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Old 02-05-2012, 06:27 AM
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Comb Chamber Design v Total WOT Spark Advance

I am removing the old cast iron 461 heads, which were run with 38 degrees total WOT spark advance. The chambers were 65.5cc and pistons have 5.3cc domes.

I am replacing with aluminum angle plug heads with 64 cc chambers as pictured, using the same 5.3cc domed pistons.

What will be the burn rate of the new v the old iron heads? Somewhat faster? Where should I expect my max WOT to be in order to optimize power?

If more information is required in order to answer these questions, please ask.

Thanks in advance.

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Old 02-05-2012, 07:48 AM
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What make of head are those exactly? Modern design heads such as the GM fast burn head effectively increase the rate at which the mixture is lit off. The angle plug design will also help this along. Less spark advance is needed due to the improvements in chamber efficiency to lite the fire.

Just depends on what you are shooting for as far as timing? max power or a conservative tune that reduces the risk of damaging parts due to detonation. Aluminum heads typically allow for more advance all things being equal in chamber design vs cast. This is due to heat dissipation rates being higher than that of cast iron. But that does not mean that you need to go there since the head is just way better than the 461.

What ever the case the heads should flow more than the 461 and this is where the power will come from not solely from where you set the WOT total timing. I would start at 34 and go from there.
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Old 02-05-2012, 09:42 PM
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Don't for get to check Piston Dome to Head clearance as most Dome Pistons used with old bathtub shaped/style Head combustion chambers don't work with the new Kidney shaped chambers. They hit the Head where the center protrudes between the Valves.
Can normally run about 1 point more compression with the Al. Heads to make up for thermal efficiency loss.

Piston Dome shape for Vortec/Kidney shaped Chambers:
http://kb-silvolite.com/features.php...n=read&F_id=16
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Old 02-06-2012, 11:38 AM
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Quote:
Originally Posted by Custom10
What make of head are those exactly? Modern design heads such as the GM fast burn head effectively increase the rate at which the mixture is lit off. The angle plug design will also help this along. Less spark advance is needed due to the improvements in chamber efficiency to lite the fire.

Just depends on what you are shooting for as far as timing? max power or a conservative tune that reduces the risk of damaging parts due to detonation. Aluminum heads typically allow for more advance all things being equal in chamber design vs cast. This is due to heat dissipation rates being higher than that of cast iron. But that does not mean that you need to go there since the head is just way better than the 461.

What ever the case the heads should flow more than the 461 and this is where the power will come from not solely from where you set the WOT total timing. I would start at 34 and go from there.
They are ProFiler 176 series All American 23 degree heads. They seem to be very similar in cc shape to Chevy Vortec heads, although the faster burn with the Vortecs seems to be attributable to the small intake runner size more so than the chamber shape. My heads, although similar in chamber shape (kidney/heart) have 203cc intake ports. The "fast burn" GM heads have a much different cc shape than the Vortecs/my heads.

I'm shooting for max power and detonation has never been a concern as the engine never showed any trace, even with the 461's. I just don't want max cylinder pressure to occur any earlier than 14-15 degrees ATDC, as is optimal.
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Old 02-06-2012, 11:44 AM
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Quote:
Originally Posted by SSedan64
Don't for get to check Piston Dome to Head clearance as most Dome Pistons used with old bathtub shaped/style Head combustion chambers don't work with the new Kidney shaped chambers. They hit the Head where the center protrudes between the Valves.
Can normally run about 1 point more compression with the Al. Heads to make up for thermal efficiency loss.

Piston Dome shape for Vortec/Kidney shaped Chambers:
http://kb-silvolite.com/features.php...n=read&F_id=16
Yes, thank you. I had completely neglected to consider this but I'll certainly check it now. The pistons are Speed Pro L2166NF, with small domes. Just eyeballing them it appears that I should be alright. At most, a very small divot taken on the domes (probably less than .2cc worth) with a bur, between the valves MIGHT be necessary. I'll do it with the engine assembled, if necessary.
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Old 02-07-2012, 05:59 PM
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The chamber shape will give good quench/turbulence providing the quench distance is good. The plug is somewhat biased towards the exhaust valve but doesn't look quite as well centered in the chamber as the L31 to me.

