Timing curve for my big-cam 451?

-
Thanks. I drive it on the street, so I need vacuum advance to have some semblance of fuel economy, and cooling ;)

Tonight I experimented some more... took the dist apart, left the super-light spring in place but removed the heavy loop spring and used the OEM blue spring. Added the FBO plate to limit at 14 crank deg.

Then I fired it up and set the initial timing to 24 deg. Without the vac advance connected, it rises to 38 and starts to retard as before, but didn't go below 35. So far so good. I'll have to get a helper tomorrow to read the tach when the advance is all in. The advance begins right off idle, which is not good for stability - so I probably need a bit more spring. I may try using a pair of medium springs from the three pairs that came with the FBO plate.

The vac advance keeps the timing around 32-36 (it doesn't always return on decel to the same spot). And it's marginal to stay advanced at idle, although I'm up to 8.5-9" vac and at least 1200 rpm (the Autometer 5" tach is not the most accurate at the low end!) Max advance in neutral is 56 which may be too much. It won't be hard to add shims to the advance arm to limit it, if I have surging at cruise.
 
I've been chewing this over for a while... did some more reading (including my thread in the Elect/Ignition forum from Feb. 2020). Since I have a high idle, I need to "cut off the bottom" (as Mattax has said) of the advance curve. That means not using the FBO plate, but instead welding the inside of the slots.

I will always need the heavy spring with loop which will provide high-rpm additional advance to offset the inherent retard in the electronic ignition and the rev limiter/nitrous timing retard unit. It appears that the OEM heavy loop spring allows 14 deg before it engages.

Tell me if this plan sounds reasonable:

Initial will be 24 deg, can't use more for hot starting.
Desired all-in is 36 deg.

Heavy spring engages at 38 (14 deg from initial to loop engagement) by which time the electronics are already starting to retard a degree or two, thus staying at 36.

Slots need to allow at least 42 deg. for ~6500 rpm shifts (it won't actually get that high due to the minimum 1 deg/1000 rpm retard in the electronics). So that is 42-24 = 18 crank degrees, or 9 dist.

I've already ordered an 18/9 tower from Halifax Hops and it should be here tomorrow... but now I'm wondering if I should take the otherwise useless stock 26/13 piece and weld up the inside of its slots to 18/9 (.390")?

The only remaining piece of the puzzle (besides the vac advance, which I'll worry about once the mech timing is right) is finding a suitable spring that will be all-in at approximately 3000 rpm. I think the OEM blue spring may be close... thoughts? :)
 
Three comments:
- if you are going to use VA to improve idle [ smart ], then you do that first. It is ALWAYS first.
- 56* at cruise for a low vac cam is not excessive. If you get surging, determine whether you are still on the T-slots or the main cct. Then richen that cct slightly. What happens with VA [ ported or man ] at cruise is that the t/blades are not open as far as they would be without VA. So engine is pulling less fuel, needs to be richened.
- you are using a lot of static init, 24*. No need to use super soft centri springs. This might be why timing does not come back to the same point each time you rev the engine, weights behaving erratically.
 
In a word... success :D

Today I received the 18/9 cam plate...decided to take the old 26/13 and weld up its slots on the inside, and grind them back out to .390 length. Unfortunately when I put it together, it was obvious that there was too much preload on the (gold) spring and not enough slot left before the OEM heavy loop spring engaged.:p

So I put the 18 deg plate in, as noted with one gold spring and one heavy loop spring, reinstalled the 8L can (16 deg max), and fired 'er up. It works great and is stable (although choppy as usual). 24 deg initial, 37 total and it maybe loses 1 degree during a rev to 4500. With the vac can connected and adjustment full CW, idle is 34 deg and almost 9" of vacuum. Rev in neutral to about 52 deg. Idle mixture on the AFR meter is almost half a point leaner. May have to tweak that a bit too. It's imperative not to put any load at all below 1100 rpm or it drops and stalls.

