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> 2056 build compression ratio dilema
jjs3rd914
post May 8 2018, 08:44 AM
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Building a 2056, want good low end torque and drivability. Car has AC. Targeting 115-125 HP

GA 2L case, stock 2L crank rods, new AA 96mm pistons, biral cylinders
New AA 2L heads with 44/36 valves
Webcam 086 cam
Weber 40IDF dual carbs

After numerous searches seem like 8.5 is optimal compression ratio (CR) and the build should get me the targeted HP.

BUT finding during assembly, with a deck height of .065" (with .010 barrel spacer) 60 cc head volume which includes 2CC for the piston valve recess, I am only getting 8.1.

So option are to leave alone and go with 8.1CR
or
Machine heads to loose about 6CC
or machine barrels to reduce deck height. Was told must use a barrel spacer and am using the thinnest I can find at .010.

To get the targetd HP do I need to get to a CR of 8.5, or will 8.1 get me there.?

Am really confused about the whole CR optimal number for what I want.

Thanks

jjs3rd914
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nditiz1
post May 8 2018, 09:27 AM
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I was running into the same issue. I'm not sure about the barrel spacers, but I had to have .5mm taken off the bottom of the jugs. Ideally I wanted to have .90 10mm of DH, but my initial measurement was off by .1mm. This got me a final DH of 1mm which is 8.4 CR. I have pretty much the same exact setup you have. Im running no spacers or gaskets from block to head.
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jjs3rd914
post May 8 2018, 11:18 AM
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Nditz1

So you have the barrels in the case with no shims, gaskets or whatever they are called. I read somewhere that that is not the way to go. Some shim must be used. Can any of you verify this, if so why? Or does it not make any difference.

If I eliminate the .010 shim then that would get me 8.3 CR, Which I might go with.

I too not using a head gasket.

Still asking members if indeed a CR of 8.5 is optimal for our builds. So would we see driving HP difference if went with 8.1 instead of 8.5. Surely would save me more machining cost's and more time. Machinists seem busier than trying to see your doctor.

Thanks jjs3rd914
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ConeDodger
post May 8 2018, 11:29 AM
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You can go higher. 9-9.5:1
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nditiz1
post May 8 2018, 12:01 PM
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Conedodger is correct you can go higher.

The dilemma you may run into is that you take so much off the bottom that your DH would cause a valve opening interference issue. With these pistons having the relief valley I don't really see that as a problem, but every setup is different.

I have never read that the base gasket is required. I was told just use some Curil T or 518 Loctite and you should be sealed and have no issues. If you are worried about cutting too much off and almost being flush with the cooling fin which happened on 2 of my cylinders you could have the shop mill a step like they cam from the factory. So they would be milling the cooling fin.

I would have them cut .010 off the bottom and not use the base shim - that would net you .045 DH which equates to 8.5 CR

OR

you could have them cut .020 off the bottom, (what I had cut) then use the base shim to get you to .045 DH
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Mblizzard
post May 8 2018, 02:32 PM
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I have used this spreadsheet to play with the numbers.

Attached File  Compression.xls ( 7.49k ) Number of downloads: 48
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McMark
post May 8 2018, 05:02 PM
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You're not going to get where you want with 8.1CR. And I don't think you're going to get there with that cam. It's not a bad cam, it's just not gonna get you there.
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VaccaRabite
post May 8 2018, 06:41 PM
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Are you using a head gasket?
If so - don't. That will bump your CR enough to get you to about 8.5 probably.

Zach
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jjs3rd914
post May 8 2018, 06:52 PM
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QUOTE(McMark @ May 8 2018, 03:02 PM) *

You're not going to get where you want with 8.1CR. And I don't think you're going to get there with that cam. It's not a bad cam, it's just not gonna get you there.



I am stuck with the cam now, so if 8.1 will not get me there what CR should I shoot for with what I have for optimal performance.

Appreciate all the replies thus far.

For more info, here in FLA we do have REC (recreation), gas for boats lawn mowers etc, which is alcohol free. It has a posted pump octane rating of 90. That's the gas I would like to use and do use in other older cars. I use it in the 914 now which is a rebuild bone stock 1.7L with original D-Jet and runs great, just not very peppy.

Thanks
jjs3rd914
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gothspeed
post May 8 2018, 06:53 PM
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I built a 2056 with 96mm AA valve relief pistons. I had to 'true' all cylinders which got my compression to about 8.6:1 , with a fresh head resurface.
I think you have plenty cylinder material to remove to get to your target. Your valve to piston clearance has plenty of room. As the Web 86 doesn't have big lift.
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Geezer914
post Jan 13 2021, 01:53 PM
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I thought the piston valve relief was 7cc's? DH calculator gave me a DH of.068 for a CR of 8.5 with 49.2cc heads 49.2=7cc's = 56.2cc's total.
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VaccaRabite
post Jan 14 2021, 08:02 AM
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QUOTE(Geezer914 @ Jan 13 2021, 02:53 PM) *

I thought the piston valve relief was 7cc's? DH calculator gave me a DH of.068 for a CR of 8.5 with 49.2cc heads 49.2=7cc's = 56.2cc's total.

Are you asking for advice or giving the OP help on a 3 year old thread?
Not trying to be a dick, I just don't know where you were trying to go here.

Zach
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Geezer914
post Jan 14 2021, 08:33 AM
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QUOTE(VaccaRabite @ Jan 14 2021, 06:02 AM) *

QUOTE(Geezer914 @ Jan 13 2021, 02:53 PM) *

I thought the piston valve relief was 7cc's? DH calculator gave me a DH of.068 for a CR of 8.5 with 49.2cc heads 49.2=7cc's = 56.2cc's total.

Are you asking for advice or giving the OP help on a 3 year old thread?
Not trying to be a dick, I just don't know where you were trying to go here.

Zach

did not realize it was an old thread.
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Jake Raby
post Jan 18 2021, 11:52 PM
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This is where a mock up of components up front comes into play.
The deck height can be altered best by cutting the case registers to reduce deck height. I am finding 100% of T4 crankcases these days needing the decks cut to flatten them. The worst of these are the GA and GC 914 2.0 cases. I am seeing .020" needing to be cut to flatten all the surfaces. This is due mostly to casting age, and not mileage.

If your cylinders measure 3.600" from case deck, the the head selfing surface, you'll have a better chance of having things install with more reasonable deck height figures to attain a CR of 8.5:1 or higher, based on the engine combo.

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