So.. If a car.. with a 288 or so degree cam. Absolutely refuses to idle with less than say 22-24 degrees of advance, at around 900 rpm.

MY assumption is either it's a hair on the lean side and it needs that advance to light the lean mixture, OR simply the cam just wants that much advance.

The tough thing I'm having is, I've had cams that were a hair larger although a tigher lsa that would idle so-so at 10 degrees.

This thing I have to play with the throttle to keep it running if it's less than 24 or so degrees.

-- Joe

 

 

give it what it likes....

what's the worst that could happen?

i like to tune idle spark via watching IAC counts when changing advance. less counts means making more power with less air(and with less air, less fuel).

 

Some cams like advance at idle...I seem to remember the cam in the 305 that is in my brothers truck liking around 16* initial advance and about 14* vacuum at idle for about 30* advance.

 

Or in other words the most vacuum.


Right now mine is idling at 36 degrees at 600 rpms at 14:1 with a LT4 hot cam. In park AC off it idles around 47-50 kpa at best. I need to play with it more and see if its overtimed or still not enough. IIRC when I first put it in I WAG'd a spark table for the initial fire up and had something like 28 degrees at idle and it hated it. 36 seems extreme to me but it seems to like it well enough. Curious to see what 38-40 would do. Sounds crazy to me but if thats what it likes...

 

I think you missed the tune on that. I've never ran a cam as small as the LT4 hot cam, yet I've seen way higher than 50kpa at idle on much larger cams.

Whats the compression ratio on that thing? It just doesn't seem right.


Anyhow, the base question and reason for my post wasn't so much what a cam liked at idle, which was the minimum advance required to idle. On XR276, CC306, and custom grinds with 280 degrees of duration I had no problem running with EST unplugged and 10* of initial. Not on this motor.

-- Joe

 

CR is 9.5:1 and I've verified with a timing light that its actually idling with that much timing. Now I'm wondering and most likely will have to go play around with the timing tonight after work.

 

Joe, I'll look where I'm at tonight when I get home. My 10:1 383, with a custom grind (more or less a 288 cam) idles around 64kpa, cam installed straight up, 6deg on the timing mark @ 25-26 total spark and 875rpms. I can go down to 22 but I get better off idle response at 25-26

 

Whats the specs on the motor in question? One in your sig? Can you verify what the fuel mixture is like? Was the cam degreed when installed?

My big cam on my 383 could idle ok at 10 deg base with EST wire disconnected but was MUCH smoother once total advance of 28 deg was achieved. That was a 286/306 advertised cam, 950-1050 rpm idle.

My 401 with a 292 advertised cam with low compression will take low initial advance, but right now I'm tuning it with 28 deg total at idle and may go even higher. 50-54KPA idle so far. 1000 rpm idle so far but I want to work it lower.

 

Alright, Camshaft: (244 exh. / 248 int. @ .500” - 113° LSA) was installed straight up, timing set at 6 (idles fine with EST wire disconnected) and has 26.72 deg at idel around 62-64kpa. AFR has been anywhere between 14.2 - 15.7

 

The 412, with the XR288HR cam. Lift is almost 600, afr heads, nearly 11:1.

The more I look into it the more I find something is screwy. I tossed the distributor in the trash today. The stator/reluctor/whatever you call it was sticking so the advance mechanism wasn't working properly.

Trying another one this week. I put this together with "stuff I had" to break the motor in before I go crazy on the EFI setup. Maybe that was a bad decision.

Anyhow, The ignition was a problem, partially. Based on the plug reading, it's running a hair rich. Which I don't quite get, cuz I'd think it would want less timing.

What is your crank advance ?

I'm still debating 4bbl TBI or port. I'm kinda either waiting for someone to bail on a HSR for short money, or the next time I see a 4150 4bbl holley setup on ebay I'll shoot for it. I don't even want to tell you what is bolted to the motor right now, it's a sin.


-- Joe

 

I'm close, but my LSA is 110, and my centerline is 106. I have to play with the throttle to get it to run around 10 degrees intitial. I'm 100% certain the timing mark is dead on, I checked the balancer and mark at TDC to zero before I put the heads on when I checked the cam.

Once I get the new dizzy in, I'll go through the whole process again.

