Can you raise compression by changing rod length with same compression height?

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Can you raise compression by changing rod length with same compression height?

Post  starsky460 on March 23rd 2012, 12:56 pm

Newbie here. Looking at doing a 532 stroker build, bore of 4.300, stroke 4.440, piston overbore of 0.080 and rod length of 6.800.
I currently have JE pistons part number 170880 which according to JE the specs are Compression Distance 1.350, Piston Head Volume (cc)+39.00cc,
which make compression 9.9 to 1 with 74 cc heads. Without a bunch of changes, I want to raise compression to just over 10.0 to 1 and was wondering
if a change in rod length from 6.8 to 6.7 would do that or would I need to change pistons? Thanks

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Re: Can you raise compression by changing rod length with same compression height?

Post  schmitty on March 23rd 2012, 1:31 pm

No, if you install a shorter rod, you are lowering the piston in the bore and lowering the C/R. If you wish to raise the C/R, you might be able to mill a little off of the heads, or deck the block, depending on how much piston to valve clearance you have, and where the piston is at in relation to the deck. Cool
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Re: Can you raise compression by changing rod length with same compression height?

Post  starsky460 on March 23rd 2012, 1:42 pm

Thanks Schmitty ! Very Happy

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Re: Can you raise compression by changing rod length with same compression height?

Post  rmcomprandy on March 23rd 2012, 7:13 pm

schmitty wrote:No, if you install a shorter rod, you are lowering the piston in the bore and lowering the C/R. If you wish to raise the C/R, you might be able to mill a little off of the heads, or deck the block, depending on how much piston to valve clearance you have, and where the piston is at in relation to the deck. Cool

You can but, not the way he is talking. You need a longer rod and machine that length difference off the piston flat area, (which changes that piston's compression height slightly), and makes the dish smaller..

Let me get this straight ... it has 9.9/1 now and just over 10/1 is wanted??? Is there a reason other than being anal??? Just mill the heads another .005".

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Re: Can you raise compression by changing rod length with same compression height?

Post  starsky460 on March 23rd 2012, 8:44 pm

No, actually with the heads I have it's closer to 9.7 to 1. I m probably just going to leave it the way it is.

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Re: Can you raise compression by changing rod length with same compression height?

Post  the Coug on March 23rd 2012, 9:06 pm

You will never know the difference in that small of a compression change....
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Re: Can you raise compression by changing rod length with same compression height?

Post  LivermoreDave on March 24th 2012, 1:18 pm

If my math is correct, your rotating assembly totals 10.37" of effective length. What's your blocks main bore center to cylinder head mating surface dimension? Also I like Randy's thinking. Is a bit under vs. a bit over worth the investment to you?

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Re: Can you raise compression by changing rod length with same compression height?

Post  rmcomprandy on March 24th 2012, 2:17 pm

LivermoreDave wrote:If my math is correct, your rotating assembly totals 10.37" of effective length. What's your blocks main bore center to cylinder head mating surface dimension? Also I like Randy's thinking. Is a bit under vs. a bit over worth the investment to you?

Dave.

I think your math is wrong.

1/2 of 4.300 = 2.15"
the rod length specified = 6.800"
piston compression height = 1.350"

That's 10.300"

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Re: Can you raise compression by changing rod length with same compression height?

Post  LivermoreDave on March 25th 2012, 1:58 pm

starsky460 wrote:Newbie here. Looking at doing a 532 stroker build, bore of 4.300, stroke 4.440, piston overbore of 0.080 and rod length of 6.800.
I currently have JE pistons part number 170880 which according to JE the specs are Compression Distance 1.350, Piston Head Volume (cc)+39.00cc,
which make compression 9.9 to 1 with 74 cc heads. Without a bunch of changes, I want to raise compression to just over 10.0 to 1 and was wondering
if a change in rod length from 6.8 to 6.7 would do that or would I need to change pistons? Thanks

Sorry Randy I didn't look around the corner, just using the information at hand!

Dave.

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