How Badly Does Decking Effect Flow?
Thread Starter
Le Mans Master
Joined: Feb 2000
Posts: 9,715
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From: Kempner Texas
How Badly Does Decking Effect Flow?
I'm in the market for a set of aftermarket heads for my 96 LT1. The ones I'm considering have 64cc chambers, which is about 10ccs more than my stock heads. That'll drop CR.
I'm considering having them angle milled to reduce the chambers to 54 ccs but am wondering how much this will effect airflow. I noticed at the bottom of AFRs info is a statement saying milling heads effects flow.
I just don't have any idea of how much.
Anyone know?
Thanks,
Jake
I'm considering having them angle milled to reduce the chambers to 54 ccs but am wondering how much this will effect airflow. I noticed at the bottom of AFRs info is a statement saying milling heads effects flow.
I just don't have any idea of how much.
Anyone know?
Thanks,
Jake
Melting Slicks
Joined: Jan 2002
Posts: 2,238
Likes: 4
From: Milan
...mmm real difficult to reply with techncal skill to JAKE 
but I think that achieving the correct cc displacement on combustion chambers is a good thing only if the milling has to be nìminimal.
The changed geometry (as you know) on intake manifold and heads can lead to milling the intake manifold too with the related problems.
-Beppe-
but I think that achieving the correct cc displacement on combustion chambers is a good thing only if the milling has to be nìminimal.
The changed geometry (as you know) on intake manifold and heads can lead to milling the intake manifold too with the related problems.
-Beppe-
Thread Starter
Le Mans Master
Joined: Feb 2000
Posts: 9,715
Likes: 27
From: Kempner Texas
If milling the heads doesn't have too much of a negative effect on flow, I'll go that route; say if it's less than 10cfm.
I'll opt for angle milling since that method removes the majority of chamber volume from the exhaust side of the heads and has little effect on the area near the intake ports.
Using the angle method maintains the intake manifold to cylinder head match so there should be no need to mill the intake manifold or elongate the bolt holes in the manifold.
I'm basically concerned about how much removing .080 from the chamber (remember this is on an angle so it won't be that much taken off the intake side) will effect overall air flow through the intake and exhaust ports.
I came up with ".080" because .008 reduces chamber volume by 1 cc. If I want to remove 10 ccs from the chambers, I'd need a cut of .080
If my current chambers are 54 ccs and the heads I'm considering have chambers that are 64 ccs, then I need to remove an amount equal to 10 ccs.
Check my math.
Jake
I'll opt for angle milling since that method removes the majority of chamber volume from the exhaust side of the heads and has little effect on the area near the intake ports.
Using the angle method maintains the intake manifold to cylinder head match so there should be no need to mill the intake manifold or elongate the bolt holes in the manifold.
I'm basically concerned about how much removing .080 from the chamber (remember this is on an angle so it won't be that much taken off the intake side) will effect overall air flow through the intake and exhaust ports.
I came up with ".080" because .008 reduces chamber volume by 1 cc. If I want to remove 10 ccs from the chambers, I'd need a cut of .080
If my current chambers are 54 ccs and the heads I'm considering have chambers that are 64 ccs, then I need to remove an amount equal to 10 ccs.
Check my math.
Jake
Team Owner
Joined: Oct 2004
Posts: 81,241
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From: altered state
St. Jude Donor '05
I would highly recommend angle milling, anytime you can change the valve angle on a 23 deg motor its all on the plus side of things. To have your intake done to match costs very little so its worth it.
Large Impressive Member
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From: Good health is merely the slowest possible rate at which one can die
St. Jude Donor '04-'05-'06-'07
How do you figure it maintains the match when their mating surfaces will no longer meet at parallel angles? Also, if you want to minimize the amount of head milling needed, use the GM Impala head gaskets. They'll give you .020" right off the bat.
Thread Starter
Le Mans Master
Joined: Feb 2000
Posts: 9,715
Likes: 27
From: Kempner Texas
I don't "figure" it. That's why angle milling is so often used as opposed to a flat cut. Within reason, nothing negative results in the intake manifold/head port alignment by angle milling the heads.
I'd say that anything .100 or above would be reason to dress the intake. I always do a dry mock-up just to see what's what in any event.
I'm not a fan of thin headgaskets. To each his own I say.
