Idle speed- any downsides to using a lower one (~750 rpm)?

[robnitro]

Hi,
My commute changed and I have a lot of stop and go and crawling.

So, I changed my ecu to run a lower idle speed- previously 840, now 756 rpm. It's actually quite smooth because I use the stock dogbone, albeit with the swiss cheese rubber replaced with the softest poly one I could find.

Group 13 looks fine too... no real difference from other idle speeds.

Are there any technical issues to running 84% (756/903=0.84) of normal idle speed?

 

[JB05]

I would be concerned with the alternator not turning fast enough to keep the battery charged as well as operating all accessories, such as glow plugs, engine or coolant gp's.

 

[robnitro]

Good point, but yeah already covered that by using a high idle set to 1200 rpm, ramping down to normal idle at around 60C coolant temp (and the high idle timeout set to 20 mins instead of the stock ridiculously low one). Also set the idle to stay at 1200 rpm as the coolant GP's are running (which in the coldest of temps I have set to turn off past 50 C or so).

I have the 120A alternator, an Autozone duralast gold import rebuild which has been nice as it puts out a bit more voltage than the stock bosch setup (14.4 at the battery instead of 13.8 or so.)

With no load, group 16 vcds shows 14.2v. Alt load around 25%
Turned on AC, rear defroster, and headlights/fogs to make a high load test.
Group 16 vcds shows 13.91 v, 64% load.
Raised the idle back to 903, shows 13.91v, 58% load. So, a bit more load on the alternator with that "torture test" but still no way close to maxed out.
I believe that % load is how much of the field current is applied. Once you hit 100%, the alternator can't push anymore. Higher rpm needs less field current to maintain the same amps x volts= watts.

 

[eddie_1]

Maybe those so called tuners should be using you as a consultant.

 

[GoFaster]

Random thoughts, some may be issues or not, just general principles ...

The alternator situation was already covered. Looks like you are okay here.

It will be easier to stall the engine on takeoff from a stop ... but you must be okay with this, since you are still driving it this way.

Bit more vibration through the engine mounts ... but again, you must be okay with it since you are still driving it this way.

The bigger concerns that I would have involve the oil films at the camshafts/lifters, and the main and rod bearings. Oil pump will be delivering less, spring pressure on the valve springs remains the same, engine compression pressure remains the same, hydrodynamic lubrication relies on relative velocity between the two parts to maintain the oil film and that relative velocity is lower so it will be trending towards lubricant film breakdown.

 

[robnitro]

eddie, you started the curiosity when you moved to germany and had issues passing TUV... digging deeper- found out why the tune was smoke free at full boost but smoky on spool up I'm never satisfied until I know WHY, and sometimes seeing how secretive or avoidant someone is about something makes me wonder! It's also tuned to give 50% power when cold and rev limited to 3500 rpm.. great idiot proofing if you have family that likes to floor it when freezing, LOL

GoFaster
Thanks for replying
Stalling is similar, its pretty amazing how strong it can be down low!

I don't know too much about the oil pump and pressure. Maybe I should tap a gauge to it and see what is the idle oil pressure. What should I be looking for?

Do you know what mean piston speed is recommended to find ideal idle speeds in turbo diesels? Trucks I see run 600 rpm idle... but their stroke is higher, 120 or so vs ours in the 90s. The newer TDI's seem to have around 820 rpm idle..
If we go by stroke alone, I figure 600 * (120/90)= 800 rpm to have the same mean piston speed. I know only that mean piston speed limits the top rpm, but can it effect idle speed ring sealing etc?

Don't know what else could be affected. It's pretty amazing at how smooth it can be at even down to 735 rpm! Less audible "bass" and a bit more vibration... It seems like that frequency is the low point. 735 rpm x 2 combustion events per rev= 1470 combustion events/min / 60s/min= 24.5 firings/s . 24.5 hz. That's around the threshold for hearing.
Just for comparison, 903rpm= 30hz.
And for the 6 cylinder truck diesel at 600 rpm - > 30 hz.

 

[turbovan+tdi]

If you have enough oil pressure, you have nothing to worry about. Many OE engines idle at 500+ rpm.