Re: cylinder heads and compression ratios  
Re: Re: cylinder heads and compression ratios -- david targhetta Post Reply Top of thread Maverick Message Board
Posted by: Fred H
01/17/2002, 22:46:40

 I think you are right. In fact. that is what my Ford Preformance book suggests. Another question I have is the book mentioned something about "rail rockers", which according the the book is a type of rocker arm that was installed on all 66-78 302's. The book said that these rocker arms are no good for a high lift cam or high rpm. I was wonder if you ever heard of this. (I dont know how accurate that books is) also when i was reading about the cylinder heads I found a part in there that told me that my compression ratio is actually rated at 8.4:1 rather than the pre '77s 8.1:1. It mentioned that the combustion chambers were actually increased BUT the book figured ford changed the type of pistons because the ratio was rated higher. I am very confused!
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:=Dave, you are right. 8:1 compression is much too low for any sort of performance. I say time and time again that compression=power. The cheapest and easiest way to bump up compression (assuming that you do not want to switch out pistons) is to install a set of closed chambered heads (like the old 289 heads w/ a 53cc chamber) and run the thinnest possible head gasket. I am not sure exactly how much that will give you, but it will surely be at least another point or so and whatever it is will certainly be streetable. I am sure someone else (like Ken or rickyracer) can give you a more accurae picture of what our final CR will be.

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There are a couple of reasons why theses rockers are not recommended for high lift and high rpm use. First, they are not designed to withstand the spring pressurs that a high RPM motor requires. Second, the factor that limits high lift is the lenght of the slot (that slides over the stud). It is typicall y said that you cannot go larger than .500 lift with a factory steel rocker arm. The benefit to a rail rocker is that it will allow you to run an adjustable valvetrain without using guideplates (which would require maching for screw-in studs) and hardened push rods. There are relativelty inexpensive aftermarket rockers available made from stronger material and with a longer slot. I would use those if your budget is tight.

BTW, how big of a cam do you plan on using?

I take it that you are going to use your original short block. In that case I would not be too concerned with .3 difference in compression as the piston is a variable that will not change. Regardless of 8.1 or 8.4 you will be well over 9:1 with the small chambered heads.


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