BBC 396 L78 Modified - Timing recommendations - Team Camaro Tech
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post #1 of 53 (permalink) Old Oct 3rd, 19, 05:38 AM Thread Starter
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BBC 396 L78 Modified - Timing recommendations

Hey guys,
this is my first thread here. I am Harold from germany and not long ago i got a real beauty, a hugger orange black striped 67 camaro convertible SS/RS with L78 4-Speed.

I am looking for a good timing setup, since i feel the responsiveness is ok, but not as crispy as it could be and there is some driveavility/hp-potential left to be woken.

My Setup:
3.42 Posi with 27" wheels
M20 Muncie
L78 all forged (TRW L2242NF 11-1 Pistons)

Mods:
Comp 270H Flat tappet; Adv.: 270°/270°; @.050: 224°/224°; Lift: .510/.510; LSA: 110°; Rpm-range: 1500-5800
77 Open Chamber head 118cc, giving 9,5-10:1 CR (with 0.040 Gasket + 0.030 Deck clearance, dont know exactly)
Headers 2" into 3.5" collectors into 2.5 pipes
Edelbrock Performer 2-O Intake (Non-rpm)
Edelbrock Performer 750CFM Carb Man. choke
MSD 6AL + Magn.Pickup Distributor (& soon the 15°-window ign. control module)

I only use pure 102 octance + pure cold air (modified cowl air induction with only outside air pulling)

Can you guys recommend me a good Initial/Curve/Total-Timing?

Since it is a Lowend-Cam, i think about this:

12° initial + 28° advance, resulting in 40° total at 2500 rpm onwards.

This is the hottest setup possible with the MSD Advance Kit.

But i will have a "from drivers seat" possibility to alter timing +-7,5°(15°total window), to relieve the strain on starter(compression) during startups and adjustments on the fly.

What do you guys say?

Last edited by Harris Roc Malouda; Oct 4th, 19 at 03:32 AM. Reason: specs
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post #2 of 53 (permalink) Old Oct 3rd, 19, 06:15 AM
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Dave
 
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Re: BBC 396 L78 Modified - Timing recommendations

Sounds like a real nice machine. Without pics we'll all doubt that you really own it though. ;-)

I'm far from a distributor curve expert, but after all my research and work I ended up with the following for a curve. Each car is different and there are ways to make the curve correct for your particular set up. Personally, I think you're pissing away your money on the 102 octane fuel. You said you have 10:1 compression.

B28 vacuum can 16 degrees @ the crank.

Total mech & vacuum advance possible: 53 degrees.

RPMs------Advance

850---------14
1,200-------19
1,400-------23
1,600-------23
1,800-------26
2,000-------27
2,200-------28
2,400-------29
2,600-------32
2,800-------34
3,000-------37 (max mech.)

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460ci Gen VI 9.75:1 AED 950 carb
241/249 @ .050" .625/.625 110 LSA

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post #3 of 53 (permalink) Old Oct 3rd, 19, 03:06 PM Thread Starter
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Re: BBC 396 L78 Modified - Timing recommendations

Hi,
thanks for the feedback! Haha you may be forgiven for doubting, since i cant belive this too every time i walk into my garage

But wow!? 53° total timing? so idle 14° + 23° mech. + 16° vacc.?

This may be dumb question (its late here), vut when exactly do the 16° vacc. adv. come it? At WOT? part throtlle? idle?

in my case there is no vacuum advance, so only idle + advance...

One big questionmark less in my System
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post #4 of 53 (permalink) Old Oct 3rd, 19, 06:35 PM
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Re: BBC 396 L78 Modified - Timing recommendations

Quote:
Originally Posted by Harris Roc Malouda View Post

Can you guys recommend me a good Initial/Curve/Total-Timing?

Since it is a Lowend-Cam, i think about this:

12° initial + 28° advance, resulting in 40° total at 2500 rpm onwards.

This is the hottest setup possible with the MSD Advance Kit.

But i will have a "from drivers seat" possibility to alter timing +-7,5°(15°total window), to relieve the strain on starter(compression) during startups and adjustments on the fly.
A good timing curve for a cammed engine like yours would be:

16-18 initial + 18 mechanical. You will want the mechanical advance all in by 3000 rpm.

Have the vacuum advance hooked up to full manifold vacuum and limit it to 10 degrees.

You said you have open-chamber heads and 396 cubic inches and you think it's 10-1 compression ?

It's very difficult to get 10-1 compression with a small 396; the piston domes would have to be huge.