AFA fast burn heads go, IMO port velocity certainly can help generate a well mixed fuel/air charge, but there's more to it than that. Don't discount the chamber shape as being a big part of the fast burn equation.

I would suggest using 30-32 degrees total to begin with. Work up carefully from there until you find the point of diminishing returns or detonation. Being aluminum I'd expect them to be able to take somewhat higher total advance than a similar head in iron, but just because they may be able to, let track testing be the judge. There's no advantage in running more advance than is needed, in fact you will see performance fall off if you have too much total advance- even w/o detonation.
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Old 02-15-2012, 04:50 PM
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yes, turn the total timing down, but maybe not a good idea to reduce initial timing. This will require reducing the amount of advance in the distributor.

what cam are you using? big cams especially like lots of timing at idle.
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Old 02-15-2012, 06:31 PM
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Quote:
Originally Posted by 454C10
yes, turn the total timing down, but maybe not a good idea to reduce initial timing. This will require reducing the amount of advance in the distributor.

what cam are you using? big cams especially like lots of timing at idle.
Comp XR286R
http://www.compcams.com/Company/CC/c...?csid=500&sb=2

Currently run 20 initial plus 17 centrifugal, all in at 2200 RPM. Also tolerates 14 degrees vacuum advance.
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Old 02-15-2012, 07:33 PM
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37 total with 17 mechanical is up there on the total IMO, set the initial at 16-17 and leave the mech alone, pull vac advance on manifold and it should be around 30 @ idle if you have the vacuum. This will be a conservative place to break the thing in with the new setup.

Then every bit of what you are giving it now may need to be put back in once you shake it down. Run the bag off it and get your total high RPM timing advance without vacuum advance at the optimal spot, maybe 36 maybe 38 maybe 34. Then if your using manifold vacuum advance at idle the initial is not as important as total but at least 18 initial I would figure and hope the idle manifold vacuum is high enough for vac advance to be included,,,maybe not though

What is/was the vacuum of this mill at idle using that cam? that is a big cam in my book. You say vac adv is adding 14 so something is happening.

Might be a good candidate for a adjustable vac advance if your manifold vac at idle and/or at light engine load is around 10 "hg. That way you can tweak the range/rate of the vacuum advance more effectively, test & tune will tell the tale.
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Old 02-16-2012, 05:33 AM
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I would try to keep 20 degrees at idle with a cam that big then plug the vacuum advance into the manifold source and add another 10-12. it will idle 30 to 32 with this setup.

That cam should have enough vacuum at 1000 rpm to use the vacuum advance at idle. (hopefully)

But first, figure out how much timing the engine likes at full power. (trial and error)
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Old 02-16-2012, 05:54 AM
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Quote:
Originally Posted by Custom10
37 total with 17 mechanical is up there on the total IMO, set the initial at 16-17 and leave the mech alone, pull vac advance on manifold and it should be around 30 @ idle if you have the vacuum. This will be a conservative place to break the thing in with the new setup.

Then every bit of what you are giving it now may need to be put back in once you shake it down. Run the bag off it and get your total high RPM timing advance without vacuum advance at the optimal spot, maybe 36 maybe 38 maybe 34. Then if your using manifold vacuum advance at idle the initial is not as important as total but at least 18 initial I would figure and hope the idle manifold vacuum is high enough for vac advance to be included,,,maybe not though

What is/was the vacuum of this mill at idle using that cam? that is a big cam in my book. You say vac adv is adding 14 so something is happening.

Might be a good candidate for a adjustable vac advance if your manifold vac at idle and/or at light engine load is around 10 "hg. That way you can tweak the range/rate of the vacuum advance more effectively, test & tune will tell the tale.
Quote:
Originally Posted by 454C10
I would try to keep 20 degrees at idle with a cam that big then plug the vacuum advance into the manifold source and add another 10-12. it will idle 30 to 32 with this setup.

