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PostPosted: Thu Sep 16, 2021 8:59 pm 
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2 BBL ''SuperSix''

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Curious how big of valves I can add to my head without cutting into the cylinders to make them work.


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PostPosted: Thu Sep 16, 2021 9:12 pm 
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Supercharged
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The stock bore size will accept a 1.75" intake valve, but it usually doesn't flow any better than a 1.72" Chevy valve. A 1.45" exhaust valve will work most of the time.

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PostPosted: Fri Sep 17, 2021 6:16 am 
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How much overbore can you do, or do you want/dare to do? 0.100" over is fine on almost all pre-80 or so Slant blocks.

Lou

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PostPosted: Sat Sep 18, 2021 8:47 am 
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Several of my race engines with a 3.500 bore use a 1.780 intake and a 1.500 exhaust


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PostPosted: Sun Sep 19, 2021 1:57 am 
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There you have it, folks. Hi Dave!

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PostPosted: Sun Sep 19, 2021 1:03 pm 
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IIRC, the biggest valves Mike used in my heads were 1.73/1.50. Mine were done for a +.045" overbore.

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PostPosted: Sun Sep 19, 2021 2:28 pm 
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If you do a small or no overbore the tops of the cylinders can be clearance for large valves as well.


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PostPosted: Sun Sep 19, 2021 8:16 pm 
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2 BBL ''SuperSix''

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The reason I'm wondering about how big you can go without boring or machine work is I've found a few programs for designing cylinder heads and at some point in the future (not exactly sure of when at this point) I'd like to make a prototype for and aluminum head and I'd like it to be a bolt on upgrade that requires no machining.


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PostPosted: Mon Sep 20, 2021 4:58 am 
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For a newly designed head the numbers you asked for are irrelevent, unless the valves are in the stock location. If you design the head, lets say for canted valves, the size could be different. My opinion, a "bolt on" head, would basically be just a stock head in aluminum, and a waist of time and money. If spending the effort, for a alloy head, it should be an improvement over the stock head, in terms of flow.

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PostPosted: Mon Sep 20, 2021 9:08 am 
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2 BBL ''SuperSix''

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I was thinking more of a four valve hemispherical head derived from one of the 3.5 heads as a starting point.


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PostPosted: Mon Sep 20, 2021 9:22 am 
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It's going to be difficult to make a 4-valve head work without it being a crossflow. But a crossflow head is definitely not going to be a 'bolt on' replacement.
Rocker arms for a 4-valve slant 6 head would be interesting...

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PostPosted: Mon Sep 20, 2021 9:45 am 
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2 BBL ''SuperSix''

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Car Model: 71 dart
I think the direction I'm going to go is to laser scan a few heads into a CAD program to see if I can basically morph the two heads together and still keep the intake and exhaust on the same side. Unfortunately this means destroying a few heads because they'll need to be cut up to scan the internals. If the four valve design ends up not being a feasible option I'll just default to a two valve.


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PostPosted: Tue Sep 21, 2021 5:35 am 
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A few have moved the int valve over toward the exh valve and then used a 1.80+ int and 1.40 or so exh. This will help. Really, the valve sizes are not that small compared to the displacement of the cylinder. A few people have reached 220+ CFM on intake, which is about like having a 300 CFM int on a 360 V8 engine (need to re-run #'s to be more exact). That is with stock ports and valve location. There is plenty to gain w/o changing port locations or even going to 4 valves/cyl.

Lou

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PostPosted: Tue Sep 21, 2021 12:01 pm 
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Dart270 wrote:
A few have moved the int valve over toward the exh valve and then used a 1.80+ int and 1.40 or so exh. This will help. Really, the valve sizes are not that small compared to the displacement of the cylinder. A few people have reached 220+ CFM on intake, which is about like having a 300 CFM int on a 360 V8 engine (need to re-run #'s to be more exact). That is with stock ports and valve location. There is plenty to gain w/o changing port locations or even going to 4 valves/cyl.

Lou


The Volvo B20 slant four has raised intake ports. A simple head with raised intake ports could have a decent flow. The B20 has a 3.50" bore, was last used in 1968. This one had fuel injection. Other than the location of the raised ports I wouldn't recommend copying the entire head. Might cause clearance problems on cars, but would be okay with Sweptlines. Its not a cool design, but it might work with minimal other changes. With quench pads and some other revisions would work best on the zero deck 170. Maybe the valves could be canted a little, but look for existing valve hardware before you consider that.

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PostPosted: Wed Sep 22, 2021 4:41 am 
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Dart270 wrote:
A few have moved the int valve over toward the exh valve and then used a 1.80+ int and 1.40 or so exh. This will help. Really, the valve sizes are not that small compared to the displacement of the cylinder. A few people have reached 220+ CFM on intake, which is about like having a 300 CFM int on a 360 V8 engine (need to re-run #'s to be more exact). That is with stock ports and valve location. There is plenty to gain w/o changing port locations or even going to 4 valves/cyl.

Lou




Is this accomplished by re drilling / boring the intake Guide /stem Center line further over toward the exhaust valve? I had never heard of this before - Intriguing, as I am sure you need a skookum machinist to re layout the seats and guides.

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