360 Idle Quality

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Earlie A

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Working on a 1965 Dart with 360. Problems with low idle vacuum, especially in gear. In gear, the most vacuum I can get is 9 in at 850ish rpm, and engine is struggling against the converter. The vacuum advance is just starting to come in at 7-8 in and would be all in at 14. I have slowly increased initial timing to 32 degrees (mechanical advance limited to 16) and I’m afraid to go farther even though every increase has helped. In park, the vacuum goes to 15 in and rpm jumps to 1400.

I have measured the cam at 278/291 duration at 006 and 0.300 tappet lift. IO at 28BTDC and IC at 70ABDC. EO at 67BBDC and EC at 44ATDC which makes the cam 106 LCA and 4 degrees retarded.

Don’t know the converter specs, but it seems too tight for the cam, at least for the retarded install.

I’m shocked at the 32+16=48 degrees total timing not causing issues under load, but it runs pretty good at mid/high rpm. Low rpm seems a little sluggish, but advancing the timing has helped.

Here’s my questions: Anyone ever seem total timing that high?
If I advance the cam, how far should I go?
I have a new B26 vacuum advance canister that comes in at 5-7 all in by 12. Any down side to installing it?

Other specs:
Looks like stock 1985 bottom end with stock dished pistons.
Compression ratio estimated at 9-9 1/4 (stock 8.4 compression, stock 73cc heads replaced with 63cc heads)
Cold cranking compression 130-140 (6 cylinders at 140)
Edelbrock RPM heads
Edelbrock RPM Air Gap
Edelbrock 650 AVS (have also tried 650 Holley)
MSD Digital 6 with Pertronix dist
New Autolite plugs gapped .045 and 8.5 mm wires
TTI headers and 2.5 dual exhaust
904 transmission with RMVB
8 3/4 with 3.91 gears
 
Sounds like it's wanting so much initial because the cam timing is retarded. I think I would look into that.
 
Would think you would want the cam installed straight up if you want it to run nice.

Most people who are trying to cheat the cam advance it by 4°.

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I don’t have a good feel for how much difference straight up (a 4 degree change from current) or 4 degrees advanced (an 8 degree change from current) would make. With the low cranking compression and idle issues, I would be inclined to go 4 degrees advanced.
 
Stock build '85 360 with iron heads and stock 340 cam (installed straight up) runs out really, really nice with the Edelbrock 650 AVS2 carb on it.

Compatible to fuel injection the way it starts, revs up quick and idles back down smooth > like fuel injection.

20230826_215423.jpg


The mopar HEI conversion kit and high voltage E-Coil is probably helping to make it run nice too.

20230826_223302.jpg


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Earlie A said:
I don’t have a good feel for how much difference straight up (a 4 degree change from current) or 4 degrees advanced (an 8 degree change from current) would make. With the low cranking compression and idle issues, I would be inclined to go 4 degrees advanced.
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A lot. Especially if the cam is a very large one with low static compression. I normally shoot from the hip with a good guess in those instances. The "normal" 4 degrees advanced is usually not enough. I wouldn't be afraid to go down around 100 ICL. Maybe even 98. I don't look at it as cheating the cam. It's more cheating the low compression. It can really help cylinder pressure and bottom end torque.
 
So that would be a 6 or 8 degree advance (10 or 12 degree change from current). What is the limiting factor on how far to advance?
 
The timing you are noticing indicates that maybe your balancer has slipped or the timing marks are wrong. Verify TCD before going any further.
 
Earlie A said:
So that would be a 6 or 8 degree advance (10 or 12 degree change from current). What is the limiting factor on how far to advance?
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When you start opening and closing valves too early or too late. You said it's retarded now. For simplicity's sake, lets just "say" it's four degrees retarded......from zero. That means it's EIGHT DEGREES away from where the cam card says. Understand? I've seen one TWELVE DEGREES retarded from zero lined up dot to dot, so throw dot to dot out the window. It means nothing without a degree wheel. I'm not saying yours is that far out, I have no idea. But you also "don't know" where it "IS", either. So there's that. Until you get the wheel on it, there's just no telling.
 
TT5.9mag said:
The timing you are noticing indicates that maybe your balancer has slipped or the timing marks are wrong. Verify TCD before going any further.
Click to expand...
Yeah, 50 year old parts can have issues...
Plus I always get a little leery when I read "360 in an early A", you never know if they switched to the early timing cover (to retain the early iron water pump and avoid clearance issues or keep the original radiator), and didn't re-mark the balancer or marked it incorrectly...
 
