Shaving heads... VG30 Minissan ZSX6

[fastclown]

I searched for the info but couldn't find it. I'm looking at just doing the top end untill I drive the car for a while to see what it does. But while I'm doing the heads, what is the max limit for shaving a NA head, for more compression of course... What then would my compression be? NA cam experience/suggestions would be welcomed as well (this will be used A LOT on the street so nothing crazy). I'm running a 500cfm 2 barrel carb with a MSD digital 6 ignition (magnetic pick up on a stock dizzy). I have a set of the stock but modified adjustable cam gears. I also have a 150 shot of NOS. The car weighs about 1,850lbs. Any help would be appreciated.


Minissan ZSX6


240SX rear minus 12" from the center...


Thanks!
Mike

[SATAN]

I'm not sure the limit at which you can take it to increase your compression however. Unless you have slide adjustable cam gears you are not going to get your valve timing correct when you bring then heads closer to the crank.

Have you considered modifying the chamber of each cylinder to increase compression? Maybe welding up certain areas in the head.

[fastclown]

Very good point. From what I understand, advancing the cam timing will give more mid-range power... The Morgan gears go in increments of +/- 3 degrees, so what I lose by dropping the head I should be able to make up and add a little with these gears. But then if it doesn't work then I'll have to break down and get the fully adjustable ones. This whole project is a crap-shoot :shock:

[84z31]

wouldn't it be easier to get higher compression pistons.

i have seen 10:1 for the Z32.

are you using the W series motor. becasue you could build one with 10:1 compression. i haven't heard of anyone shaving vg heads. another very minimal gain in compression would be running a thinner head gasket. but then again you'll run into the problem of getting proper cam timing with the shorter distances.

[fastclown]

The engine is from a '92 Maxima with '80s ZX pieces swaped to make it RWD. So I would assume the casting would be the same as the W series engines. While at this time I'm not really looking at doing the bottom end, where would be a good source for 10:1 pistons?

Is there a difference in the heads over the years... 80's ZX vs. the Maxima heads???

[SATAN]

The problem with running the 10:1 pistons out of a z32 is that when you put them in a z31 the compression ratio is no longer 10:1 but now closer to 9:1. Which doesnt help him at all. He would have to get a custom set of pistons made for increaced compression over that of a stock NA piston.

never mind the z32 and the z31 view. instead use the vg30de and the vg30e view.

[Steve88t]

Also consider that using thinner gaskets or shaving the heads brings the valves closer to the pistons. I am not sure how close they are stock, but I would be cautious of making drastic changes using either method.

If there was a way to get 10-10.5:1 on a vg30e for less than the cost of a custom set of pistons I'd be all kinds of happy. I've got one engine that I'd love to go up on and have had little to no luck.

[Chris86NA2T]

the factory service manual says that 8 thousandths is the limit.. the head is physically able to be milled much further before you have uncorrectable issues.. if you have fully adjustable cam gears then you can mill the head as much as you want provided your valves clear the pistons.. all you would have to do is re-degree your cams with a degree wheel.

[SATAN]

For you higher compresion type's, you guys should seriously look into adding material to the combustion chamber. If you are willing to go as far as pulling the heads and porting them, then why not build up the combustion chamber in certain spots to gain compression and then smooth it out?

Here's what to do. get the vg30de pistons which are 10:1 (i think) in a vg30e i think it makes it slightly higher than 9:1. More like 9.2:1 roughly. Shave the head .020"(or do the math to figure out how much exactly to take out) to gain that extra little bit. Build up the combustion chamber with weld and smooth it out. Doing this I could see someone easily getting to 10:1.

To me this seems like a lot of work when you could drop 800 bucks or less on a custom set of pistons made to achieve the compression you want. You could run 12 or 13:1 if you felt the need to. You would be limited to race gas, but if its a race car then... :roll:

If somoene does do this, make sure you research what it would so to the quench area so you dont get negative effects. All of cource assuming you have slide gears to adjust timing correctly and not negate any progress you made with poor valve timing.

[fastclown]

Thanks for the replies/info. A friend is trying to sell me some revalved '80s VG30 heads, is there any difference between those and the '92 Maxima heads? Or are VG30's pretty much the same throughout other than the W series castings which I would assume would be the block...

[Andrevas]

VG30DE pistons are 11:1 w/ a DE head

you could go VG34 by picking up a VG33 and boring it for VH45DE pistons and DE rods which give ~9.7:1 and by using DE rods, direct drop in after boring to 93mm

[craZed]

DE = 10.5:1 cr

[Steve88t]

OK but do you know the chamber volume or piston dish on a DE?

Math is our friend...

Or I can do the math backwards to get them I guess.

[MachZ]

VG-DE(TT) about 44-45 cc

[Steve88t]

OK so doing the math as:

Bore 87mm
stroke 83mm
gasket thickness .040 *probably wrong*
0 deck height *again prolly wrong*
piston volume 1cc *for valve reliefs*
combustion chamber 55cc and 45cc

Even if the numbers are wrong, when you change the chamber volume you get 9:1 or 10.5:1. The difference being 10cc less in a DE head. So I can have a set of heads welded up 10cc's and get roughly what I'd want for the engine.

Does anyone have more specific numbers to correct the math?