"Decking" or shaving layers off cylinder heads
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Old January 24th, 2011, 08:41 AM   1 links from elsewhere to this Post. Click to view. #1
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"Decking" or shaving layers off cylinder heads


so i was told that if you shave thousanths of mm in layers off your cylinder heads you could theoretically increase compression ratio thus increasing hp. is this true or was the machine shop blowin smoke? and if it is possible, would it be worth it to "deck" the npi heads and port and polish the ports to increase power?
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Old January 24th, 2011, 10:28 AM   #2
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Decking the heads will increase the CR, as will the use of a headgasket with a shorter installed thickness. As far as whether or not it would be worthwhile, it depends on what you're setup is, and what you're trying to acheive.
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Old January 24th, 2011, 10:32 AM   #3
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machine shop aint lying to you. it will increase cr.

but with it being a mod motor would it really be worth tearing the heads off just to yield a few more horses
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Old January 24th, 2011, 10:33 AM   #4
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remember that we have sohc engines, so decking will retard your cam timing (the exact numbers i cant recall) if youre degreeing your cams later though, no problems.
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Old January 24th, 2011, 11:07 AM   #5
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so decking will increase CR but affect cam timing, so getting a custum cam won't yeild any hp?? i mean with my understanding, if you increase CR you increase power? couldn't decking heads be worth while for a budget to increase power? sorry for my ineptness, i'm not real Knowledgeable on these things, but i was thinking of decking my heads at some point.
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Old January 24th, 2011, 11:53 AM   #6
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Originally Posted by delgadog View Post
so decking will increase CR but affect cam timing, so getting a custum cam won't yeild any hp?? i mean with my understanding, if you increase CR you increase power? couldn't decking heads be worth while for a budget to increase power? sorry for my ineptness, i'm not real Knowledgeable on these things, but i was thinking of decking my heads at some point.

Getting a custom cam will give you more hp regardless of whether or not you deck the heads.

As far as decking the heads, if you have them off the car already for some reason, then yes decking them is worthwhile. You can increase compression ratio but your cam timing will be off 1-2 degrees max depending on how much you take off. You will gain horsepower the higher the compression. These motors love high compression.

With that said, decking heads is extremely expensive if the heads aren't already off. Also if you have NPI heads, converting to pi heads that are decked .020 thousandths would yield quite a horsepower increase. Doing cams would be smart at the same time. That way you can do the heads and degree the cams all at once.

What are your goals for the car and what is your current setup?
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Old January 24th, 2011, 12:13 PM   #7
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doing pistons with a smaller cc dish would probably be in your best interest if youre serious about getting into this. stock 4v pistons will up the compression ratio as well as swapping to pi heads.
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Old January 24th, 2011, 01:30 PM   #8
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Mach 1, my goal is to do a buget build like everyone here, but my thought process is to deck my NPI heads to increase the compression, add PI intake and custom cams and gain close to what a PI H/C/I set up would yeild. also, since the compression ratio would increase, i figured it would increase the CR but not as much as a PI set up to keep it boost and NO2 friendly. i'm completely stock...
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Old January 24th, 2011, 03:25 PM   #9
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The money spent to take the heads off and getting new gaskets and bolts is not worth the gain IMHO. However, if you built up a shortblock or did the typical use a 4 valve shortblock, then it would be worth it. 10 horsepower isn't worth the cost in my opinion if it's not necessary to take them off.

