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There are currently two P90 heads on ebay:
<http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1&item=7927293706&category=33696&sspagename=WDVW>
<http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1&item=7927236616&category=33617&sspagename=WDVW>

Both auctions advertise the heads as giving horsepower gains even when put on a N/A engine. Here's my question: won't it reduce the compression ratio and thereby reduce the engine's power output? I mean, that's one of the special things about the turbo heads isn't it? That they have lower compression than N/A heads designed for the turbo engines?
Why would they claim something that isn't true? Or is it true?

SO, will the P90 head (instead of say a N47 head on my N/A) reduce compression? yes/no
And, will this increase or decrease horsepower? yes/no
 

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if you have dished pistons then yes it will lower compression dramatically and reduce power output. However if you add flattops with that p90 you'll raise the compression to a reasonable level for an N/A and the better flowing head may give you some power gains.
 

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False all around. A p79 and P90 have the same size combustion chambers. The later ZX engines used flat top pistons on he NA to get more compression. On your N47 engine, it has dished pistons so you don't want one of these heads, unless you're going to install flat top pistons.

Also 1 fast Z here did some flow testing and with VERY minor port mods (smooth the area between the valve seat and the liner), a round port head flows as good as a square port so why bother unless you plan on doing SERIOUS port work to the P90.

As far as why someone who is selling something would claim things that aren't true? You just fall off the turnip truck?
 

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P90 and P79 CR ratios

P90 head will not just give you the compression you need if you just slap it on

Here is the compression chart that I will provide such that everyone can search it

Here it is for L28 eninge and 1mm head gasket

Shave Dished Flattops
.000" 7.52 8.71
.040" 8.13 9.59
.080" 8.61 10.28

Remember those are approximatelly and should not be take as gospel, but those are fairly close and the reason you want to shave those is for rasing the CR but might as well get the 84 maxima head the N47 which has the same combusion chaimber shape and big valves but will not require a crazy amount of shimming the head towers because the chaimber is 40cc vs 56cc on the p90 P79 heads
 
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