What's the deal with the revs?
- Thread starter krly79
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Roger Ramjet
New Member
I noticed the RPM hangup between shifts right away. I also don't like the freewheeling feeling (lack of engine braking) when decelerating in the lower gears. I have driven other cars that do the same thing. I suspect it is done for emission reasons.
True Blue - how big a hole in the copper cap?
True Blue - how big a hole in the copper cap?
Roger Ramjet said:
I just checked it and I drilled the hole to 5/32". I guess you can start out smaller if you want and keep making it bigger until you get it to where you like it. I was also wrong about the size of the cap, it's closer to 3/4 ... not 1/2". It's the size of most of your common household plumbing. There was a thread about this whole procedure, that's how I found out about it ... You might have to dig in the archives to find it or maybe try a search on "hanging idle" or something.
I might have to try putting mine back in and see if I can find out why I didn't leave it in.
jstreet0204
Active Member
This is mostly controlled by the computer. The EEC has a function called dashpot_decay_rate. If you get a dyno-tune you can have them adjust this to control how fast the revs drop. I use my tweecer to adjust mine.
Roger Ramjet
New Member
jstreet0204 said:
And, I assume, implemented through the IAC (an orfice on the inlet to the IAC can limit the airflow).
jstreet0204
Active Member
Roger Ramjet
New Member
OK - this is what I have done, so far. I removed the entire IAC air supply hose assembly from the engine (hose from the intake duct, the little plenum, and the hose to the IAC). I then pulled the IAC side hose off the plenum. I pushed a 1/2 inch long piece of 3/4 inch OD hose midway into that hose. I then pushed a 3/4 inch diameter disk with a 5/32 inch hole into the plenum side of the hose. I used a small socket to push the disk in flat against the internal smaller hose.
I wanted the restriction downstream of the plenum to minimize the air available to the IAC. I also wanted the short internal hose closer to the IAC so the disk couldn't enter the IAC.
Started the engine to go on a test run but I didn't like the closed loop idle operation when I turned the A/C compressor on - not enough authority.
I removed the 5/32 orfice and inserted a disk with a 5/16 inch hole (actually a standard 5/16 flat washer). Idle authority seems fine. And, on the test run the engine seems to decel much better.
I'll wait until morning to see if the fast idle function is satisfactory.
I wanted the restriction downstream of the plenum to minimize the air available to the IAC. I also wanted the short internal hose closer to the IAC so the disk couldn't enter the IAC.
Started the engine to go on a test run but I didn't like the closed loop idle operation when I turned the A/C compressor on - not enough authority.
I removed the 5/32 orfice and inserted a disk with a 5/16 inch hole (actually a standard 5/16 flat washer). Idle authority seems fine. And, on the test run the engine seems to decel much better.
I'll wait until morning to see if the fast idle function is satisfactory.
The slow RPM decay is done for emission reasons. Most new engines do this. I belive the reason is if you're winding the thing out and suddenly close the throttle you end up with a bunch of un-burnt gas in the cylinders that causes real rich running for a second or two.
The '05s will most likely be LEV's (Low Emission Vehicles) and will have even more things to annoy you guys who still drive 1967 289 Mustangs. One of them will be that when you cold start it, the engine will probably rev real high for 20 - 30 seconds (based on temperature) to heat the cats rapidly.
However with the ETC on the '05s (Electronic Throttle Control) you may find the revs will drop quiker when you take you foot off the gas as ETC gives you better overall control of the engine. One strategy to do this with ETC is to cut the fuel flow quick and delay the throttle flappers from closing a few milliseconds to clear out the cylinders quicker thus after a very short delay the revs. would drop faster.
The '05s will most likely be LEV's (Low Emission Vehicles) and will have even more things to annoy you guys who still drive 1967 289 Mustangs. One of them will be that when you cold start it, the engine will probably rev real high for 20 - 30 seconds (based on temperature) to heat the cats rapidly.
However with the ETC on the '05s (Electronic Throttle Control) you may find the revs will drop quiker when you take you foot off the gas as ETC gives you better overall control of the engine. One strategy to do this with ETC is to cut the fuel flow quick and delay the throttle flappers from closing a few milliseconds to clear out the cylinders quicker thus after a very short delay the revs. would drop faster.
Roger Ramjet said:
I think that might be the reason I took the copper cap out of mine. I'll have to drill it to 5/16" and give that a try. I probably got pissed before and just took it out instead of playing with the size of the hole. When it stops raining I'll put mine in and let you know how it works.
Wayne S
Founding Member
I’ve had all the cars/engines you mentioned and I love the sound of my Mod motor better.krly79 said:
If you don’t like it stick an H-pipe and some flowmasters on there and get that generic sound that almost every v8/v6 sounds like with them.
Even the 5.0 hangs its revs like you are talking about in your post above.
One thing to keep in mind for the occasional drag tracker who only granny shifts/speed shift. The way the revs are setup you will get a better E.T since the RPMS don’t drop off on you so fast. An OHC will hang longer in the RPMS then a push rod engine will anyway.
That has to do with gearing and the rpms the engine is at. Has nothing to do with what this thread was started about. If it did do like you mentioned, it would be the other way around since the engine is still at a higher rev.Roger Ramjet said:
My guess is your low in the rpms for the gear your going into when you down shift.
Roger Ramjet
New Member
351CJ said:
This is my first modular motor. I've had 3 LS motors since '98 - none of them did the idle hangup thing. I did drive a BMW last year that had worse RPM hangup between shifts than the Mustang.
Back to the fix! Cold start fast idle works fine with the 5/16 inch orfice. The car noses over right away when the throttle pressure is reduced. No down side yet.
I'm going to re-drill mine right now, put it in today and see how it works and let you know later. I just hope it does't screw with the idle too much when the A/C is on.
Roger Ramjet
New Member
I did use defrost this morning - causing the A/C compressor to run. Closed loop idle picked up the RPM like normal.
v8thunder1
New Member
- Dec 7, 2003
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Ok, I wanna do this trick, but Im a little confused as to where exactly it goes, could someone please explain? Its for a 99 GT. Thanks !!!
Mine worked out great, the car does back-off the throttle faster now and I didn't have any other problems from it. Fast idle is good and it still works fine when the compressor kicks in. I drilled mine to 9/32".
v8thunder ... It goes in the big hose running from the plastic black box to your IAC valve. You just take the hose off and put it in ... that's it, no clamps or anything needed. The coppper cap we're talking about fits perfect so get one of those and drill a hole in the center, put it in and you're done!
v8thunder ... It goes in the big hose running from the plastic black box to your IAC valve. You just take the hose off and put it in ... that's it, no clamps or anything needed. The coppper cap we're talking about fits perfect so get one of those and drill a hole in the center, put it in and you're done!
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