Earlie A said:
So that would be a 6 or 8 degree advance (10 or 12 degree change from current). What is the limiting factor on how far to advance?
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When valves and pistons start to try and occupy the same place at the same time.
 
That timing for idle is not high at all for that combo. My engine idles with 48*. You may need more than 32*. The people saying check for bal slippage do not understand the requirements. A 440 I set up two weeks ago, man trans, 13-14" of vac likes 44* at idle. Your low comp is a big player, not helping, but it does make the engine want more timing.
Advancing the cam might make a small difference, but not worth the trouble in my opinion.
Use an adjustable VA unit connected to manifold vacuum to get the reqd idle timing. On the 440 mentioned above, I found the spring in the VA unit was too stiff, even on the softest setting, to work reliably. I took a softer spring from a HEI VA unit & fitted it to the Chrys VA unit & it now works reliably. If you use MVA to provide more timing at idle, you MUST make sure that the VA does not start to drop timing when put into gear...& cause erratic timing/operation. When I hear MVA didn't work for me, this is the number one cause!! NOT MVA!! Easy to check. Just put a Tee in the vac line so that the VA is connected & watch the plunger move.
More on the importance of idle timing...

img267.jpg
 
Common Mistakes >
Selecting too much camshaft for your compression ratio.



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Yep, that'll do it.....
Then you have two choices: change the cam or tune around the existing cam as best you can.
 
TT5.9mag said:
The timing you are noticing indicates that maybe your balancer has slipped or the timing marks are wrong. Verify TCD before going any further.
Click to expand...
Before measuring the cam, I verified TDC on the balancer with a piston stop. Rotated the crank by hand to 20 ATDC, installed the stop snug against the piston, rotated until it stopped. It was right at 20 BTDC.
 
RustyRatRod said:
When you start opening and closing valves too early or too late. You said it's retarded now. For simplicity's sake, lets just "say" it's four degrees retarded......from zero. That means it's EIGHT DEGREES away from where the cam card says. Understand? I've seen one TWELVE DEGREES retarded from zero lined up dot to dot, so throw dot to dot out the window. It means nothing without a degree wheel. I'm not saying yours is that far out, I have no idea. But you also "don't know" where it "IS", either. So there's that. Until you get the wheel on it, there's just no telling.
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Not 100% on the understanding, no. But you guys are helping that.

To measure the cam, I used a piston stop to verify TDC on the balancer was correct. Then I made a 360 degree timing tape on the computer and attached that to the circumference of the balancer. Then I removed the valve cover and rocker arms and attached a dial indicator to measure lift from #1 intake and exhaust lifters (pushrod was temporarily replaced with a carbide pilot to keep the indicator centered). Everything was pretty rigid. I measured both lobes twice. Everything was repeatable to within 1 degree and 0.001.
 
Earlie A said:
Not 100% on the understanding, no. But you guys are helping that.

To measure the cam, I used a piston stop to verify TDC on the balancer was correct. Then I made a 360 degree timing tape on the computer and attached that to the circumference of the balancer. Then I removed the valve cover and rocker arms and attached a dial indicator to measure lift from #1 intake and exhaust lifters (pushrod was temporarily replaced with a carbide pilot to keep the indicator centered). Everything was pretty rigid. I measured both lobes twice. Everything was repeatable to within 1 degree and 0.001.
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If you went to that much trouble to verify, why did you set the cam retarded?
 
Ricks70Duster340 said:
If you went to that much trouble to verify, why did you set the cam retarded?
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I bought the car this way. I'm troubleshooting the poor idle issues. Changed carbs, changed plugs and plug wires. Playing with timing. Nothing was working so that's when I started trying to figure out the cam specs.
 
George Jets said:
Common Mistakes >
Selecting too much camshaft for your compression ratio.



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I believe this is exactly the problem. Too much cam for the compression ratio and converter. I bought the car this way. Question is - what to do about it now.
 
Earlie A said:
I believe this is exactly the problem. Too much cam for the compression ratio and converter. I bought the car this way. Question is - what to do about it now.
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you already found the issue is too much cam... easy fix would be to put in a smaller cam that will fit the combo perfectly for what you want..
 
Earlie A said:
I bought the car this way. I'm troubleshooting the poor idle issues. Changed carbs, changed plugs and plug wires. Playing with timing. Nothing was working so that's when I started trying to figure out the cam specs.
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Got it. If it was my situation, I would definitely try advancing the cam like RRR suggested. That may do it for you for minimal cost. Other than that, the next step I'd take is to replace the cam with something more streetable, as bewy mentioned. The heads you have, intake, rear gears are fine for street performance.
 
Deck the heads and use a thin head gasket.
 
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