boring/overbore - is it worth it?

[DenZel]

hey guys, i was going to get pistons later down the track, but have found some at a very good price, so might grab them now and hold on to them until i use them.

now im a nooby, so please be patient.

current compression ratio is 10:1 , and thinking of going 11.5:1, but i need to know what i should do about overbore

should i bother? only pros i can see is the VERY little extra displacement, and to freshen it up a little while matching the bore size.

if i just go same bore-size, will my higher compression ratio pistons just be a straight drop in?

or i could go 20thou overbore, what else would i have to do when installing these pistons, whats invovled? and HOW MUCH?

either way, when installing new pistons, most people clean up/machine the block either way, hence they go oversize pistons?

thanks guys.

[Lump]

depends on the condition of the bores now & the size of the new pistons (or can you get them standard size or oversize?)

are you experienced enough & have the equipment to asses the condition of the bores?
how do they look? any lip?

they might be fine with a ridge-cut & light hone if dimensionally ok, but if in doubt may as well overbore.
imo the extra displacement is not a reason alone for a overbore unless $ are not important.
bore condition & ring/piston performance is, so yes generally safer to just go with bore/oversised pistons

[DenZel]

i can get the pistons to whatever spec i want, so im wondering what to do, standard size or go 20thou.

wont be looking at the bores, i wont be putting in these pistons in for awhile, just thought id take advantage of this deal.

[Lump]

how many kms has the motor done?

might be safe just to buy o/s now

[DenZel]

been told 120,

what i was thinking, better off to just go bigger?, as im not going to know the condition until i plan to do the mod.

as if i have to do the bores either way, then the advtange of going the 20thou would just be the slighty bigger capicty?

why is it generally safer to just go with oversized pistons?

why wouldnt it just be better getting standard size pistons, and just cleaning up the bores a bit if i had to, compared to going oversize? then it would just be the increase in capacity difference?

[joshg123]

You wont notice a difference in power due to capacity.

You will however, be able to clean a mildly scratched bore or a bore with a lip with a 20 thou piston, remember this is 0.5mm across the diameter, its not much!

And you give your pistons to your machine shop before they do it anyway as each piston and ring with have different clearences depending on brand and application.

I go 40 thou with most of my motors as blocks are easy to come by, however if blocks are expensive for whatever engine you are doing, go 20 because with most engines pistons will come STD +0.5(20thou) and +1mm (40 thou), depending on wall thickness etc etc.

[matty12]

i'd get them in a oversize,its easier to bore the block to suit pistons than to possibly have to find a block with very little wear to slip in standard sized piston.

It will cost little more for a bore/hone but atleast you will know they will be right.

Sounds like your going for a killer n/a.Start saving for a set of cams,yourve got four to save for...

[DenZel]

cant i just bore/hone to standard size again? or not possible if damage already done?

sounds like 20thou is the best bet.?


also is going 11.5:1 to high on 98 fuel? or should i stick with 11:1.

i want to go a high compression, but still safe and durable. i want to go as high as i can, but still last for years and years.

how much does it cost to bore/hone the block?

[matty12]

you basiclly gain size through wear so the bores get larger and larger.You may get away with a hone to refit standard piston but its to much of a risk to spend 1k or so on pistons that may not fit so you cant use.

11.5:1 is only one1/2 steps up from factory(and only 1 from the 95 motor onwards),im cheap,i run 91 octane on 10.1:1 comp ratio that it comes with from the factory although there may be abit of timing pulled out so it doesnt ping.I dont think its like the old days,i thought 10.1:1 was huge on my 308 but shes cast iron which i think makes a difference over ally block as its also able to remove heat far better to stop preigniting etc.

[DenZel]

yeh i know the later models used 10.5:1, so im thinking 11.5:1 should be safe, and last for years to come? you only run 91 lol, says 95 is recommended.
its also more the fact i want it to last aswell. they say anything like 12.5:1 should be used on avgas.

[swordfish]

cant delete post?

[Smoked]

^ that was me by accident im at a mates house it logged me in as him..

if your going compression up then i suggest the oversize.. its basically like giving it a brand new bore surface..

but my mates (swordfish) N/A 31 skyline he put 20 thou over ACL "high compression" pistons in there,, with stock rods.. and the compression wasnt really affected that much as far as we know because the idles not very angry and the performance didnt really increase much. and thats with a crow cam as well.
stock ecu though with turbo injectors so.. tune would probably help. resizing rods might increase the compression more i dont know.

its got me beat

[Simon]

denzel easiest way to increase cap in 1uz is to ft 2uz crank and crown rods with oe 1uz pistons, gives you capacity of 4.4l and 10.5:1cr
then if u want face the block and heads ( not sure how much)
or buy bullet cars race heads and cam package to take it to 11.4:1cr
skids

[Sciflyer]

increasing the CR by a significant amount is a waste if you dont get cams to take advantage of it

plenty of off the shelf cams available now too (assuming we're talking about the 1UZ)

[matty12]

Smoked,are you talk compression fiquires(with a compression tester)or compression ratio as the are to different things.

[DenZel]

the reason im getting higher comp pistons is becuase to match the cams i want later down the track. i dont care much for capicity gains, just wondering what were the pros/cons.

sounds like 11.5:1 should be fine on 98 fuel, could i go any higher? or that will reduce my durabilty and not be too safe.

ill be going 20thou oversize, cheers for that info guys, now how high is too high for comp on 98 fuel haha.

[Simon]

itd be best to stay around 11.0cr. stock 10.9:1 1uz will ping on a hot day (30+) with 98 and chew a fkload of it

gotta watch cr vs octane, i had an old honda (cough, shame)
bored and stroked to 1.8L, 12.2:1 cr, and dirty cams and i had to run a mix of Avgas and 98 all year round. the economy was fkd worst was 5L got me 16ks

98oc is a minimum in a stock 1uz as they desinged to run on super rich japo fuels.

cams ul def need higher cr ratio and some qualtiy tuning prob kiss any chance of any economy goodbye

best bet speak to bullet cars being they design and sell all the good stuff they will no wats wat for 1uz street cars

[Simon]

OR just stay standard bore+stroke, change from crappy low cr bowl pistons to a set of quality flat tops, get some decent head studs, gaskets and a tune
that would be easiest and the most reliable option.

[DenZel]

yeh will fuel economy wont be too much of an issue, as once the build is finished will only be a weekend car.

cheers for info

[Smoked]

Smoked,are you talk compression fiquires(with a compression tester)or compression ratio as the are to different things.

i was talking about the compression ratio,, the figures we got with the compression tester were actually pretty low though..