idle speed and engine setup
Discussion
I may answer my own question here. I've been tinkering with my car and have been setting it up using the software and an airflow meter. So far so good. I've also replaced a number of components like coils and leads and have replaced the throttle linkage ball joints with tiny rod ends.
Anyway, the car is running ok and is showing low values for the adaptives. Throttle pot settings at idle are 18.0/18.4% when the engine is hot. Airflow is equal for both banks.
The idle speed is around 1150rpm and I've noticed the ignition timing is about 14 degrees BTDC. I've read it should be around 17 degrees at idle. My question is, does the MBE ECU have a 'target' idle RPM of 900? If so, am I confusing it, as it'll be trying to lower the idle speed by retarding the timing, but it'll never get it down as I've got the idle screw wound out too much?
Also, will the higher idle speed cause any other gremlins?
Finally, I did all this without blocking off the purge pipe - does that make me a bad person?
Anyway, the car is running ok and is showing low values for the adaptives. Throttle pot settings at idle are 18.0/18.4% when the engine is hot. Airflow is equal for both banks.
The idle speed is around 1150rpm and I've noticed the ignition timing is about 14 degrees BTDC. I've read it should be around 17 degrees at idle. My question is, does the MBE ECU have a 'target' idle RPM of 900? If so, am I confusing it, as it'll be trying to lower the idle speed by retarding the timing, but it'll never get it down as I've got the idle screw wound out too much?
Also, will the higher idle speed cause any other gremlins?
Finally, I did all this without blocking off the purge pipe - does that make me a bad person?
Although the 941 ECU has a target idle function its not enabled on the Cerbera.
Whih is why Trackcar does a trick with the below idle speed ignition advance to make the idle more stable.
He also doesn't block the purge pipe. And he does the final tweaks with airboxes on.
He reckons the best way to get the t pots sorted is with the car in the state it will be when its on the road.
There is a thread floating about where he decribes how he does it (his handle was joospeed then).
Makes for interesting reading
Whih is why Trackcar does a trick with the below idle speed ignition advance to make the idle more stable.
He also doesn't block the purge pipe. And he does the final tweaks with airboxes on.
He reckons the best way to get the t pots sorted is with the car in the state it will be when its on the road.
There is a thread floating about where he decribes how he does it (his handle was joospeed then).
Makes for interesting reading
Thinking about it I believe the post where Joo describes how he does it was put into the Cerbera manual.
www.tvrfreak.com/Albums/Auto/cerbera/Manual/Owner's%20Manual.htm
www.tvrfreak.com/Albums/Auto/cerbera/Manual/Owner's%20Manual.htm
This is the bit here. I guess equal adaptives close to zero, throttle pots reading the same each side and equal injector times for both banks means balanced airflow.
"If the banks are balanced, then the adaptives should be equal. It really is that easy.
The garage should have an airmass meter ... it comes supplied in the cerbera kit that every dealer got along with their computer / software / cam timing gear. I very rarely use the meter and just balance it from the values on the screen which is much quicker, but if you had a SP6 then I'd be worrying...that engine has individually adjustable butterflies for each cylinder and you cannot balance this without the meter.
The basis for setting the V8 is to firstly slacken the linkrod between the banks, then by interpreting the adaptive value relative to the throttle pot value bank to bank you can deduce which bank is sucking more air (assuming equal t-pot values this is the bank with the higher adaptive value) there is no other way of interpreting the values .. it's as simple as that. You just adjust the link / throttle pots and idle screw to get the adaptives equal, but also AS CLOSE TO ZERO AS POSSIBLE! If one bank of your engine has adaptives of around 30 % then this is running about on the rich limit ... it won't enrich any more than that. So this means your engine needs a good tune up.
It's also wrong to say that the ECU throws up spurious fault codes in the garage such as the AFR error ... there's a logged fault because there IS a fault ... it's the difference in airflow that's causing it. The imbalance is more noticeable at small throttles / lightcruise conditions and getting it right can give much better town driving - there's also a Joospeed mod for the 4.2 version to cut out the slop in the cross link that gives a lot of poor running at part throttle."