That cam should have enough vacuum at 1000 rpm to use the vacuum advance at idle. (hopefully)

But first, figure out how much timing the engine likes at full power. (trial and error)

Those advance numbers are with the old 461's, you understand. I am using 1.6:1 roller trunnion rockers on both int/exh with this cam. The idle vacuum is about 8 in-hg @ 1000 RPM. I idle it at about 1100-1200 and about 9 in-hg. I'm using an adjustable vac advance can with this setup, and, believe me, it's at the low limit of its adjustment. The VAC is controlled by manifold vacuum. It cruises with 20+17+14, for a total of 51 degrees and idles with 20+14.

I will take a few degrees static out of it and test from there.

Last edited by 1965tripleblack; 02-16-2012 at 06:02 AM.
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Old 02-16-2012, 09:27 AM
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If it were mine, I'd verify the inertia ring/damper hub integrity while I had the heads off....
Use a piston stop like the one pictured or make one....
http://www.summitracing.com/parts/CCA-4933/
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Old 02-16-2012, 11:02 AM
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Quote:
Originally Posted by techinspector1
If it were mine, I'd verify the inertia ring/damper hub integrity while I had the heads off....
Use a piston stop like the one pictured or make one....
http://www.summitracing.com/parts/CCA-4933/
Thanks. That's a good idea. I can tell you that I'd done that at least once on this build. Most recently was last summer. It's an original, "olde fashioned" finned harmonic balancer vintage 1965, SHP SBC. It's dead nutz on.
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Old 02-16-2012, 11:06 AM
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Quote:
Originally Posted by SSedan64
Don't for get to check Piston Dome to Head clearance as most Dome Pistons used with old bathtub shaped/style Head combustion chambers don't work with the new Kidney shaped chambers. They hit the Head where the center protrudes between the Valves.
Can normally run about 1 point more compression with the Al. Heads to make up for thermal efficiency loss.

Piston Dome shape for Vortec/Kidney shaped Chambers:
http://kb-silvolite.com/features.php...n=read&F_id=16
There's about .040" clearance between the valves, where the apex of the piston's eyebrows project between. This was checked WITHOUT the .038" head gasket installed, so total clearance in this area looks to be about .080".
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Old 02-16-2012, 01:04 PM
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Quote:
Originally Posted by 1965tripleblack
Those advance numbers are with the old 461's, you understand. I am using 1.6:1 roller trunnion rockers on both int/exh with this cam. The idle vacuum is about 8 in-hg @ 1000 RPM. I idle it at about 1100-1200 and about 9 in-hg. I'm using an adjustable vac advance can with this setup, and, believe me, it's at the low limit of its adjustment. The VAC is controlled by manifold vacuum. It cruises with 20+17+14, for a total of 51 degrees and idles with 20+14.

I will take a few degrees static out of it and test from there.
Vacuum advance goes away as the throttle is open because the vacuum in the manifold reduces toward atmospheric pressure unless you're running a super charger of some sort.

With an idle vacuum of only 8 inches you wouldn't be getting much vacuum advance unless the advance can is adjustable or has been reworked with a different return spring pressure.

With 8 inches of idle vacuum, this is pulling a pretty good cam, big cams like a lot of initial advance because the effects of late closing the intake valves reduces cylinder pressure which causes a slower burn which needs more time, thus advance to get across the chamber. I don't think dialing back the base timing is the way to go, at least not if you don't put it into the centrifugal and perhaps bring it up faster.

Generally Vortec type chambers like about 34-36 degrees max. Aluminum often likes more advance and more compression to make up for the cylinders faster temperature loss because of aluminum's much faster heat transfer rate into the cooling system than iron. You can also stand to run the coolant temp 15-20 degrees to slow the heat transfer rate a little.

All this is tuning for the new conditions, it can take a while to sort out what works best.

Bogie
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