length encroaches too close to the top of the plenum cover. This causes the trumpets on induction cycle to create a negative pressure in the vicinity of the top of those trumpet which causes air starvation to adjacent trumpets. People who have installed plenum spacers to lift the plenum and thereby open up the tight air space report complete elimination of low engine speed shunting.

I would not believe that one considering it happens at very low throttle openings so the air flow is very low over the top of the trumpets, so plenum height wont cause a restriction.

[ChimpOnGas]

I'm a huge fan of what's possible with after market engine management systems, the 3D mappable ignition advantage alone puts it streets ahead of the ancient distributor setup. But then I jump in my mate's bone stock 5.0 litre Chimaeraa with its 14CUX fuelling & distributor/single coil ignition, and boy its smooth.

Conclusion... if a standard car is running right there is no shunting.. period!!!!

Or to put it another way.... if its shunting its broken.

However... it's certainly true the Lucas 14CUX ECU can make some odd instructions to the stepper motor at times (normally sensor related) and the stepper motor itself is not the most reliable thing in the world, not to mention those dreadful spark plug extenders that should be eliminated in favour of a more reliable solution to protect the HT leads from the heat TVR inflicted on them with their exhaust manifold design.

Get it all right (base idle, stepper motor function, throttle pot adjustment, ect ect) and honestly it shouldn't shunt, replace the highly inappropriate shrouded electrode (non projected) NGK B7ECS plugs chosen by TVR with a set of of NGK BPR6ES or better still NGK BPR6EIX iridium projected electrode hotter rated No6 plugs and you will feel further improvements still.

I am still an after market ECU advocate for sure, but I have to admit if everything is spot on with the standard setup... a Chimaera really shouldn't shunt

[bobfather]

blitzracing said:

bobfather said:

There are several stories relating to low engine speed shunting. The current front runner is that the trumpet length encroaches too close to the top of the plenum cover. This causes the trumpets on induction cycle to create a negative pressure in the vicinity of the top of those trumpet which causes air starvation to adjacent trumpets. People who have installed plenum spacers to lift the plenum and thereby open up the tight air space report complete elimination of low engine speed shunting.

I would not believe that one considering it happens at very low throttle openings so the air flow is very low over the top of the trumpets, so plenum height wont cause a restriction.

I agree with that, my thoughts exactly but evidence has shown many many times that increasing the head space in the plenum eliminates shunting. Perhaps it has nothing to do with adjacent trumpet starvation, I am certain though that there is some physical change that associates greater plenum head space with smoother 'on power' low rev operation

[ChimpOnGas]

With everything I've just said.... this was my post back in October 2009.

http://www.pistonheads.com/gassing/topic.asp?t=764...

That spacer definitely made a big difference for me, and its still on my car to this day

[TV8]

ChimpOnGas said:

I'm a huge fan of what's possible with after market engine management systems, the 3D mappable ignition advantage alone puts it streets ahead of the ancient distributor setup. But then I jump in my mate's bone stock 5.0 litre Chimaeraa with its 14CUX fuelling & distributor/single coil ignition, and boy its smooth.

Conclusion... if a standard car is running right there is no shunting.. period!!!!

Or to put it another way.... if its shunting its broken.

However... it's certainly true the Lucas 14CUX ECU can make some odd instructions to the stepper motor at times (normally sensor related) and the stepper motor itself is not the most reliable thing in the world, not to mention those dreadful spark plug extenders that should be eliminated in favour of a more reliable solution to protect the HT leads from the heat TVR inflicted on them with their exhaust manifold design.

Get it all right (base idle, stepper motor function, throttle pot adjustment, ect ect) and honestly it shouldn't shunt, replace the highly inappropriate shrouded electrode (non projected) NGK B7ECS plugs chosen by TVR with a set of of NGK BPR6ES or better still NGK BPR6EIX iridium projected electrode hotter rated No6 plugs and you will feel further improvements still.

I am still an after market ECU advocate for sure, but I have to admit if everything is spot on with the standard setup... a Chimaera really shouldn't shunt

Agree with this. There are a series of books that pull together all of the road tests from when these cars were new and shunting and poor low speed running are not really mentioned, let alone almost accepted/expected as they are today. There are a couple of comments about traffic not being the cars favorite place but nothing to dramatic from memory

[ChimpOnGas]

TV8 said:

ChimpOnGas said:

I'm a huge fan of what's possible with after market engine management systems, the 3D mappable ignition advantage alone puts it streets ahead of the ancient distributor setup. But then I jump in my mate's bone stock 5.0 litre Chimaeraa with its 14CUX fuelling & distributor/single coil ignition, and boy its smooth.

