Help: knocking her t*ts off…
Discussion
Did the Rolling Road today. Good stuff, nice power output BUT was told she is knocking her t*ts off (which is apparently a technical term 
) as well as running too lean. Shame on her.
So, she is parked now waiting to be sorted. Has anybody here had the same problem? Which dealer/garage would people recommend to get it sorted properly?
Thx
>> Edited by reichejh on Saturday 6th May 17:55
) as well as running too lean. Shame on her. So, she is parked now waiting to be sorted. Has anybody here had the same problem? Which dealer/garage would people recommend to get it sorted properly?
Thx
>> Edited by reichejh on Saturday 6th May 17:55
I wouldnt necessarily jump to the conlusion that the servicing agent is at fault or would have been aware of this problem as you dont state under what conditions it's occuring. High speed pre-ignition under load may be anything but obvious to the non-expert especially without rolling road facilities, as would be lean mixture under load. These are not things that would ordinarily be examined in normal servicing.
Clearly gross pre-ignition would be obvious but its not really a ticking sound (that sounds more like valve gear noise or a hundred and one other things which may or may not be a problem depending on the cause) rather it is usually an irregular metallic rattling sound under acceleration. There are some very simple problems that could result in lean mixtiure (which would make pre-ignition very likley) such as any air leak up stream of the air flow meter, such as a crack in the flexible elbow or any air leak in the plenum (Check the bolt securing the PAS reservoir to the plenum base, the stepper motor etc are secure and sealing along with all other bolts and tappings). Check the vacuum advance system for leaks (very simply done by taking off the tube at the throttle body end and with the engine idling suck hard on the tube which should cause a brief rise in engine revs as ignition timing advances - but which then gets compensated for by the stepper motor. Whilst sucking, seal the end of the tube with your tongue and see if the vacuum is maintained for a few seconds to rule out a perforated diaphragm on the advance capsule).
Incorrect timing would be a possibility but this would have been identified on the RR and very easily rectified, but I dont think this would cause lean mixture CO values. Servicing errors such as the wrong heat value / spec spark plugs could cause pre-ignition but again not likely low CO.
Basically there isnt enough to go on to narrow it down much but I would put an air leak high on the list of suspects.
Clearly gross pre-ignition would be obvious but its not really a ticking sound (that sounds more like valve gear noise or a hundred and one other things which may or may not be a problem depending on the cause) rather it is usually an irregular metallic rattling sound under acceleration. There are some very simple problems that could result in lean mixtiure (which would make pre-ignition very likley) such as any air leak up stream of the air flow meter, such as a crack in the flexible elbow or any air leak in the plenum (Check the bolt securing the PAS reservoir to the plenum base, the stepper motor etc are secure and sealing along with all other bolts and tappings). Check the vacuum advance system for leaks (very simply done by taking off the tube at the throttle body end and with the engine idling suck hard on the tube which should cause a brief rise in engine revs as ignition timing advances - but which then gets compensated for by the stepper motor. Whilst sucking, seal the end of the tube with your tongue and see if the vacuum is maintained for a few seconds to rule out a perforated diaphragm on the advance capsule).
Incorrect timing would be a possibility but this would have been identified on the RR and very easily rectified, but I dont think this would cause lean mixture CO values. Servicing errors such as the wrong heat value / spec spark plugs could cause pre-ignition but again not likely low CO.
Basically there isnt enough to go on to narrow it down much but I would put an air leak high on the list of suspects.
Thanks. 
reichejh said:
Called a garage who suggested a remapping to narrow down the problem. Guess this is a resonable thing to do, as not cheap??
There's no point in remapping to get around the problem if the root of the problem (lean running) isn't fixed. If the problem is an air leak, sticky stepper, timing out of adjustment, gummed up injectors, weak fuel pump, dodgy fuel pressure regulator - all of which could cause lean running - I'd get that identified and fixed. Then you fix the problem not the solution. Any reputable remapper won't remap unless the rest of the car is 100% anyway.
I was at the same rolling road session as Reichejh and was also running lean. The difference is that mine has a 404 cam and Charlie reckoned the problem was probably caused by that.
Essentially the fuelling is all over the place until about 2500rpm (which one would expect given the power based nature of the cam).
Up until about 5000 the fuelling seems ok (represented by the lower line on this graph) but then it goes very lean and apparantly knocks at the top end.
Charlie at Surrey RR said that I need a Mark Adams chip but that I may be able to solve the problem with higher fuel pressure and that it was likely that the engine simply wasn't supplying the amount of fuel that the cam required.
Initially I would rather do that as a new chip would invalidate my (Fernhurst) warranty. Anyone got any experience of upping fuel pressure or know how to fix this without going down the chip route??
Planning an MA chip sometime towards the end of this year anyway
>> Edited by george 500 on Monday 8th May 22:14
Essentially the fuelling is all over the place until about 2500rpm (which one would expect given the power based nature of the cam).
Up until about 5000 the fuelling seems ok (represented by the lower line on this graph) but then it goes very lean and apparantly knocks at the top end.
Charlie at Surrey RR said that I need a Mark Adams chip but that I may be able to solve the problem with higher fuel pressure and that it was likely that the engine simply wasn't supplying the amount of fuel that the cam required.
Initially I would rather do that as a new chip would invalidate my (Fernhurst) warranty. Anyone got any experience of upping fuel pressure or know how to fix this without going down the chip route??
Planning an MA chip sometime towards the end of this year anyway
>> Edited by george 500 on Monday 8th May 22:14
Given that the hotwire system measures mass air flow, I've never really understood why minor induction or cam changes require remapping every time. Maybe understandable if the upgrade takes the engine outside the fuel/air flow region defined in the original map, but not mid range. Clearly they do need remapping every time, I just don't get why.
