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#1
07-17-2009, 03:54 PM
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zero decking, how important
After reading recent threads mentioning zero decking, I realized my block wasn't done. I didn't ask the machine shop to do it and they didn't recommend it. It is a .40 over 1969 400 with Speed Pro forged. The pistons have a small dish (pistons prepped by Mr. Pbody) intended to reduce compression to about 9.2 with non-ported 1969 62 heads that measured at 75cc's. The shop did not measure cylinder volume.
I've already assembled it with a CC XE268 hydraulic flat tappet cam. Cam choice based on lower compression. The installation into my 69 bird is pending other work. One serious concern of mine is a thread I read here regarding how Speed Pro forged pistons sit far down the hole on an block not zero decked. I didn't take the measurement as a rookie mistake. The tranny is a TH400 with a Hughes GM 20 converter, and while it currently has 4:11 gears, I will be getting 3:23s installed soon after engine install. Will this thing be a dog with the piston dish and no zero decking combo? Last edited by grivera; 07-17-2009 at 04:14 PM. Reason: forgot to add head cc |
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#2
07-17-2009, 04:04 PM
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You should have cc'd the heads and cylinder when you had it apart during assembly so you would know where you are at.
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#3
07-17-2009, 04:08 PM
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The heads were CC'd by the machine shop and came in at 75cc's.
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#4
07-17-2009, 04:20 PM
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As stated in your first post you have approx 9.2 compression..
Zero decking is more desireable the closer you are to max compression for pump fuel.. At 9.2 you will be fine as is.. The closer you get to 10.1 its more important to reduce the possibility of detonation.. It should have excellent low and mid range torque... but it might have a shortened top end due to cam selection.. Should NOT be a dog tho...
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If your not at the table you're on the menu A man who falls for everything stands for nothing. Last edited by Formulas; 07-17-2009 at 04:26 PM. |
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#5
07-17-2009, 07:38 PM
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Quote:
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#6
07-17-2009, 08:15 PM
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Most of the time the TRW pistons sit around .020" down on the ones I've checked (at least a half dozen or so different 400 engines).
Not too big a deal IMO,though were I going to sweat details,and I really did'nt wanna go back and change/machine anything in the bottom end,I would just run the thinner (.027" C.T.) cometic headgaskets to bring the head to piston distance a bit closer to increase the quench to some degree (.047" vs. .067"),albeit with the piston down in the hole some. The bonus there is you get to choose a better bore size for the head gasket as well (I would get the 4.160" bore size gaskets w/the bore reliefs). They are'nt cheap,but they are about the best headgaskets you can find. But even that is getting a bit overly cautious. But I digress. BP. |
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#7
07-17-2009, 11:24 PM
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Screaming Chief is right.
You should shoot for about .030-.040" quench whenever possible (with steel rods). If your budget doesn't include decking, fine. But with low compression, the ideal quench will allow you to run LESS octane or MORE timing and still be fine. But even combos that are borderline as far as detonation go will become LESS prone to detonation as you get your quench into this area. Obviously, with a .027 cometic gasket, you can actually leave the piston .013 in the hole and hit this figure. FlatOut Gaskets has a .025 gasket, I believe also. With a TRW piston, you are going to be pretty far off. I wouldn't take more than 030 off the deck to achieve these figures. My block and Ross pistons measured out at 020-028 down. I had my block cut to make them an even .010 all around and am using the thinner MLS gaskets.
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greg 1976 SE 455 twin turbo austin, tx |
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#8
07-18-2009, 12:17 AM
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"I've already assembled it"
You people are suggesting a tear down wasting time, effort, gaskets in the original build just to throw $140+ worth of head, intake gaskets and labor to fix a problem that doesnt even exist.. there will be no negative effects that this guy would even realize the way its built now.............very strange.. Grivera.... run it the way it is then change head gaskets then kick yourself in the butt for the money spent... Dont even know why i waste my time here.. I`am done..
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If your not at the table you're on the menu A man who falls for everything stands for nothing. Last edited by Formulas; 07-18-2009 at 12:34 AM. |
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#9
07-18-2009, 08:38 AM
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Not tearing down
Quote:
Thanks Formulas. No way I'm tearing it down; that being said I feel like a jack ass not having checked into this issue earlier. I just didn't know about the Speed Pros sitting so far in the hole. |
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#10
07-18-2009, 12:42 PM
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no need to feel bad about not having the block zero decked. Most performance related builders and machinists want everything 'perfect'. nothing wrong with that. But, for a street car? Do not worry about it at all. Go and have fun with your new motor.
Steve Hosting - * Apache County Motorsports day Sat August 29 St Johns, Az www.apachecountymotorsportsday.org * Maricopa Motorsports Day Sat Octorber 17, 2009 Maricopa, Az * 12th annual Pontiac Heaven March 27, 2010 Phoenix Speedworld Dragstrip * 8th annual Nostalgic Show and Go! Sunday March 28, 2010 Speedworld Dragstrip www.nostalgicshow&go.org
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