-- Joe

 

I'm real interested in this too. My 350 with Sportsman II, XR276HR 112 LSA, Should be on a 108 but I installed on 104 for a bit of DCR boost.
About 9.3:1 static running right at 27* at idle, 58 Kpa MAP using MSD 6A box.
Same types of conditions you describe. I've always thought it to be too much SA but I can't get it to run smooth or clean any lower. Still have no detonation at 38* WOT running 92 gas. Rich smell at idle taken out by adjusting idle O2 constants by 45 mV and using the 6A box. Idles pretty clean at 750-800 RPM.
I've had a theory that it could be due to the HSR and the 200 cc intake runners having low velocity at idle. Can't confirm or deny that or if the quench being a bit more than factory has anything to do with it as well. I do not remember it being that high with the L98 083 heads with shim gaskets on it with the TPI.
Confirmed TDC to balancer with heads off as well so the numbers are correct.
The plot thickens...

 

Just a general observation here but you guys all seem to be idling at what I would consider to be very high. Heck, on cold start mine might see 800 rpms if its cold enough out but once warmed up I like to keep it under the 600 mark. My converter is extremely tight (stock 6.5 diesel 4L80E unit) and it works but without a low idle speed its like trying to come to a stop with my foot on the gas.

That being said does anyone know off hand the correlation between idle speed and idle advance? Meaning does a higher idle speed mean less advance is needed to make it idle at a decent amount of vacuum? I want to say that it does because at a higher speed there is less load on the engine. At a lower speed the engine is working harder to sustain an idle and pushes the vacuum lower. The general rule of thumb with timing is as vacuum goes down the timing should go down but does that translate to idle timing as well? If I try to idle at 600 rpms should I expect to run more timing then say idling at 750 rpms? The plot thickens.

 

In my experience, the combination of higher idle rpm and timing makes for best vacuum. Big cams seem to want 900-1000 rpm idle.

All my timing curves go UP as rpm increases and vacuum increases (or KPA MAP in speed density gets lower)
MAF cars its LV8, lower LV8 is more vacuum *just less load on the motor*

you can play with your base timing with EST disconnected and watch what happens as you give it alot of timing advance. Give it 30 deg timing EST disconnected and slowly bring it down to 10. Listen/watch the motor rpms die down.

Piston speeds increase as rpms increase so you need more timing advance to start the combustion process in the chamber before piston reaches TDC. There is just less and less time to give spark as rpms go up. Then, as engine load increases, cylinder pressures rise so less timing is needed to prevent detonation. Its a two way street. Timing is increasing as rpms rise, but decreasing as you go from vacuum condition to no vacuum/WOT condition.

You refering to my base timing for my 401? Its at 11-12 deg now but I may lower it as low as I can get. I've played with my 401 for MANY days chasing an overheating issue that i thought was due to timing at idle. I tried 15-16 degs TOTAL EST connected, all the way to 30 at idle. Idle seemed to stay about the same, with 30 deg being slightly higher rpm wise and a touch smoother. Headers seemed to get about same temps, so I"m not sure timing really was making a difference.

I did notice my vacuum went from about 13" Hg to 15" Hg with 28-30 deg timing instead of 16 or so. I'm going to keep playing with it. PLugs looked right when timing was in the mid high 20's and wideband air fuel was saying mid high 14's to 1 at most times, with cold starts in the 12-13 range.


I was playing with a 280xfi L98 HSR motor that has AFR 190 heads on it. Runs strong, mostly ARAP bin with abit of modification. Idle likes near 30 deg timing with 10 as base. Idles well around 950-1000 rpm with a leaner 16 to 1 air fuel in Park, but in Drive it idles well around 800 with air fuel in the high 14's to 1 low 15's to 1. Still working on it but I'm happy with where its at. Spark ramps up rather quickly to 40-41 deg in the mid range cruise areas. Car likes it there but I originally thought that would be WAY to much.

Fairly big cam but certainly not outrageous for a 412. I'd expect it to idle ok at 10 deg base timing if the air fuel is suitable. I dont think its out of the ordinary to have it idle in the 12-15 deg base timing range with overall timing of closer to 30, if that is what the motor wants.

Piston shape/valve reliefs and combustion chamber shape on the heads will have some influence on this as well.