the h75 I had didn't turn out that well, so i got hold of a complete gx head, i'm going to fit the h75 valves as they are larger than the gx ones. how much must i mill off the head for a good streetable compression? would 10.5:1 or 11.0:1 be fine on normal lrp 93? the cam is standard a14 atm, but my profiled cam will be back in about 3 weeks.

Rather than mill your rare GX head, perhaps look into flat top pistons or dome-top pistons. These increases in the volume of the crown will affect the compression ratio significantly. Some people argue that the dome tops do not provide optimum combustion characteristics, but they were offered by Datsun Competition Dept and they were used extensively throughout the racing groups, so they are worthy.

Also, a thin head-gasket will influence the compression ratio significantly, meaning that if your intended comp ratios are too high or you have limited access to high octane fuel, you can always resort to a thicker head gasket and not have any regrets about milling too much off your head face (it is practically impossible to replace the material you have machined off).

Ignition timing will also dictate what compression ratios you can successfully run. Personally I think 10:1 is about as far as you'd want to push it with 93 octane (ron?) fuel.

I wish I could offer you one of my copper 0.7mm head gaskets but I haven't even made them yet so I cannot help there. Bprojects offer a proven race head gasket that is probably better tan a copper equivalent, not sure.

What went wrong with your H75 head? They get a lot of power out of them.

Flat tops with the extra-thin stock GX head gasket give 10:1 on the A12.

Thin gasket helps burn better (more power + more economy) by increasing the quench.

Yea, I agree. Put in the bigger valves,etc but don't mess up a GX head by milling it to the max. They are very rare and just to say you have a stock one is good enough in my books. If you start to mill it to the max, it will loose it's value and also become useless for future usage.

the guy i sent the h75 to made a mess of the ports and i'm not willing to modify the su intake manifold as these are more rare than a gx head. i will get a set of flat tops, the mazda 1.6 pistons works as far as i know, just need to modify the conrod to make it fit on those. thanx for the info.

i was told by someone 11:1 compression is very high and to only run 10.5:1 compression or thing start to #OOPS# up? i dont no could be wrong

First......yes don't mill the head (even Nissan Motosports said this is not the way to get compression)

For reference my Volvo 142E sported 10.5 - 1 compression and on 92 octane pump gas it tended to ping accelerating up freeway on ramps during the height of the summer. It's 107-112 F that time of year in Vegas. So I'd say 10-1 would be max for readily available pump gas.

Tom

guys, a question here, little bit out of topic.


lets say i overtorque the head, using the stock head gasket, does that means the CR is increased a little? any harm doing this?

Overtorquing the head bolts will not increase compression. It my lead to bolt failures.

yer it is not a good idear to overtorquing the head bolts they might brake or stretch,