Loss of power above 80% throttle..
Discussion
I recently realised I get a drop in power on my 4.5 when pushing past 80-85% of throttle travel.. I checked the throttle butterflies to see if they are over opening, but they themselves are only opening to around 80%.. I can unwind the throttle stop but it'll be pointless until I fix the power drop...
Lambdas are relatively new but I imagine it's open loop at that point anyway...
I was thinking it must be a fuelling issue across the board, perhaps a lazy fuel pump?
I hooked up the ecu diag software and noticed around when the software registers the throttle approaching 80% the injector values on both banks begin to decrease.. Is this normal?
I'm not sure how clear the image will be but here's a screen shot of the trace with the engine off.. (I've not yet tried taken an ecu trace whilst on the move)
Heres the same trace with throttle highlighted in red and injectors in yellow
This is one of the butterflies at max pedal travel!
Lambdas are relatively new but I imagine it's open loop at that point anyway...
I was thinking it must be a fuelling issue across the board, perhaps a lazy fuel pump?
I hooked up the ecu diag software and noticed around when the software registers the throttle approaching 80% the injector values on both banks begin to decrease.. Is this normal?
I'm not sure how clear the image will be but here's a screen shot of the trace with the engine off.. (I've not yet tried taken an ecu trace whilst on the move)
Heres the same trace with throttle highlighted in red and injectors in yellow
This is one of the butterflies at max pedal travel!
Edited by Ice_blue_tvr on Sunday 5th September 19:53
Your throttles should obviously open fully .. on a sports car why has full throttle not been set correctly .. sack your service agent.
But .. the drop in power towards full throttle at lower revs is very normal, even on a remapped c ar you get this trend to some extent.
This is because of 3 things mainly .. firstly at low revs the cylinder filling at full throttle is actually worse than at part throttle. At part throttle the part closed butterfly acts as a wave interrupter .. it disrupts any standing waves - some of which at certain revs benefit cylinder filling, and some of which at certain ravs hinder cylinder filling. At low revs it's better to have the butterfly part closed as this actually disrupts the standing wave which at low revs is reducing cylinder filling. This isn't a cerb specific thing, it's a factor on most (all?) engines with wild cams and ITBs.
The second thing is that the ignition advance at full throttle on the std map is very retarded compared to part throttle, so you get a double whammy of poorer cylinder filling and retarded ignition at full throttle.
Also the mixture goes very rich at full throttle on the std map .. further reducing power. This makes the fuel map look odd to the casual observer because the fuel numbers at full throttle on lower revs are less than at part throttle, because of the poorer cylinder air filling at full throttle of course.
But .. the drop in power towards full throttle at lower revs is very normal, even on a remapped c ar you get this trend to some extent.
This is because of 3 things mainly .. firstly at low revs the cylinder filling at full throttle is actually worse than at part throttle. At part throttle the part closed butterfly acts as a wave interrupter .. it disrupts any standing waves - some of which at certain revs benefit cylinder filling, and some of which at certain ravs hinder cylinder filling. At low revs it's better to have the butterfly part closed as this actually disrupts the standing wave which at low revs is reducing cylinder filling. This isn't a cerb specific thing, it's a factor on most (all?) engines with wild cams and ITBs.
The second thing is that the ignition advance at full throttle on the std map is very retarded compared to part throttle, so you get a double whammy of poorer cylinder filling and retarded ignition at full throttle.
Also the mixture goes very rich at full throttle on the std map .. further reducing power. This makes the fuel map look odd to the casual observer because the fuel numbers at full throttle on lower revs are less than at part throttle, because of the poorer cylinder air filling at full throttle of course.
Edited by spitfire4v8 on Monday 6th September 08:26
Thanks for the info guys.. I'll get some info while on the move once I sort out a laptop (that doesn't die as soon as the power is disconnected!).
Just to be clear, it's a very noticeable loss of power, not just a feeling of flatness/no more power.