Conclusion... if a standard car is running right there is no shunting.. period!!!!

Or to put it another way.... if its shunting its broken.

However... it's certainly true the Lucas 14CUX ECU can make some odd instructions to the stepper motor at times (normally sensor related) and the stepper motor itself is not the most reliable thing in the world, not to mention those dreadful spark plug extenders that should be eliminated in favour of a more reliable solution to protect the HT leads from the heat TVR inflicted on them with their exhaust manifold design.

Get it all right (base idle, stepper motor function, throttle pot adjustment, ect ect) and honestly it shouldn't shunt, replace the highly inappropriate shrouded electrode (non projected) NGK B7ECS plugs chosen by TVR with a set of of NGK BPR6ES or better still NGK BPR6EIX iridium projected electrode hotter rated No6 plugs and you will feel further improvements still.

I am still an after market ECU advocate for sure, but I have to admit if everything is spot on with the standard setup... a Chimaera really shouldn't shunt

Agree with this. There are a series of books that pull together all of the road tests from when these cars were new and shunting and poor low speed running are not really mentioned, let alone almost accepted/expected as they are today. There are a couple of comments about traffic not being the cars favorite place but nothing to dramatic from memory

And how many £35k TVR Chimaeras & Grffs would they have sold after a test drive if they shunted?

[sheel]

Prior to my upgrades on my 4.5 chim to include the fabulous canems system I must have been lucky as I never suffered shunting and could happily drop her down to 1000rpm in 5th with no detrimental effect,with the canems she is just as or more so happy and totally flexible
Rich

[ClassiChimi]

Sheels car is a bit special you know,,
It was rather impressive on the old CUX, I remember that,
Now if Rich's car has never shunted, it might account for why his drive train is smooth and tight,,, no play whatsoever,

I've often tried to work out if the drive train is not matched up well but in fact Rich's car contradicts than in spectacular fashion,
It's the easiest Tvr I've driven and possibly one of the nicest, that drivetrain does really add to the enjoyment, mine in comparison is loose,
Oddly driving hard mine seems rock solid and direct.

I've actually tried turning a rear wheel on Rich's car when it was up in the air, no free pay, no movement just tight.
Mine has free play of about an inch,,, just enough to cause shunting if I'm not careful with the throttle, slack!

Just shows you that if the CUX is working well, it has added benefits!

[Jon100p]

I am a bit of a novice on these cars so tell me if you think I am talking rubbish.

As I understand the shunting problem, it occurs when applying a just little throttle at low speed and rpm, after the throttle has been closed. Things that could influence engine behaviour in this instance and have already been mentioned such as stepper motor, base idle setting also dirt in these and on the throttle butterfly. But what about the vacuum advance - has anybody tried blocking this to see if it makes a difference?

My reason for suggesting this is because the tapping is just ahead of the throttle seat so there is no vacuum applied when the throttle is closed and then a lot of vacuum as soon as the throttle is a little open past the tapping. Could a faulty vacuum advance delay valve cause problems?

[griffdude]

ClassiChimi said:

^^^^^^ What he said
After market Ecu will sort it!
Couldn't resist,,,
Failing that a trip to someone who can adjust the map after making sure you've current tested everything and ensured all the other associated parts are in fully working order when hot.
Or just buy Rover gauge and start from there.

Edited by ClassiChimi on Tuesday 9th August 17:36

Jools remaps the standard ECU aswell.

[ClassiChimi]

griffdude said:

ClassiChimi said:

^^^^^^ What he said
After market Ecu will sort it!
Couldn't resist,,,
Failing that a trip to someone who can adjust the map after making sure you've current tested everything and ensured all the other associated parts are in fully working order when hot.
Or just buy Rover gauge and start from there.

Edited by ClassiChimi on Tuesday 9th August 17:36

Jools remaps the standard ECU aswell.

And very well to so I hear

[blitzracing]

I can add that I can turn shunting on and off at the flick of a fuel map between the white catalyst map (white tune) that shunts and the green non catalyst map that runs beautifully. In my eyes you are simply getting the air fuel ratio where you want it on the green map without the big expense of an aftermarket ECU to do exactly the same thing. Mind you you cant use the standard green map as its Range Rover plus you cant run catalysts. I dont know if the aftermarket ECU boys faff about with all the emission compliance, or just get the car running right.