Compression ratios

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pthomas

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« on: December 07, 2015, 08:10:36 AM »
Hey fellas got a question, I've got a .030 over 454 running flat top pistons, and a set of 291 107cc closed chambered heads. Looks like I'm going to have to do a rebuild, I'd like to put some higher compression pistons back in it. But I'd like to get an idea of where it's at now compression wise and at what compression ratio do you need to step up to above 93 octane?


GT Jets

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« Reply #1 on: December 07, 2015, 10:28:03 AM »
There are some pretty extreme variables to answer your questions.

As for the octane requirement for example,  camshaft selection alone will determine at what point you will be out of the pump gas category. 10.5:1 is typically the max. Heard of more and less.

You really need to look at the dynamic C/R, not just the static.

If you are working with a machine that does high performance engines, they should be able to set you up in the right direction.

Another thing to consider would be if your current heads will have the ability to take advantage of more compression.

I am on a job site right now having a much needed cup of coffee, but will be able to pay with some numbers later and give a rough idea of where your at now.

I'm thinking off the top of my head your in the 8.5:1 ballpark, but don't hold me to it.

Corrected from 9.5 to 8.5:1

Pretty typical actually.

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« Last Edit: December 07, 2015, 02:29:17 PM by GT Jets »
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« Reply #2 on: December 07, 2015, 01:05:20 PM »
Ok cool thank you GT I greatly appreciate your info, basically I'm wanting to be able to gain as much as I can without having to run race fuel and still be reliable.

pthomas

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« Reply #3 on: December 07, 2015, 01:12:40 PM »
I'll give you the run down of what I'm running, it's .030 over 454 4 bolt block, closed chambered 291 rectangle port 107 cc heads, the pistons are Keith black flat tops with single intake valve reliefs, standard felpro head gaskets, the cam is advertised duration of 284/284 and lift at .544/.544, the intake is a port matched team G, and running a Holley 800 DP and I think the jets are #85 in the front and #87 in the back.

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« Reply #4 on: December 07, 2015, 01:59:36 PM »
Actually... If the block hasn't been decked... Typically the piston lands about .020 in the hole... You're looking like about the 8.6:1 range

Gonna need to run some small bump pistons to get up on compression. As far as how high... What kind of boat? Load will also play a role in what compression you can get away with.


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« Reply #5 on: December 07, 2015, 02:09:26 PM »
That's kind of what I was thinking I was going to have to do, it's in a 19 ft hornet with a stock Berkeley E pump, ride plate and a manual place diverter.

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« Reply #6 on: December 07, 2015, 02:28:05 PM »
Actually... If the block hasn't been decked... Typically the piston lands about .020 in the hole... You're looking like about the 8.6:1 range

Gonna need to run some small bump pistons to get up on compression. As far as how high... What kind of boat? Load will also play a role in what compression you can get away with.


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You are correct. 8.5:1.  A little typo. Told you I needed the coffee break.

Lol

I corrected my original post.

GT
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« Reply #7 on: December 07, 2015, 02:42:00 PM »
A "standard", as in most common, Fel Pro BBC gaskets are 9.700cc.  Those KB pistons have a valve relief volume of 3.0cc and a 1.645" compression height.

If your block has not been decked, that would put your compression ratio at 8.58:1.

If your block was zero-decked, your compression ratio would be 8.88:1.

Really need the Lobe Separation Angle (LSA), AKA Lobe Centerline Angle (LCA) (not to be confused with intake lobe centerline), or intake/exhaust duration overlap in order to calculate the dynamic compression ratio.  The intake valve opening and closing points would be ideal.  Also assuming by lift and duration that this is a flat hydraulic lifter cam.

Throwing out a guess, assuming a 112 LSA installed "straight up" and a zero-decked block, your dynamic compression ratio is about 7.11:1.  Advancing the cam 4-degrees will bump it up to 7.31:1.

If you stepped up to a 12cc dome volume piston, your dynamic compression ratio would be a safe 7.98:1 and advancing the cam 4-degrees will bump it to a still safe 8.22:1 with a 10:1 static compression ratio.

If you wanted to push the limits, careful attention to detail of the piston domes, combustion chamber, quench distance, and mixture motion could allow you to squeeze a bit more.

IMHO, your compression ratio with the flat tops is way too low for 93.  At the performance level you are at, I would zero-deck the block and run the 12cc dome volume pistons with a good tune and enjoy.

Cheers,

Joe
"I want to roll with my brother Joe" - Joe Bateman - January 29, 1950 ~ November 27, 2013

 


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