915 heads on a 68 bottom end

[etauljr]

I'm sure this has been hashed out before but I'm new here. Anybody used closed chamber 915 heads on a stock 1968 440 bottom end before? What would the resulting cr be? Thanks

 

[GTX MATT]

The 68/69 pistons will end up .050-.060 below deck on a stock block. Actual CC on a 915 head is generally 80-84.

.060 below deck with a .051 gasket at 4.59 bore and you're 9:1 with an 84 CC chamber.

.050 below deck with a .039 gasket at 4.41 bore and you're 9.5:1 with an 84 CC chamber.

.050 below deck with a .021 steel shim gasket at 4.41 bore and you're 9.9:1 with an 84 CC chamber.

.050 below deck with a .021 steel shim gasket at 4.41 bore and you're 10.25:1 with an 80 CC chamber.

 

[Meep-Meep]

More compression, which isn't such a bad thing. I have 915 heads on my 68 383 and on a 77 440. Both engines have zero or nearly zero deck pistons. Do it!

 

[chargin']

Agree with Meep-Meep,done it,worked great!

 

[Cranky]

What cam are you going to run? You don't want a cam that builds a lot of cylinder pressure in the lower rpm ranges with 10+ compression.

 

[Garys1969RR]

I have found that these motors love compression. Especially at 5400 feet where the air is already thin. Theory says that the more you compress the charge, the stronger the expansion force is. That results in more torque and horsepower. Plus when the piston starts down on the intake stroke, a smaller chamber volume will result in a greater pressure differential sooner in the downstroke. And when the ex gases are pushed out, there will be better scavenging simply because there is less area for the residual ex gas to reside in. That's my thoughts on C/R. And my 451 at 13.3 to 1 C/R runs very well with the 509 cam and stock valve gear.

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Put those 915 heads on your 68 440 using the .020" steel shim gasket and you should have a great running combination. As long as the rest of the motor is built and tuned properly. Make sure the pistons are at least .017" below the deck.

 

[etauljr]

What cam are you going to run? You don't want a cam that builds a lot of cylinder pressure in the lower rpm ranges with 10+ compression.

As far as a cam goes I'm not sure just yet. That's why I was trying to figure out the compression ratio before I chose one. The motors going in a 65 coronet with a 727 with a 2800 stall speed gears probably 3.55 with headers a tm7 eddie intake and will only be driven probably less than a thousand miles a year. It doesn't need to be a race car. Just run good sound good and make decent power. I just got the car and it came with these heads I'm talking about fresh from the machine shop still in the bag with the spec tag on them. Theyve been ported a little and fitted with 2.14 1.8 inch valves. I would prefer to use them since I already have them.

 

[Garys1969RR]

Based on a bore size of 4.32", 80 cc head volume, .020" gasket with a diameter of 4.4", Looks as if your C/R is going to be in the 10.25 to 1 area, as GTX predicted. You can go to the United Engine and Machine website, click on calculators, and come up with various C/R figures by changing piston height, head cc, etc. For instance using these same figures but reducing chamber volume to 73 cc, you come up with 11.03 to 1. And raising the piston below deck distance to .015" you would have 12.04 to 1 C/R. By the way, the factory states that on a 915 head the volume is 73.5cc. But in reality they check at 78 to 80.

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You may want to cc your heads to be sure what cc they are at. They may have been milled down. Once you have your exact figures in, we can help you choose a cam.

 

[Meep-Meep]

Little things like a huge bore chamfer (like on my engine) will effectively increase chamber volume by about 5 CC's. Also valve reliefs can add another 5 CC's. The head gasket isn't round and is larger than the bore so you really need to CC it with the chamber or cylinder. And before you know it your 11:1 calculated ratio has become 10:1.