Higher compression ratio...
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From: mt. shasta california
Higher compression ratio...
I would like to know what steps need to be taken to achieve a higher compression ratio. I am assuming i would need new pistons. But, what else would i need and how does it acheive higher compression?
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From: PacNW
Higher compression ratio...
>I would like to know what steps need to be taken to achieve
>a higher compression ratio. I am assuming i would need new
>pistons. But, what else would i need and how does it acheive
>higher compression?
The key to increasing C.R. is reducing the amount of volume above the piston at TDC. This can be achieved a number of ways including head milling, block decking, changing to flat top or "pop-up" pistons and installing new heads with smaller combustion chambers or a combination of all or some of the above. Increasing the C.R. the "right" way also involves being aware of the "quench" effect to avoid detonation and making sure that you don't run into mechanical interference between the pistons & valves.
I realize this is a pretty "general" answer but your question was not very specific as to what exactly you were looking for.
>a higher compression ratio. I am assuming i would need new
>pistons. But, what else would i need and how does it acheive
>higher compression?
The key to increasing C.R. is reducing the amount of volume above the piston at TDC. This can be achieved a number of ways including head milling, block decking, changing to flat top or "pop-up" pistons and installing new heads with smaller combustion chambers or a combination of all or some of the above. Increasing the C.R. the "right" way also involves being aware of the "quench" effect to avoid detonation and making sure that you don't run into mechanical interference between the pistons & valves.
I realize this is a pretty "general" answer but your question was not very specific as to what exactly you were looking for.
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Senior User
Joined: Sep 2002
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From: mt. shasta california
Higher compression ratio...
Thanks! I was just trying to understand how achieving a higher compression ratio works. What is the most simple way to get a higher compression ratio?
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From: PacNW
Higher compression ratio...
I'd have to say the simplest way is to swap in a set of heads with a smaller combustion chamber. With the 385 series thats not tough to do once you find a set of early (68-71) 429 or 460 heads.
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Higher compression ratio...
Or Just a thinner head gasket. Reduces the space above the piston.
Hay Erik #l85, You ask good questions.
CAN anyone explane "quench" effect ? Is this something I should know about before milling my heads ?
Hi Erik , Milling / mechineing or shaving the heads is removing some of the metal surface of the head that go against the block. This will reduse the valume (combustion area) in the head. Leaving a smaller space for the gas/air to be squeezed in to. Cousing higher pressure. you see, higher compression ratio.
The more the gas is compressed, the faster the gas burns, the faster the gas burns the more heat it makes. The hotter the air its the more the air(gases)expands, the more the gases try to expanded the more force is placed on the piston. something like that. the hotter the gas burns the more pullutance are made. back to low compression for the Goverment. Sorry for the long post.
Hay Erik #l85, You ask good questions.
CAN anyone explane "quench" effect ? Is this something I should know about before milling my heads ?
Hi Erik , Milling / mechineing or shaving the heads is removing some of the metal surface of the head that go against the block. This will reduse the valume (combustion area) in the head. Leaving a smaller space for the gas/air to be squeezed in to. Cousing higher pressure. you see, higher compression ratio.
The more the gas is compressed, the faster the gas burns, the faster the gas burns the more heat it makes. The hotter the air its the more the air(gases)expands, the more the gases try to expanded the more force is placed on the piston. something like that. the hotter the gas burns the more pullutance are made. back to low compression for the Goverment. Sorry for the long post.
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Senior User
Joined: Sep 2002
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From: mt. shasta california
Higher compression ratio...
Thanks for the long post! It cleared up alot of mis comprehension for me. I have been learning so fast recently thanks to this board. How does one mill heads? Is it advisable? Sorry for the flurry of questions on this board, i am just curious and fastly becoming a motor head. I appreciate your help. Oh how come you and a few others type my name as Erik #l85? It does not bother me in the least bit i am just curious.
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From: PacNW
Higher compression ratio...
>Or Just a thinner head gasket. Reduces the space above
>the piston.
Going from an .040 head gasket to a special .035 head gasket isn't really going to net you much compression increase.
>CAN anyone explane "quench" effect ? Is this something I
>should know about before milling my heads ?
Here's a link to the KB site which explains the "quench" effect:
[link:www.kb-silvolite.com/page05.htm|Engine Building]
Milling open chamber heads isn't going to net you much of an increase in the "quench" effect because of the nature of the heads. It will decrease the size of the combustion chamber and net you an increase in the C.R. but it may actually be counterproductive in terms of net HP gain because of the tendency for detonation in open chamber heads. IOW you may have to back the timing down so much to prevent detonation that you actually lose power.
>the piston.
Going from an .040 head gasket to a special .035 head gasket isn't really going to net you much compression increase.
>CAN anyone explane "quench" effect ? Is this something I
>should know about before milling my heads ?
Here's a link to the KB site which explains the "quench" effect:
[link:www.kb-silvolite.com/page05.htm|Engine Building]
Milling open chamber heads isn't going to net you much of an increase in the "quench" effect because of the nature of the heads. It will decrease the size of the combustion chamber and net you an increase in the C.R. but it may actually be counterproductive in terms of net HP gain because of the tendency for detonation in open chamber heads. IOW you may have to back the timing down so much to prevent detonation that you actually lose power.
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Posting Guru
Joined: Aug 2000
Posts: 2,416
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From: PacNW
Higher compression ratio...
>Thanks for the long post! It cleared up alot of mis
>comprehension for me. I have been learning so fast recently
>thanks to this board. How does one mill heads? Is it
>advisable? Sorry for the flurry of questions on this board,
>i am just curious and fastly becoming a motor head. I
>appreciate your help. Oh how come you and a few others type
>my name as Erik #l85? It does not bother me in the least bit
>i am just curious.
Milling heads comes with it's own set of complications depending on the type of head you're working with. In order to significantly reduce the size of combustion chamber you have to mill a lot of material (.015+) off of the heads. This changes the geometry of the top of the engine and will mean you'll probably have to switch to a variable geometry valve train, i.e. studs & guideplates. You'll also need to get different size push rods and very probably also have to mill the intake manifold to accomodate the new angles.
>comprehension for me. I have been learning so fast recently
>thanks to this board. How does one mill heads? Is it
>advisable? Sorry for the flurry of questions on this board,
>i am just curious and fastly becoming a motor head. I
>appreciate your help. Oh how come you and a few others type
>my name as Erik #l85? It does not bother me in the least bit
>i am just curious.
Milling heads comes with it's own set of complications depending on the type of head you're working with. In order to significantly reduce the size of combustion chamber you have to mill a lot of material (.015+) off of the heads. This changes the geometry of the top of the engine and will mean you'll probably have to switch to a variable geometry valve train, i.e. studs & guideplates. You'll also need to get different size push rods and very probably also have to mill the intake manifold to accomodate the new angles.
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Postmaster
Joined: Aug 2001
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From: Nevada
Higher compression ratio...
Eric, I think this has been covered in one of your prior posts. Any '68-78 385 series block and stock pistons, '68-71 heads with stock rail rockers, stock pushrods and a '68-71 timing chain assembly will get you 9.5-9.7:1 c/r, plenty for a cam up to .520 lift, 210/220 duration at .050 with mildly ported heads. Beyond that, you'll need to look at increasing your budget.
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