Timing and TDC

[Chryco Psycho]

I would use 8* , 28-36*

[Aar1064]

I'll be doing some tuning on timing and carbs this weekend and was wondering how do you determine if vacuum reading is correct for your car?

[71-440]

Depends on your setup I believe.
Chryco Psycho would better explain it.

[HP_Cuda]

Vacuum is directly related to your cam and how much compression you are running.

The cam manufacturer should be able to let you know a vacuum range that cam should fall in.

I'll be doing some tuning on timing and carbs this weekend and was wondering how do you determine if vacuum reading is correct for your car?

[Chryco Psycho]

Vacuum is ambiguous  , there is really no right vacuum spec .
There are a number of factors the biggest is the camshaft design , more duration = less vacuum type & size of the ports in the heads [small or large , swirl or straight],this will affect the velocity inside the engine  , type of intake [single or dual plane] & size of the plenum & ports , simple tuning can make a big difference , if you adjust the timing to factory spec the vacuum will suffer , advance the timing & you may find 3" of vacuum as demonstrated by the original person posting , idle RPM makes a difference , TQ converter load if auto , all affect vacuum

[Aar1064]

Thanks for the replies.

340 TA Engine was built to stock specs if that helps. No torque converter, a four speed.

Wish I knew more about this. Thanks for the help.

[Chryco Psycho]

Keep working on the engine & you will gain experience & knowledge just as we did , I was not born with any knowledge either 

[71-440]

Update on my original post about timing. Had some time to mess around with it. Balancer marks were ok with the pointer.
Changed out the Mopar Box, which is a MSD-6, and installed a FBO box. Bypassed the ballast resistor.
Ran heavier gauge wire down to the box and wrapped the wires in that harness with a heat reflecting wrap.
Got them off the manifold with two cable clamps.
Installed the 10* advance bushing in the distributor. Cleaned the pickup and reluctor.
Set initial timing at 25* with the heavy springs installed on the advance otherwise the timing would jump around.
The heavy springs held it steady at idle. 900 RPM.
3000 rpm the timing went to 30-32.(with the heavy springs still installed) Should have went to 3500 RPM but will check that tomorrow.
Vacuum was between 12 and 13 with  it fluctuating between those two numbers.

Took it out for a test drive but was only able to get on it once. Too many cars on the road.
From about 10 MPH jumped on it and it lit up the tires going a little sideways but not too bad.
Ran it through 3rd. gear with no hesitation.
Rev limiter is set at 7000 RPM.

Torque converter, from what I understand, is a 2600-2800 stall. Might have to eventually go to a 3000.

That's where I am now. Still more playing to be done. The carb will be next after I fine tune the timing.
The fluctuation in the vacuum has got me a little puzzled. 12-13 With the needle fluctuating between them.
Not rapid but up and down.

[Chryco Psycho]

not enough varience in the vacuum to worry about IMO

[71-440]

not enough varience in the vacuum to worry about IMO

I was hoping you would say that...lol

Thanks.

[headejm]

@Chryco Psycho I have a basic ignition timing question. My factory timing spec. on a 383 auto engine is 2 1/2 * btdc. Most of the recommendations I've seen is to run somewhere around 10 - 15* initial timing and around 34* total timing. Question is: why are we not running the 2 1/2* btdc initial timing that Chrysler recommends?

[Chryco Psycho]

There are a number of factors
Most builds do not use the factory cams most aftermarket cams use 110or 112 * lobe separation allowing more overlap where both valves are open at the same time , the factory cams were 114-115* .
Second believe it or not emissions .
With lower manifold vacuum using larger cams the engine responds much better with more timing & generally runs poorly set to factory specs .
I have built & tested a number of engines at the track & on dynos , setting the timing far higher works , it runs better , starts better , idles better with more vacuum , responds better with less pump shot from the carb , increases economy etc .
it is not uncommon to idle near 20* with a 14-16 degree advance curve for best results .

[71-440]

Update on my Timing. Started to get some pre-ignition. Don't hear any knocking or pinging though. Going to back off the initial from 25* to 22* and leave the 10* advance bushing in and see
how that goes. I have a 14* advance bushing which would give me 36* total I may try that also. I ran one heavy and one light spring. But I may have to experiment with that also.

I don't know what compression I am running but from what I understand that also plays a roll in your timing. Correct me if I am wrong Neil.
I have to get a compression gauge and check it.

[Chryco Psycho]

if you leave the timing as is & just install a heavier spring where the light spring is & the pre-ignition should stop . If it performs better set at 25* initial leave it there , if you lose no performance at 22* you are probably better there .
Pre-ignition is detonation / pinging

[71-440]

Ok I'll do a little experimenting.

Thanks Neil!