Calculate compression

CochinoFilipino · Nov 30, 2004

Someone please help my figure out compression ratio.

If I have 9:1 with 64CC chamber heads and a .030 overbore on a roller 5.0 block, what would the compression be by switching to 61CC chamber heads?

Thanks,

D.Hearne · Nov 30, 2004

3cc MIGHT get you a 1/4 point increase

Route666 · Nov 30, 2004

I get 9.3 assuming a 3" stroke.

EDIT: Here's how I arrived at that:

(4.03 / 2)^2 * Pi * 3 = cylinder swept volume = 8 times the combustion chamber volume (the 9 in 9:1 minus 1).

= 38.2667...

Divide by 8 to find out what combustion chamber volume is

-> 38.2667... / 8 = 4.783...

Multiply by 2.54^3 to convert to CC

-> 4.783... * 2.54^3 = 78.385...

Minus the 3CCs for change of head chamber = 75.385...

divide by 2.54^3 to get back to CI = 4.6002...

Add that to 38.2667... (the swept volume) = 42.866...

Divide that by the combustion chamber volume

-> 42.866... / 4.6002... = 9.318

therefore new CR ~ 9.3:1

D.Hearne · Nov 30, 2004

did the math, it'll get you 3 tenths increase. Not hardly worth the effort just for the compression increase.

D.Hearne · Nov 30, 2004

How'd you come up with 10.4 ? I get 9 to 1 with 64cc heads same as he said, and 9.3 with 61 cc. 627 cc cylinder swept volume, 61 cc head, 9.5 cc for the gasket, and 5cc for the piston volume , and that's assuming he's got a zero deck, which he most likely doesn't have, unless he has the right pistons and decked the block

Fostang · Nov 30, 2004

Instead of trying to guess.

Find out what your deck height will be, what type of piston will be used + or - CC, head cc, type of gaskets that will be used etc.

Changes in the deck height or gaskets make for some huge differences if they are enough.

Route666 · Nov 30, 2004

I edited my post, you don't need to know what deck height is, you can work out total combustion chamber volume by knowing that the 9 in 9:1 is going to be the swept volume. You can then modify the combustion chamber volume (take 3 off) to change to the 61cc heads chamber volume and add that to the swept volume, and divide by the combustion chamber volume to arrive at the ratio.

EDIT: Ok that's wrong - the 9 includes the combustion chamber as well. Well then you know the swept is 8 times the combustion chamber then. I'll change my result.

CochinoFilipino · Nov 30, 2004

Thanks you guys are fast.

FYI, I'm not just trying to increase compression. I just found out the Aluminum GT40Y303 heads I have for my rebuild kinda suck. I thought they were very close to the X303 heads but they are not.

GT-40 Y's: Aluminum, 1.94 int./1.54 exh. 160cc intake runners. 53cc exh. runners. 64cc chamber
GT-40 X's: Aluminum, 1.94 int./1.54 exh. 178cc intake runner, 62cc exh. runner. 64 or 58cc chamber.

I was looking through the Ford Racing catalog and found that the Y-headed crate engine was rated at 315HP while the X-headed engine was rated at 340HP. Cam, compression etc was the same, just diferent heads.
http://www.fordracingparts.com/crateengine/302smallblock.asp

I was basically copying the the crate engine specs. So I was contemplating putting the Y303s on ebay and buying some TrickFlows which are 61CC.

I suppose I should see how much the machineshop wants to port the Y303's.

CochinoFilipino · Nov 30, 2004

I was initially wondering about compression because I wanted to run regular crap gas.
Then I realized that more than compression will determine what gas I use (i.e cylinder pressure, timing etc).
Then I got to thinking about how more compression can make the engine more efficient.
Then I was wondering how the cost of using premium gas in a more efficient engine would weigh against a less efficient engine on regular.
Then thought forget it, get heads that perform the way I want and use whatever gas is required.
So expect a "what gas" thread once I figure out what I'm going to do about the head situation.

And thanks again, Now I can save that cool formula Route666 posted and figure it out for myself in the future.

Fostang · Nov 30, 2004

Don't forget that aluminum heads will allow you to run pump gas with a little more timing/compression.

Route666 · Dec 1, 2004

Filipino I still didn't change the 9 in my first post up there. You should take whatever your compression ratio is and minus one - THAT is the swept area (pi * r^2 * h)

I'll change it now, if you copied it, please make sure it's original CR - 1. EDIT: Or you'll balls up like I did.

D.Hearne · Dec 1, 2004

With a 9 to 1 compression range it should run on 87 octane. Might require more depending on how much timing you give it. My 10.4 to 1 331 stroker will run on 89 if I need it to, simply by backing the timing down.

CochinoFilipino · Dec 1, 2004

I've never done any porting. Do you guys think opening up the runners on aluminum heads would be too big an undertaking for a beginner? I would stay away from the valve seats.

I'm wanted to portmatch everything anyway.(i.e. TB to intake manifold, upper to lower manifold, lower to head and head to eexhaust.) But I was not comtemplating going into the runners until now.
Thanks for the help with the brainstorming.

D.Hearne · Dec 1, 2004

What cam are you running? As for porting, I think you'd be better off leaving them be, except for port matching. With iron heads, that's a different story, there, you'd be working on a $50 set of heads, so if you screwed up, you'd not be out the cost of the aluminum heads. You could also mill the Y heads about .030-.040 to get the comp ratio up, that would add some power too, and that in addition to the port match could conceivably get your gain to what the X heads would.

66P51GT · Dec 2, 2004

Here is a compression ratio calculator I created in Excel.

Enjoy.

CochinoFilipino · Dec 2, 2004

I'm using Ford's E303 cam: Duration @.050; 220*/220*, Lift; .498"/.498". Like I said, I was basically copying their crate engine.

I'll be using an AOD with an aftermarket TQ converter (expect a "what stall?" thread) and either a shift kit or valve-body. And 3.50 gears in the rear.

And yeah, I think I won't touch the heads. I already have alot of work to do on this thing.

D.Hearne · Dec 2, 2004

You should be able to use a stock or near to stock stall with that cam. Don't get to radical or you'll forfit the whole purpose in using an AOD for the mileage increase. An E cam should be able to idle at less than 1000 rpm's in gear, so a stall for that purpose shouldn't be necessary. The idle on my 331 stroker with it's Z303 is surprisingly low, it idles ( this is with a 4 speed) at 6-700 rpms.

CochinoFilipino · Dec 2, 2004

D.Hearne said:
You should be able to use a stock or near to stock stall with that cam. Don't get to radical or you'll forfit the whole purpose in using an AOD for the mileage increase. An E cam should be able to idle at less than 1000 rpm's in gear, so a stall for that purpose shouldn't be necessary. The idle on my 331 stroker with it's Z303 is surprisingly low, it idles ( this is with a 4 speed) at 6-700 rpms.

Cool. That'll save me a few ducats.

66P51GT, Thanks for the spreadsheet

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