Lowering compression ratio and head gaskets

[RobertoX]

Hi all...

my 4ac is still going strong, but i would like to build a 'more sikker' motor for a few more killerwhales

i have another 4ac which cost - $0

im 99.999% sure 4age headgaskets work with 4ac

i have a hks metal thing 2.0mm which owes me fuck all

i was wondering if any one had any suggestings for comp ratios (i want to run about 0.8 bar)

4ac factoty comp ratio = 8.8?

what will it go to with the 2.0mm?

is 2.0mm too thick? bearing in mind i want decent response and the car gets run on good fuel etc



any other ideas for mods while it is apart

bearing in mind this is a super super low budget project

thanks

[mattysshop]

Dave - yes.. HG will work with 4ac

also, if you really wanted to lower the CR.. pull the head off, and give the combustion chamber a but of a grind.. there should be some flats etc you can carve out.. even just a few mm here and there can lower it a bit, plus practice porting a cylinder head is an added bonus.. especially when you can get them a dime a dozen..

[RobertoX]

thanks, you could prolly actually get them for a dime a dozen

[mattysshop]

lol i sold 2 complete 4ac's running from fan to flywheel for $50 on the weekend.. so i wouldn't doubt it.. also.. if your intersted i have a multipoint EFI setup for a FWD 4A...E...?? bolt on, just cut and shut had injectors, t/B, and tps.. ready to roll

[RobertoX]

scratches chin, i will reply here when some more ideas etc come through my head lol


for now post away ideas

[psychofox]

why bother doing anything to the internals of the 4ac. just run the 0.8bar boost and see how long it lasts, given that the stock comp ratio is pretty low at 8.8:1, as long as you can give it enough fuel and you don't rev it above the factory redline, I reckon it will probably hold together for a fair while.

[Dems]

dave when you gonna come pick up the sick 4AC. it is waiting.

[Medwin_3sGTE_AE86]

0.8bar... fark that..! 4ac's are tough as... did u know that RB26N1 are not really RB26s... they are really just one and half 4ac's welded together with some nissan markings slapped on... she'll handle 1.5bar no worries...!

[RobertoX]

im not 'doing' anything to the internals, just a quick freshen up so it holds together to some serious thrashing and considering the shit caning it has had so far 220,000kms, pinging, 20psi, 7000rpm etc i wouldnt mind a bit of piece of mind

what CR will the motor be if it is 1.5mm head gasketed?

[Big T]

Hey Dave,

If you're planning on using a metal hg, you'll need to deck the block/head so you'll be shaving off 0.1-0.2mm (depending on how much machining it needs) I don't know what the stock 4ac hg thickness is but you'll have to work it out from there using one of those comp ratio calculators. Im sure there are head specs for the mighty 4ac floating about somewhere.

Alternatively, there is a test you can do AFTER the motor has been put together. My cousin was getting paranoid about my comp ratio thinking it'll be too high when putting my engine together so he had it tested. With GZE 8.9cr pistons and a HKS 1.6mm hg, my actual cr is about 9.08:1. This is due to the decking of head/block. I could've got the 2mm hg but at the expense of having worse squish areas so i decided against it. It'll make for a better motor off boost anyway.

Eddie.