CR Variables
04-20-2018, 12:02 AM
Post: #1
CR Variables
Hi all,

Can someone give me accurate variables for my compression ratio calculator please?

I need everything including CC of the 8.0:1 & 8.9:1 pistons for the 4AGZE.

I'm not getting a correct value using old numbers I've found on this forum, i'm wondering if there are any updated values I could use? I cannot reach 8.9:1 or 8.0:1 without changing the piston volumes to reach the target CR.

Many thanks in advance
Alex
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04-20-2018, 11:02 AM
Post: #2
CR Variables
MRP Ltd have said that the 8.9 pistons have an volume of 4.8cc - that does not work at all with the calculator.

Clarification is required.
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04-20-2018, 05:14 PM
Post: #3
CR Variables
Post all the data you have and I'll have a look later

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04-20-2018, 10:53 PM
Post: #4
CR Variables
Bore: 81mm / Stroke: 77mm
So V: 396.8 cc

Head: ~36.3 cc, HG: 6.6 cc (82x1.25mm) Piston: ?

If (V+v)/v = 8.9, then v = V/7.9

So piston = ~7.3 cc

With 4.8 cc you get 9.3:1
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04-21-2018, 08:39 AM (This post was last modified: 04-21-2018 08:42 AM by Bean.)
Post: #5
CR Variables
Looks about right to me[Image: AEU86 AE86 - CR Variables]

mpsracing.com

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04-21-2018, 11:01 AM (This post was last modified: 04-21-2018 11:04 AM by Alex170984.)
Post: #6
CR Variables
So it makes these variables for volume wrong then

http://www.aeu86.org/forum/Thread-piston-dome-cc-figure

When has deck clearance ever been .5mm?
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04-21-2018, 02:32 PM
Post: #7
CR Variables
Deck clearance is the distance between the piston and the top of the block.
On the 4A-GE with STOCK pistons it is 0mm (the piston is flush with the block) meaning the squish gap is just the thickness of the head gasket, and 1.25mm is already a big squish...
If you add 0.5mm of deck clearance, that means a squish gap of 1.75mm. That is awful !

(I mean, sure that is one way to add 2.6ccc and reduce the compression ratio, but god it is bad)

On a side note it's almost impossible to MEASURE the piston dome volume alone.
Normally you measure it together with the deck clearance. (then you can do the math to separate them if you really want to)
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04-21-2018, 05:38 PM
Post: #8
CR Variables
We're looking at a 4agZe not an NA.
The pistons are dished no dome.
I can only assume that a few of the measurements are guessed.
Head volume as well as deckheight may not be stock anymore depending if it was skimmed before or not.
The only way to know all this for sure would be to measure everything properly.
Honestly it shouldn't be an issue if he ends up with 8.9 or 9.3 - with proper fuel and stock pressure it'll be fine.


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04-22-2018, 11:04 AM
Post: #9
CR Variables
Hi all,

My block is assembled with my new OEM 8.9 Pistons and mine are 0.000127 (5 thou) above the deck!
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04-22-2018, 11:38 AM
Post: #10
CR Variables
so you'll end up with about 9.3:1 assuming your 36.3 are correct. I can only guess that both head and block were skimmed before. with some headwork you could bring it down a little but not much more than 0.1. every 0.1mm of additional HG would lower CR too by about 0.1. Just live with it and use good fuel.

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