GreenV8S said:
Given that the hotwire system measures mass air flow, I've never really understood why minor induction or cam changes require remapping every time. Maybe understandable if the upgrade takes the engine outside the fuel/air flow region defined in the original map, but not mid range. Clearly they do need remapping every time, I just don't get why.
Not totally sure here Peter but I have a pet theory on this one.
The map is scaled to the 5AM AFM load (voltage) which has a maximum of 5volts
. My lowly 275hp pretty well maxes out my 5am at 5Vfsd at 6Krpm. So unless the map scaling is extended to take it above the 5V afm reading Vs rpm, to extrapolate beyond the 5v point, then you need to get the map set up to suit, not only matched to your engine but scaled to your AFM (within a tolerance band). I'm not so sure standard maps increase fueling above 5Vfsd out of the AFM regardless of revs tbh.
Just a theory.
HarryW said:
Given that the hotwire system measures mass air flow, I've never really understood why minor induction or cam changes require remapping every time. Maybe understandable if the upgrade takes the engine outside the fuel/air flow region defined in the original map, but not mid range. Clearly they do need remapping every time, I just don't get why.
Not totally sure here Peter but I have a pet theory on this one.
The map is scaled to the 5AM AFM load (voltage) which has a maximum of 5volts
I'm not so sure standard maps increase fueling above 5Vfsd out of the AFM regardless of revs tbh.
Just a theory.
Sounds very viable,
That only explains why it might be necessary to extend or rescale the map to increase the peak power supported, but even when that isn't the case they seem to need remapping.
The only theory I can think of is that the AFM is producing a signal that depends on average mass air flow, and on 'pulsiness'. According to this theory, any change that affects the pulsiness of the intake air flow will change the relationship between true mass air flow and the AFM signal - just like conventional carb tuning. I'm not completely convinced by this theory though; with eight cylinders breathing through a single intake I would have thought that the pulses would average out. Maybe not completely, though.
One interesting consequence predicted by this theory, is that when the AFM is upstream of a supercharger this completely masks any pulsing from the engine so it should only need tuning once regardless of the power levels. Must test that theory one day.
The only theory I can think of is that the AFM is producing a signal that depends on average mass air flow, and on 'pulsiness'. According to this theory, any change that affects the pulsiness of the intake air flow will change the relationship between true mass air flow and the AFM signal - just like conventional carb tuning. I'm not completely convinced by this theory though; with eight cylinders breathing through a single intake I would have thought that the pulses would average out. Maybe not completely, though.
One interesting consequence predicted by this theory, is that when the AFM is upstream of a supercharger this completely masks any pulsing from the engine so it should only need tuning once regardless of the power levels. Must test that theory one day.
Not sure what the pulses would look like on a scope, but they probably are there. However, I can certainly say on a DVM, hooked up on the move 
, the output is very linear from 1.7v to 5v as you go up the rev band and the load increases, if that helps
. Even if it did pulse excesively it would be easy enough to filter and slug it, its only electricery after all, to be linear.
With the afm gauze removed you certainly get 5-10hp more out of a RV8 motor, however the low load/rpm/shunting and hence manners can suffer. I think this is because the gauze smooths out the flow (pulses) before the hot wire senses it iykwim.
, the output is very linear from 1.7v to 5v as you go up the rev band and the load increases, if that helps. Even if it did pulse excesively it would be easy enough to filter and slug it, its only electricery after all, to be linear. With the afm gauze removed you certainly get 5-10hp more out of a RV8 motor, however the low load/rpm/shunting and hence manners can suffer. I think this is because the gauze smooths out the flow (pulses) before the hot wire senses it iykwim. HarryW said:
Even if it did pulse excesively it would be easy enough to filter and slug it, its only electricery after all, to be linear.
I wouldn't expect tha AFM readng to pulse significantly, but what I mean is that the cooling effect that the hotwire uses to measure air speed will be highly non-linear with air speed so for a given average air flow you could expect to get different readings from the AFM depending how much pulsiness there was in the air flow. With the AFM the smoothing occurs due to the thermal inertia of the element. It's exactly analogous to carb tuning where depression is proportion to V squared and mass flow is proprtional to V.
GreenV8S said:
That only explains why it might be necessary to extend or rescale the map to increase the peak power supported, but even when that isn't the case they seem to need remapping.
Until you get the air signal maxing out you don't have to do anything (in fact after it's maxed, as long as the mixture was goodly safe beforehand you can get the airflow higher without the mixture leanig out to be a problem), I've only ever seen weak running cars when there's a fault somewhere, even high hp cars can have a perfect fuelling trace if all components are working properly.
Just to raise another issue about fuelling limitations and lean mixture on modified engines:
On the 14CUX / Rover V8 injection system is there likley to be any problem related to injector saturation, with the injectors being the limiting factor (with standard fuel pressure) when constantly open?
I realise that this is more likely to occur around peak torque rather than peak power. The question is really up to what state of tuning are the standard injectors going to cope, without having to raise fuel pressure and remap the fuelling to prevent over rich running elsewhere?
On the 14CUX / Rover V8 injection system is there likley to be any problem related to injector saturation, with the injectors being the limiting factor (with standard fuel pressure) when constantly open?
I realise that this is more likely to occur around peak torque rather than peak power. The question is really up to what state of tuning are the standard injectors going to cope, without having to raise fuel pressure and remap the fuelling to prevent over rich running elsewhere?
radzoule said:
Interesting Joo ... do you think that my lean spot is indicative of something else, and a remap wouldn't necessarily sort it out?
before you do any remapping you would want to eliminate physical problems or you just end up disguising the symptom not the fault .. if everything checks out ok then remapping is the way to go.
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