Jake
Safety Car
Joined: Apr 2005
Posts: 3,624
Likes: 431
From: Dale City VA
Angle milling the heads will actually increase the intakes flow by a small margin, althou this is not the only reason angle milling was concived. The flow will increase because you move the intake valve away from the outside edge of the cyl wall and more to the center of the bore. When it (the intake valve) opens it still moves toward the outside edge of the bore but not as much as it would stock, due to the valve angle being shallower... You are in effect "rolling" the valve angle over a few degrees and have less shrouding of the valve from the cyl. As cruisinartvette said anytime you can change the valve angle on a 23degree SBC its all good.
We orginally angle milled heads because there wasn't enough material in the bottom of a low port SBC intake floor to flat mill them down far enough (you would run out of material) to get any compression without running huge dome pistons.
As stated the intake manifold will have to have a "correction angle" milled into it to fit properly with angle milled heads. Its a lot of work to angle mill heads and intakes properly. Any good machinest with a broach or Berco type surface grinder can do it but its pretty time consuming to set everything up properly, expect to pay more for it.
Will
We orginally angle milled heads because there wasn't enough material in the bottom of a low port SBC intake floor to flat mill them down far enough (you would run out of material) to get any compression without running huge dome pistons.
As stated the intake manifold will have to have a "correction angle" milled into it to fit properly with angle milled heads. Its a lot of work to angle mill heads and intakes properly. Any good machinest with a broach or Berco type surface grinder can do it but its pretty time consuming to set everything up properly, expect to pay more for it.
Will
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From: Good health is merely the slowest possible rate at which one can die
St. Jude Donor '04-'05-'06-'07
I don't "figure" it. That's why angle milling is so often used as opposed to a flat cut. Within reason, nothing negative results in the intake manifold/head port alignment by angle milling the heads.
I'd say that anything .100 or above would be reason to dress the intake. I always do a dry mock-up just to see what's what in any event.
I'm not a fan of thin headgaskets. To each his own I say.
Jake
I'd say that anything .100 or above would be reason to dress the intake. I always do a dry mock-up just to see what's what in any event.
I'm not a fan of thin headgaskets. To each his own I say.
Jake
I don't consider .029" to be a thin gasket. I used the Impala gasket with 11.3:1 compression and had no problems. With my current combo, I use the .039" Felpro because it puts my quench right at .041"
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Thread Starter
Le Mans Master
Joined: Feb 2000
Posts: 9,715
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From: Kempner Texas
Reher-Morrison did the machine work on the last three engines I built. I spent some time at their facility in Arlington a few years ago and had lunch with some of the guys.
We talked about lots of different things - how valve spring material is the bottle-neck to their race engines. How NASA has some spring material that's not available to the public and if they could get their hands on some it would solve their spring problems, etc.
They also told me about the steam holes in the 400 block and whether or not they're needed in after-market street heads installed on a 400 block.
One of the guys gave me to .100 angle milling spec for a street engine. When those guys talk, I listen. Just passing it on.
Jake
We talked about lots of different things - how valve spring material is the bottle-neck to their race engines. How NASA has some spring material that's not available to the public and if they could get their hands on some it would solve their spring problems, etc.
They also told me about the steam holes in the 400 block and whether or not they're needed in after-market street heads installed on a 400 block.
One of the guys gave me to .100 angle milling spec for a street engine. When those guys talk, I listen. Just passing it on.
Jake
Large Impressive Member
Joined: Jul 2003
Posts: 65,811
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From: Good health is merely the slowest possible rate at which one can die
St. Jude Donor '04-'05-'06-'07
Thread Starter
Le Mans Master
Joined: Feb 2000
Posts: 9,715
Likes: 27
From: Kempner Texas
Another thing that really amazed me was how open, friendly and forth-coming the guys were. No, they didn't share any trade secrets, but they were willing to talk about Chevy engines, in general, non-stop.
That reminds me, a couple of years before that, I called Hendrick Motor Sports on another SB question (ring gaps) and I couldn't get the guy off the phone. A couple of years later Speed Pro began advocating the exact same information the Hendrick guy had told me years before.
These guys LOVE to talk engines!
Jake
That reminds me, a couple of years before that, I called Hendrick Motor Sports on another SB question (ring gaps) and I couldn't get the guy off the phone. A couple of years later Speed Pro began advocating the exact same information the Hendrick guy had told me years before.
These guys LOVE to talk engines!
Jake