8 cylinder front engine iconic vehicle
  • User avatar
  • User avatar
  • User avatar
User avatar
By Crumpler
I have only known the S3 set up which uses an adjustable throttle bypass port (ie screw) in which stock idle is set and AFR manipulated with MAF adjustment.
I don’t think later 928’s do it this way and I don’t think earlier models do either?

My current project involves VEMS conversion with a S4 IAC for “simplicity” ;).
What I’m getting into is the throttle bypass port has the ability to override the IAC until higher duty cycles( percentage of open IAC valve). Meaning finer adjustment of IAC don’t move the rpms around at idle. For background, the software lets you “lock” IAC at any given opening or duty cycle. This in turn should adjust idle speed.
Because of the SC I have to assume the forced induction is a factor here as well, even with a BOV I’m sure bypass port gets more air then designed.

So what am I really achieving?
The idle screw is an IAC right? Just manually set.
The actual IAC, I assume is to deliver or not deliver additional air for fine tuning at different engine demands correct?

I guess the point is how hard do I have to try to perfectly manage this bastardized IAC if current idle rpm is fine and car doesn’t stumble after throttle blips?

I can close the port and rely on IAC exclusively, and is that essentially how the S4 would do it?

Opinions please.
User avatar
By hans14914
You are correct. The 87+ engines are configured in a way that the IAC is the only device controlling the idle makeup air.

There is a throttle stop, which is adjusted to the blade does not bind in the bore. This may allow some nominal amount of air to pass around the blade, but the majority of air ingested by the engine at idle is being passed through the IAC.

However, using the metering screw is something standard on many other fuel injection designs. The S4 idle valve is fairly quick, so you can certainly close that screw and rely on the valve to do all the work.

Just make sure you look at all the other "adder" tables like temperature compensation idle-up. You may have another table fighting against you like the AC idle up if not properly configured.
User avatar
By worf
Yeah. What Hans wrote -^.

In general you need ensure good idle at all combinations of:
- engine temp
- A/C on/off
- Auto transaxle in D, R, N/P

That's a fairly extensive Karnaugh Map. And it may take a year to dial in while you wait for temperature swings (e.g. good performance in summer may be accompanied by no-start/no-idle in winter...)

You'll also probably want a higher idle (~100 rpm) at cold start that settles down gradually until the 02 loop kicks in.

It's also important to look at idle timing and then timing in the load/rpm cells that are active just-off idle. Big changes in timing between idle and off-idle can result in "interesting" behavior especially in a 5-speed where there's no viscous coupling to act as a "capacitor."
User avatar
By hans14914
VEMS does have some earlyish implementation of "scatter spark" to help aid in RPM lock/idle control. You may want to dig into that as well, or make sure its disabled if you are trying to just rely on the valve only:


I dont have any experience with using the adaptive ignition on any of my previous VEMS builds. I can say that this feature does work well on more modern hardware/software packages.
User avatar
By Crumpler
Thank you again Hans and Worf.
I wouldn’t be doing well at all without being able to use you guys as a sounding board.

The VEMS website seems to have some quirks as well. I finally found the IAC help pages, but had to do an independent search to get to it. It does not come up when navigate the main web site?
I messed around for awhile today.
What seems to work the best in this set up is leaving the screw “a little open” to avoid hard starts and hit target idle. While also changing DC range on the IAC to 50 min and 90 max, which is the number range that actually open the IAC gate.
Idle seems passable now.
I will try and work on reference table next.
Modified Gauge Cluster

Well, I should come clean on these shenanigans to […]

The 928 Photo Thread

2 928s at CNC this morning. I heard rumors of a th[…]

Formula 1 2020 And Beyond

Agreed, I was yelling at my screen during SQ3. V[…]

Rear Brake line Seized

Just watched a video on that tool...that's bad a[…]