by Michael Decipha Ponthieux
Posted: 2013-07-27
Last Updated: 2018-11-13

Dashpot is the ISC "Idle Speed Control" valve (as known in the tuning world), also known as the IAC "Idle Air Control" valve's control function that causes the RPM's to drop slowly to a stabilized idle. Reducing RPM's slowly has 3 primary benefits:
1> Allow the vehicle to maintain momentum to prevent bucking
2> Ensure a stabilized idle return
3> Reduces emissions thus you keep all the tree huggers erect

Dashpot is one of the most significant factors in creating a 'streetable' tune.

Dashpot relies on the idle airflow modeling to be accurate, thus any dashpot tuning must be done AFTER Idle Air is dialed in, which must be done after Idle Fuel is dialed in

Dashpot can be simplified down to 2 major functions.
FN882 - Dashpot Pre-Position Airmass
FN879 - Dashpot Decay Rate

all 93+ strategies have additional functions that will be simplified later on, the following applies to all 1983-2004 EEC-IVs and EEC-Vs.

The objective when dialing in dashpot is to get AS MUCH air into the engine to maintain an RPM just a cunt hair less than the actual RPM. If done correctly, the result will be a very smooth tip-in and tip-out transition with an RPM that decay's back to idle at a controlled and slow rate. Not enough dashpot at low loads WILL result in bucking. Typically, decay from high rpm should last 3-4 seconds before reaching idle rpm. As noted in the
Idle Air Write Up , I cannot stress enough the importance of having the ISCDC practically off at a warm stabilized idle, this will ensure you have enough overhead to allow for proper dashpot control. As well as enough overhead to allow for cold idle and startup when airflow requirements are greater.


When the throttle is applied and the part throttle flag is set, the ecu multiplies the relative throttle position by DASPTK to come up with an airmass. The ecu then looks up the pre-position airmass function FN882 to calculate a maximum dashpot airmass clip at a given RPM. That value is then ADDED to the BASE idle airmass value to come up with an ISC airmass flow. The BASE idle airmass value is the actual isc airmass flow (FN875N-ITHBMA). The ISC transfer function FN800(0) is then looked up to correlate the isc airmass to an isc duty cycle. When the throttle is released, the dashpot airmass then decay's at the rate in the Decay Rate function FN879.

...In other words...

There's a few things you need to know about dashpot:
1> You want the MAXIMUM amount of airmass flow possible as soon as you can do so without causing cruise control.
2> You do not want the IAC valve to exceed 98%, this will shorten the life of the IAC valve as well as often times inducing a "humming" noise from the valve.
3> At 2000 rpm above idle, most engines can tolerate maximum isc airmass flow without causing cruise control (typically 2.0 lbs/min).
Within' 500 rpm of idle, I find most engines like the airmass flow required to hold the rpm just above idle. (typically 0.30 - 0.50 lbs/min).
6> You want decay to be as fast as possible without causing engine drag during tip-out's and without dropping rpm's too fast that you overshoot idle.
7> You would typically decay very slow at high rpm increasing the decay in the mid range and then reducing the decay significantly as it approaches idle.

Sounds like a lot of work but its really simple, I'll break it down step by step.

Dashpot Scalars

First we will want to simply the dashpot scalars so we can dial in dashpot airmass and decay first.
PIDParameterRecommended ValueComments
-DASCTLDashpot Max for RPM Control0.500this scalar must be set high enough to allow idle spark control to begin while dashpot is still decaying for stable idle return
***DASMINDashpot Moving Minimum Clip0.150This is the minimum dashpot clip while the vehicle speed exceeds DASMPH, replaced by FN894 on newer strats
DASMPHDashpot Min Clip Disallow Speed0.5Minimum vehicle speed to keep DASMIN active
DASMHYSTDashpot Min Clip Enable Speed12Minimum vehicle speed +dasmph to enable the DASMIN clip
DASPTODashpot Offset0.5dashpot when throttle is applied and rpm is less than dsdrpm, replaced by FN830 in newer strats
DASPTKDashpot Gain0.062Gain rate that per relative tp count that dashpot is incremented to reach FN882
MINMPHIdle Control Min MPH25This is an idle related scalar but relevant to dashpot, this is the minimum mph to allow idle isc control
SPKCTLFBS DSDRPM Adder Max Clip2000max dsdrpm adder to base idle (nubase or drbase) in order to permit feedback idle spark control
-TR_DELTAFBS TQ Ratio Max Delta0amount of spark retard when entering idle spark feedback

Note: ***If dashpot is clipped too high during extended decels, DASMIN will need to be adjusted to reach your desired dashpot.
- == Not all of these parameters exist in GUFX and older strats.

Dialing in PREPOSITION / Dashpot Airmass - FN882(A)

Before proceeding you must have fuel, idle, and spark dialed in before dashpot can be properly adjusted. If you haven't dialed in fuel, idle, and spark, but you need to make an adjustment to dashpot now, this function can be simplified by setting 3500 = 2.0 and everything below that to all 0s. Then adjust as needed.

