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#1
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going from open chamber to closed chamber head
Here is my engine combo..
1977 400 stock piston, reringed and honed 494/292 cam, M-1 dual plane intake, 3310 holley vaccum secondary 72/74 no power valve carb, windage tray, high volume oil pump, baffled oil pan, Pocket ported and exhaust ported 452 heads 1 3/4 B/E body hedman headers 3000 converter 3.91 gear 26 inch tire 2 inch cowl scoop through a velocity stack shift at 6000 rpm goes through the traps at 6000 rpm Has run 12.80 @ 107 in a 3800 lb Aspen Makes about 368 hp to the back wheels My question: I have a set of stock closed chamber 516 heads.. they have the smaller exhaust valve Will I pick up any performance by swapping on the closed chamber heads even though the heads have the smaller exhaust valve.. I was told that I would have to shift at a lower rpm because of the restriction in the exhaust flow... I'm just looking for the best bang for my buck... Should I pocket port the 516's even though I'm leaving the stock exhaust valve in? |
#2
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You will be gaining around 3/4 of a point in compression. On the other hand you will be loosing head flow also. Opening the bowls up wnt help much with the same valves maybe a little. I would say your gain in compression, will be offset by the head flow. You wont see much gain if any. Also you now will be running a soft exhaust seat head made for leaded fuel. I will say take the 452's if thats what you are running pocket port them, deck them .040 for a 4 cc reduction, run a steel shim head gasket and have the best of both worlds. The steel head gasket is worth close to 1/4 point in compression at around .020 compressed thickness, compared to the felpro at .038-.040 thick.
Good Luck
__________________
68 Coronet 69 Super Bee......new 500 cid comin soon! 73 Duster witha missing 440/727 |
#3
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I am at a loss as to a suggestion.
I use the same heads, with the small valve. Power is extremely good. The smaller valve will improve your throttle response. Pocket port WILL help. Running the numbers through the compression calculator at www.kb-silvolite.com, gets you up to about 8.6:1 if you use the .020 gasket. You won't see the benefits of the "quench" area, because it's still too big for an effective quench. Too much deck clearance. The reason I'm not sure where I stand on the whole thing is simple. You have gotten waaaaayyyyy better results than you should have with what you've got. Don't get me wrong, I'm GLAD to see it. It's just that , to get an aspen into the 12's with a 7.5 to one 400, just using bolt ons, is an amazing thing. You've stumbled on to a good combo. I don't know if you should mess with it. As for valve size, have you read the thread on valve size that cuda66273 started? |
#4
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thanks for the reponse...
I do think I'm doing pretty good with this motor.. Not quite as good as my buddy who runs 12 teens with a stock piston 400 with a 509 cam.. even with a iron intake... but he runs the closed chamber heads with the bigger exhaust valve.. So you think I should have my 452 heads milled... I'm a little confused thought... this is from my Mopar performance Engine book Quote: " the '72 through '78 400 and 440 compression ratio is approximately 8.5:1. milling the heads .060" increases the 400's compression ratio to almost 10.0:1 " Mopar Performance Engine book 1989 page 271 I thought these things barely made 8 to 1 compression.. or is this for an engine that is blueprinted to Mopar specs like the one for NHRA stock elim engines.. If I do mill the heads I will have the intake side of the head milled too so I can run an unmodified intake... One more question... Do I have to completely disassemble the heads to have them milled? |
#5
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If you actually measure the deck clearance of the engine, you will find that these engines were only 7.5 to 7.7 to one
I measured mine, and was amazed, but not surprised. After reducing the deck clearance to .015(from .100)by milling the block, using a .039 head gasket and 78 cc 516 heads it's up to 10 Try playing with the compression calculator at www.kb-silvolite.com |
#6
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I certainly wouldn't do the swap. The 452 heads are way better performance wise than the 516 heads. They flow better stock, and they flow better ported. The difference in combustion chamber sizes may not be that great; the last 516's measured were 84 cc's while the 452's are typically at around 90. If your true CR now is 7.8, a 10 cc chamber size reduction would bring it to about 8.4:1. Milling the 452's will remove more cc's than milling the 516's the same amount. I would stick with the 452's, mill them .060 and if your funds allow install new nail head valves in to them. If possible, go with 2.14/1.81 valves and have them ground so that they sit pretty high in the chamber to reduce it's size and gain short side even more.
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