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need some help about compression 

 

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 Post subject: need some help about compression
Posted: Sun Jan 07, 2007 6:29 pm 
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hey guys im building an au motor to go into my ef, i am looking at a 546 cam from a mate. the head is getting port and polished and heavy springs put in. the cam requires around 10.5-11:1 compression. if i put in acl race series pistons seen here http://www.aclperformance.com.au/6MKR9400EF.htm (the 20 thou oversized ones) with 20 thou off the head, and leave the block as is with an au head gasket, will that bring the compression up to that 10.5-11:1 compression mark without any problems with valve clearance?
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Posted: Sun Jan 07, 2007 6:36 pm 
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i jsut put those pistons in had 20thou taken off the head, the pistons come up flush with the top of the block and a acl gasket and the engine builder worked out my compressoin to be 9.8...

 

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Posted: Sun Jan 07, 2007 6:54 pm 
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aren't 20thou oversize pistons 20 thou bigger diameter to suit a re-bore ? flat tops raise compression. i was under the impression oversize pistons are for flogged out recondition jobs.
and shaving the head won't bring the pistons any closer to the deck height of the block, closer to the cumbustion chamber yes, increasing compression, yes, but no difference in position inside the block unless you get that decked.

 

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Posted: Sun Jan 07, 2007 6:58 pm 
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increasing the size of the piston increases the displacement

there are a couple of different types of pistons u can get..but for ef/el the ones we have got raise the compression to 9.6, they dont make flat tops to suit ef/el cons rods

the pistons increase the compression by having a smaller dish in the tpo of the piston... if i recall correctly these ones have a 5.8cc dish and standard have about 11cc dish

so no they arent just for flogged out applications...

if u sahve the head u are decreasing the size of the compression chamber so it will increase the compression....

u really dont have any idea about much, u should read up on things b4 giving ur opinion

 

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Last edited by Pulco on Sun Jan 07, 2007 7:02 pm, edited 1 time in total.

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Posted: Sun Jan 07, 2007 7:01 pm 
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Oversized pistons are for when you want to square the bores up. An engine with oversized pistons is generally much better than one that has been honed and had the stock pistons thrown back in. Also if you are blueprinting an engine you dont have much choise other than to go oversized.

The pistons you have there are for pre-AU engines. You could use them, but you would have to get a set of rods from an e-series. ACL have a near flat top piston with .25cc dish to suit AU rods (part number 6MKRY9414S) that brings the compression up to 10.2 without skimming the head.

 

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Posted: Sun Jan 07, 2007 7:03 pm 
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http://www.aclperformance.com.au/Ford4. ... istons.htm

well considering its an au block, with an au crank and undecided rods that should fit? being flat top for more compression, should be able to get the fly cut and take 10 thou off the block 20 thou off the head, with an au head gasket? or are factory au pistons flat top?

edit: posted too late. so if i take 20 thou off the head, with an au gasket, will the pistons need to be fly cut for a cam with 524" lift and 290 duration (.075" = 230 in and ex)
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Posted: Mon Jan 08, 2007 3:03 pm 
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so the smaller dish (read cc) increases compression, which is why i said flat tops raise compression, since you can't get flat tops u get smaller dish. 20thou over diameter of a bore nearly 4" round equates to exactly how much more displacement ? 4.0 litre to 4.5 or something ground breaking ? it's only slightly more displacement for slightly more compression with the smaller dishes ? i was only askin a question not giving opinion thats what the ?'s are for.

 

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Posted: Mon Jan 08, 2007 3:23 pm 
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Final displacement for a 4.0 with oversized pistons is around 4.1-4.2L depending on how many oversized you go. But of course that is not why you go oversized, its all about preparing a quility working surface and clearances for the new pistons. And using the pistons from above the compression ratio will go from around 9.6 to 10.5, which is quite a big jump.

efgiar - you will have to collect all the parts and do a test assembly to measure the p/v clearance to work out if you need to enlarge the valve reliefs. My guess is that you will have to enlarge the reliefs.

 

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Posted: Tue Jan 09, 2007 1:48 pm 
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4.2 is more than i thought. nice. thanks stockstandard.

 

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