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fugg it
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Discussion Starter #1
has anybody had this done? any reccomendations as to how much to take off? i'm thinking half a millimeter tops, but that's just my guess. it's a usdm sr20 btw. what's the compression ratio on those, 9:1?

thanks
brendan
 

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SE-R Geek / Nissan Tech
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From the FSM-- "Do not remove more than .008" (.20 mm) from total cylinder head or block deck surface"

I'll get back to you on the new CR. I have some calculations to do.

EDIT: it started at 9.5:1
 

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fugg it
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Discussion Starter #3
any word on this?
 

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edit: I had #s, but....

i used .02mm instead of .2 mm for the mill distance. I'll get back to you....again.
 

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fugg it
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Discussion Starter #5
haha bummer. alright, lemme know. i wish i knew how to calculate that.
 

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The DET is in!
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Trepanation isn't worth it.
 

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fugg it
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Discussion Starter #8

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merely a flesh wound
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Trepanation isn't worth it.
Damn, you're not kidding!

Trepanation (also known trepanning, trephination, trephining or burr hole) is a form of surgery in which a hole is drilled or scraped into the skull, thus exposing the dura mater in order to treat health problems related to intracranial diseases, though in the modern era it is used only to treat epidural and subdural hematomas, as an extreme body modification, and for surgical access for certain other neurosurgical procedures.

C
 

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OK here goes...

(under the assumptions of a stock motor)
Bore: 86mm
Stroke: 86mm
Combustion Chamber: 46cc
Avg. Head Gasket Compressed Thickness: 1.25mm
Avg. Deck Clearance: .500mm

Starting Compression with those #s: 9.89:1 (probably shoud have assumed a larger HG, cause the FSM states that its 9.5:1, although I didnt compensate for valve reliefs, if there are any in the stock pistons, i dont remember...


So, if you remore .2mm from the deck clearance (now .3mm) and the head (now 44.838cc combustion chambers), the new compression ratio should be: 10.27:1

Now....to protect my ass...

these are just example numbers and your actual results are almost guarnteed to vary, especially since nissan uses different sized pistons per each cylinder, the headgasket thickness is bound to be different, the actual combustion chamber is a bit larger, but i dont recall the size, and i dont remember if there are valve reliefs in the stock pistons.

anyway, if you are to do this, get everything blueprinted to be sure of piston to valve clearances and max camshaft allowances, among other things.

so if you have any other questions, just let me know, i'll see what I can do. happy machining!
 

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fugg it
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Discussion Starter #12 (Edited)
OK here goes...

(under the assumptions of a stock motor)
Bore: 86mm
Stroke: 86mm
Combustion Chamber: 46cc
Avg. Head Gasket Compressed Thickness: 1.25mm
Avg. Deck Clearance: .500mm

Starting Compression with those #s: 9.89:1 (probably shoud have assumed a larger HG, cause the FSM states that its 9.5:1, although I didnt compensate for valve reliefs, if there are any in the stock pistons, i dont remember...


So, if you remore .2mm from the deck clearance (now .3mm) and the head (now 44.838cc combustion chambers), the new compression ratio should be: 10.27:1

Now....to protect my ass...

these are just example numbers and your actual results are almost guarnteed to vary, especially since nissan uses different sized pistons per each cylinder, the headgasket thickness is bound to be different, the actual combustion chamber is a bit larger, but i dont recall the size, and i dont remember if there are valve reliefs in the stock pistons.

anyway, if you are to do this, get everything blueprinted to be sure of piston to valve clearances and max camshaft allowances, among other things.

so if you have any other questions, just let me know, i'll see what I can do. happy machining!
haha thanks man. just a few questions.

my head is getting a p&p and blueprint right now, what is the "deck clearance"? i figured i'd just be getting the combustion chamber side of the head milled, or is that the deck clearance?

also, there are no valve reliefs on my pistons (i'm guessing those are the little slots on the top of the pistons, like honda's have) so should those numbers actually be a little higher? the motor is stock.

and if i was running around 10.2:1, should i get a high performance HG or should a stone brand HG be fine?

thanks for the numbers!

brendan
 

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deck clearance is the amount the pistons are "in the hole" when they are at TDC.

you need a certain minimum clearance between the piston crown and the cylinder head to all for rod stretch etc when at the redline. typically about 30-40 thou
you seperately need some clearance between the piston crown and the valves to allow for valve bounce AND rod stretch at the redline. typically about 60 thou?

what cams do you want to run, that will dictate how much flycutting you might need. if you want to copy the cut that an SR16 piston has you will be removing about 1.5cc's

regards,

Mike
 

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deck clearance is the distance from the top of the piston to the deck of the block at top dead center I have no idea why i added that.....apparently i thought you mentioned getting the block milled, also. im retarded.

anyway, i didnt know that it removes more than just a few CC's from the head (see post #11) cause I havent had mine done, i just know the math from school and assumed the removal would be a perfect cylinder, which in retrospect is not realistic.

SO, with all that corrected...

Bore: 86 mm
Stroke: 86 mm
Deck Clearance: .50 mm
Combustion Chamber: 40.6 cc
Head Gasket: 87 mm Bore x 1.25 mm compressed thickness (~7.43cc)

Compression Ratio: 10.84:1

as for the HG question, I would use a performance one, and if youre going for the maximum compression ratio without modding the bottom end, get the head milled all .008"/.2mm and get the smallest compressed thickness headgasket with the smallest bore (thickness is the most important)..... i think that wiseco's is 88.5mm x .762mm, which is 4.687 cc, its one of the smallest ones available, IIRC. HKS's 87 x .8 is 4.755cc, just as a comparison.

Edit: I just looked on GSpec (GregV's company) and he can get a Cometic thats .686mm (he also has alot of others available) compressed thickness in a number of bore sizes, so if you got a .686mm (.027") x 86.5mm it would be 4.030cc's.

That would bump the compression up to 11.45:1

Edit 2: I recomend getting valve reliefs cut into the piston and possibly having the engine blueprinted to get the most out of it

Edit 3: This does not take into account piston dish/dome volume or valve relief volume.
 
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