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Old 29th-October-2007, 01:53 PM   #1
WhitecatD
 
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Big Block Heads

I have a set of Big Block GM 781 casting # oval port heads sitting in the basement and a set on my 475 big block. The set on my motor have the stock valves but have been pocket ported. In the never ending quest for more power I was thinking about having the 781s in my basement extensively ported and larger valves installed then swaping them on to my motor.

What do you guys think?
ported 781s with bigger valves (experience? ideas?)


Keeping in mind that i do not have enough money for aluminum heads right now

I was also keeping in mind that the combustion chamber will be the same shape and size because i do have domed pistons
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Old 29th-October-2007, 02:07 PM   #2
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from what I have researched enlarging the valves in those heads ARE THEY WAY TO GO. I have a set being cut as we speak, but have not run any with the larger valves.
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Old 29th-October-2007, 02:54 PM   #3
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True... the absolute best thing for the 781 or any of the large oval port head is the 2.19/1.88 valves. it will really wake it up for the average street driven motor.

disassemble the heads, have the head plunge cut to the larger valve, then do your porting. After the porting is done, then take them back to the shop for the final valve work.(if your doing the porting your self). if not... BIG valves are the ticket
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Old 29th-October-2007, 03:55 PM   #4
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A set of those heads done up exactly like Veno said in an 86 SWB Cheby truck went DEEP in the 11's on a MILD 150 shot tune.. I'm not sure how heavy it was but it was a FULLY stock truck with a/c, power steering, full interior including a box behind the seat with a pair of Kicker 12's, etc... 3200 converter in a th400 and 4.10 gears. The motor had a MILD solid flat tappet and a Weiand X-L-R-8 intake (yuk) and a worked 750 dp..
Those heads can work if they are prepped right!!
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Old 29th-October-2007, 04:06 PM   #5
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The late great John Lingenfelter used to put 2.25" on the intake side of these heads an if you do the machine work recommended by veno and get the short side nicely rounded up, they will work pretty good
Of course, nothing you can do to those will come close to the iron Merlin ovals and they don't cost nearly as much as aluminum!!
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Old 29th-October-2007, 04:20 PM   #6
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Sweet, that is what I was hoping to hear. I will probably have the heads milled down for a little more compression too. Right now it is 9.5:1 What can I do with 91 to 93 octane and an octane booster?

(I dont have a problem putting the octane booster in, i was originally going to use this as a street car but things changed and it is now going to be more of a drag car)

I was thinking 11:1 ? maybe if the heads will allow without changing pistons?



This is a really fresh motor only half a summer running on it (400 miles) but...

I WANT MORE POWER!!! .......

and it's a good excuse for a winter project
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Old 29th-October-2007, 04:29 PM   #7
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How high of a compression ratio you can get by with "Depends" on a LOT of things....but more importantly, what's your p-to-v (piston -to- valve) clearance. If you start milling your heads, that clearance will start getting tighter too.. That could make for a bad day if you're not sure.
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Old 29th-October-2007, 04:46 PM   #8
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Quote:
Originally Posted by rhorne12 View Post
The late great John Lingenfelter used to put 2.25" on the intake side of these heads an if you do the machine work recommended by veno and get the short side nicely rounded up, they will work pretty good
Of course, nothing you can do to those will come close to the iron Merlin ovals and they don't cost nearly as much as aluminum!!

True, I will have to take a good look at those Merlins. The only thing I am a little concerned about is piston clearance with those domes.

As D-Man said I will also look into the P-to-V clearance. As for right now i'm just shootin out some ideas.

