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Old 07-12-2011, 01:35 PM   #1
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Default Milling piston dome to lower compression ratio?

Looking at building a reliable B20Vtec for daily use and i was considering RS machine pistons.

The only problem is that they would put me at too high of a compression ratio for 91 pump.

The question is, is it an acceptable practice to machine the dome/valve clearance portion of the piston down to reduce compression?

I havent been able to find thicker head gaskets for the B20.
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Old 07-12-2011, 01:41 PM   #2
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Default Re: Milling piston dome to lower compression ratio?

I thought those pistons are only in the 11's? 12:1 can definitely be done on pump without a problem.
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Old 07-12-2011, 01:48 PM   #3
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Default Re: Milling piston dome to lower compression ratio?

cometic makes thicker head gaskets. give them a call.

as far as compression goes, buy the pistons first and then see what you get. you should be within the compression range that they provide.

from my research, getting "exact" specs for these pistons are not as easy as most think. rs machine themselves are no help. you can try using different inputs that would yield more or less the number they say you will have but it would be better off if you did everything yourself to figure out exactly what your working with.
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Old 07-12-2011, 04:53 PM   #4
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Default Re: Milling piston dome to lower compression ratio?

Plenty of people use these pistons on 91 oct... Has the block been decked to much or the head milled??? Only real reason I can see why you can't use pump... If all else fails get a cometic or u can add a layer or 2 to the headgasket
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Old 07-12-2011, 05:34 PM   #5
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Default Re: Milling piston dome to lower compression ratio?

I have those pistons and I'm tuned on 91.
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Old 07-12-2011, 08:06 PM   #6
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Default Re: Milling piston dome to lower compression ratio?

main things to be reliable are.

12.0 or less on 91 octane
Good tune
Arp rod bolts,type r oil pump and water pump

anything past taht would be preference on how much power you really need.
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Old 07-13-2011, 06:01 AM   #7
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Default Re: Milling piston dome to lower compression ratio?

I was hoping to just be in the 11.0:1 -11.3:1 compression.

They state that i would be 11.7-12:1 (depending on head, so with b16 head i assume 11.7. gsr being the 12.0)


I dont want to push the limits on 91 incase for some reason i get a bad batch... for example on the drive to my cottage in some strange little town.

i will contact cometic but i think i need a 1.4mm headgasket to get the compression i want, which would change the cam timing fairly signifigantly.

is there any differences of the typeR oil pump to the EM1 b16 oil pump?


Edit.... down the road i would like to add some skunk2 pro2 cams (i think that is the best for a 2.0L?) and dont want to have any issues with piston to valve clearance, so maybe the headgasket would be best...
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Old 07-13-2011, 11:43 AM   #8
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Default Re: Milling piston dome to lower compression ratio?

I would leave compression where it is. If you're worried about a bad batch of gas killing your motor at 11.7:1 ratio; 11.0:1 ratio isn't going to save you.
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Old 07-13-2011, 11:55 AM   #9
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Default Re: Milling piston dome to lower compression ratio?

Yeah a bad batch of gas is going to kill ya either way. Also don't fill up in some strange little town if they have el cheap o gas. I literally will only put Shell 93 octane in my car. If I am not in a familiar area and I am running low, I GPS Shell and will drive 10 miles out of the way to fill up.
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Old 07-13-2011, 07:05 PM   #10
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Default Re: Milling piston dome to lower compression ratio?

You're over thinking it.
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Old 07-13-2011, 07:41 PM   #11
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Default Re: Milling piston dome to lower compression ratio?

just build it with those pistons and enjoy 0.5 point of compression isnt going to make any real difference
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Old 07-14-2011, 12:51 AM   #12
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Default Re: Milling piston dome to lower compression ratio?

Quote:
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You're over thinking it.
x2.
my friends b20v with rs machine itr 84 mm pistons, s2s2cams on base map calli **** 91 oct put down 204 whp and ran 13.3s on slicks on a base map.
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