Reducing Compression?
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Road Captain
Joined: Sep 2007
Posts: 742
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From: Twin Cities, MN
Reducing Compression?
I've got a riding buddy looking at a set of heads with the combustion chambers set @ 79 cc's.
Obviously this raises his CR for the 103" build, but it would be up to ~10.8:1 with a 0.030" head gasket and flat-tops.
Question: Is there a way to re-work these heads to reduce the CR/ccp, or should he look into re-working his stock heads?
He's running SE251's and wants lower compression than 10.8, but also he's getting a great deal for these heads.
Re-working his stock heads will run him from $500-$700 roughly.
Any help would be great... Thanks!
Obviously this raises his CR for the 103" build, but it would be up to ~10.8:1 with a 0.030" head gasket and flat-tops.
Question: Is there a way to re-work these heads to reduce the CR/ccp, or should he look into re-working his stock heads?
He's running SE251's and wants lower compression than 10.8, but also he's getting a great deal for these heads.
Re-working his stock heads will run him from $500-$700 roughly.
Any help would be great... Thanks!
HDF Community Team
Joined: Jun 2006
Posts: 12,763
Likes: 2,597
From: san antonio
The "good deal" heads may be a not so good deal. Those heads have been decked .060"-.065" to get to 79cc chambers and you can't add material back. Additionally, if decked that much and a larger intake valve has been installed, valve to piston clearance should be checked on assembly. As I see it, your buddy has four options:
1. Pass on the good deal heads and work the stock heads.
2. Consider different cams with later intake close to lower corrected and cranking compression but he will still have to deal with static at 10.7 or thereabouts which meand compression releases if not already installed in the good deal heads. It will take cams with 48*-50* intake close to lower corrected and cranking compression to a manageable level. Those cams are relatively high lift and long duration cams that are intended more for performance above 3000-3500 rpms which may or may not fit your buddy's riding style.
3. Consider changing pistons to a dished piston with a -6cc, or thereabouts, dome volume and he can still run the SE251 cams. However, that will mean pulling the cylinders, buying .010" OS pistons and boring to fit.
4. A combination of 2 and 3.
So, the good deal heads, IMHO, are not such a good deal when you consider the additional cost to put lipstick on a pig. Option 1 makes the most sense as it will cost less in the end and the reworking the stock heads he can tailor them to his current configuration. JMHO.
1. Pass on the good deal heads and work the stock heads.
2. Consider different cams with later intake close to lower corrected and cranking compression but he will still have to deal with static at 10.7 or thereabouts which meand compression releases if not already installed in the good deal heads. It will take cams with 48*-50* intake close to lower corrected and cranking compression to a manageable level. Those cams are relatively high lift and long duration cams that are intended more for performance above 3000-3500 rpms which may or may not fit your buddy's riding style.
3. Consider changing pistons to a dished piston with a -6cc, or thereabouts, dome volume and he can still run the SE251 cams. However, that will mean pulling the cylinders, buying .010" OS pistons and boring to fit.
4. A combination of 2 and 3.
So, the good deal heads, IMHO, are not such a good deal when you consider the additional cost to put lipstick on a pig. Option 1 makes the most sense as it will cost less in the end and the reworking the stock heads he can tailor them to his current configuration. JMHO.
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Road Captain
Joined: Sep 2007
Posts: 742
Likes: 4
From: Twin Cities, MN
Thanks for the excellent, detailed response!
I wasn't sure, but that seemed like pretty high compression numbers to me.
I'll pass that along to him, and maybe even print this out... afterall, it's January in Minn, so he's got time to get his own heads re-worked and save the hassles assoicated with these.
I wasn't sure, but that seemed like pretty high compression numbers to me.
I'll pass that along to him, and maybe even print this out... afterall, it's January in Minn, so he's got time to get his own heads re-worked and save the hassles assoicated with these.
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I've got a riding buddy looking at a set of heads with the combustion chambers set @ 79 cc's.
Obviously this raises his CR for the 103" build, but it would be up to ~10.8:1 with a 0.030" head gasket and flat-tops.
Question: Is there a way to re-work these heads to reduce the CR/ccp, or should he look into re-working his stock heads?
He's running SE251's and wants lower compression than 10.8, but also he's getting a great deal for these heads.
Re-working his stock heads will run him from $500-$700 roughly.
Any help would be great... Thanks!
Obviously this raises his CR for the 103" build, but it would be up to ~10.8:1 with a 0.030" head gasket and flat-tops.
Question: Is there a way to re-work these heads to reduce the CR/ccp, or should he look into re-working his stock heads?
He's running SE251's and wants lower compression than 10.8, but also he's getting a great deal for these heads.
Re-working his stock heads will run him from $500-$700 roughly.
Any help would be great... Thanks!
A Bob Wood 8 thrives at that compression, and is where we set them.
You'll want a good head with that, and it'll really roll.
Scott
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