hi everyone, want to know what would be a good scheme for my h75 head, could not get hold of a gx or h89 (i know hey are much better) but have to work with what i got. i want to get compression good for street use, say about 10.5:1 if anyone could tell me how much to take off the head. i've already got double valve springs for it and getting it flown and match ported for the twin su manifold. another question is how light should i go with the flywheel, i know it loses some power, but i need the thing to rev up fast. any help is greatly appreciated

The H75 head has a big cylindrical dish around the combustion chamber, I can calculate it's capacity but that'll have to happen tomorrow.

I've recently gotten rid of my race GX head in favour of a smaller port (but ported) H75 head. It has been machined to remove the large dish so the combustion chamber shape is similar to that of a GX head. Machining this off brings the comp ratio right up, I can't help you with calculating what you'll end up with until you give some more details over; pistons dish, or fly-cut cc; piston height @TDC relative to deck height; head gasket type (standard supplier or thinner custom item), bore size, stroke, comb chamber cc etc, valves (stock or dished type or flat-top) etc etc

10.5:1 is perfect and should behave on most pump fuels (slightly dependent on spark timing) unless you get some real low grade stuff.

Flywheel? I can't remember the spec of mine. You don't really lose power but I get the inertia is somewhat affected. I have a little trouble with my motor getting off the mark, especially on a hill, everything is lightened and the cam doesn't wake up until about 3500-4000 so a hard start is somewhat sluggish with the cam and lightened internals + flywheel. Look up user name lemonhead and have a look at one of his recent posts where he talks about his flywheel, pretty sure it's on his race car but worth a look as I think he details it's weight.

thanks, i'll look into that. i've seen a few threads and most people seems to be going for around 5kg's on the flywheel, just want some more input before i jump into it. as for the head i have no idea if 2mm will be enough as i saw someone taking 2mm off an h89 which compreesion is already a lot more than the h75

had a look at lemonhead's thread. his is just under 3kg's, which is too much for a street car i think. guess 5kg's should do the trick

The H75 is only 2cc bigger in each chamber (which equates to about one half point (0.5)) of a compression ratio, so they are different but certainly comparable. SO for every 2cc you remove you drop 0.5 of a CR unit.

I think that shaving the H75 will achieve much more CR than a H89 if you were to shave it by the same amount because alot of the H75's combustion chamber CC's are within the volume of the cylindrical dish. The big key is to get rid of that ghastly cylindrical dish as this does no favours for an effective combustion process.

Here is a link:

True Or False

do you maybe know how much shaving is 2cc 's? eg 1mm removes 1.5cc

No you'll have to (and hope for) a mechanics response. I'm a student!

The 2CC/0.5CR unit is approximate so don;t go by those figures.

Seeing as the cylindrical dish is a cylinder we have a constant shape from which we can calculate volume (as you reduce it's size). Simple mathematics really, calculate the volume of the cylinder by determining it's radius and depth. And then go from there.

I have the calcs for my H75 cylinder head but I've left my book with the machinist.

Check Dodgeman's profile for all the pics he posted. There is a scan for the lightening of a flywheel to the specs of the Datsun factory racing team's specs. Much safer than a guess.
Also, try and get a A12 flywheel which is already a bit lighter and get it lightened to these specs.

i checked his thread, most people is talking about 3 kg's. i'm settling on 6 kg's, have to cut the 1400 flywheel as i can't get hold of a 1200 one and this thing needs to get done asap

I think the one in his profile of Datsun racing team is something like 5,5kg.