Idle RPM question

[theholycow]

Your ideas are no more likely than the lame one I came up with. Erik's is only slightly better. Why would it second-guess driver input for the sake of smooth shifts then but never any other time?

Also, it's probably not lugging, just "growling". That's still worth avoiding but it's not going to destroy your engine.

[R.I.D.E.]

Not sure of my idle speed, never checked it, but it is very low. On the tach it is just above the small mark above the 0 mark. Not really smooth, slow enough where you can feel each combustion event. I would guess its below 500, maybe closer to 400.

Mine will drop to the same idle speed as long as no accessories are engaged.

I can feel the difference when you let the clutch out in neutral, even when you apply the brakes when sitting still, the load of the brake light bulbs affects the idle. First blower speed does not increase idle, but second poistion will kick it up to about 750.

My VX has no radio either.

As far as speed in gear I generally do not go below 1000 RPM, 30 MPH in 5th gear. Even with the AC on (rarely used) it will easily accelerate from 30 MPH in high gear.

It will drop to the low idle speed at any vehicle speed, but it does take a few seconds.

regards
gary

[1993CivicVX]

Originally Posted by theholycow

Your ideas are no more likely than the lame one I came up with. Erik's is only slightly better. Why would it second-guess driver input for the sake of smooth shifts then but never any other time?

Also, it's probably not lugging, just "growling". That's still worth avoiding but it's not going to destroy your engine.

Okay, thanks for the clarification. My dad told me I was lugging it when it was "growling". So lugging it would be trying to accelerate in 4th gear going 12mph. But one mechanic I went to said that it wasn't good to accelerate even at 1000rpm in 5th. Most people I drive with seem to think I should down shift when I'm trying to accelerate at those low rpms. I don't think I'm convinced 800rpm in 5th isn't bad for the transmission.

[R.I.D.E.]

The shift indicator light tells me to upshift to 5th at 30 MPH. No reason for Honda to do that if it was damaging the car. The gear i choose to accelerate is the highest gear that is adequate for the desired rate of acceleration.

regards
gary

[suspendedhatch]

Originally Posted by theholycow

That breaks every explanation I can remember that's been proposed to explain why cars with automatics behave that way...and I can't think of any explanation for it at all. Can anyone else?

Was it originally an automatic, and still tuned as one?

Can you measure throttle position in each of the described conditions?

Actually I explained it already. The VX has a strategy that takes into account VSS and uses the IACV to hold the idle a few RPM higher above a certain speed. Then when your speed drops below a threshold, you'll notice the RPM smoothly drop to 600.

No VX and no D15Z1 was ever mated to an auto from the factory.

[theholycow]

Originally Posted by 1993CivicVX

So lugging it would be trying to accelerate in 4th gear going 12mph.

No, probably not, unless it was an old car that couldn't detect knock and adjust spark timing. Lugging would be a really terrible ping/detonation problem (which could be caused by the driver trying to accelerate in 4th gear going 12mph, but probably not on a modern car).

But one mechanic I went to said that it wasn't good to accelerate even at 1000rpm in 5th. Most people I drive with seem to think I should down shift when I'm trying to accelerate at those low rpms. I don't think I'm convinced 800rpm in 5th isn't bad for the transmission.

I'm no expert (then again, many mechanics aren't either), but I don't think you're damaging anything if it's growling; you're just causing extra vibration. Extra vibration, of course, could rattle something loose if it's almost rusted through, or could put more wear on something, or could prematurely fatigue a material that has to flex during vibration.

I try to point it out when someone talks about lugging not because I would encourage driving like that, and not even to be semantically pedantic (that's a mouthful), but because the word "lugging" is associated with the danger of causing damage that would not be caused by what's actually happening.

[theholycow]

Originally Posted by suspendedhatch

Actually I explained it already. The VX has a strategy that takes into account VSS and uses the IACV to hold the idle a few RPM higher above a certain speed.

Ah, I missed (or forgot) that. Did you say why it was designed that way?

[dkjones96]

Also remember that tachometers in cars aren't generally all that accurate to begin with. My Cressida used to tach an in-gear hot idle of 400-450rpm but with a real engine analyzer on it the idle was really 550. My current car is similar. According to the tachometer I hit fuel cut at exactly 6500 rpm(the red line) but when it's hooked up to the dyno using a Smart-Tach or OBD2 streaming I actually hit fuel cut at 6655. Just remember to hook up a REAL tach before you go messing with your idle.

As for the lugging. The only reason that's supposed to be 'bad' for an engine is it can cause overheating. In an air cooled engine as long as you are at speed it's alright but a water cooled engine has to rely on engine speed to get water really flowing. Also, on older engines they had a mechanical fan without a shroud so you had to REALLY get the engine moving to get good air flow. Like in the picture below, the fan stands by the engine with no shroud and about 4" of space between the fan and the radiator.


Another note on EFI vehicles, you don't need to mess with your idle. The ECU will adjust the idle to where it is supposed to be. If you move a set screw or anything it will still idle the same unless you mess with the IACV or you've adjusted the set screw(if there is one) so much that the IACV can't adjust anymore.

[1993CivicVX]

Originally Posted by theholycow

Your ideas are no more likely than the lame one I came up with. Erik's is only slightly better. Why would it second-guess driver input for the sake of smooth shifts then but never any other time?

Also, it's probably not lugging, just "growling". That's still worth avoiding but it's not going to destroy your engine.

Because at 65mph the tach would lurch to 2100rpm whereas at lower speeds of say 30mph, there is basically no lurch because shifting into 5th gear from neutral at 30mph means the rpm would jump to 1050rpm. No reason to have the idle be as high as 1000 at 30mph....

[R.I.D.E.]

dkjones that flathead engine compartment looks like my 37 Ford.

regards
gary