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Discussion Starter #1
Trying to put a parts list together for a head swap and am wondering which head gasket would work out the best.

Old setup has 64cc chambers with dome pistons. 8.3ish DCR .064 quench. It actually runs fine on 91 octane.

New setup has 67cc chambers with same dome pistons.

1) 8.05 DCR and .064 quench with the same .039 gasket
2) 8.48 DCR and .044 quench with .018-.020 gasket

My question is would the tighter quench offset the increased DCR enough to avoid detonation?

If I were to always stay close to home I'd run the tighter quench, mix in some race gas and not give it a second thought. I'd like to be able to go on road trips and not have to worry though.

Or a 3rd 'middle of the road' option would be .028 gasket for a DCR of 8.28 and .053 quench.

Thanks for any insight or opinions.

Mark
 

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I assume you are checking quench and your piston is 0 deck? if it is, I would go with the .039. We are talking quench here . You did not mention deck. If you want to run on the edge with steel rods , you could use the .028 gasket , but thats close.

if the block had never been decked, I would also guess your piston can be in the hole .020-.040., so best you can do here is steel shim at maybe .015-.017.

You need to know what cc your piston is. You already know what the head volumes are so maybe you can juggle something around .
here is something that may help you do that. http://members.uia.net/pkelley2/crc.htm
 

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Discussion Starter #3
Don

I'm assuming, yeah I know, that the pistons are .025ish in the hole. That's what I'm doing my calculations with. Never been decked, but haven't measured it yet either. Using +6cc for piston dome volume.

I wanted to cut the quench down to where it should be, but it jacks my compression up significantly. I wanted to know if the better quench is going to offset the increased compression enough to go with the shim gaskets.

Mark
 

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Don

I'm assuming, yeah I know, that the pistons are .025ish in the hole. That's what I'm doing my calculations with. Never been decked, but haven't measured it yet either. Using +6cc for piston dome volume.

I wanted to cut the quench down to where it should be, but it jacks my compression up significantly. I wanted to know if the better quench is going to offset the increased compression enough to go with the shim gaskets.

Mark
You are basically where I was and I ran the thin gasket, .020-.030 in the hole with 6cc pistons and 64 cc heads. I think keeping the quench down is the idea to controlling detonation in your case. Maybe Pdq will drop by soon.
Your cam overlap also comes into play here too. Too much will bleed off excessive compression, not so much will help you retain.
 

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Discussion Starter #6
You are basically where I was and I ran the thin gasket, .020-.030 in the hole with 6cc pistons and 64 cc heads. I think keeping the quench down is the idea to controlling detonation in your case. Maybe Pdq will drop by soon.
Your cam overlap also comes into play here too. Too much will bleed off excessive compression, not so much will help you retain.
Thanks for the quick replies Don. "Maybe pdq will drop by soon." Yeah, he'll chew me out for not going with bigger heads. ;)

Mark
 

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Discussion Starter #7
I got about 10.2 comp using stock 350 specs and your heads with .017 gasket. You did not say what material the heads were, if alloy, you can get away with a lot.

Cast iron heads. About 10.2:1 static (8.05 DCR) with the .039 gasket and huge quench. 10.7:1ish (8.48 DCR) with shim gaskets and tight quench.

I think I just talked myself into the 'middle of the road' option...

Mark
 

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Cast iron heads. About 10.2:1 static (8.05 DCR) with the .039 gasket and huge quench. 10.7:1ish (8.48 DCR) with shim gaskets and tight quench.

Mark
which cam? specs?
 

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Well, thats close to what I had, 282HR with thin gaskets and pistons like yours on my old 355 with old darts. My guess was the cam was bleeding off some cly pressure but quench was tight, i ran pump prem, I ran altitude from 2ft above to 8500 ft sea level ,37° timing . no issues. I see your pain. I would go with thin gaskets, if you have issues, you can back off a little on timing, if it gets real bad, well, pull of cheap thin gaskets and replace with .039 gaskets at $25 a head. Final answer. Lets see if someone else disagrees. Paul? Tim? Home from work yet?

