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Compression Ratio + Rebuild Kits

dezrik

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Hey guys,

I am in the process of rebuilding a blown 308 bottom end from a VN (I know they were 304's but this one came with ACL stamped pistons, a 308 crank, and a 40 thou oversize so I'm assuming it's a 308? It was assembled as such in my Berlina anyway and appears to have blown from a lack of servicing.)

Anyway, I'm not too up-to-date on bottom end stuff such as compression ratios and the like and I am looking at an ACL rebuild kit.

I want to up the bottom end power wise and work on all the extraneous bolt-on shmaltz later on.

As far as I understand the stock compression ratio was 8.4:1 and the pistons in the ACL rebuild kit are flat top (what's the difference with flat top... all the more expensive ones seem to be flat top) and have a compression ratio of 9.3:1. Do the pistons in the kit basically give me an instant compression ratio increase?

How do I get my compression ratio to 9.5:1 - 10:1?

I am not too familiar on ways to increase the compression ratio, besides having read about head shaving and the like.

Anyway, there's a collection of ACL rebuild kits, from the basic kit to the ACL Race Rebuild Kit (which is about $940 or so) at Gimmecarparts.com.

Is the Race Rebuild kit worth the extra dosh, and in what way? I take it is, especially for future applications, but I'd like to now the 'how' as some of these things are a little out of my depth.

Anyone know of any other options I have with rebuilds/kits/pistons etc. and any other places to find them cheaper?

Phew that was a long winded bloody question without about 20 mini questions. Thanks guys.
 

geardys

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the piston that i think u'll need is the ALC 9490
this is a flat-top H.comp piston that will give a comp ratio of 9.3:[email protected] its also weighs 564gm and the Assembly with pin & pre-gapped rings part number is 8MKRY9490.

the comp ratio is based on the std head and nominal bore diameter.
the weight of the std old pistons are 765gm (part number 9088) so u save 100gms when using the 9490's.
also there ALC std replacement pistons is 90gms (675) lighter then the std holden one.

with less weight this will also make it easier to balance the bottom end.

also if its 40tho is would be more like 311 0r 312. i think.

the 9490 flat tops have a thicker/ different compression height at the top of the piston

the only real ways of increasing the comp ratio is as u said, shaved heads or pistons.
the type of camshaft will also effect the comp ratio by the way in which it opens/and valve overlap. usually a 'bigger' the cam will lower the comp ratio slighty. also turbo's or blower will also up the comp.
and i guess that if u use water injection that this will also up the comp by a very very very small amount seeing as though water is not compressable.

ACL Race Rebuild Kit are good value and if u can go the race kits.

also with a blown application and high boost try for at least cast pistons if not forged pistons and maybe h-beem rods or shotpeeded ones.

have fun and good luck
 

dezrik

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Thanks a lot mate, I'll go the Race Rebuild kit then... From memory I believe they were cast pistons when I was trying to read through all their literature up on the ACL site.

Some good info there, stuff I've been unable to find elsewhere, so I'll take some time to ponder over it all. :)
 

dezrik

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Another quick question, I've honed all the cylinders, and the block seems fine, but one of the cylinders has the tiniest lip on it after honing (and only around about a quarter of that one cylinder's edge.) It'd have to be a fraction of a mill if that.

Will such a tiny thing make any sort of major difference to integrity/performance of the block? As I said I'm not too knowledgeable on the bottom end, and I'm trying to build with price matched with performance and don't feel like getting another block, and am just wondering how much of a difference that would make. (Hopefully very minute.)

Oh, and while I think of it, anyone got any estimates on basically what upping the compression from 8.4:1 to roughly 9.3:1 as Geardys stated above would do in terms of horsepower/etc.? 'cos I have no idea what it'd do in quantifiable terms, all I know is that it should make it 'go harder.' ;)
 

geardys

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dezrik said:
Oh, and while I think of it, anyone got any estimates on basically what upping the compression from 8.4:1 to roughly 9.3:1 as Geardys stated above would do in terms of horsepower/etc.? 'cos I have no idea what it'd do in quantifiable terms, all I know is that it should make it 'go harder.' ;)
it may give u 5-10kw's more but it will also allow the engine to rev freely, especially @ higher rpm and also allow u to use bigger cams then stock comp.

as for the lip, try to make it as smooth as possibly. most limits are set to .o of a mm limits e.g .03 difference up to .07mm difference.

also clean and scrub the bores down after the honing to maximise the life of the engine.

race builder will use wet/dry (600) sand paper on honing stones then hand polish the bores to get an excellent finish. Allowing for maximum life.
anyway good luck.
 
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