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Lower Compression for Turbo 

 

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Posted: Sat Jul 01, 2006 5:16 pm 
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one word and that is yuck.

 

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Posted: Mon Jul 10, 2006 5:34 pm 
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wat comp. is the EL 4lt... sorry its prob posted already i just thought i'd ask here first
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Posted: Mon Jul 10, 2006 7:38 pm 
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Shum's EL wrote:
wat comp. is the EL 4lt... sorry its prob posted already i just thought i'd ask here first



GLI engine 9.3:1
XR6 engine 9.35:1

 

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Posted: Mon Jul 10, 2006 7:39 pm 
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tickford_6 wrote:
Shum's EL wrote:
wat comp. is the EL 4lt... sorry its prob posted already i just thought i'd ask here first



GLI engine 9.3:1
XR6 engine 9.35:1


thanks... so if i'm to go turbo wat comp should i be aiming at??
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Posted: Mon Jul 10, 2006 8:47 pm 
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Shum's EL wrote:
tickford_6 wrote:
Shum's EL wrote:
wat comp. is the EL 4lt... sorry its prob posted already i just thought i'd ask here first



GLI engine 9.3:1
XR6 engine 9.35:1


thanks... so if i'm to go turbo wat comp should i be aiming at??


whats the plan for the engine?
how much boost? what turbo? what intercooler?
give as the details of the setup and the planned power you are aiming for.

you don't realy want to lower it unless you realy have to.
keeping the compression up help with power off boost, give better economy and better drivability off boost

 

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Posted: Mon Jul 10, 2006 9:49 pm 
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Posted: Mon Jul 10, 2006 10:37 pm 
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at that boost 9.3:1 will be fine providing the tune is good.

If you ever have to take the head off, use a thinner AU gasket and rework the chambers slightly and drop it back to about 9.1:1 the tighter squish will help stop detonation just as much as the lower comp will.

word of warning though, don't use thicker gaskets/head shims to drop compression. it opens the squish up to much

 

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Posted: Mon Jul 10, 2006 10:38 pm 
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sweet thanks heaps
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Posted: Mon Jul 10, 2006 10:52 pm 
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What sort of tuning are you going to use???
Tune saves engines... Imo no more than 9 to 1...

 

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Posted: Mon Jul 10, 2006 11:47 pm 
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t_6 whadaya mean by 'squish'?

piston deck clearance, overall compression? something else.

 

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Posted: Tue Jul 11, 2006 2:51 pm 
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data_mine wrote:
t_6 whadaya mean by 'squish'?

piston deck clearance, overall compression? something else.


i think he's talking destance between the head and the block...

and on reworking the chambers, they're made in the shape they are for a reason, screwing around with them isn't a great idea IMO

 

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Posted: Tue Jul 11, 2006 6:55 pm 
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he's refering to the distance from the piston to the head, the smaller the gap the more turbulance is created, and the charge is directed into the chamber near the plug, promoting better burning and lesser chance of detonation...

without actually doing too much reshaping work, you can still safely remove 1-1.5cc on a 4l head, and about 2-3cc on a 3.9 head with the swirl ridge.

 

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Posted: Tue Jul 11, 2006 7:43 pm 
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Slabz wrote:
data_mine wrote:
t_6 whadaya mean by 'squish'?

piston deck clearance, overall compression? something else.


i think he's talking destance between the head and the block...

and on reworking the chambers, they're made in the shape they are for a reason, screwing around with them isn't a great idea IMO



squish is piston to head clearnce. deck hight and gasket thickness added together = squish. 35thou is regarded as the safe limit, but i have seen engines as tight as 30thou. you have to remember this will close up when the engine gets to temp.


there is nothing wrong with reworking chambers. they are not perfect.
and there are gains to be had in low lift flow aswell.

it not like we are talking about huge changes. just a slight touch up and deshroud the vales a little.

 

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Posted: Tue Jul 11, 2006 9:08 pm 
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remembering also, most 4valve heads have basically zero squish pad area, and, without back to back, real world testing on your particular chamber, you wont know whether youve done good or bad with regards to squish area's and chamber shapes.

there comes a point when chasing big power, where the squish pads start to work againts you, causing more detonation, things like sharp edges become hot spots, this is where a hemi chamber is the best.

tickford_6 wrote:
not like we are talking huge changes. just slight touch up and deshroud valves a little

thats the sort of work i was refering to with regards to the amount of cc's to be gained, 1.5cc is a fair bit of meat and is about what you can get out without too much trouble

 

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Posted: Wed Jul 12, 2006 10:58 am 
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The correct or another term is quench . Opening up the chambers on the outer edges of valves is a good way to lower comp as it's bassically relieving valve which increases flow... Quench is not such a big deal with forced induction as it is with hi po n/a engine... But the idea is to get one big flame front and not many, as this causes detonation... Note the 351m / 400m motor in the States is notorious for detonation yet it has approx 8.5 compression and Clevo style open chamber heads.. The main reason for bad detonation?? The piston is designed to be way down bore .. The fix to this was to fit pistons with pin height lower essentually bringing piston closer to deck. Even though it raised compression it prevented detonation...

 

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