Tunnel Ram Tune

[440Duster]

I'm looking for some direction in a lean condition at cruise. I suspect it is related to the transition between the end of the transition slots and the beginning of the mains coming on. I can feel and hear the engine going lean and flat in the cruise range of 50-60mph.
MPH RPM AFR
35 1500 14
45 1900 15.5
55 2250 16.5-17
65 2800 15
70 3050 14.5
It runs and feels good in the 14-15 afr cruise range.

The car/engine specs:
440 Dodge, 10.0 CR, iron 915 closed chamber heads with chambers cleaned up and mild porting. 231/237 @ .050 110 lobe separation 106 ICL, Weiand 1987 Hi-Ram Tunnel Ram w/2 ProForm 750 DP, 3000 stall and 3.23 gears.
It runs really well and am very happy with it overall. Its just that lean area that bothers me at this point, it just sound and feels "off" there. I'll throw in that the timing is 19* at idle and has a fast curve, all in by 2300 rpm. I am running vacuum advance (ported at this time). I've tested with the vac can disconnected thinking maybe there was too much timing at those cruise speeds but the change (if any) was really unnoticeable. I have the secondaries wired closed as I am working on the cruise and PV circuit at this point. I will note that this is a 100% street driven car that occasionally gets a little romp on back country roads.
The carbs:
Primary Secondary
.031 IFR .031
.078 IAB .078
.085 TSR .080
.046 PVRC ---
.036 MAB .036
#74 MainJet #80

I'll have the carbs apart this weekend to change PV's from 10.5 to 8.5 i believe the PV's are coming in a tic early (unrelated issue) so any suggestions are welcomed and appreciated.

[GTO Geoff]

The 440 has a karge rod/stroke ratio & does not tolerate cam duration/overlap like long stroke engines like a BB Chev.
It should have a lot more idle timing, probably in the 25-35* range for that cam. You get that by connecting the VA to a manifold vac source. Chrysler missed the boat on VA....
To test for optimum idle advance, simply turn the dist slowly with the engine idling [ in gear if auto ] until you get the highest idle rpm. Idle vac will also be highest at this point. Then, check what the timing is. Say it is 32*. You can use any combination of initial + VA to give you 32*.
This needs to be done BEFORE any carb adjustments; while it may not totally fix your lean condition, it will help because it is making the engine more efficient & better able to carry the load.

More info on MVA, scroll down to post #6:

www.hotrodders.com/forum/vacuum-advance ... 47495.html

[rgalajda]

Not enough overlap between idle and main circuit? Try reducing IAB by .003"
Maybe someone else can pipe in.

[Right hand drive]

[440Duster]

Thanks guys, I’ll do some testing and let you know how it goes.

[440Duster]

Update: I pulled the IAB's and found out they were .072 not the .078 that I had written down. I now remember doing that, but for the life of me can't remember if i actually tested afterwards. Then i went and broke the ever so important rule. I changed more than 1 thing at a time. #1 Seeing that i used to run my vac advance with manifold vac i went back to that but left the fast curve in it, #2 i put .070 IAB's in, adjusted mixture and idle, so the tune for most recent test is this:
P/S
.031 IFR .031
.070 IAB .070
.085 TSR .080
.046 PVR -
.036 MAB .036
74 main 80
10.5 PV
Emulsion stack from the top:
1) .028
2) blocked
3) .028
4) blocked
20* Idle timing 34* all in at 2300rpm and 14* of vacuum advance @ 15" of vac. vacuum can starts to add @ 6" of vacuum.
The results:

MPH RPM AFR VAC
35 1500 14 15"
45 1850 15 16"
55 2250 15.5 16"
65 2800 15 16"
70 3000 15 15.5"

Now there wasn't a big change but it was much better sounding and feeling. No longer feels flat in that transition area. The only negative thing was a popping out of the exhaust from say 40-60mph. I suspect that the timing is coming in too fast and is almost maxed out at 50mph. my next test will be to slow the curve back down and retest. It's definitely much better by the ear and butt dyno.

[Right hand drive]

Maybe dial back the vac advance first before the mechanical advance. Your getting the full 14* @40-60mph now. Quick test back to ported might tell you something.

[440Duster]

Good call, I’ll definitely try that out.

[440Duster]

UPDATE: I believe i have my timing squared away. So i'm looking to get back to dialing in the cruise and then WOT. The combo again is: The car/engine specs:1971 Plymouth Duster with a straight axle (gasser inspired) 440 Dodge, 10.0 CR, iron 915 closed chamber heads with chambers cleaned up and mild porting. 231/237 @ .050 110 lobe separation 106 ICL, Weiand 1987 Hi-Ram Tunnel Ram w/2 ProForm 750 DP, 3000 stall and 3.23 gears. 100% street car. Here are the updated carb specs and readings as of this morning:

.031 IFR .031
.072 IAB .072
.085 TSlot .080
10.5 PV ---
.046 PVR ---
.036 MAB .036
#74 Main #80


AFR Vac Rpm
12.5-13 14 950 in park
13.0 12 850in gear
12.0 14 1100 @ 20 mph
12.0 15 1300 @ 30 mph
13.5 14 1700 @ 40 mph
14.7 16 2100 @ 50 mph
14.7 15 2600 @ 60 mph
14.1 15 3100 @ 70 mph
12.0 0 WOT

Light acceleration keeping vacuum over 10.5 (out of PV) 14-15.5 AFR
Moderate acceleration Vacuum under 10.5 PV active 10.5-11 AFR
Decel 20+ vac 12 AFR

The car runs really well overall, in fact if it wasn't for the AFR readings I would probably call it good. But I know its fat @ low speed cruise and under WOT. Where I have struggled in the past is when I get the cruise leaned out it really gets lean in the transition between the slots and the mains. Any tips on getting it leaned out a bit at low speed cruising without pissing her off in the transition to the mains? I can address WOT after the cruise is squared away as this car really doesn't see WOT very often.

[440Duster]

So after staring at the "data", My initial thought is to drop the t-slots from .085 to .080 to clean up the low speed cruise. Am I thinking clearly?