"If the banks are balanced, then the adaptives should be equal. It really is that easy.
The garage should have an airmass meter ... it comes supplied in the cerbera kit that every dealer got along with their computer / software / cam timing gear. I very rarely use the meter and just balance it from the values on the screen which is much quicker, but if you had a SP6 then I'd be worrying...that engine has individually adjustable butterflies for each cylinder and you cannot balance this without the meter.
The basis for setting the V8 is to firstly slacken the linkrod between the banks, then by interpreting the adaptive value relative to the throttle pot value bank to bank you can deduce which bank is sucking more air (assuming equal t-pot values this is the bank with the higher adaptive value) there is no other way of interpreting the values .. it's as simple as that. You just adjust the link / throttle pots and idle screw to get the adaptives equal, but also AS CLOSE TO ZERO AS POSSIBLE! If one bank of your engine has adaptives of around 30 % then this is running about on the rich limit ... it won't enrich any more than that. So this means your engine needs a good tune up.
It's also wrong to say that the ECU throws up spurious fault codes in the garage such as the AFR error ... there's a logged fault because there IS a fault ... it's the difference in airflow that's causing it. The imbalance is more noticeable at small throttles / lightcruise conditions and getting it right can give much better town driving - there's also a Joospeed mod for the 4.2 version to cut out the slop in the cross link that gives a lot of poor running at part throttle."
Simon, sounds like your engine setup is exactly the same as mine, with an idle of 1150rpm. As has been said, it doesn't look like the ECU trims the idle by adjusting the ignition advance, it's just a function of the idle screw and pot settings. Although I have a unique problem with my engine, and I can only guess it has one cause. If I unscrew the idle stop fully, so the throttles are supposedly fully closed, my engine still idles at 1000rpm!! So, I think that polishing the intakes has thinned out enough material to allow sufficient air around the butterflies to keep the engine ticking over. Funny thing is that when I polished the intakes, I remember not touching the throat area for this exact reason. Anyway, mine idles highish, and seems happy enough. The car also feels faster now than it ever has done before, so the setup must be good.
Dave, I've been thinking over this problem with your car. As the adaptives reduce to a decent level as the rpm increases, I can only assume that the ECU map is not entirely accurate/proportional to the airflow past the throttles at small throttle angles. So what I'm thinking is that if the base map is not entirely suited to the engine, the adaptives will try to correct for this. Remember the airflow past the throttles at idle is tiny in comparison to fully open, so mapping it must be very difficult. If you drop the pots to 12ish on idle, I think you'll run lean at higher rpms. Just a thought. The 4.5 map is supposedly pretty poor as standard anyway. Short induction and a remap, you know it makes sense!
Dave, I've been thinking over this problem with your car. As the adaptives reduce to a decent level as the rpm increases, I can only assume that the ECU map is not entirely accurate/proportional to the airflow past the throttles at small throttle angles. So what I'm thinking is that if the base map is not entirely suited to the engine, the adaptives will try to correct for this. Remember the airflow past the throttles at idle is tiny in comparison to fully open, so mapping it must be very difficult. If you drop the pots to 12ish on idle, I think you'll run lean at higher rpms. Just a thought. The 4.5 map is supposedly pretty poor as standard anyway. Short induction and a remap, you know it makes sense!
copilot said:
Is this the answer? Just get the adaptives to zero on tickover @ around 1000 whatever the value of the T-Pots?
Think so, the the T-pot values need to be pretty much the same too. When the T-pot is reading, say 18%, then the fuel map will be supplying fuel from the '18%' load site. If that bank is flowing more or less air than that, then the O2 sensor will feed this back as an 'adaptive' value - i.e. it'll be adding or taking fuel away from the base load site value in order to get the mixture correct.
So if both banks have T-pot readings of 18%, then each bank is getting fuel from the 18% load site,and the adaptives are equal and close to 0%, then no fuel needs to be added or taken away to get the correct mixture.
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