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OHC 4L max compression 

 

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 Post subject: OHC 4L max compression
Posted: Tue Jun 09, 2009 11:08 am 
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Hello , trying to find out the max comp people have achieved on the 4L with flat tops with no clearance issues. Im looking at putting one together on methanol and 4 barrel carb, any info will be appreciated . thanks
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 Post subject: Re: OHC 4L max compression
Posted: Tue Jun 09, 2009 10:19 pm 
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your problem is going to be timing chain length not 'clearance'

you will need to have notches cut in the pistons to provide clearance for valves regardless of the piston being a flat top or not. even stock dished pistons have one notch on each piston.

you would be using an AU steel head gasket, that measure between 18-20thou depending on who measures it and with what.
you'll need to have the pistons about 15-17thou down.

then it's going to be a mater of shaving the head until you get the compression you want.
most people call 50thou off the head about as much as you want take off.

you now have all the info you need to spend 30mins with a calculator to work how much compression you can get.

 

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 Post subject: Re: OHC 4L max compression
Posted: Wed Jun 10, 2009 12:24 pm 
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thanks for the info , so it sounds like any more than 50 thou of the head and you run out of tension adjustment on the timing chain . Do you know roughly what chamber volume you end up with 50 thou shaved , i think they start around 65cc . I know there are 4L around on methanol but it doesent sound like they are running the same sort of comp you can achieve with x-flow. thanks.
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 Post subject: Re: OHC 4L max compression
Posted: Wed Jun 10, 2009 9:05 pm 
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you should be about to get in the low 50s with chamber cc
You'll still make more power with meth regardless of the compression.

you could have dome top pistons made and run 13:1, it all comes down to $$$$$

personally i believe there is more power in careful reshaping of the chamber and suffering the slight loss in comp.

if you had the time and money, you could reshape/fill the chamber using plasticine and flow testing it. When you find a shape and volume that works, weld fill and grind the final shape.

there is no reason old methods can't be used on a newish engine,

 

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 Post subject: Re: OHC 4L max compression
Posted: Thu Jun 11, 2009 1:57 am 
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I'm currently building an engine ( have been for several yrs....LOL ), and I'm using domed pistons, std bore, no decking of the block or head. I have machined valve reliefs in the pistons.
I'm yet to dummy fit the engine and check what clearance I will have ( piston to valve ). I'm looking at anywhere from 10:1 to 10.5:1, only because of the cam i'm running.
In the pic there is an EL piston and the BA LPG piston.

 

 

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 Post subject: Re: OHC 4L max compression
Posted: Thu Jun 11, 2009 11:05 am 
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The rules the engine has to remain within dont allow for anything more than .0 deck hieght , and the other problem is you cant add material. I think for now I will stick with my crossflow , the ohc is somthing I have thought about for a while because it looks as though it would have plenty of potential with a four barrel and custom intake on methanol but also has limitations . My x-flow runs 13.5 comp ,4l crank ,200 rods custom 42mm id intake and methanol , makes around the 340 rwhp. thanks for your info.
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 Post subject: Re: OHC 4L max compression
Posted: Thu Jun 11, 2009 3:50 pm 
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Thats some awsome power out of x flow engine

Unfortunatly there isent any super high comp pistons available and u would have to get them made up which costs abit but maybe worth it as its a better engine to start of with but love the old push rod x flow engine.

 

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 Post subject: Re: OHC 4L max compression
Posted: Fri Jun 12, 2009 7:04 pm 
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What slugs does your x-flow run/are they off the shelf?

Also if you were using a ohc head you might wanna think about changing to solid followers....I'm not exactly sure how it's done as I have only seen one with rocker cover off, some of the guys we used to race against did it (wanting to rev with monster cams). We never went there though as didn't rev it past 6000 anyway.

 

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 Post subject: Re: OHC 4L max compression
Posted: Fri Jun 12, 2009 11:04 pm 
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61cc head, ACL flat tops, 0.005" deck height, AU MLS head gasket =11.3:1 in an AU engine
61cc head, Hypatec dish tops, +0.005 hanging out the deck, AU MLS head gasket=10.6:1 in same engine

 

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 Post subject: Re: OHC 4L max compression
Posted: Sun Jun 14, 2009 11:21 am 
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macxr8 wrote:
61cc head, Hypatec dish tops, +0.005 hanging out the deck, AU MLS head gasket=10.6:1 in same engine



that sort of piston to head clearance is asking for trouble

 

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 Post subject: Re: OHC 4L max compression
Posted: Sun Jun 14, 2009 1:12 pm 
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So whats the max comp u can get without having timing chain problems and having at least 20-30 thou just in case u need to do a headgasket and need to machine head???

 

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 Post subject: Re: OHC 4L max compression
Posted: Sun Jun 14, 2009 7:32 pm 
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MICKYYYY wrote:
So whats the max comp u can get without having timing chain problems and having at least 20-30 thou just in case u need to do a headgasket and need to machine head???



about 11:1, but you would need flat top pistons and a 40thou over bore.

 

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 Post subject: Re: OHC 4L max compression
Posted: Sun Jun 14, 2009 10:19 pm 
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tickford_6 wrote:
macxr8 wrote:
61cc head, Hypatec dish tops, +0.005 hanging out the deck, AU MLS head gasket=10.6:1 in same engine



that sort of piston to head clearance is asking for trouble


Yup, pistons will have a much bigger valve relief machined into, and the motor dummy assembled again to check it all. May even go back to 0 deck height for added safety

 

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 Post subject: Re: OHC 4L max compression
Posted: Mon Jun 15, 2009 10:52 am 
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So without machining valve reliefs into pistons and doing a more easy build as in high comp flat tops the comp would be about 10.2 or there abouts.

probably saving abit on dummy assembly and valve reliefs machining or would it be worth wild for the extra comp???

 

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