Took it out for a quick low-speed test (up to 40 mph) and immediately noticed a great improvement in off-idle and part-throttle response. No pinging anywhere, although it would be hard to hear over the 3" X-pipe dual exhaust. Will have to take a close look at my plugs after a highway run, but shouldn't have pinging there either. :)
 
Great news!
And it is a great example of [a] how perseverance pays off in getting the idle timing correct & doing this BEFORE touching the carb because it can affect carb idle calibration.
 
At the most you need about 5° advance in the distributor and that's only for starting. Your blowing smoke up your own butt thinking you need vacuum advance. Iron heads consistantly need 38+ degrees of total timing.
 
Oldiron440 said:
At the most you need about 5° advance in the distributor and that's only for starting. Your blowing smoke up your own butt thinking you need vacuum advance. Iron heads consistantly need 38+ degrees of total timing.
Click to expand...
I think you're blowing smoke up my butt. :p

Remember, this is a street-driven car on pump gas, sometimes in hot weather. I would rather give up a little power than run the absolute max timing and risk knocking under full throttle at high rpm. That tends to be hard on ring lands and piston tops :eek:

My engine does not like to start hot with 28 deg initial (which it would be with your suggested 38, and 5 distributor degrees).

With my big cam and low cylinder pressure at low speeds, plus the lean cruise mixture and poor volumetric efficiency at part throttle, more than 38 degrees of timing is desirable. 50+ degrees is usual at cruise! The fuel economy will be better, the engine will run cooler, the plugs and oil will stay clean longer, etc. Why do you think every street car has a part-throttle advance, whether old-school vacuum can, or built into the EFI map?

I agree that up to 38 degrees can be run with the mildly ported iron heads. This winter I'm finally going to put my PRH-massaged aluminum heads on (293.5 cfm), which may change the required advance.

Anyway, your approach seems to be for a strip-only car that happens to be street-driven occasionally. All that engine needs to do is have the right timing at WOT, and start.
What I have here is a mostly street car that may be on the strip once in a while :)

Edit: 38 deg of timing is only going to be optimal at one setting - full throttle and above 2500-3000 rpm. It is wrong at all other conditions of load, throttle setting and rpm unless that timing requirement just happens to coincide with 38 degrees. Even my cam doesn't need that much at idle, and certainly not during a typical departure from a stoplight (not a clutch-sidestepping dragstrip launch). Lean mixtures burn slower...
 
Last edited:
I've got a 452" 440 with the mopar 557 mechanical cam mildly ported heads an original M1 intake and a 850 dp and 9.5 cr.
It's been flogged on the dyno after hundreds of passes and thousands of street miles.
You talk about your big cam that's actually to big for a 9.5 motor so not much of a street cam.
 
It's not too big for my 10.3:1 CR with the iron heads, or soon to be 10.6:1 with the aluminum heads. With the four-speed, 3.91 gears and 26.7" tall tires, anyway. Does just fine on the street, thank you :p

I'm sure your combo runs well on the dyno and on the strip, although that 850 is a little small for max power. You've probably also wasted hundreds of gallons of fuel running on the street without a vacuum advance, too.
But you do it your way, I'm doing it my way. :thumbsup:
 
Bewy said:
Great news!
And it is a great example of [a] how perseverance pays off in getting the idle timing correct & doing this BEFORE touching the carb because it can affect carb idle calibration.
Click to expand...

What about cruise and WOT? My low-speed cruise AFR seems to be a bit leaner too (since I'm on the t-slot and there is a significant contribution from the idle circuit). Haven't had a chance to drive out to the highway and do some short blasts...
 
Oldiron440 said:
I've got a 452" 440 with the mopar 557 mechanical cam mildly ported heads an original M1 intake and a 850 dp and 9.5 cr.
It's been flogged on the dyno after hundreds of passes and thousands of street miles.
You talk about your big cam that's actually to big for a 9.5 motor so not much of a street cam.
Click to expand...
Not to hijack, but I'm curious as to what kind power numbers this set up had.
 
Dr.C,
Once you have the idle timing sorted, then you dial in timing for WOT & cruise, reply to post #36.
Take no notice of the fool in post #32.
 
No need for name-calling... I make it a point to respond with information instead :) I'm not too old to learn something new, maybe the others are too!
 
You must log in or register to reply here.
-

Back
Top