396 bbc timing help

[fdbdw]

Hi all, I could use some help. 396 Mark IV BBC, mild cam. New cam, carb, dizzy, manifold & mid length headers. Im in the process of timing and tuning and I’m having trouble finding consistent timing numbers. In my research I’ve found everything from initial 4 degrees to 20 and everything in between and all in at widely variable rpms and degrees. Does anyone have solid numbers I can shoot for.
The other variable is 6,500 feet. In the past I’ve added 2 degrees per 1500’. Any added info on elevation would be great also. Today I’m going to verify TDC then start tuning n timing.
Thank you

[weq92f]

.
I started at about 12* BTDC on the crank and another 16 or so via the dizzy with two light springs so all in before 2500.

Then I started driving it and advancing manually during the drive until it started pinging under load and backed it off a bit from there.

Haven't checked with the light to determine where it ended up but I suspect it's around 18-20 degrees BTDC on the crank.

Hth,

-klb

[geezer#99]

You need to experiment and see what your motor likes.
I’ve run the same 396 and had 20 initial at 4000 feet.
Here’s a timing link.
http://www.badasscars.com/index.cfm/...d=76/prd76.htm

[Mike C]

Read this. It's long. There are some disagreements in it. But it is worth your time. And no reason to have everybody re-write this stuff everytime.

https://www.corvetteforum.com/forums...ng-post-2.html

[Greasey Harley]

Use a dial back timing light.
Shoot for 34-36 total timing with the vacuum advance disconnected and the RPM's around 2800.
Your total timing is way more important than your initial timing.

[fdbdw]

I appreciate the help, here’s where I’m at so far. Confirmed TDC, carb is pretty dialed in, still need some fine tuning. Set initial timing at 24, all in 40 @ 2500 rpm. Seems pretty good, adjusted valves on right bank, took it for a test drive, ran well no pinging. Adjusted the valves on the left bank, at that point had to many beers to go for a test drive. Will continue tomorrow.

[geezer#99]

Originally Posted by Greasey Harley

Your total timing is way more important than your initial timing.

Not where he lives.
At 6500 feet you need to start the burn way way way earlier.

[geezer#99]

Originally Posted by fdbdw

I appreciate the help, here’s where I’m at so far. Confirmed TDC, carb is pretty dialed in, still need some fine tuning. Set initial timing at 24, all in 40 @ 2500 rpm. Seems pretty good, adjusted valves on right bank, took it for a test drive, ran well no pinging. Adjusted the valves on the left bank, at that point had to many beers to go for a test drive. Will continue tomorrow.

How did you confirm tdc?
What carb you got?

[Greasey Harley]

Originally Posted by geezer#99

Not where he lives.
At 6500 feet you need to start the burn way way way earlier.

I didn't see that. Add another 4 to 7 degrees to the timing ...what ever you can get away with.
As long as it ain't pinging or hard to start.


At 40* it sounds like you're in the ballpark.

[fdbdw]

Originally Posted by geezer#99

How did you confirm tdc?
What carb you got?

I used a wood rod in the #1 spark plug hole turned the motor with a socket wrench, repeated multiple times to be sure . Luckily the previous zero marks were dead on.

AVS 2 650. 98 primary jet 65x37 rod, 92 secondary, accelerator pump on bottom setting, yellow spring.

[geezer#99]

How’s the fuel pressure with your avs?
Did you lower the floats a bit?
How did you recurve your distributor?

[fdbdw]

Originally Posted by geezer#99

How’s the fuel pressure with your avs?
Did you lower the floats a bit?
How did you recurve your distributor?

5.5-6psi. Edelbrock fuel pump, a little pricey but I don’t have to use a regulator and gives the right psi for their carbs. I recommend it to anyone running an edelbrock that doesn’t plan on switching . Had a 1406 on a 383 stock fuel pump and had constant issues and alway adjusting, went to the Edelbrock fuel pump and never had another pressure issue.

Set the floats out of the box and re-check them every time I break it down to change jets. Used the specs out of the manual. I think it’s 7/16” inverted and 1” upright.

Running stock springs & weights, I have a Mr. Gasket spring and weight set on order , the curve comes in pretty fast so I’m gonna even it out

[geezer#99]

When you’re out cruising listen closely for any low rpm or sudden throttle closures noises. You can get detonation with too much timing that’s in too fast.
I had that once on a 396. Spent a thousand bucks finding what was rattling. It was detonating!

[fdbdw]

Originally Posted by geezer#99

When you’re out cruising listen closely for any low rpm or sudden throttle closures noises. You can get detonation with too much timing that’s in too fast.
I had that once on a 396. Spent a thousand bucks finding what was rattling. It was detonating!

Oh man, I’ve been chasing a rattling sound, kinda sounds like pinging but I wasn’t thinking that route due to it being at low RPMs. Also led me away from that thought because it sounds like it’s coming from under the cab/bed area. I wonder if it’s echoing in the mid exhaust or off the O2 sensor. I’m glad you brought this up, I’m gonna look into it.

[geezer#99]

First test is plug off your vacuum advance.
When you slam the throttle shut the vacuum spikes, your timing gets way advanced from the vac pot adding too much timing.

[fdbdw]

Originally Posted by geezer#99

First test is plug off your vacuum advance.
When you slam the throttle shut the vacuum spikes, your timing gets way advanced from the vac pot adding too much timing.

Right on, thank you. What did you end up setting your curve at?

[geezer#99]

IIRC around 16 initial, 14 from the dizzy plus 10 from the vac can.
I used a vac can limiter to keep the vac amount down.
Mind you this 396 I had was a 10.25 to 1 motor with a cam similar to an L78.

[fdbdw]

Just got done messing with it some more, I’m getting 24 from the vac can which Is way to much. Ordered an adjustable can gonna set it around 10 and go from there

[geezer#99]

Originally Posted by fdbdw

Just got done messing with it some more, I’m getting 24 from the vac can which Is way to much. Ordered an adjustable can gonna set it around 10 and go from there

Better to install a stop plate like in this link.
http://www.crankshaftcoalition.com/w...EI_distributor

[BossHogg69]

Originally Posted by geezer#99

Better to install a stop plate like in this link.
http://www.crankshaftcoalition.com/w...EI_distributor

This ^^^ I wouldn't even mess with the adjustable can, I always just order the $2.50 Crane limiter plate and use a regular can. I'll take my MityVac hand vac pump, pull on the can to figure how much it moves per degree of timing it advances and set the plate by that. I also mount the plate on the opposite side of the arm than what the directions say and it works great. That way the vac can arm still starts at '0' and just pulls to wherever the plate stops it. The way the directions have it, you are limiting the spot were it starts pulling from and that always seemed a little weird to me.

http://67-72chevytrucks.com/vboard/s...d.php?t=755690

Pics in post #6

[fdbdw]

Prior to seeing geezer and bosshog’s post I had already ordered an adjustable vac can, what a waste of money, I was wanting 10 from vac can and I could get no less than 20.. After reading their post I ordered a stop plate which was exactly what I needed. Finally on the right track and now just doing some fine tuning.
Here’s where I’m at, remember I live in high altitude and I’m adding 4 degrees to accommodate.
20 initial, advance starts in at 1500rpm
40 all in at 3200rpm
Vac advance stop at 8.
Thank you for the advice on the vac advance stop.