Somewhere I have a little program that figures your fuel with cam and eng specs, tells you what you can get away with. I will look for it and post it later this evening.
 

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Discussion Starter #11
Well, thats close to what I had, 282HR with thin gaskets and pistons like yours on my old 355 with old darts. My guess was the cam was bleeding off some cly pressure but quench was tight, i ran pump prem, I ran altitude from 2ft above to 8500 ft sea level ,37° timing . no issues. I see your pain. I would go with thin gaskets, if you have issues, you can back off a little on timing, if it gets real bad, well, pull of cheap thin gaskets and replace with .039 gaskets at $25 a head. Final answer. Lets see if someone else disagrees. Paul? Tim? Home from work yet?

Somewhere I have a little program that figures your fuel with cam and eng specs, tells you what you can get away with. I will look for it and post it later this evening.
Thanks Don

My current setup is about 10.5 static and 8.3 dynamic. It runs on 91 octane. My hope was to tighten up the quench but that puts me at 10.7 static and right at 8.5 dynamic. Pretty high for cast iron.

Mark
 

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Thanks Don

My current setup is about 10.5 static and 8.3 dynamic. It runs on 91 octane. My hope was to tighten up the quench but that puts me at 10.7 static and right at 8.5 dynamic. Pretty high for cast iron.

Mark
Your cam numbers and altitude will play a part, I will post this little link as soon as i got home, bout 5 pm. The numbers sound high but i think the main issue is controlling quench.
The 8.5 dynamic is at the high side of Vizards recommendations but its in the range he talks about , not over into the 9.0
 

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Dang.....that's some confusing stuff right there......:clonk::clonk::clonk::clonk:
Yes, and the engine efficency factor plays too, less efficent your motor is , lowers you fuel requirement , as I am finding out.
 

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Detonation has a long list of contributors. Your temperature and load are huge factors. Low engine speed vs high load with high temp is the killer in automotive. If you have a manual tranny and low gears, or a loose converter and your engine doesn't run hot and you have cold intake air... I say cram the dcr up there a leetle and get your moneys worth. And like Don said, you can always back down the timing some if your on a road trip and it's bitch hot and clickin at you some. Advance it again when it's cool outside.
 

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Detonation has a long list of contributors. Your temperature and load are huge factors. Low engine speed vs high load with high temp is the killer in automotive. If you have a manual tranny and low gears, or a loose converter and your engine doesn't run hot and you have cold intake air... I say cram the dcr up there a leetle and get your moneys worth. And like Don said, you can always back down the timing some if your on a road trip and it's bitch hot and clickin at you some. Advance it again when it's cool outside.
How true. I've got an old MSD timing controler right next to the 8 track. I can vary the timing up to 15°, depending on the baseline. Very handy. No pun intended.:yes:
 

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Discussion Starter #18

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Your cam overlap also comes into play here too. Too much will bleed off excessive compression, not so much will help you retain.
Overlap occurs at the end of the exhaust stroke. How does it affect the compression stroke?
 

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Discussion Starter #20
Detonation has a long list of contributors. Your temperature and load are huge factors. Low engine speed vs high load with high temp is the killer in automotive. If you have a manual tranny and low gears, or a loose converter and your engine doesn't run hot and you have cold intake air... I say cram the dcr up there a leetle and get your moneys worth. And like Don said, you can always back down the timing some if your on a road trip and it's bitch hot and clickin at you some. Advance it again when it's cool outside.
Temp. usually runs right at 180. 4-speed and 3.42's out back.

I was starting to lean toward a compromise and go with a .028ish gasket which would put the quench in the .050's, but may just go for shims now. You're right. Gaskets won't break the budget and it's only a Satuday's worth of labor.

Doesn't better quench allow you to back off the timing a bit anyway?

Mark
 
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