Ignition timing for Supercharged 408

[crazyfireman]

Can anyone give me a good starting point for my ignition timing with my B&M supercharged 408 with a HEI dist? I am not to familiar with initial, and other timing terms but I would like a good number with my vacuum advance disconnected and at approximately 2000 rpms, what number?? I want to be safe..

[68Formula]

You need to give a detailed description of your engine (compression, displacement, camshaft specs, fuel octane, cylinderheads, boost) before that can be answered. Also, any Turbo should use a boost retard system to pull additional timing out.

[crazyfireman]

408 with 8.5-1 compression, 91 octane, Trick Flow heads and 6-8 pounds boost and this link is my camshaft
http://www.compcams.com/Cam_Specs/CamDetails.aspx?csid=87&sb=0

[68Formula]

That is a really poor camshaft for a boosted application. Not only is the duration too long, but the lobe seperation at 110 is too tight. It's going to hurt your ability to create boost, and probably increase your tendency to detonate.

Try a single pattern camshaft around 212 to 218° duration @ .050" and lobe seperation of at least 114°.

[Procharmo]

CrazyFireman, this is an area I've been toying with recently. Regardless of your cam search over on hotrod.com or speedtalk.com for the ball park figure BTDC setting with no vacuum connected at idle (assuming idle speed has been set and vacuum port plugged). Adjust the distributor to suit. Then check your full advance is in at by about 3000 rpm using a dial gun timing light. I think its approximately 8-12 degs BTDC at idle no vacuum for most Gen I sbc's depending on compression, heads etc. This is a function of the distrubutor module, older points type allowed for tuning the timing curve but I think the HEI has only one module. Then either purchase an MSD BTM or back off the distributor a few degrees. How many degrees depends on fuel octane, boost level and intercooler/supercharger efficiency.
Here's some useful reading.
http://www.superchargersonline.com/hp_calculator.asp
I think that if you time it as an N/A motor then back off enough to avoid detonation you'll be fine. If you have a small blower and relatively low boost you may not need to back of timing from the usual N/A levels.
Below is the most complete simple explanation found on Yahoo answers,
"Your research sounds about right. The small block chevy runs best with 34 to 38 degrees total timing.

Ok let me break it down for you..

You have initail timing which is the timing at idle.. Then you have the mech. advance in the dizzy.. Most dizzy gives 18 to 22* of advance..

Total is timing at idle, + mech advance..

The right way to check total is with a dail back timing light..

Remove and plug/cap off the vac advance hose, Rev engine to 3500 rpm and hold there.. Shoot timing light at damper, and turn knob on back of gun (or push button) untill the line on damper lines up with the 0/TDC mark on timing tab... Now read the number the gun's knob is sitting on... That is your total timing.

Now if you don't have a dail back light, you can degree your own damper, or put timing tape on it, and use a std. gun... Just rev engine to 3500 rpm, shoot gun on damper and what ever # on timing tape around damper that lines up with 0/TDC mark on tab is the amount of total you have.

You you can use math to get close.

As I said most HEI's will give 18-22* of mech. So split it and go with say 20*

36* total = 20* mech + X (16 in this case)... So that means you will need to set timing to 16* BTDC at idle with vac advance line un hooked
Source(s):
Performance/Race engine builder, 12+ years"
This is not a definitive answer so engine guru's please comment.

[crazyfireman]

Thanks!!

[DeltaT]

That's actually not an awful cam for your application. It is very similar to what BDS would recommend at about a Stage II. If you change anything and get another cam, widen the LSA to 113 or 114. For a base ignition timing, no vacuum, I'd start with 12-14 and play around. But Procharmo's answer is way more scientific! :)

Jim

[68Formula]

Then either purchase an MSD BTM or back off the distributor a few degrees.

If he was to not use a boost retard controller (which still is the best choice), with that particular combination he could run 20° with the mechanical advance locked. Then use an adjustable vacuum can connected to a ported source with set to the lightest tension.

Remember too much timing coupled with boost = big kaboom.

[crazyfireman]

Thanks for all the input, I am learning!!!