Back in our isc airmass transfer function FN800, we see the isc is capable of flowing a maximum of 2.00 lbs/min of airmass flow at 100% duty cycle. Remember, you don't want the ISC to exceed 98% duty cycle, this will shorten the life of the iac valve and often times cause a "humming" noise from the valve. So instead of cross calculating and making sure we never exceed the maximum ISC airmass, we can very simply just clip the maximum ISC duty cycle to 98% in the ISC transfer function fn800(0). just simply change the maximum ISC duty cycle to no more than 98% in the function.. done! While your in there, I HIGHLY recommend setting 0.100 isc airmass to 10% duty cycle (detailed in the mechanical idle control write up), as well as setting 0.000 isc airmass = 0 duty cycle to disable the ISC during overheat protection; otherwise when the engine leans out with overheat protection the isc duty cycle will spike very high to prevent the engine from shutting off.

Since we want the MAXIMUM amount of airmass as possible, without inadvertently causing cruise control from too much dashpot, We can limit the maximum dashpot airmass in the pre position function FN882 so that 3500 rpm = 2.0 lbs/min.

Next we can dialed in the dashpot decay when we fire the engine up in the next few steps.

If you simplified the dashpot pre-position function it will look like this:

FN882(A) - Dashpot Airmass (Pre-Position)

Setting up Dashpot Decay - FN879

Now we want to do a basic setup of the dashpot decay rate function FN879 like so:
16.00 0.0025

Now we will need to set the minimum required isc airmass to decay at a rate that doesn't overshoot idle, this can only be ascertained from actually playing with the values to get a stable dashpot, time to disable the remaining dashpot functions before we fire it up

Simplifying Dashpot

This applies to all 93+ strategies, this does not apply to the foxbody ecu's

NOTE: These functions can easily be found due to being less than 7 row functions.
FN891 VSBar Multipliermaximum dashpot multiplier for vehicle speed the y-axis should be set to all 1s to disable for now. This is often mislabeled as AM multiplier on the y-axis, it is not it is in fact vehicle speed.
FN894 Min Clip Low Gearsminimum dashpot clip when not in overdrive the y-axis should be set to all 0 lbs/min to disable for now
FN830 Pre-Position Offsetthe y-axis can be set to all 0.5 lbs/min as a base starting point
The following are only available in EEC-5 ecu's
FN894H Min Clip 4th Gearmin dashpot in overdrive the y-axis should be set to all 0.25 lbs/min for now
FN894L Min Clip 4th Gear Coastingmin dashpot in overdrive when not deceleratingthe y-axis should be set to all 0.100 lbs/min for now
FN882A Pre Position Max Clipthis function replaces FN882 in newer stratssee above to set it up correctly


Now that you have the base dashpot control setup, its time to fire the engine up and let it get to a warm stabilized idle. The engine must be at a warm stabilized idle for dashpot to be dialed in properly.
First, fire it up and see how dashpot functions, if its acceptable to you then great your done, otherwise...

You will first need to dial in FN882 by giving the engine the highest airmass you can that doesn't cause the rpms to spike up when the throttle is relased. Once you have found that sweet spot you will then move on to dialing in the the decay rate.
Manipulate the Dashpot Decay Rate function's lbs/min "Y-axis" to have the rpm's fall at the rate you desire.

Be advised this is a tedious process depending on your level of pickiness and can last several days weeks months or even years.

Ideally you want dashpot dialed in so that the rpm's fall back down at a controlled rate and as it nears 500 rpm or so of idle, the decay should slow the rpm back to idle without overshooting.
A slight hang just above idle for a few moments is ideal in maintaining a stabilized decay, especially with really choppy cam's

The rpm fall "decay" is adjusted in the Y-axis of the decay function
If the rpm's fall too quickly, you must reduce the decay in the Y-axis of the decay function so dashpot reduces slower
if the rpm's fall too slowly, you must increase the decay in the Y-axis of the decay function so dashpot reduces faster

once the rpm's fall down "decay" to idle at your pleasing now is the time to go for a slow creep
with the shifter in 1st slowly slip the clutch and let the isc valve move the vehicle forward

If theres too much DASMIN present you will get a hanging idle or 'cruise control' resulting in excessive speeds ( > 10mph)
this can be resolved by adjustming DASMIN (or FN894) and DASMPH to get desired results

If while driving during decel conditions you experience 'bucking' then you will need to reduce the dashpot decay rate at that dashpot airmass.
If you do not have enough dashpot present to prevent bucking, it is neccessary to increase the dashpot airmass at that rpm in FN882(A).
This is typical with big blocks and higher displacement engines.

Time to take her for a high rpm cruise, If you experiencing a 'cruise control' during high speed coasting,
you can correct this by INCREASING the dashpot decay rate at that dashpot airmass.

The secret to dialin in dashpot perfectly is to have a reference RPM with a reference dashpot value. So for example at 1500 rpm you want to always end up with the same dashpot airmass regardless of what rpm above that you started from. This is best achieved by logging dashpot and verifying when the rpms fall and reach 1500 rpm the dashpot airmass is always at the same value. You can then adjust the decay and airmass up top in the higher rpms to make that happen.

Continue reading on to the
Ignition Timing Write Up.

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