top of your list should be a pertronix electronic ignition conversion or a straight across conversion to an 85 Isuzu 1.9L distributor goes right in place of the stock one and has no points to wear out or need adjustment, and handles more power
the aftermarket intakes are sold here on the forum the sellers name on here is mokadeth in the search bar enter offy intake or offenhauser and it should come up
sleeves idk about, the stock block isn't sleeved so that would be extra machine work and custom sleeves
forged pistons there are no off the shelf ones would be a custom set, but the stock cast pistons are very strong for what they are and can actually stand up to 10psi and more of turbo boost so really no need for forged ones.. check out rockauto.com for stock rebuild parts
head work for mine including having the surface milled and valve ground was $250, if you bring your engine block down to the machine shop they can hot tank it along with your head and get any rusty crap and any gunk out of it inside and out
delta cams and crow cams are the ones who people usually go to for cam grinds not sure of the price i never went that route
if you plan to port the head be VERY careful there isn't a whole lot of material you can remove before you get into the water jackets, maybe smooth things out a bit but don't expect to make the ports much bigger. you can however pay to have bigger valves installed, the 1.9L valves can be installed as can the 2.3 I believe but those are probably overkill without a turbo and may be a pain to get in there
if you are looking for the info about pulling more power out of these engines feel free to send me a PM and you should go join ozgemini.com it is an Australian forum for the holden Gemini which is an Isuzu car made the same time and with the 1.6L version of our engine but those guys do all sorts of crazy stuff. so far the record over there is decently north of 600whp with the 1.8L and a BIG turbo
there isn't a lot of off the shelf performance parts for these motors but the aussies have it figured out pretty well using the parts from other misc Isuzu's, you can but a full bolt on efi kit, full bolt on turbo kit, full bolt on turbo and efi kit.... lots of misc from Australia. not much here other than oe stuff and the stuff you already have , somewhere in AU they know of double valve springs you can install
as far as sbc swaps go there are bolt in mounts and trans mounts to make it fairly straight forward, overall cost depends on how you do it but you would be looking at mounts, trans mounts, headers from the same company that made your headers either powerbyace or hooker, you cant get one set of mounts and the other set of headers. you will also need a bigger aluminum radiator, full exhaust system, and a few misc small things to make everything work. my 4.3 swap cost me a few grand but I went a bit overkill with the engine build I could have done it a lot cheaper if I just went with a stock carbed 350 but I was being stubborn about saving weight... as far as the swap kits go powerbyace's owner is a member here on the forum and is still active on here so that is where most of us go and I got my stuff real quick from him when I ordered it and the fitment was perfect
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1980 Mikado-Sold.
1993 Lexus SC400 my Daily
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