Dashpot 101

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Dashpot 101

Unread postby cmlomax » Sun Sep 21, 2014 4:03 pm

I'm tuning the dashpot in my car and I've read decipha's "Dashpot 101" but I'm still not getting the decay the way I'd like. One question I have, when setting up preposition, why put in an rpm value that is less than the desired idle speed? For example, dashpot 101 has RPM values of 16384, 2000, 1500, 500, and 0. If desired RPM is 800 (or 750, or stock 672, etc) then what purpose does the 500 value serve? Should that value be set to slightly above the desired rpm value if it's used to clip the isc? Just trying to understand the function in order to better understand the table.

Another question is decay. How does the function work in order to understand how much air is flowing and how much to reduce the flow? What I'm shooting for in my own tune is an airflow function that minimizes rpm drop during shifting and also doesn't let the rpm's overshoot the desired rpm when coming down to idle. The way my current decay works is the rpms drop down towards idle (800 desired) but the isc can't catch it and they drop to 500-ish before they come back up, but sometimes the isc can't catch it and the engine conks out (usually during a hard stop or while turning).
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Re: Dashpot 101

Unread postby decipha » Sun Sep 21, 2014 4:12 pm

thats not rpm its delta rpm which means (RPM - DSDRPM) so 500 would be 800 (nubase) + 500 = 1300 rpm

that means you have it decaying too fast up across the board, reduce the decay rate
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Re: Dashpot 101

Unread postby cmlomax » Sun Sep 21, 2014 4:21 pm

decipha wrote:thats not rpm its delta rpm which means (RPM - DSDRPM) so 500 would be 800 (nubase) + 500 = 1300 rpm

that means you have it decaying too fast up across the board, reduce the decay rate


Great info, thanks.

Another question is, what does preposition really do? What I think happens is when the tps shows a closed throttle and the rpms are above the preposition value in the table then the isc is held to the table value until the rpms drop below it. That's what I think I've observed just cruising around town. So if I'm cruising around in 2nd gear and 1000 rpm I'd want a preposition value of 200 (nubase (800)+200=1000 actual rpm) in order to take care of any bucking issues. Am I getting that right?
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Re: Dashpot 101

Unread postby decipha » Sun Sep 21, 2014 5:04 pm

second part yea

when the tps goes ct the dashpot thata already in from the pre position function then decays its not a clip function
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Re: Dashpot 101

Unread postby jbirken » Tue Sep 23, 2014 7:26 am

Daspot is a function of TP and DASPTK, clipped at FN882. FN882 is not a 'lookup' function. GUFB document and attached datalogs support this.

I also have datalogs that suggest that the RPM value in FN882 is a RPM, not adder to NUBASE. In other words, pre-position max clip = air from FN882 @ RPM - throttle body leakage.

Daspot begins to decay immediately after tip out. So after a long decel, FN882 is not a factor in combating 'conk', because daspot has long since decayed back to dasmin. The solution for me was to calibrate DASMIN to a useful value. I want my idle to hang 300 rpm above base for delcutch. So, from the idle air function: 800 rpm idle = 1.3lb and 1100 rpm ~= 1.9 lb air. So DASMIN = .6lb. DASPTO = DASMIN -.1 This gives me a perfect 1100 rpm 'high idle' above DASMPH, and completely eliminated the 'conk' after long decel.

Also, make sure your injector breakpoint is set in GUFB units (lb/rev).
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Re: Dashpot 101

Unread postby decipha » Tue Sep 23, 2014 11:01 am

fn882 is the dashpot airmass, they consider it a "clip" function in the doc because dashpot works it way up to it, its not instantaneous so in essence it 'clips' dashpot.

what you set in there will be your actual dashpot airmass, if your logging a different value then something is off

dasmin is the correct scalar to prevent rpm drops after dashpot has already finished decaying, dasmin is only needed on long decels while the clutch is out, it sounds like the OP has too fast of decay across the board not just long decel
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