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post #5 of 53 (permalink) Old Oct 3rd, 19, 06:39 PM
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Re: BBC 396 L78 Modified - Timing recommendations

Try and get as much initial timing as you can get, especially if you are not using vacuum advance. I run 20* initial and 20 mechanical for 40 on the street, bump it up to 44 for the track.

You should not need more than 38* total. Unless you want to make a custom bushing, the black is the biggest and will get you the least amount of advance . Set the total and let the initial fall where it is.

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post #6 of 53 (permalink) Old Oct 4th, 19, 03:43 AM Thread Starter
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Re: BBC 396 L78 Modified - Timing recommendations

Quote:
You said you have open-chamber heads and 396 cubic inches and you think it's 10-1 compression ?

It's very difficult to get 10-1 compression with a small 396; the piston domes would have to be huge.
i added some specs. I know its somewhere between 9.5 to 10:1. based on open chambers having about +10cc-ish compared to closed chambers

Quote:
Have the vacuum advance hooked up to full manifold vacuum and limit it to 10 degrees.
I want to keep it only mechanical advance for several reasons. But for certain situations where i need different timing i will use the ign. control.

Quote:
I run 20* initial and 20 mechanical for 40 on the street, bump it up to 44 for the track.
Wow, thats much of initial. The black gives 18° advance. Thank you for your thoughts.


You guys are the best. So much good info with real life experience. I am just gettin into all this.

So i guess the direction i will be heading is:

18/20° of initial plus 18/20° of mechanical advance. (+-7,5° extra adv./ret. through the ign. controle module i will add to the system.


Just to give you guys something back: I asked a comp cams tech the same question, he came up with:

"I would recommend idle timing between 8 and 14 degrees advance and total timing at 30-34 degrees advance."

So its in the ball park of what you guys say, maybe a little more conservative.

Last edited by Harris Roc Malouda; Oct 4th, 19 at 04:02 AM.
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post #7 of 53 (permalink) Old Oct 5th, 19, 09:30 AM
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Dan
 
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Re: BBC 396 L78 Modified - Timing recommendations

Quote:
Originally Posted by NashvilleCat View Post


You said you have open-chamber heads and 396 cubic inches and you think it's 10-1 compression ?

It's very difficult to get 10-1 compression with a small 396; the piston domes would have to be huge.
I agree, the short stroke doesn't provide much squeeze. Might check cranking compression, I suspect its much lower than 160-ish psi, which would make it a little soggy on the bottom. If its low, more lead advance or thinner head gaskets ( like .020 steel/copper shim) would help.

Last edited by HarrisFD; Oct 5th, 19 at 09:32 AM. Reason: I Kant Spel
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post #8 of 53 (permalink) Old Oct 6th, 19, 05:16 AM Thread Starter
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Re: BBC 396 L78 Modified - Timing recommendations

Quote:
Originally Posted by HarrisFD View Post
I agree, the short stroke doesn't provide much squeeze. Might check cranking compression, I suspect its much lower than 160-ish psi, which would make it a little soggy on the bottom. If its low, more lead advance or thinner head gaskets ( like .020 steel/copper shim) would help.
Thanks, i added some info about the pistons in the initial post, but the pistons are - to my understanding - the original shaped closed deck pistons, offering +38,3cc domes.

I used a calculator and with 0.040 gasket, 0.030 deck clearance (both i just assume to be near stock values) i end up with 9.6:1 CR.

A 0.020 gasket would give 10:1 and is on my list.
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post #9 of 53 (permalink) Old Oct 6th, 19, 11:02 AM
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Re: BBC 396 L78 Modified - Timing recommendations

NO, 16 to 18 INITIAL timing is too much.

The 12 you now have is adequate, but, you need to enhance it by setting the degrees to no more than 12 degrees of VACUUM ADVANCE degrees, then, use full manifold vacuum to engage the vacuum advance can to give 24 degrees of IDLE timing. This will give you easy starting, and the IDLE timing the engine wants after it gets started.

From what it sounds like, you state you have 40 degrees total, and 53 overall, which makes me think the vacuum advance is on ported vacuum, completely the wrong way to do the vacuum advance, should be degrees set and full manifold vacuum, NEVER ported vacuum.
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post #10 of 53 (permalink) Old Oct 6th, 19, 12:07 PM Thread Starter
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Thumbs up Re: BBC 396 L78 Modified - Timing recommendations

Quote:
Originally Posted by dave ray View Post
NO, 16 to 18 INITIAL timing is too much.

The 12 you now have is adequate, but, you need to enhance it by setting the degrees to no more than 12 degrees of VACUUM ADVANCE degrees, then, use full manifold vacuum to engage the vacuum advance can to give 24 degrees of IDLE timing. This will give you easy starting, and the IDLE timing the engine wants after it gets started.

