Lowering idle RPM?
 

Hello all, I finally got Kpro for my K24A2 swapped 04 Civic, so I can now configure how the engine operates to my liking.:) Right now, I'm messing with the idle RPM. For now I set the idle to 700 RPM, which seems to actually be more like 650. Anyways, it seems quite happy at that RPM. I'm not sure how much fuel dropping the idle by ~100 RPM actually saves (if any), but I could easily test that.

My main concern is whether idling too low could in any way damage or reduce the longevity of the engine. The main ways I could see this being a problem is if the oil pressure was inadequate at a lower RPM or if the oil splash to non pressure lubricated parts like the valvetrain was significantly reduced, but then again at a lower RPM I would expect that not as much lubrication would be needed in the first place, so how much of a problem is this realistically?

Looking forward to hearing opinions and experiences on this one!

You're welcome to my K24A2 tune, if you'd like to use it as a starting point.

I found my engine got rough below ~745rpm. Granted, I had removed the balance shafts for both fuel economy and power reasons, but the Insight has no subframe, the engine is bolted directly to the chassis (with poly mounts in this case), and the stiff and lightweight aluminum chassis transmits those vibrations well, so I didn't go any lower to avoid harshness.

I don't see a low idle as hurting it. There's not much load on the engine. K's have great oil pressure, especially off VTEC, because they're designed to have extra pressure due to so much of it being bled off to feed the VVT and VTEC system, which are oil-pressure powered.

Low idle RPM shouldn't hurt it. What hurts an engine at low RPM is when you put load on it (i.e. lugging the engine). Long as it has oil pressure, you should be fine.

Cool, thanks. What mods is that tune set up for? For now I'm using the stock TSX basemap in Kmanager and as far as I can tell not being able to monitor fuel trims and air/fuel ratios since I don't have an exhaust and oxygen sensor yet it seems to be working reasonably well. I got 42.2 MPG last tank despite everything being wrong, so that's not too bad for something so quick. That should improve a bit when the brakes aren't dragging, I get a hood, exhaust, oxygen sensor and taller geared 6 speed trans, suspension is fixed and alignment is correct, etc.

Mine doesn't have balance shafts either and it still idles smooth enough at 600-700 RPM that I usually can barely feel it running over the vibrations from the road, tires, etc while I'm coasting and it never vibrates so much that it annoys me. Since I don't have an exhaust system yet, I actually prefer the lower idle because it's quieter.

As for oil pressure, I'm not sure what it is since I don't have a gauge yet. I imagine it is probably pretty good since the chain driven oil pump spins 1.6 times faster than the crankshaft. I was more concerned about inadequate oil splash at a lower RPM, but I wouldn't think 600-700 RPM would be so low that I would have to worry much. It's not like I'm trying to get it to idle ridiculously low, plenty of cars idle at that RPM with no problems.

Thanks, that's pretty much what I'm thinking. Now that I lowered the idle I don't take off or creep forwards/backwards without using the gas anymore, I always bring the RPM up to 800+ before letting the clutch out. I'm not sure taking off from low idle without bringing the RPM up is really "lugging", but it doesn't seem to like it so I don't do it.

Here is a link to my repository of tunes:

https://drive.google.com/file/d/13Dz...ew?usp=sharing

The motor was a K24A with the 04-05 intake, Accord cable throttle body, stock header, 2.5" exhaust, RSX oil pump, and 06-08 TSX cams. I had a 50° VTC with a 40° limiting pin, but it should work fine even if you have only a 25°.

The only thing I'd caution is to listen for knock, as the tunes were very much maximizing ignition advance for each octane level - the 93 tune is essentially MBT everywhere but at WOT below ~2250rpm, where I still had to pull some advance. You can, of course, simply flash a 91 tune if you're running 93 and still getting knock on the 93 tune, but I'd be happy to help adjust the ignition tables to fix that.

There are lean burn and non-lean burn variants for each fuel grade. You'll want to tick or untick the boxes based on what sensors you have - mine was complete, with upstream and downstream O2 sensors and full emissions set up. You'll also want to adjust the transmission ratio table.

42mpg is great. I found gearing was absolutely key, and gained another 5+ mpg when I got my 3.4 final drive and 9th gen 6th installed.

I don't know anything, but I'd think main concerns with low idle have to do with alternator power output, AC cooling ability, and NVH concerns.

On my motorcycle, I lower the RPM to the point that I end up stalling it too frequently, then bump it up slightly.

Awesome, thank you so much! I'll try those and see how they work for me. I have a very similar setup- JDM K24A, USDM 06 TSX cams, RSX oil pump, OEM oil cooler, Accord cable throttle body, custom cold air intake, and stock header for now although I will eventually need to change that because the collector is directly above the back of the subframe and it would be impossible to get an exhaust on it. I'll probably get a long tube 4-2-1 header eventually since it should be best for fuel efficiency and overall torque. Out of curiosity, how did you tune yours? Did you street tune it or was it dyno tuned?

Yeah my base RSX transmission's 5th gear is definitely shorter than ideal considering the torque of the K24. I think the ideal transmission for my setup would be an 02-04 RSX-S 6 speed with 04-08 TSX 5th gear, 9th gen Si 6th gear, 4.0 final drive, and OEM LSD. I'm sure the 3.4 final drive works great in an Insight, but I think it would be a bit too tall for my heavier Civic.

I think you are right and none of those are much of a concern in my application.

Alternator works great. Even with my lower idle and all electrical loads on, the voltage always stays above 14V since the Acura TSX alternator is so overkill for the small electrical demands of my Civic, especially since it has all LED lighting.

I don't have AC, so that's not a concern to me. But as weak as some AC systems are at idle, I could definitely see an even lower idle being a problem on some vehicles.

NVH is somewhat of a concern to me, but not much. It's plenty smooth at around 650 RPM and realistically a 4 cylinder on stiff mounts is never going to be as smooth as something like a straight 6 or V8, which is fine with me.

Actually at least the first of those two problems could easily be solved by simply programming the ECU to increase idle speed when the electrical load is high and/or voltage drops too low or the AC is on. As for why they don't do this, my best guess is probably because a changing idle speed could be annoying and might make people think something is poorly designed or malfunctioning.

I suspect the limiting factor is rotating mass. Compare a race-bred engine with a chopped flywheel and lots of valve overlap vs those 1930s British stationary diesels I can't recall the name of. Or a Model T Ford.