Discussion
Guys,
Just completed an eighty mile round trip and found that the Cerb was rather jumpy when up to temperature and at low speed. Foot down it was sweet, but after hitting traffic she was uncomfortably jerky! Any suggestions on why this is happening? Tick over revs was still a steady 1000 when static.
Any comments or suggestions would be very much appreciated.
Thanks in advance
GG
Just completed an eighty mile round trip and found that the Cerb was rather jumpy when up to temperature and at low speed. Foot down it was sweet, but after hitting traffic she was uncomfortably jerky! Any suggestions on why this is happening? Tick over revs was still a steady 1000 when static.
Any comments or suggestions would be very much appreciated.
Thanks in advance
GG
I tend to find that urban speeds find me in 2nd or 3rd and revs hovering around 2000. This seems to be a sour spot (as opposed to a sweet spot!) in the rev range for the AJP. I have a Jools tweeked chip and it is still like it.
The only solution is to run around town at 2500-3000 revs scaring the local populace or change up as soon as you hit about 1800!
The only solution is to run around town at 2500-3000 revs scaring the local populace or change up as soon as you hit about 1800!
When I first got mine it was really bad at 1500-1500 revs. Took it to APM for a shim, balance and service/tune. It's now a lot better but still a little 'hesitant' in traffic. Could it be a function of under-bonnet temperature when not moving fast? I'm looking at ceramic coating my manifold to try and move the heat out of the engine bay.
Not sure about with the MBE but with my Emerald the solution was to pull a few degrees of timing at part throttle and low RPM. That cured it immediately and it has never returned. It's a bit of a duff resonance in the inlets between valve back and throttle plate at very low throttle angles. Pulling timing means you don't cruise around that throttle plate angle. Of course if it's a standard MBE running standard timing that's probably not the problem, unless someone has remapped it badly. The optimum timing for BHP at 1400-1800 rpm as indicated by a rolling road is not the optimum timing for a car that doesn't shuck, shimmy and clank from the diff and box at that engine speed when in traffic so it's not uncommon for a well intentioned rolling road operator to turn a V8 cerbera into an undriveable pain in the arse by accident. The problem is not just one that affects Cerbs, pretty much anything with ITBs can get a nasty dose of this if not set up sypathetically and road tested after the rolling road session.
Sadly it's not as simple as that Jon. The 1800-2200rpm stutter exists in both the 4.2 and 4.5 versions of inlet design, with butterflies at opposite ends of the inlet tract, so it's not a simple induction resonance issue. It also exists with different exhaust designs, so it's not an exhaust resonant issue either.
Tvr already have the ignition map well retarded around the bad drivability area .. it's a good 10degrees retarded from where best power is at those small throttle openings. Retarding it further (under lambda control, see below) doesn't help and indeed brings about another unwelcome problem.
The engines are under lambda control which is a cyclical fuelling event as you know (alternating rich and weak either side of lambda=1). As you reduce the ignition timing you need more throttle to maintain any given power output. More throttle = more airflow obviously, but then the cycling of the fuelling rich and weak means the power output even on a steady throttle is increasing and decreasing with the mixture swings. As you retard the timing this power surge gets worse and worse. So to eliminate the power surge you need to add advance.
The bad drivability can be vastly reduced / eliminated by taking the engine out of lambda control and richening the load sites that are problematic .. but then you run into the problem of not being able to meet the lambda=1 section of the MOT fast idle test.
There's much more to it than just retarding the ignition.
Things which do help are wider valve clearances and richer mixtures out of lambda control.
Tvr already have the ignition map well retarded around the bad drivability area .. it's a good 10degrees retarded from where best power is at those small throttle openings. Retarding it further (under lambda control, see below) doesn't help and indeed brings about another unwelcome problem.
The engines are under lambda control which is a cyclical fuelling event as you know (alternating rich and weak either side of lambda=1). As you reduce the ignition timing you need more throttle to maintain any given power output. More throttle = more airflow obviously, but then the cycling of the fuelling rich and weak means the power output even on a steady throttle is increasing and decreasing with the mixture swings. As you retard the timing this power surge gets worse and worse. So to eliminate the power surge you need to add advance.
The bad drivability can be vastly reduced / eliminated by taking the engine out of lambda control and richening the load sites that are problematic .. but then you run into the problem of not being able to meet the lambda=1 section of the MOT fast idle test.
There's much more to it than just retarding the ignition.
Things which do help are wider valve clearances and richer mixtures out of lambda control.
Never had any luck getting rid of it by advancing the ignition, even to the point of mild detonation. I am quite convinced that the reason the V8s pop on the over run is that TVR had to retard to silly levels in order to get the car driveable at low revs and high vacuum. On mine the point at which the pops and bangs stop is precisely the point at which the shucking and shunting take over. The Lambdas on mine are disabled so they are out of the equation. The chaps at Emerald have reached the same conclusions, not a marketing ploy at all, a necessity due to unfortunate inlet geometry which makes vast power but is not road friendly. I have experienced this on several cars with short inlets and the cure has always been the same.
morebeanz said:
Tanguero said:
Thinner shims.
Not questioning your input here, but if that was it, why wouldn't everyone "do it"?A lot of people do. Joolz has been recommending opening up the clearances for years. Many of us have taken his advice. If you understand how the valve clearances on the AJP work, then how else could you do it? The shim is what controls the clearance between cam lobe and the bucket.
morebeanz said:
I tend to find that urban speeds find me in 2nd or 3rd and revs hovering around 2000. This seems to be a sour spot (as opposed to a sweet spot!) in the rev range for the AJP. I have a Jools tweeked chip and it is still like it.
The only solution is to run around town at 2500-3000 revs scaring the local populace or change up as soon as you hit about 1800!
Now I never make a blinding, out of the box suggestion without a lot more infoThe only solution is to run around town at 2500-3000 revs scaring the local populace or change up as soon as you hit about 1800!
BUT
Get her warm, clear the adaptives with the ECU Diagnostic software then run her around town for 10-15 mins.
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