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I am currently tuning my distributor for my LS6 clone.
Thanks to the great advice from this forum, Lars and others in this prior thread: https://www.corvetteforum.com/forums...requested.html
I came up with my combo as a starting point as I have no running engine to pre-test it in.
I thought I would share this ""tuning chart" in case it is helpful to others.
BTW my BB has a 230/240 degree solid roller, but should idle like a L71, L72 or LS6,
Your preferred "spec" may need to vary if your engine combo is different.
My "530" distributor cam in my Vette tach drive distributor has too much mechanical advance (the 30 means 30 degrees)
Larrywalk identified for me that a "722" cam from a 1965 L76 327/365, has a much preferred mech advance of 22 degrees.
And believe it or not I found one on E-Bay!
So after going thru Lars papers, and buying a few parts, this is what I came up with.
722 distributor cam for 22 degrees mechanical advance
B26 vac cam for 16 degrees vac advance all-in at 12 inches.
LARS VAC limiter to limit that vac advance to 10 or 12 degrees.
At that point the vac can should be all-in at around 10 inches and hopefully my engine idles at 12-14 inches Vac.
If not, further adjustments will be required.
Here is a chart with the range of possibilities for tuning with the above parts.
I'll start with combo #1 or 2 for Dyno day, because it is the mildest and requires the least parts modification.
Other combos require further welding or modifying of the distributor cam and I'll wait until I actually drive the car to do that level of tuning.
BTW we just scheduled DYNO DAY for Nov 7th or 8th. It should roar soon!
On Dyno day I'd like to dial in carb jetting, total and initial timing and maybe idle or cruise timing.
what combination of springs are you using in that chart? You typically have 3 choices silver, black and gold, and combinations of the three. That adds a whole new dimension to your test matrix.
Lars recommends mechanical advance all-in by 2800-3000.
So I'll start with the Gold Mr Gasket springs and see if that works.
If not I'll have spares with me on dyno day.
From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
Yes.
You are tuning for what the engine wants. It may not want 20* initial but may want 22* or 17* for best vacuum/idling which relates to peak cylinder pressure. Something to look for on dyno day. Have fun!
From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
The same reasoning for a 160F ts, 0.040" quench, aluminium heads, cai. Removing heat from the cylinder and burning the fuel more efficiently can be achieved by reduced timing reducing the risk of detonation. Aluminium heads on there own could allow a small reduction in total timing. But then again, maybe it'll end up at or higher than 36* total. Waiting to see the results in a few weeks.
Last edited by resdoggie; Oct 21, 2019 at 08:39 PM.
From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
Originally Posted by leigh1322
Does it like the 22 initial Plus Vacuum? or no vacuum?
Have you considered a programmable ignition box? They are pricey but if you make any engine mods in the future, you can make your own timing curves really easy. I gave up on fooling with springs, reducing mech'l and vacuum advance. My Mallory crapped out but I'll more than likely get a MSD programmable box over the winter.
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Originally Posted by resdoggie
The same reasoning for a 160F ts, 0.040" quench, aluminium heads, cai. Removing heat from the cylinder and burning the fuel more efficiently can be achieved by reduced timing reducing the risk of detonation. Aluminium heads on there own could allow a small reduction in total timing. But then again, maybe it'll end up at or higher than 36* total. Waiting to see the results in a few weeks.
From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
Better heat removal over iron heads from the combustion process. Like an aluminium radiator over a brass/copper rad. So rather than light off the fuel with 36* advance with iron heads and on the verge of detonation at say 37* due to heat in the cylinder, then reduce the heat in the cylinder and light off the fuel at say 34*. You still have the same power but less chance of engine damage. This is one reason why aluminium heads are popular along with weight savings. Maybe aluminium heads by itself won't permit reduced timing,,,,or maybe it will for his engine build.
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Originally Posted by resdoggie
Better heat removal over iron heads from the combustion process. Like an aluminium radiator over a brass/copper rad. So rather than light off the fuel with 36* advance with iron heads and on the verge of detonation at say 37* due to heat in the cylinder, then reduce the heat in the cylinder and light off the fuel at say 34*. You still have the same power but less chance of engine damage. This is one reason why aluminium heads are popular along with weight savings. Maybe aluminium heads by itself won't permit reduced timing,,,,or maybe it will for his engine build.
I gotta disagree with that. Aluminum heads will certainly conduct heat better from the chamber surface than iron heads. This will reduce the temperature of the A/F mixture/mass in the chamber periphery (and also the bulk mass a touch). Lower temperature mixtures burn slower than hot mixtures (chemical reaction speed issue). This means a cooler mixture in an aluminum chamber needs more advance to get the peak combustion pressure point to arrive at the correct time/crank-angle (LPP: Location of Peak Pressure). If you reduce the timing advance with aluminum heads (compared to iron heads) you will cause the LPP to be retarded from its optimal location (15-20* ATDC), causing a reduction in torque/power.
Most aluminum heads have a Modern combustion shape that doesn't require as much timing … A great combustion shape will be happy with 30 to 32 degrees total timing and you can't do that with a old Iron factory head
Assuming the chambers are the same......you need to remove total timing for cast iron heads and add for aluminum. There are a ton of variables here but that is the overall consensus.
Cast iron heads hold heat......so you run less total advance to curb the onslaught of detonation......and opposite for the aluminum units.....they dissipate heat seven times the rate of cast iron so one can potentially run more lead before knocking on detonations door (pun intended).
I run 20 degrees initial on my 406 with a 244@.050 camshaft......it is what it likes.
Distributor Tuning Chart
