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  #16 (permalink)  
Old 05-23-2008, 04:04 PM
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lower cc heads = higher compression.

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Old 05-23-2008, 04:12 PM
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I was looking on summit and there is not many 74 or 76 cc heads.
there is plenty of 72 cc aluminum heads.
would 72 cc put my compression to high with the flat top pistons?
or would these heads work in my application?
thanks
Stuccoman
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Old 05-23-2008, 04:26 PM
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That's right. Not many big cc chambers out there. The afr's are the biggest aluminum heads that I have seen.

Not sure of your current head cc. Only assuming they are 76cc by the year model heads (mortec.com didn't list cc for those heads). You really need to find out what cc they really are before making any choices.

If you use aluminum heads then you can use less octane as compared to iron heads. Also, new style heads have a fast burn design which allows you to use less total ignition timing, which also allows the use of lower octane.

So, running 72 cc heads will rise your compression ratio if you are currently running 76 cc heads. However, new heads require less timing and the aluminum head are less prone to detonate due to better heat control. I'm not saying it will work, but it may. To be completely sure, get the same size head in aluminum (195cc afr with 74cc chambers)
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Old 05-23-2008, 04:44 PM
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If your heads are 75cc, and the piston is below the deck 0.015", and the pistons are 6cc flat tops (2 relief), and you are using a standard 0.039" head gasket then your compression is around 9.9:1.

then if you drop to 72cc heads then the cr with go up to 10.2. Not a big jump.

Then you could drop total timing from 38 to 34 with the fast burn heads.
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Old 05-23-2008, 04:44 PM
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I have been on line trying to find out what cc heads they are also and there is no info on these stock heads except that they are 350 heads.
Assuming that the heads I am running now are 76 cc. What would I have brought them down to by shaving them .030?
74cc?
I have found a 76cc head at summit.
http://store.summitracing.com/partde...5&autoview=sku
what do you think about me running those heads or should I chance it and go to 72 cc.
I dont want to loose power by lowering compression to much, but I dont want to have it any higher also.
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Old 05-23-2008, 04:46 PM
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Quote:
Originally Posted by 454C10
If your heads are 75cc, and the piston is below the deck 0.015", and the pistons are 6cc flat tops (2 relief), and you are using a standard 0.039" head gasket then your compression is around 9.9:1.

then if you drop to 72cc heads then the cr with go up to 10.2. Not a big jump.

Then you could drop total timing from 38 to 34 with the fast burn heads.

so your saying I should be ok ( educated guess ) with a set of 72 cc aluminum heads than?
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Old 05-23-2008, 04:48 PM
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454 I appreciate all your help
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Old 05-23-2008, 04:55 PM
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10.2:1 isn't that high with your cam and aluminum heads. However, I'm not sure about your current compression ratio. For example, if the deck height is 0.000 then cr would be at 10.2 now with 75cc heads. And if the head gasket is 0.020" then cr would be at 10.6 now. So your cr and vary depending on what was done to the block and what gasket is being used.

Those 76cc heads you found are good but pricey.

I personally would have no problem using 10.2 or even 10.5 with your cam with aluminum fast burn heads. But make sure to set the ignition timing up right. 20 degrees initial with 34 total would be a good place to start. This kind of curve requires a distributor that is adjustable or if you know how to shorten up a stock hei curve.
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Old 05-23-2008, 05:18 PM
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so here is what I am figuring.
if I go with 76 cc heads I can run on regular unleaded
if I go with 72 cc I will probably still need premium.
if I choose 76cc I will have less power than the 72cc.

I would like to go with the 72cc heads, my only concern would be the octane I need to run with,
should I still be able to run no problem on premium with a compression ratio with 10:1 - 11:1 compression with aluminum heads?
if so I will just get a set of 72cc heads and see what happens.
thanks
Stuccoman
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Old 05-23-2008, 05:36 PM
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Your figuring seems about right.

The difference in power between 10.0:1 and 10.5:1 is small and I doubt you will notice it.

Running lower octane fuel is nice, but the difference in price isn't that much either.
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Old 05-23-2008, 11:59 PM
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Chamber size

This site has Info on your heads.
http://www.chevymania.com/partsid/pa...p=head&block=S

458624- 350ci-1969-1984 - 1.94/1.50 76cc
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Old 05-24-2008, 05:53 AM
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Quote:
Originally Posted by SSedan64
This site has Info on your heads.
http://www.chevymania.com/partsid/pa...p=head&block=S

458624- 350ci-1969-1984 - 1.94/1.50 76cc
458642 is the casting we are looking for. I believe you found it, they must have misprinted the order of the #s.
thanks
Honestly the strange thing is my heads only say 45864 thats it...no 2 at the end.
when I went searching my only conclusion was they forgot to print the 2 on my heads.
it said GM 62 over the casting #
and on the left side of the head it said 4287 which I am guessing is the code for manufacturing date.
I was told when they were machined that they were 1.94/1.50 and I pulled them off a 350 engine so I was pretty sure we found the match at 458642.
anyways
thanks a lot Sedan64
Stuccoman
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  #28 (permalink)  
Old 05-24-2008, 07:23 AM
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mortec.com says

458642.....75-84...350..........350


doesn't spec cc but those years point to 76cc.
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  #29 (permalink)  
Old 05-24-2008, 04:15 PM
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If you really want to operate this motor on 87 octane regular fuel all the time without needing to compromise the ignition timing to avoid pinging @ WOT, you should build the motor with less than 10:1 compression ratio.
Big bore motors like the 400 are more sensitive than small bore motors are.
I would build it with not more than 9.4:1 with conventional iron heads and 9.6:1 with aluminum heads for 87 octane every day.
Otherwise plan on mid grade 91 or better at a minimum.
10.5:1 will require 92 octane or better.

The theoretical (paper) power difference between 9.5:1 and 10.5:1 compression is 4%. A motor that has excessive compression for the fuel will not make full power and will not last long if operated in detonation.
If you want more than 10:1 cr don;t even think about 87 octane.
You'll be fine at light load part throttle cruising (possiblely requiring less than optimim timing), but full load WOT with full power timing, will knock.

If you operate a motor under detonation for long, any power advantage you should , would or think you got or was trying to get will be lost because the piston ring seal quality will be reduced by the excessive shock and heat of detonation eating at the piston ring lands. We won't even got into preignition. Your fresh motor will soon be "old" way before its time.

A GM vortec head (which is 64cc) with its high combustion chamber swirl and fast flame burn speed can/may have a bit more compression ratio limit and knock tolerance (mostly because it often requires less total spark timing to reach full power than other heads) But the difference is small.
The stock old open chamber heads you have now are the worst of the worst as far as real spark knock tolerance goes.
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Old 05-25-2008, 08:01 AM
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thanks guys I am going to go with a set of brodix heads.
http://store.summitracing.com/partde...5&autoview=sku
Price difference between these and twist wedge are only a few hundred bux.
twist wedge are 2.020 and these are 2.080, twist wedge are 72cc and 195 cc intake runner and these ones are 76cc and 210cc intake runner.

my thought here is brodix will run on 87 octane no problem and will flow much better that twist wedge = lower compression + more HP

am i correct?

thanks guys
Stuccoman
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