You really should decide if your doing nitrous or Supercharging before you do this. Nitrous works good with high compression but supercharging is entirely dependent upon the blower picked.
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Old January 24th, 2011, 03:41 PM   #10
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i am going to go nitrous. i was invisioning decking my heads, hopping to up the compression to 9.5:1 (i'm not sure of the stock compression) which will raise the hp close to what the PI heads would make then add a PI intake and custom cam and throw a 100 shot at it (of course, adding the supporting mods CAI, pullies, gears, exhaust etc..). i'm not sure if thats a good plan or not, but i thought decking would be cheaper then doing a whole PI swap.
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Old January 24th, 2011, 10:55 PM   #11
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Decking the heads would be cheaper for sure then buying another set of heads. You will be able to get higher compression than that. I would shoot more for 10.5 or 11-1 if you don't mind running 91-93 octane on a daily basis. 9.5-1 is basically stock.
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Old January 25th, 2011, 12:48 AM   #12
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If you do up the compression too much and run nitrous you also run the risk of blowing head gaskets alot more than keeping a stock CC combustion chamber. I have all that you have said besides ported and polished heads and cams and am barely pushing 250 pathetic horses, plus side + when I do get PI heads which is worth looking for than dumping money into npi heads I will have a almost bullet proof motor. Pi heads from my research adds almost 60 horses just because of the smaller chamber and better intake runners. SO I would just look for a set of PI heads and get them shaved to make sure they seal if your gonna go through all the trouble.
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Old January 25th, 2011, 07:28 AM   #13
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well mach, i see what you're saying, though, i didn't think i'd have to up the compression to 10.5-11:1 to make decent gains. i was just trying to stay away from the PI swap to keep compression down to NO2 or FI friendly levels. then again, i don't know that much and i'm sure most people here are running high compression setups. guess i'll just save for a PI swap.lol. thank you for all the help guys.
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Old January 25th, 2011, 12:09 PM   #14
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I'm planning on running 93 octane with 12-1 compression and a 150 shot on race gas.

JME, While the pi heads, cams, and intake add around 60 horsepower, very little is actually due to the heads. The increased compression is worth 10 HP, the intake 15, and the cams another 15. The heads account for very little but that is just my opinion.

My personal opinion is this. Taking the heads off to add less than one point of compression is not the most effective use of money. Do it when you need to build a shortblock or when there is something wrong with the heads.
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Old January 25th, 2011, 01:51 PM   #15
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thank you for your opinion. the only reason i'm hesitant on going the PI route directly is because the high compression scares me. i hear 10.5 compression and then i will not be running 91 or better octane so that has me worried. then again, are there any safe combos out there running NO2 with 10.5 compression or higher not on 91+ octane? i have a family and money doesn't go that far, especially in the military...
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Old January 25th, 2011, 09:08 PM   #16
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Originally Posted by delgadog View Post
thank you for your opinion. the only reason i'm hesitant on going the PI route directly is because the high compression scares me. i hear 10.5 compression and then i will not be running 91 or better octane so that has me worried. then again, are there any safe combos out there running NO2 with 10.5 compression or higher not on 91+ octane? i have a family and money doesn't go that far, especially in the military...
If you can't afford to run premium fuel on a daily basis, don't deck the heads. That is the trade off. You could pull lots of timing and then only run premium fuel at the track. I suppose that would work.
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Old January 26th, 2011, 12:06 AM   #17
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Why would you "deck" a set of npi heads? While they are off throw them away! Since it is already apart just do the PI swap! 10.5 isn't a lot of compression, certainly not enough to scare you, decking the heads will put you around 10.5 anyway and you are still stuck with the small port/small valve heads with no potential. You can get a tuner , which you'll need one anyway, with multiple octane tunes, and sounded like you are planning all the other stuff /intake etc. Just do a PI swap-Mod motors are pretty bullet proof, unless you get too greedy with the no2!
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Old January 26th, 2011, 12:10 AM   #18
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^ i was waiting to read someone suggest PI heads to bump compression. One thing about decking the heads of a mod motor is you have to worry about the front cover fitting properly also.
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Old January 26th, 2011, 12:25 AM   #19
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Intake too! shave heads and it changes the distance between the heads, gotta shave intake, can you do that with plastic? safely? And, the valve are that much closer to the pistons, so if you cam it, gotta be careful
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Old January 26th, 2011, 01:15 PM   #20
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so 10.5 compression and a 75-100 shot won't hurt the motor?? thats all i'm worried about, i'm not opposed to do the PI swap, i was just saying if decking would yield similar results close to a PI swap. i plan on going full bolt on with a 100 shot, i just don't want to blow the engine when i use the juice.lol.
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