guys just after some advice please, do you have to change the desired IAC steps vs desired idle effective area when changing to a bigger throttle body? I've read stickies but just after a simple answer cheers. oops on cable set up
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guys just after some advice please, do you have to change the desired IAC steps vs desired idle effective area when changing to a bigger throttle body? I've read stickies but just after a simple answer cheers. oops on cable set up
For an electric throttle, yeah. For a bigger cable throttle,
the problems seem to be in the undersized IAC channels
of some big-TB castings, making the table bogus at higher
IAC counts. The air received is less than the table predicts
and then the control loop has to try and compensate but
ends up "winding up" and going unstable.
In my limited experience, you need the IAC channel to be
at least 3/8" diameter (or equiv) to not affect IAC airflow
vs steps or limit IAC airflow prematurely.
Maslic says not to modify the IAC counts table if you are using the stock IAC motor. I have had throttle bodies
where the off idle table had to be reduced a little to keep the car from driving itself (automatic transmission).
If you compare an early LS6 table to one for an early LS1, you will see where GM did this themselves.
In all cases I have found that you need to open the butter fly enough so that at warm idle in park your IAC counts
are around 45 to 55 so that you have sufficient room left for the IAC motor to open enough for the extra air needed
to start motor at cold temperatures (winter temps 32 and below). If you still do not have sufficient air for cold temp
starts you could go as low as 20 counts at warm idle in park. I would not change the IAC curve until I had tried to
tune idle without doing so first.
What size and brand of throttle body did you go with?
It's not about the IAC motor, if the IAC motor is not
changed. It's about an upstream restriction that makes
the IAC lose authority over airflow (comes up short vs
expected).
In my case the 85mm Professional Products TB has a
plenty big enough channel. Where I ran into trouble
was when I ported, filled & drilled the stock TB and
'net "wisdom" at the time was to drill a 3/16" hole.
Not big enough. I logged and plotted airflow vs IAC
counts and overlaid it with the table data, back when;
with the filled IAC port & 3/16" hole you could see the
airflow line "lay over". Repeated after drilling out to 3/8"
and they matched as far up as the IAC counts went.
Jacking the blade open can take you out of idle-range
TPS voltage / %. Drilling the blade is a one way trip and
adds air the IAC has no authority over. Better to just
enable the IAC to fully do its job.
james went with a nw 102mm set up, did take a while to get everything idling well etc...did what you guys were saying left the desired area stock and ran Russ idle config that worked a treat just took about 2 cold runs each park/gear but seems to be working great..cheers mate and also thanks to Russ for config set up that is worth every bit towards tuning:hi: also forgot to mention had to play with the throttle cracker/follower tables to get drivability up to liking..
All the IAC Counts vs Effective area table does is translate airflow into a stepper count for the IAC. I modify this table on all cars, even stock cars. The reasoning is due to alot of things. The calibration is setup for a stock air tract, intake manifold, cylinder heads, camshaft, displacement. When you add a camshaft that prevents the intake from drawing as much vacuum you effectively loose airflow capability of that same orifice size. A cam'd engine draws less vacuum which is why there needs to be more "area" to allow the lazier air a path to reach similar airflow to the stock engine with more vacuum and higher velocity of airflow.
An easier way to thing about it is nitrous jets. You will get more nitrous through a certain jet with more bottle pressure correct? Same thing for vacuum on a intake manifold trying to maintain idle through an orifice.
How does this correlate to this table you ask because the orifice isn't changing? Because you need to modify the opening of the throttle blade rest point to allow additional airflow around the blade. When doing this manipulating the IAC counts vs Effective area will directly skew what base running airflow results in a certain IAC Count. Its the work of getting that table plotted out for various loads and temperatures that is key!
So then because of reduced vacuum and reversion from cam overlap, there is less air coming through the idle circuit than the table indicates whether it's cable-driven or electronic? That would mean fewer zeros at the beginning of the table?
Thank you James; your explanation makes sense. Rescaling table to adjust for the fact that intake manifold vacuum
at idle is less with a bigger cam which results in less air being pulled through the IAC. Does one just need to increase
the IAC counts in the cold and warm idle areas, or is it necessary to modify the entire table? One of you guys should
sell a book just dealing with idle tuning on LS motors. I feel that both the Maslic and the La Sota tuning books (which
are the best two on the market IMHO) are a little light on idle tuning. Even Greg B's tuning DVD which is excellent
guide to wide band VE and MAF tuning does not give any guidance on idle tuning.
Anyone who is not familiar with James Short, he definitely knows what he's talking about when it comes to tuning.
He is not speculating, but giving you solid information that you can rely on.
James, if you read this, I wanted to ask you if you have experienced any LS motors which even after the MAF and VE
tables were dialed in using wideband, they still ran better left open loop. Mine has perceivably better throttle response
in open loop than when the loop is closed and LTFT are active. Even after changing stoichiometric to 14.22 for 7%
ethanol gasoline in the fuel, general tab, this was still the case. I have a DashDaq unit and I could see the fuel trims
adding fuel in certain areas EVEN though the MAF and VEs were dialed in perfectly - and I must conclude the fuel trims
were not doing their job correctly since throttle response and acceleration were better with PCM in open loop. Maybe the
OEM narrow band oxygen sensors installed in long tube headers are not close enough to the exhaust ports to provide
accurate corrections? Not sure? I have Kooks long tube headers with their Y pipe.
Does adding to effective area slow down IAC movements because you're telling the pcm that each step is moving more air and therefore doesn't move as far? I haven't messed with this yet but it's on my to-do list whenever the snow is gone.
if you log IAC counts you can see what columns in the IAC table are being used under specific conditions, such as when idling with car stopped,
coasting at a given speed with throttle closed, etc. If you want the IAC to provide more air you would increase the counts at that point
in the table above what is already in a specific column, and if you want to reduce air (car driving itself) you could decrease the count
value for the column or columns where the problem is occurring. I have found that this is a last resort for me because I do all of my
tuning in the throttle follower and throttle cracker tables (air quantity & decays), and adjust the position of the throttle blade at idle in park so that
the vehicle will idle okay when the IAC connector is disconnected when engine is at normal operating temperature (ECT). I always zero out
the throttle follower DELAY in-gear because there is no need to delay decay of this air when you let off the pedal. Leaving this at factory
settings on a modified engine causes an undesirable delay of the car slowing down when you let off the throttle and coast making it
necessary to hit the brakes for cornering at fast speeds, etc.
I agree with this. Only difference is I don't bother to disconnect the IAC. As long as my hot counts are in range, my TPS % reads 0 and idle trims are correct, I know I'm good to go.Quote:
Originally Posted by JamesLinder
Good point; this is just what I use to get the IAC into the correct range; the point being that if you do not have
your counts low enough at warm idle in park, then you will run out of table at the coldest temperature starts. once
you reach the highest value in the IAC table, which I believe is about 310, if that is not enough air then you would
have to open your throttle blade further in fully closed position or drill larger hole in blade (last resort) or modify
the IAC pintle (which doubt most of us would be able to do correctly).
Now that I have couple more years experience I have learned that everything you have stated here is spot on JimmyBlue.
I took your approach to getting a 102 mm PTM to work correctly recently; you can find that discussion on a recent thread.
I have been messing around with an excel tool to support our tutorial narrative....Still a work in progress
Ed M
Ed, I see the spreadsheet you posted, but where are you gettig Dynamic Airflow A and B and IAC Steps A and B? Can you give an example of how to use the spreadsheet?Quote:
Originally Posted by mowton