rsc350

I did a search on this and could not find anything, maybe wrong terminology...

I want my engine to idle down faster than it does.

For example you have the car in neutral and rev the engine, I want the engine to cut rpm's faster on the way down, it seems like they linger longer and take forever to de-rev.

Anything I can play with to change that?

Slow89Iroc-Z

do a search on throttle follower or IAC steps. etc.

Kevin91Z

iTrader: (1)

I know how to change the idle follower in the constants in a speed density TPI, but not in a TBI, sorry.

JPrevost

It would help if you would post the ecm you're using. I'm no psychic so I'm just guessing but is your ecm an 8746? If so, check out these locations in the chip;
There's a lot to play with right there. There are factors for when in park/neutral, when not, below a mph, imbetween, and above. There is also a max TF steps! Have fun, I know I did when I was playing with these values.

Rick King

a fast drop in rpms on a push rod motor can cause other unwanted issues--ex bent push rods


later and

GB

JPrevost

Could you explain how the deceleration would bend push rods. I can't seem to figure it out. I do know that rappid deceleration and engine braking is really bad for your rotating assembly but I had never heard of it effecting the valvetrain. If you could tell me your sources I'd really appreciate it.

Grumpy

Actually, it's the first instant the throttles snap shut. The sudden increase in manifold vac., can literary tear a rod apart, in tension.

Rods fail either in Tension, or Compression (loads). Compression rod failures *generally* wind up with a bend in the mid section of the beam area. A *tension* rod failure, is a *clean* break without much bending. If you get to see enough rods that break at the top end of the track, vs most any other time, you'll quickly get to see/understand the difference.

It can be more complex, and involve other issues, but in a nutshell, this should help explain what's happening.

JPrevost

I knew that, that's why I stated rotating assembly. What I don't understand is how it effects the pushrods like he said it does.
BTW; I've broken a couple rods at about 8000rpm but it was a clean break at the top neck of the rod and it was under steady high load . None of the ME's at the C.A.R. could figure out why it failed but it did and completely ruined our chances that year of coming in 1st place. Came in 10th which was pretty good considering we couldn't do jack squat in the acceleration department with a 3 year old engine down 20% of our intended hp.

Rick King

my tpi motor, experince frist hand. had it at wot doinging about 110 let off the gas completly to catch my exit and well the rest is history 3 bent pushrods and 2 bent valves

later and

GB

rk

rsc350

Wow... lots of good information

Where would you consider TF too low and be risking engine damage?

DM91RS

Are these values accessable in Tunerpro or Tunercat? I ASSume that they have to be in the xdf/ecu or tdf.

Thanks DM

JPrevost

That doesn't tell me why they bent. If anything I would have assumed the high revs (lifter and rocker inertial compressing the pushrod) where to blame! I've never noticed a bent pushrod because of the acceleration or decel of an engine's RPM and I've beat'n on a bunch of built Lt1's (7200rpm with blower) and Ls1's (7500rpm nitrous). Neither of them bent a pushrod and they weren't using exotic pushrods . You're actually the first person that I've heard of that's said deceleration has bent their pushrods. Having more than zero lash on most hydralic lifters now-a-day's can also bend pushrods when the oil pressure gets really high (again, dictated by rpm, not acceleration). Don't think I don't believe you that you broke those parts but I'm having a hard time believe it's for the reasons you stated.
Does anybody know WHY (or if) the pushrods are more acceptable to bending failure during decel because I'm seriously stumped. Please don't sight experiences, I want a reason.
They don't have to be but it could be added easily enough. Or just use a hex editor like the one built into TunerPro and edit those locations.

dimented24x7

iTrader: (2)

Even with the IAC all the way open it isnt going to break the vac too much at high rpms where youll be risking damage to the rod bolts. Best thing to do is try and remind yourself to slowly roll back on the throttle. Im trying to break the habit of snapping it shut myself when at high rpms.

RBob

iTrader: (1)

Possibly due to minimal valve to piston clearance. With the high manifold vacuum trying to rip the piston off the top of the rod, the valve to piston clearance closes up and they make contact.

At high RPM with high vacuum the rod/piston assembly is going to stretch.

RBob.

Rick King

I don't know why it happened just that it did. the piston tops where unmarked, so there was no hitting, I do know that shutting down a push rod engine (sharp drop in rpms very fast) can be detritmantel to the health of the motor. you have mechanical parts that are moving very fast and the ask them to almost stop--the energy has to go some where and that would be what ever part is the weakest link for what ever reason mine was the pushrods and vales

later and

GB

rick