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Old 09-15-2008, 10:36 AM
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454 w/ 781 Heads - What can I expect with 290's?

Have a 454 with flat top pistons, 781 heads currently around 8.5:1 compression. Picked up a set of 290 heads over the weekend for what I thought was a deal I couldn't pass up. Looks like there is about a 18cc reduction.

Currently running a TH350 with a 1700 stall and 3.73 gears. What can I expect in the way of performance change if I make this swap? What would you recommend I have done to the heads while they are in the shop?

Godspeed, Dan Solomon
"It is a mistake to allow any mechanical object to realize you are in a hurry."
http://www.freewebs.com/chevelles1970/

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Old 09-15-2008, 01:51 PM
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Hi,
The 781s have hardened exh seats & breath well w/the semi open chambers, have 2.19 & 1.88 valves put in & have them milled, these heads will out breath your closed chambered heads
Rich
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Old 09-15-2008, 02:34 PM
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cc the 290 heads to see the current chamber volume. They do vary.

You want a 96cc finished volume. Install with a thin .022 LS6 head gasket
(1970 454/450hp chevelle) Stock bore +.022" piston deck clearance will give you 9.34:1.
overbore will be slightly higher cr.

While the semi closed chamber of the 1970 454/360 390hp LS5 290 casting heads do not flow quite as good as a open chamber 780-049 head in stock form
They do respond veryb well to typical street porting and bowl work and larger valves ( with some chamber deshrouding). The short side radius need particular attention to improve flow. Same job as doing the 781's.
The ports are essentually the same.
While the chamber is more shrouded than a 781, the gain incompression will more than offset this on a mild moderate BBC with flat tops.
You'll never get a 781 chamber down any where near 96-100cc.
You cr now is potentialy as low as 7.5:1 depending on chamber volume and gasket thickness.

I had this same build. The 290's when modded properly will work very well for you.

You'll have to CC the heads first and go from there.

What are the details of the build and purpose?

A piston swap is another alternative. Total cost is comparable.

Last edited by F-BIRD'88; 09-15-2008 at 03:00 PM.
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Old 09-15-2008, 03:28 PM
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Thanks F-Bird,

Just plan to have fun with the car. maybe a couple passes at the track and a whole bunch of rubber at the local stop light. I really like the torque now as I just put the engine in as a replacement for a 350. It only has about 200 miles on it. The 290's would be correct for the car which is a small plus but I would like to bring up the horsepower as much as possible and still stay on high test.

Godspeed, Dan Solomon
"It is a mistake to allow any mechanical object to realize you are in a hurry."
http://www.freewebs.com/chevelles1970/
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Old 09-15-2008, 10:02 PM
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Just an idea.
Why don't you sell both sets of heads & buy yourself some aftermarket aluminum heads???

The weight savings would be Phonemonial & the flow would far surpass anything the backyard wrencher could ever muster.

Even POWERHOUSE's El Cheapo generic aluminum heads would be far better than OEM's

If you have pop-up pistons you might run into trouble changing from a closed chamber to an open chamber & vice-versa due to dome shape.
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