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PostPosted: Thu Feb 04, 2021 5:26 pm 
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Falcon67 wrote:
The way to get some turbulence in open chamber heads is to cut them down some - 70~74cc, less if possible - then run flat top pistons at zero deck.


We took .050 off a set of open chambered 4v heads and it dropped them down to 67 cc's.


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PostPosted: Thu Feb 04, 2021 8:32 pm 
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Thank you. I think this is the route I'm taking. Couple of questions:
I need to make 1 change already to the piston order- .020" over. Can I make another change- increase the compression height to get zero deck or .005" over like suggested instead of decking the block? According to my calculations that would be 1.680" compression height.
second question- would that together with decking the heads .050" bring the compression ratio too high for pump gas (91 octane here in CA)? would the engine still run well on pump gas with zero deck and 67cc combustion chamber?


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PostPosted: Fri Feb 05, 2021 6:36 am 
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jacek wrote:
Thank you. I think this is the route I'm taking. Couple of questions:
I need to make 1 change already to the piston order- .020" over. Can I make another change- increase the compression height to get zero deck or .005" over like suggested instead of decking the block? According to my calculations that would be 1.680" compression height.
second question- would that together with decking the heads .050" bring the compression ratio too high for pump gas (91 octane here in CA)? would the engine still run well on pump gas with zero deck and 67cc combustion chamber?


I sell a full custom Racetec piston for $650. You can do anything you want to it for that price.

The issue is that you really do want the decks to be surfaced. When blocks are surfaced correctly, the decks are surfaced parallel to the crankshaft centerline. I have seen factory blocks be out .020" from end to end. So you may aim for your pistons to be at zero deck, but one will be at zero, and another will be .010" under, or even .020" under.

The machine work is the foundation of the build. If you don't spend money anywhere else, make sure the block is correct.

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PostPosted: Fri Feb 05, 2021 10:27 am 
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kv racing wrote:
We took .050 off a set of open chambered 4v heads and it dropped them down to 67 cc's.


That's some serious cutting LOL. The OCs that I've had started in the 76~78cc range.

Follow Brent's advice. Echo - get that block prepped first, then start taking measurements or working the plan for the custom pistons. Custom units fix all issues vs working with off the shelf units. Get the block true, then using a prepped crank and rods you can go from there to get the pistons as needed. When I ordered my 393C kit, I measured my prepped/decked block and asked for .005 deck clearance. 3.85 arm + 6.00 rods + custom pistons, all landed at .005. Nice when a plan comes together. Brent is doing the same for the new 427C except the block is chasing the other parts this time LOL.

Pump Fuel - that's a hard question to answer. Depends on lots of factors including cam timing. Vehicle weight, gearing, converter, etc all play a part. You'll want to stay in the 9~9.5 range IMHO and let Brent spec in a cam to work with everything. You'll want to add as much initial timing as the combo can stand for helping with carb calibration and "drive-a-way" performance and limit total at a reasonable RPM.

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PostPosted: Fri Feb 05, 2021 12:31 pm 
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Thank you. Off to the machine shop for trueing the block. I'm thinking to use the original pistons to measure and calculate the compression height on the new ones after I get the block deck squared out. Should I wait with decking the heads till after the block is squared and new 0.020" over pistons with zero deck obtained?


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PostPosted: Fri Feb 05, 2021 1:31 pm 
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All machine work needs to be done and then you order the pistons last. If something happens and you need to cut the heads, or a different valve job is needed, it will affect your compression ratio.

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PostPosted: Fri Feb 05, 2021 1:37 pm 
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Thank you. so it looks like it's .005" off the heads per cc. Should I go for .050" to get them down to 66 or so cc?


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PostPosted: Fri Feb 05, 2021 2:02 pm 
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If these are open chamber, I think it's more like .008/cc. Closed are around .005~6. As far as how much I have no opinion. Our 355C 2V engine made around 400 HP at 9.5:1, 74cc chamber head with a flat tappet hydraulic cam and 91 pump fuel. Depends on your power goals and fuel requirements/limitations.

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PostPosted: Fri Feb 05, 2021 2:29 pm 
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Falcon67 wrote:
If these are open chamber, I think it's more like .008/cc. Closed are around .005~6. As far as how much I have no opinion. Our 355C 2V engine made around 400 HP at 9.5:1, 74cc chamber head with a flat tappet hydraulic cam and 91 pump fuel. Depends on your power goals and fuel requirements/limitations.

Oh for me that would be ideal. Mine 1970 was 9.5:1 stock. Does that mean I don't need to deck the heads since they are 76cc stock? Or deck them 0.020" ? Stock cast iron heads? Stock 2v valves? Can I pm you for the cam specs?


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PostPosted: Fri Feb 05, 2021 6:18 pm 
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You don't have any idea of the compression until you measure everything - deck clearance, chambers, etc. A 351C 4V from 1970 was advertised at 11:1. IMHO - I don't believe it. From what I have seen using stocker crank, rods and old school TRW pistons: 4.00x3.500, 68cc closed chamber head, .012 deck clearance, .039 x 4.1 Felpro = 9.8:1. I think in the true 11:1 engines they had to run a 4 ~ 6cc dome to even get close to the mark, unless specific heads were cast with smaller chambers. None of the D0 or D1 heads I've checked were less than 68cc if they were stock. Hard to find ones that haven't been cut a little on a rebuild.

With a 78cc open chamber head, you're at 8.8:1.

Mine: 4.030 x 3.500, 70 model closed chamber heads 65 cc chamber (down from stock measured 68cc), .039 x 4.1 Felpro gasket, flat top pistons with 3cc reliefs at zero deck = 10.54:1.

The engineering term is "nominal" - which means "That's what we shoot for". Manufacturing tolerances and casting changes result is figures well off from "nominal". Which is one reason why "blueprinting" an engine can make a lot more power.

Mock it up, measure deck on all cylinders, CC the heads, pick a spec for the gasket (I typically use FelPro 1013s) and figure it up. Do not rely on published factory figures.

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