Help please! Struggling with timing on modified 318...

[Flathead38]

For those of you who saw my intro, I have some progress to report, which brings new challenges. I am helping to resurrect a long dormant '69 Charger which has a modified 318 built in the early eighties... I just did a valve job, and replaced head gaskets, valve seals, etc, and the engine is back to running. I'm now trying to determine what ignition timing should be. Here's my basic setup:

1971 318, stock compression, "675" heads, Edelbrock LD4B intake with Carter AFB carburetor, new Doug's headers. The aftermarket cam is unfortunately a bit of an unknown. The P.O. installed it in the early 1980's, before we acquired the car. I have tried to measure the cam in the engine, and can provide cam specs as well as I can measure them. Slightly more lift than stock and a little longer duration, but not radical. The timing chain & sprockets were also replaced with a Cloyes double row set about 10 years ago. I don't think it's likely that the cam chain jumped a tooth.

I know that Chrysler calls for initial timing at TDC for this engine in stock form. I first set the distributor by ear to idle smoothly and then connected a timing light. Turns out a "happy" INITIAL timing (vacuum advance disconnected) is about 28 degrees BTDC, which seems very wrong to me. I tried to retard the timing, and when I get to about 20 degrees BTDC initial advance, the idle starts to fall off and it runs rougher.

I did at least a rough check that the outer ring on the harmonic balancer hadn't slipped. It seems close, if not exact.

Using my adjustable timing light, I collected the following data to try to define the mechanical advance:

Idle (800 RPM) 28 deg BTDC
1500 RPM 34 deg BTDC
2000 RPM 43 deg BTDC
2400 RPM 48 deg BTDC
3000 RPM 53 deg BTDC (!)

The distributor is a cheap stock rebuilt unit, and the mechanical advance I measured is more than what the manual calls for. Oh, the last detail... The car actually seems to run well, other than the idle being a bit "lumpy". Last detail... the car seems to accelerate well and run fine at higher RPMs...

Simple question: What do I do now? Thanks in advance for any advice.

 

[Kern Dog]

An engine with no load on it can tolerate a LOT of timing. Once you drive it, it may knock like marbles in a glass jar.
Throw the factory specifications out of the window. The engine is no longer stock.
Detonation may be hard to hear. You may be knocking, causing damage and not even know. What you CAN hear is when it is really bad.
You can mimic what others have done by setting the timing to what others have or you can custom tailor your timing to match your engine's needs.
That takes time and effort.
The mechanical advance curve can be way too long in some distributors. In these cases, when you set the timing at idle to say...20 degrees, the mechanical timing adds another 24-28 degrees leaving you with 44-48 degrees at say...3000 rpms. That is way too much timing for today's fuel. Even with 110 octane, the timing is more than that engine would want or need.
The fix for a distributor with a l o n g advance curve is to disassemble the distributor and shorten the slots that allow the long curve.

There are threads here and elsewhere that can explain it in detail. In short, the horizontal slots get welded up a small amount to limit the travel of the advance mechanism. There may be a formula as to how much to weld up but I've just welded up a little more than needed, then file it back open to get the advance to approximately half of stock.
This distributor in these pictures allows 18 degrees at idle and 32 degrees fully advanced. The engine can take more than that if you want to set it to 20 at idle and 34 total.
Just an example.....Many Mopar V8s run strongest at 33 to 35 degrees total timing. Vacuum advance is not factored into that number, this is strictly initial at idle along with the mechanical advance.

 

[Flathead38]

Thanks, Kern Dog. I'm not afraid to put in the work to make it right. For instance, welding the distributor shaft to limit max mechanical advance is well within my capabilities. What I'm not so confident about is how to define what "right" is...

 

[Flathead38]

One other "simple" question. I measured our cam using a dial indicator, and took measurements at .050" valve lift. The 1969 Dodge factory manual lists OEM cam duration and valve timing, but does not define the reference frame. Anyone know for sure if the numbers in the manual are absolute, or .050" valve lift, or something else...?

 

[RemCharger]

That's a hell of a big cam for a 318 (@.050)
Circle track or cheater s/s cam..?

 

[PRHeads]

If your 244/[email protected](at the lifter) numbers are accurate, the cam is way too big for just about any street friendly 318 combo.
215-220-ish is more like what most would be looking for.

Long duration reduces engine vacuum....... which affects the carb signal.
More initial timing increases the vacuum.
Your situation is the carb isn’t happy with the low vacuum of the long duration cam, and you’re using the way too much timing as a crutch.

That being said, if there are no hot start issues with the initial timing at 34-35*, then just lock out the distributor.
That would not be uncommon to do with that sized cam in a 318.

Edit- I see you took measurements at .050 valve lift.
I don’t really have a good way to equate that to readily available info from the various cam suppliers.

 

[Flathead38]

If your 244/[email protected](at the lifter) numbers are accurate, the cam is way too big for just about any street friendly 318 combo.
215-220-ish is more like what most would be looking for.

Long duration reduces engine vacuum....... which affects the carb signal.
More initial timing increases the vacuum.
Your situation is the carb isn’t happy with the low vacuum of the long duration cam, and you’re using the way too much timing as a crutch.

That being said, if there are no hot start issues with the initial timing at 34-35*, then just lock out the distributor.
That would not be uncommon to do with that sized cam in a 318.

Edit- I see you took measurements at .050 valve lift.
I don’t really have a good way to equate that to readily available info from the various cam suppliers.

Thanks for taking the time to look at my problems. Your response highlights a point of confusion. I originally measured duration at .050" cam (lifter) lift. Then I was told by my machinist that standard is to measure at .050" valve lift, so I went back and re-measured. My measured duration at .050" cam/lobe lift is 223/245. Anyone know for sure what the "standard" is, and equally important, can anyone confirm the basis for the published cam specs in the OEM Manual?

 

[WileERobby]

Me, I would've taken the opportunity to replace the cam with a known entity, when you had everything apart.

 

[RemCharger]

these are from a 72 manual

 

[theman440]

Way too much timing, small blocks don't like as much lead as a BB or HEMI. 30 degrees total is about the max, if you cant get it to idle you may have to lock the distributor and use a start retard.

 

[Geoff 2]

Flathead,
A lot of misinformation in some of the above posts by people who do not understand ign.
What YOU did to find the best initial timing was correct & ...very clever. Your engine likes 28*. End of story, 28* it is....& should be. My engine has a bigger cam, likes 48*....

You have choices here. I do not know what those heads & CR normally have at WOT, but say for example 36*. You can have your dist recurved to provide an 8* centri curve with 28* init. Some starters will object to 28*, but a high hp starter [ over 3.0 hp, such as MSD Dyna Force] will crank that all day long. With a high HP starter, you can also lock the dist @36, but you might get detonation.

Another method which is probably better is to use vac adv connected to manifold vacuum [MVA] because it is load sensitive.
You will need a dist that has an adj VA unit. You can then use any combination of VA + init to get the desired 28* at idle. You will probably have to fabricate a stop for the VA arm to limit travel. This is ALL part of tuning, no different than adjusting the mixture screws.
Read more about MVA benefits, post #6:
www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html


Also from David Vizard, author of 30+ auto books

 

[Geoff 2]

More on MVA

 

[toolmanmike]

Did you disconnect the vacuum advance and plug the hose when you checked the timing?