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?
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
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
starsky460- Posts : 8
Join date : 2012-03-22
Re: Can you raise compression by changing rod length with same compression height?
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.
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schmitty- Posts : 4538
Join date : 2008-12-02
Age : 54
Location : Holdrege, NE
starsky460- Posts : 8
Join date : 2012-03-22
Re: Can you raise compression by changing rod length with same compression height?
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.![]()
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".
Re: Can you raise compression by changing rod length with same compression height?
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.
starsky460- Posts : 8
Join date : 2012-03-22
Re: Can you raise compression by changing rod length with same compression height?
You will never know the difference in that small of a compression change....
the Coug- Posts : 3055
Join date : 2008-12-02
Re: Can you raise compression by changing rod length with same compression height?
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.
Dave.
LivermoreDave- Posts : 972
Join date : 2009-09-27
Location : North of the Equator.
Re: Can you raise compression by changing rod length with same compression height?
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"
Re: Can you raise compression by changing rod length with same compression height?
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.
LivermoreDave- Posts : 972
Join date : 2009-09-27
Location : North of the Equator.
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» 351m or 400?.
» Compression Height
» compression height
» forged .030 over pistons 460 stock length rod and stroke . for dove heads high compression
» I've shared these pictures elsewhere, thought I would share them here. Set of DO0E's that we had welded up and hope to someday have ported to raise the ports.
» Compression Height
» compression height
» forged .030 over pistons 460 stock length rod and stroke . for dove heads high compression
» I've shared these pictures elsewhere, thought I would share them here. Set of DO0E's that we had welded up and hope to someday have ported to raise the ports.
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