From what it sounds like, you state you have 40 degrees total, and 53 overall, which makes me think the vacuum advance is on ported vacuum, completely the wrong way to do the vacuum advance, should be degrees set and full manifold vacuum, NEVER ported vacuum.
Hi dave ray,

thanks for the input. My distributor doesnt have a vacuum port (MSD 85551), so there is no vacc. advance and i want to keep it that way.

Actually i dont know the total timing of my engine yet. I got all the measurement and adjustment eqiupment on the way to me right now.

But it is interesting to see the differences in how the people time their (Idle) ignitions.

Obviously many factors come into play (static compression of engine, dynamic compression through cam timing, the displacement, leaner/richer-mixtures differences at idle up to max rpm or part-throttle vs. WO-Throttle etc. etc.)

What i will definitely do is to install the control module to give me some adjustment room for starting, cruising and WOT-situatuions.

Thanks for the warning though!
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post #11 of 53 (permalink) Old Oct 6th, 19, 01:13 PM
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Re: BBC 396 L78 Modified - Timing recommendations

Quote:
Originally Posted by Harris Roc Malouda View Post
Thanks, i added some info about the pistons in the initial post, but the pistons are - to my understanding - the original shaped closed deck pistons, offering +38,3cc domes.

I used a calculator and with 0.040 gasket, 0.030 deck clearance (both i just assume to be near stock values) i end up with 9.6:1 CR.

A 0.020 gasket would give 10:1 and is on my list.
A thinner gasket could be possibly a problem. Depending on how flat you block and heads are it may not seal. Another thing to consider is quench. If not right and with higher compression you may get detonation which will require backing of timing and actually losing power.

Assuming clearances can get you in trouble. When you pull the heads I would get actuate measurements before deciding in head gasket.

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post #12 of 53 (permalink) Old Oct 6th, 19, 01:35 PM
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Re: BBC 396 L78 Modified - Timing recommendations

Quote:
Originally Posted by Harris Roc Malouda View Post
Hi dave ray,

thanks for the input. My distributor doesnt have a vacuum port (MSD 85551), so there is no vacc. advance and i want to keep it that way.

Actually i dont know the total timing of my engine yet. I got all the measurement and adjustment eqiupment on the way to me right now.

But it is interesting to see the differences in how the people time their (Idle) ignitions.

Obviously many factors come into play (static compression of engine, dynamic compression through cam timing, the displacement, leaner/richer-mixtures differences at idle up to max rpm or part-throttle vs. WO-Throttle etc. etc.)

What i will definitely do is to install the control module to give me some adjustment room for starting, cruising and WOT-situatuions.

Thanks for the warning though!
Hi Harris. Welcome to the club! I did the Quench thing that John is talking about and I did the recommendations that Dave Ray is talking about. Both these gentlemen are quite versed in these areas and I followed their direction. John is right, you need to actually take measurements rather than guess. Detonation is the penalty here.
The one thing I did add is the AFR monitoring with dual bank O2 sensors. This enables you to "See" exactly what is going on with your air/fuel ratio. Very helpful.
Good Luck!
Brett.......

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post #13 of 53 (permalink) Old Oct 7th, 19, 05:12 AM
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Dave
 
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Re: BBC 396 L78 Modified - Timing recommendations

Quote:
Originally Posted by Harris Roc Malouda View Post
Hi,
thanks for the feedback! Haha you may be forgiven for doubting, since i cant belive this too every time i walk into my garage

But wow!? 53° total timing? so idle 14° + 23° mech. + 16° vacc.?

This may be dumb question (its late here), vut when exactly do the 16° vacc. adv. come it? At WOT? part throtlle? idle?

in my case there is no vacuum advance, so only idle + advance...

One big questionmark less in my System
I'm not sure why everyone is so surprised about 10:1 on a 396. My stock 396 is 10.5:1. I will say, by your specs you posted I thinks it's doubtful your at 10:1. Probably 8.5:1 which is common, but that's just a guess. Like has been mentioned, guess is no good on that. You need to measure everything. Even if you had 10:1, I still say you should be just running regular unleaded. That's what I run with 10:1 without any issues. Yes, I rip it up plenty too...
Everyone has a lot of opinions on everything. The hard part is wading through all of them and deciding what you want to do. What I posted works well for me and my set up. If you are planning on driving on the street, you really should have a vacuum advance. Your motor will thank you for it.

