What all must be done to an engine if it has been boared at all. I'm assuming the same things need to be done whether .03 over, or .06 over. Just in a different scale. Anyway, I know new pistons are needed, but what about the heads? Or are those also boared a little?

Also, about how much do good flowing heads, a cam, lifters/ect. and intake cost? Would there be any way to fit most of that in for about $1k? I already have headers/Y-Pipe, and later on a cat-back.

 

Hmmm, lotsa questions.
Alright, from the top. No, there's no problem with the heads when you bore an engine. The heads remain unchanged. Most times, they're checked to make sure they're still true (flat), and milled if they fail. A 3-angle valve job is also a good idea if the heads are worth keeping. The head have their own optional rebuilding process, but that's a whole other story. I think the comp ratio goes up a little when the engine is bored, but we're not talking much.
As for new head, expect to pay close to $1000 for a complete head. You can get iron heads for cheaper, but I'd need more info before I could start throwing around brand names and part #'s.
Cam is usually about $175-280 for a brand name roller. Lifters are about the same price. Again, need more info before I can be more specific. You can get used parts (LT1) for way cheaper.
Intakes go for less than $200 if you want carb, and over $200 if you want TBI. The difference is one needs an adapter plate, the other doesn't.
$1000 should go a long way if spent wisely.

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No guts, no glory.

 

incorrect info was posted. sorry.

[This message has been edited by zerogauge (edited November 29, 2001).]

 

Goes up when you mill the heads. So, if you want to keep the same compression while boaring, you have to mill the heads. But I would think that would actualy lower your cubic inches again, which would put you back where you started. Or would it not? The only way I can think of bringing your compression back up after boaring would be to get a little longer rods. But that is tricky due to the smashing of the sparkplug/valves. :-(

 

i don't think that longer rods effect compression. it seems they would, but i thought i read somewhere that they do not. milling the heads will not effect the cubic inches of the motor either. the only way to raise compression is by using a domed piston or milling the heads because that will make the combustion chamber on the head smaller.

compression goes up with a smaller area.
compression goes down with a larger area.

i think people think that the more room you have the more compression you have, but it doenst work that way, you have more room to squeeze the same volume of air and fuel into which equals less compression.

correct me if i am wrong...but this is just how I understand it.

zg.

 

um no. Since the pistons are bigger as they move up they are compressing extra atmosphere that wasn't there before which increases the compression ratio.

as for longer rods, they don't effect the compression ratio but they do keep the piston at top dead center and bottom dead center a little longer. This helps create a more powerful explosion when the spark hits the mixture because it is held longer.


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-Tas
'89 Formula WS-6

305, TBI, 700R4, P.A.W. 14x3 open element with K&N, Milodon 160* thermo, functional Formula hood, cross-flow Flowmaster, '99z28 rear pipes and tips, Hooker 1-5/8" 50 state legal headers, Dynomax 3" I pipe (PN 44063 and 43248)
Super GRK_Taz World
F-Body Dual Exaust
EFI & Intake Options

[This message has been edited by Tas (edited November 27, 2001).]

 

i see. i wasnt thinking straight. the piston is going to go all the way to the top of the cylinder so the actual size of the cylinder is not really taken into account for the compression ratio. well it is...like you said the larger the cylinder the more air that will be pushed up...

thanks for correcting me.

 

WTF are you guys talking about. It's obvious that some of you aren't doing enough research but it's all good, at least there is some chatter.
Displacement (bore changes) and cylinder compression are 2 different things. Displacement is how much volume of air the piston moves. You can't increase displacement by changing the heads combustion chamber cc! Without any heads the engine still displaces x amount of air.
A larger bore will move more air and so the same heads installed will increase compression IF the pistons and stroke are the same (pistons being oversize obviously).
This is why most 350 heads installed on a 305 have to be milled to get normal compression ratios. If you're boreing out an engine the heads can usually be left alone.
Below is a scetch to prove that same heads on a bored engine will increase, not decrease, compression.
The math, try and follow, sit back and enjoy the show, lights please, thanks :
Test engine is a 350 block with a 3.484 stroke bore
The constants when calculating compression ratio:
Stroke, head gasket volume, piston dome volume, and combustion chamber volume.
The variables: cylinder bore.
The formula for compression ratio:
compression ratio=
[(combustion chamber volume)+(head gasket volume)-(piston dome volume)+(1 engine cylinder displacement in cc, NOT ci!)]/(combustion chamber volume)+(head gasket volume)-(piston dome volume)
To calculate the cylinder displacement in cc:
(Diameter/2)^2 x 51.49 x stroke=single cylinder displacement=V
This is variable since diameter changes after you bore the block!
Now that we have all this jazz, let's simplify it with abbreviations:
CR=compression ratio
C=combustion chamber volume=(78cc)
P=piston dome volume=(-4.2cc)
G=compressed head gasket volume=(7.57cc) this is if you use a 4.1"x.035" gasket
V=single cylinder displacement (aka swept volume)
We can now rewrite the CR formula as such:
CR=[C-(P)+G+V]/[C-(P)+G]
Let's start with a standard 350 with a 4" bore and 3.484" stroke.
Calculate the volume with the first formula and you get 717.6cc=V
Now we have all the numbers and just need to plug them into the CR formula:
You should get 9:1 if you did everything right.
Now lets increase the bore to 4.03", calculate the new volume and you get 722.9cc=V
Plug in all the numbers and you get 9.05:1
Now lets increase the bore to 4.06", 728.3cc=V
CR= 9.1:1
So you can see that as the bore increases the compression ratio increases.
You can try the math with your own numbers from a 305 and even play with different heads, you'll still notice the compression ratio increasing.

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, Jon (350 TBI!)
91 Red My website

 

I think that's what Tas and I are saying.


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No guts, no glory.