Goals: In order of priority

1. Better Flowing Heads
2. Different Cam
3. Higher compression
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Old 29th-October-2007, 04:57 PM   #9
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The Merlins will clear any piston your stock heads will. I strongly recommend not more than high 9's cr with any pump gas BBC. You may find a time you need gas and have no additive or race gas and you can ruin a good motor quickly!!!
Also, if you maximize everything, you will get more power with 9.5cr and 34* timing than with more compression and less timing, not to mention better cooling, economy, etc.
Also, make sure the quench clearance TOTAL (deck plus gasket thickness) is about .035".040". This will make a big difference!!!
If you have less than 9:1 now, I like to angle mill the heads. This will give better valve angles, many more advantages. But then you better know your valve to piston relationship
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Old 29th-October-2007, 05:16 PM   #10
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Quote:
Originally Posted by rhorne12 View Post
The Merlins will clear any piston your stock heads will. I strongly recommend not more than high 9's cr with any pump gas BBC. You may find a time you need gas and have no additive or race gas and you can ruin a good motor quickly!!!
Also, if you maximize everything, you will get more power with 9.5cr and 34* timing than with more compression and less timing, not to mention better cooling, economy, etc.
Also, make sure the quench clearance TOTAL (deck plus gasket thickness) is about .035".040". This will make a big difference!!!
If you have less than 9:1 now, I like to angle mill the heads. This will give better valve angles, many more advantages. But then you better know your valve to piston relationship
I usually shoot for .038" to .045" on the quench. And as for angle milling the heads...Make sure you've got a competent machinist because the intake may need machining to all 3 surfaces to match!! Otherwise the intake-to-cylinder heads surfaces won't be parallel..
Also, be sure and check the RADIAL clearance around the valves. I'm sure some others can chime in and give you the exact minimum #'s there (I can't recall off the top of my head) but they are just as important as the p-2-v clearance!!
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Old 29th-October-2007, 05:17 PM   #11
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Hello,

Here a pic of some 049 casting heads that I machined for 2.19s--1.88 a while back.

I think they make resonably good performance street type head for the money.

Last edited by stock z/28; 18th-January-2008 at 07:06 PM.
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Old 29th-October-2007, 06:40 PM   #12
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I usually shoot for .038" to .045" on the quench. And as for angle milling the heads...Make sure you've got a competent machinist because the intake may need machining to all 3 surfaces to match!! Otherwise the intake-to-cylinder heads surfaces won't be parallel..
Also, be sure and check the RADIAL clearance around the valves. I'm sure some others can chime in and give you the exact minimum #'s there (I can't recall off the top of my head) but they are just as important as the p-2-v clearance!!
Your the man, D-Man!! I have ALWAYS made more power, all else being equal, with angle milled heads, but the intake side MUST be done right as well. You may need 1/8" intake gaskets or mill the intake ends as well.
I have run .030 quench with great success with the right bottom end and piston to wall clearance. In fact, the tighter the better. I have run .025 on a round track 348 inch SBC and went faster each time we ran less!!
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Old 29th-October-2007, 07:07 PM   #13
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Your the man, D-Man!! I have ALWAYS made more power, all else being equal, with angle milled heads, but the intake side MUST be done right as well. You may need 1/8" intake gaskets or mill the intake ends as well.
I have run .030 quench with great success with the right bottom end and piston to wall clearance. In fact, the tighter the better. I have run .025 on a round track 348 inch SBC and went faster each time we ran less!!
Quench is a funny thing. I've gotten away with some rediculously tight clearance before in a SBC too but in a BBC the larger pistons rock a bit more in the bores (especially with the domes) and may bite ya.. Then don't even get started in the power adders because it seems the trend there is to open the quench back up!! I know of a FEW Outlaw 10.5 guys that are running close to .090" quench in their nitrous motors!! I guess with 4 foggers blasting in there, the mixture is pretty homogenized already. Then there's those that are saying it's because they're spraying soooooo much that the quench (squish) can actually cause too much of a pressure wave in the chambers and either pre-ignite (like a diesal--compression fire) or simply blow the flame out. Who really knows?? It's a different world out there when your making 2700-2800 HP with 123 octane gasoline!!! At that level I'll just continue to sell them all of the nitrous refills they want, sit back with a and enjoy the show. Not very many people can afford to compete with them boys and their 870" Kasse and 820" Fulton mountain motors!! Let alone the Bruno's, G-forces, multistage clutches, 3 passes on a set of slicks, etc.... Them's-a-lot-o-thick checkbooks!!!
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Old 29th-October-2007, 08:43 PM   #14
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Also, be sure and check the RADIAL clearance around the valves. I'm sure some others can chime in and give you the exact minimum #'s there (I can't recall off the top of my head) but they are just as important as the p-2-v clearance!!

Stupid question

Is Radial clearance the valves clearing the edge of the cylinder bore?
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Old 29th-October-2007, 09:39 PM   #15
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Stupid question

Is Radial clearance the valves clearing the edge of the cylinder bore?
Its the radius of the valve relief in the piston.

a pair of arcs in in the same segment each on the same radi to each other with equal space between
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