Excerpt from Lars Grimsrud's paper which you should search for and read. Well worth it. He has it posted at www.TunedByLars.com

"The vacuum advance control unit on the distributor is intended
to advance the ignition timing above and beyond the limits of
the mechanical advance (mechanical advance consists of the
initial timing plus the centrifugal advance that the distributor
adds as rpm comes up) under light to medium throttle settings.
When the load on the engine is light or moderate, the timing can
be advanced to improve fuel economy and throttle response due,
in part, to the slower flame travel in the combustion chamber
under these lean conditions. Once the engine load increases, this
“over-advanced” condition must be eliminated to produce peak
power and to eliminate the possibility of detonation (“engine
knock”). A control unit that responds to engine vacuum
performs this job remarkably well."

'69 SS396 X66 L34 08D M21 BS
460ci Gen VI 9.75:1 AED 950 carb
241/249 @ .050" .625/.625 110 LSA

Semper Fi!

"The galleries are full of critics. They play no ball, they fight no fights. They make no mistakes because they attempt nothing."
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post #14 of 53 (permalink) Old Oct 7th, 19, 10:21 AM Thread Starter
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Re: BBC 396 L78 Modified - Timing recommendations

Thanks for the info everybody.

@John: I will definitely measure the engine once the heads are off. This will happen because i will probably do a roller conversion some day..

@Brett: I already got my Dual AFR Monitor here, i will make use of it a lot

@Dave: Thank you for the Vacc-Advance Info! And also: The "only" difference to the stock 1969 L78 375hp version (rectangular closed chambers) are the heads. This is why the previous owner told me he expects it to be around 10:1 instead of 11/11.5:1, due to about 118cc oval port open chambers instead of 108cc rectangular closed chambers. Oval closed chambers were even smaller (101cc) to my knowledge. And this is a chart from an engine builder, regarding my pistions:

Compression figured with .020 deck clearance and .028 gasket.
*102CC HEAD = 11.58 : 1
*109CC HEAD*= 11.00*:1
*117CC HEAD*=*10.00 : 1
*119CC HEAD*=*9.80 : 1
*124 CC HEAD =*9.25 : 1 ....
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post #15 of 53 (permalink) Old Oct 7th, 19, 10:39 AM
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Re: BBC 396 L78 Modified - Timing recommendations

"so there is no vacc. advance and i want to keep it that way." So, yet another Top Fuel engine on the street.

Did I read you are adding a B28 vacuum advance? How can that be if the distributor has no place for it?

Then, what Lars says is true, but too many people interpret it as needing to bring the vacuum advance timing in along with the mechanical curve, creating TWO acceleration advance curves, DEAD WRONG TO DO. The way it needs to be done is as a supplement to INITIAL timing, controlled by vacuum loading, as Lars says, and then, NOT added during acceleration, but when the engine can tolerate it, from full manifold vacuum changes as engine loading changes. Acceleration, NO vacuum advance, light to NO load, high vacuum, add the vacuum advance timing into the mix, add throttle, drop it back off until load reduces again, and on, and on. An added benefit to doing it the right way is that idle timing, which likes to be 22 to 24 degrees, can be done easily by adding the vacuum advance degrees directly to the initial timing number, to get there, without over advancing the timing so far the engine doesn't like to spin over and start.

An engine, even most stockers, can tolerate up to 12 or so initial degrees before they get into starter problems, bigger cams make it a bit higher. Setting 'initial to the moon high' timing isn't the way to do it. Using the vacuum advance to supplement the 12 initial, to get the 22 to 24 IDLE timing the engine wants....IS, and, that makes the vacuum advance work as both myself, and Lars describe it, NOT the way the 'Top Fuel for the Street' top tuners do.

Don't mistake ported vacuum sourcing vs full manifold vacuum sourcing, world of difference in it.

No vacuum advance, not set up right, LOSER, vacuum advance, set up correctly, applied correctly, great benefit. top tuner ignorant and arrogant about lack of knowledge on vacuum advance is a serous LOSER.

"The vacuum advance control unit on the distributor is intended
to advance the ignition timing above and beyond the limits of
the mechanical advance (mechanical advance consists of the
initial timing plus the centrifugal advance that the distributor
adds as rpm comes up) under light to medium throttle settings.
When the load on the engine is light or moderate, the timing can
be advanced to improve fuel economy and throttle response due,
in part, to the slower flame travel in the combustion chamber
under these lean conditions. Once the engine load increases, this
“over-advanced” condition must be eliminated to produce peak
power and to eliminate the possibility of detonation (“engine
knock”). A control unit that responds to engine vacuum
performs this job remarkably well."


Another top notch "must read" article is here on these boards, in the 'Performance' section as a "sticky", it is by another former GM engineer, John Z, " Timing & Vacuum Advance 101".

Last edited by dave ray; Oct 7th, 19 at 11:28 AM.
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