If, for example, I get to around 3krpm and gradually floor it, the rate of acceleration increases, but when I pass 80% throttle it feels like I am taking my foot off when I'm not.. If I DO take my foot off the throttle and go from 100% to 80% acceleration increases before it decreases as I come down closer to 0%
Just to be clear, it's a very noticeable loss of power, not just a feeling of flatness/no more power.
If, for example, I get to around 3krpm and gradually floor it, the rate of acceleration increases, but when I pass 80% throttle it feels like I am taking my foot off when I'm not.. If I DO take my foot off the throttle and go from 100% to 80% acceleration increases before it decreases as I come down closer to 0%
Yours might be worse than most, and might even have a fault .. but often on the dyno I go to start a power run, and as I put my foot towards full throttle I can feel the car pull hard against the straps as I'm applying throttle then as I get to full throttle the pull against the straps decreases and the power reading drops .. it's a pretty substantial change in feeling of the car.
I think that what you are feeling is still the std cerb mapping trend, but might be far more aggressive on your car for whatever reason.
Who services your car and what have they done about it for you so far ?
I think that what you are feeling is still the std cerb mapping trend, but might be far more aggressive on your car for whatever reason.
Who services your car and what have they done about it for you so far ?
The only way to fix this is by getting to a known good starting point. Get the throttles adjusted so they fully open, then check that the ECU sees the correct opening at idle and 100% at full throttle. Until you have this working your are pi55ing into the wind.
The ECU uses very little info to work out fuelling. It uses throttle position, RPM, air temp for a small correction and lambda at cruise and idle. RPM will be correct or it wouldn't run at all, air temp correction would affect slow running much more than high throttle and is more of a trim than a major calculation factor. Throttle position is absolutely critical.
If throttle position is inaccurate the lambda sensors will apply some correction but only below 75% throttle so the symptoms fit inaccurate throttle position sensing very well. The lambdas dealing with it until you reach a throttle point where they stop being used then it dies on its backside. Of course it could be a fuel supply problem but start with the area where you can already see a clear problem to be dealt with then dig deeper if necessary.
The ECU uses very little info to work out fuelling. It uses throttle position, RPM, air temp for a small correction and lambda at cruise and idle. RPM will be correct or it wouldn't run at all, air temp correction would affect slow running much more than high throttle and is more of a trim than a major calculation factor. Throttle position is absolutely critical.
If throttle position is inaccurate the lambda sensors will apply some correction but only below 75% throttle so the symptoms fit inaccurate throttle position sensing very well. The lambdas dealing with it until you reach a throttle point where they stop being used then it dies on its backside. Of course it could be a fuel supply problem but start with the area where you can already see a clear problem to be dealt with then dig deeper if necessary.
Throttles are set for 14.8 at idle iirc and the car behaves well and pulls well up until it doesn't.. I'll adjust the pedal to allow for full butterfly opening but I imagine it won't help with the issue (as the situation now is the same as me simply pressing the pedal up until the same point and stopping)
One thing that also springs to mind, which may be relevant or not.. The previous owner had switched over to the short induction and new map/chip.. How can I be sure its not some knock off version as I don't have any documentation for it?
FYI, other than some work being done on it about a year ago, and and oil change recently carried out by myself it's not yet been looked at.. I'd like to understand the issue myself first if possible!
One thing that also springs to mind, which may be relevant or not.. The previous owner had switched over to the short induction and new map/chip.. How can I be sure its not some knock off version as I don't have any documentation for it?
FYI, other than some work being done on it about a year ago, and and oil change recently carried out by myself it's not yet been looked at.. I'd like to understand the issue myself first if possible!
Will try and dig out a better pic as I don't have any of the clips, but they have a 6mm nut on the jubilee clip iirc 60mm? I think I've seen other cars with 16 clips, I only have 8 attached on the throttle body side, but might just be that they have not been fitted on the airbox side as the angle they join at seems awkward..
Adaptives fwiw...
Adaptives fwiw...
Those are my MK2 hoses so you've at least got the right ones there. The fit onto the airbox is improved on the Mk3 versions. Your car probably never had the airbox hose clips as I didn't supply them because people just overtighten them and destroy the airbox spigot tubes .. like this ..
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