[Shaun @ Street or Track]

I'm planning a hotter cam, intake and a mild head port job on my motor next month. I've been digging out old receipts from the PO and found the engine rebuild parts list. When I bought the car 2 years ago, it had even compression in the 170 range on all cylinders. It still runs good with no smoke so I was planning on leaving the bottom end alone.

My distressing findings related to the pistons he ordered. Part number FED-273AP40. I researched this through Summit and found this:

http://store.summitracing.com/partdetail.asp?part=8359

The only difference is that mine has a '40' on the end referring to the overbore.

I'm hoping you engine guru's can tell me that the compression I should have is higher than they state here at 8.2:1 with my stock heads. That would be terrible! My car runs like crap on anything other than premium but would explain why it's down on power compared with other stock 289's...

If I'm wrong, what might my compression be with this piston and stock cc heads? Or how can I work it out?

 

[Dinosawnj]

This is really a big depends (on what heads your running). I think the quoted 8.2-1 is based on 68-70 heads with a 63 cc combustion chamber. Depending on your heads, the combustion chamber volume varies from 54.5 (most early heads through 67) to 69 cc for late 70's heads.

If you really want to know, your going to have to check your heads cc and the cylinder cc at TDC a run thru an available calculator. Some one posted a link to one recently. I would have to dig out my paper notes to do it the old fashion way.

Bob.

 

[Shaun @ Street or Track]

I have stock 66 289 heads. 54.5cc's... If that is calculated on a higer cc head, you just saved me from many sleepless nights! Thanks!

 

[sharkmouth]

if you get all the numbers off your heads, some of our brothers here can tell you what cc heads you have

8.2 to 1 is a perfectly accepable comp ratio.......according to what I've read, having 9 to 1 would only give you about 3.2 percent additional power

As for leaving the bottom end alone......if it were mine, knowing that it has good ring seal, I'd pull the engine and put a crank kit (newly turned crank and bearings), especially if I were camming it up. Crank kits are relatively inexpensive, and the crank bearings control oil pressure throughout ford small blocks. Just the way I do things!!

 

[Dinosawnj]

Here's a link to a calculator:

http://www.rosspistons.com/CR%20Calculator.htm

Bob.

 

[Shaun @ Street or Track]

How do I work out my deck height and the cc's of the 4 vavle reliefs in the pistons???

Got some high numbers from that calculator! I know I'm doing something wrong!

I entered

Bore - 4.04
Stroke - 2.87
Head cc's - 54.5
Gasket Thickness - .039 using FelPro
Deck Height - .0001 (don't know what this is...)
Dish - 0 as I don't know how to work out what 4 vavle reliefs might be...

Compression ratio of 10.61:1

I've got something wrong...

 

[Dinosawnj]

I don't think so. I'm running similiar pistons with 54.5 cc heads and if I remember correctly, I'm in the 10.2 or 10.3 to 1 range. To get the cc on relief's and deck height, you could put modelling clay around the piston (usually done in mock up stage before final assembly) a cc the total volume. Otherwise, if your a good friend of a mathamatican, your can measure deck height and calculate.

Bob.

 

[cam65fb]

Running thru the calculation I came up with ~10.5:1 for flat top pistons. If you assume about 4 cc for the valve relives the your compression ratio works out to be ~ 10.0:1. The 54.4 cc heads pumps up the compression ratio quite a bit. But these heads are also know for their small valves which hurts performance more than the higher compression ratio helps. A nice (but a little expensive) performance hop up is the retro fit larger valves. A hotter cam should help a lot with the smaller valves.

BTW the standard crush thickness for head gasket is 0.04, which I used.

 

[Shaun @ Street or Track]

Hey thanks a lot! New valves aren't in the budget at this build up but I will be adding full length headers and porting my heads... Should flow much better.