I recently purchased a set of aussie heads for my 400. What I have read about these heads is that they will bump the cr up by 1.5. The heads have a combustion chamber of 59 cc's. But the High Performance Ford Parts Interchange book States that 351C-4v heads with a 61-64 cc chamber (i'm comparing cc's, not the heads themselves!) will push the cr to 11.0:1 with stock pistons. Are these numbers based off flat tops or dished pistons. I will keep the dished pistons in my 400, but possibly bore it 30-over. I plan on running a weiand intake, 670 cfm holly, 2" spacer, comp cams 265 deh, headers, and dual exhaust, with an msd ignition. What would my compression be and how would this perform in a 4x4 with a c-6, 205 t-case, 5.13's and 44's? We only have 91 octane around here, so I figured I might have to run an additive. It is stricktly a weekend toy, so gas mileage is not an issue. Any insight is greatly appreciated. Thanks.

 

According to my calculations you will get 10.04:1 with 10cc dished pistons, 3cc valve relief, 0.0565 deck clearance and 59cc heads. Replacement pistons sometimes have a lower compression height than stock pistons. This will increase the deck clearance slightly.

The 265DEH has an IVC of 58.5 degrees. The Dynamic Compression ratio will be 8.39:1. That is very marginal to use 91 Octane gas. You could retard the cam 4 degrees and the DCR drops to 8.15:1. If your replacement pistons are down to 0.0665, then your DCR drops again to 8:1. At 8:1, I think that you will be OK.
You could use a 275DEH with a small loss of torque at low RPM, but your DCR drops to 7.94:1, with the cam at zero degrees and 0.0665 deck clearance.

 

How much loss in torque would I be looking at? I want to make as much power as I can, considering my small budget. Im all for lots of horsepower, but I want the motor to pull at low rpms as well.
Also, what is dynamic compresion ratio? I have never heard of that. And how do I know how much the pistons are dished? Thanks for your help.

 

The following is a brief list of the torque and HP from a 400+0.030, 59cc heads, 10cc dished pistons, 0.0665 deck clearance, 3cc valve relief.

• 265DEH 265DEH/-4 Degrees 275DEH
• 452 448 425 Torque@2000 RPM
• 453 448 433 Tq@2500
• 450 443 433 Tq@3000
• 440 438 433 Tq@3500
• 421 422 424 Tq@4000
• 330 336 343 HP@4500 RPM

Here is a site which will explain DCR, also there is a downloadable program that will calculate DCR.

http://cochise.uia.net/pkelley2/DynamicCR.html

You should be able to get all the information from the piston manufacturer, as to the ccs of the dish, valve relief and Compression Height. You need all of this information to calculate your Static Compression ratio. You should also check the ccs of the heads.

I hope that you have enough in your budget for porting work. If you get a book on porting and a few hand tools, you should be able to improve the head flow. Improved head flow will boost those numbers significantly.