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Discussion Starter · #1 ·
A friend has a connection to a machine shop and can get my head decked/milled for $50. Question is, if I raise my compression at all, how "bad" would my emissions be?? How dirty will my car be?
Also how much should I get shaved for maximum hp "boost" without making the head less reliable?
 

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Discussion Starter · #2 ·
Re: Raising compression.........how much???? (RacinJetta)

BTW I spoke to Jeff Moss and he suggested .0020" Anyone agree? Disagree??
My setup will be Autotech 270, Autotech Adj. cam gear, Autotech H.D. valve springs, GIAC (in near future), EuroSport coolflo intake and the exhaust........headers are "possible".
Also could anyone make a "guesstimation" on my "final" power output?? (To the wheels of course......)
Any help is much appreciated. Thanks guys!
 

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Re: Raising compression.........how much???? (RacinJetta)

I'm hearing conflicting statements regarding this. I was/am getting my head decked to give me around 11.2:1 CR, which is about the max my VW tuner guy said I'd want to go in my car. However, others have told me that isn't feasible to run......that around 10.5:1 is about the max you'd want to do for a street car.
 

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Re: Raising compression.........how much???? (VW97Jetta)

Generally on a naturally-aspirated motor, the max you really want to run is about 10 to 10.5:1 compression ratio on 93 octane. Any higher compression and you won't be able to escape detonation problems (i.e. holes in the piston) without bumping up the octane. If you've got access to 110 octane or Av gas, go for it, but plan on tripling your current gas expenses. And no, I don't think Super 104+ is going to do it.
 

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Discussion Starter · #5 ·
Re: Raising compression.........how much???? (FrameRotBlues)

quote:[HR][/HR]...about 10 to 10.5:1 compression ratio on 93 octane.[HR][/HR]​
Is that what millin' off .0020" will get me??
Another thing.......what CR can I get away with running 91 octane.......it's the highest I've seen locally in West L.A.
 

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Re: Raising compression.........how much???? (RacinJetta)

First off, I assume you meant 0.020" not .0020" as .0020" would have not decernable effect at all.
You already have about 10:1, .020" would give you about 10.25:1, definitely not worth the bother unless you have taken the head off for some other reason. FWIW, there's alot of meat on an 8V head, it is be possible to take off as much as .100" without serious compromising the structural integrity of the head.
 

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Re: Raising compression.........how much???? (RacinJetta)

RacinJetta,
You mentioned that you are "rebuilding the head" and I assume you mean also that you perhaps recut the valve seat and lapped in the valves. IF so, you need to cc each chamber for volumn to determine where your compression ration really is. After the above valve work, it will actually be less than it was, not much, but some.
It is a close guess, but usually, about a .033" milling cut will get you close to 10.5:1, assuming you started with a chamber volumn of 30cc, which is what I measure with most stock 8V heads. You can get by on 91 octane if your cooling system is in really good shape. I would stay with the stock timing. I would maybe only go .028", around 10.3 with 91 as you will typically pick up about .2 from carbon buildup.
Remember, the ONLY way to know for sure what you compression ratio is, is to cc the chambers after all you work is completed. Then match all the volumns up to the chamber that has the greatest volumn by carefully removeing material from the chambers to get them to equal to about +/- .1.
 

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Re: Raising compression.........how much???? (RacinJetta)

M
Also could anyone make a "guesstimation" on my "final" power output?? (To the wheels of course......)
Rule of thumb is, for every full point you raise the compression, your hp gain is somewhere between 5 - 7%



[Modified by jettafahrer, 4:14 PM 1-27-2003]
 

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Re: Raising compression.........how much???? (jettafahrer)

just as a point of reference.
the acura rsx type-s has a compression ratio of 11.0:1 and it only requires premium.(91 octane or higher)
so 11:1 is feasible for a street car... maybe not necessarily a VW 8v engine but if the fuel/fuel delivery is good and the ignition is tuned well you should be fine. SHOULD I said, not will (thats my disclaimer)
 
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