11.6 cr / 9.9 Dcr - 360 build

I'm currently building a street-only 360 and shooting for low rpm torque / high efficiency engine.
Would like to hear thoughts about the high dynamic compression ratio, cam choice and octane on this 'low rpm' engine.

Am I good to go, or pushing the limits too much with the cam?


Engine specs
- 360ci +0.030"
- Stock, but balanced, rotating assembly except for .030" KB 0-deck pistons.
- Iron heads, '302' casting (closed chamber). 60cc polished chambers and valves
- Edelbrock aluminium Performer intake, w/ blocked heatriser ports.

- Shooting for a very tight quench-clearance of around 0.025-0.030".
- Average rpm-usage: 600-2500rpm. Max. rpm around 5000 rpm.
- Fuel is Propane/LPG (±104 octane)
- Cam is CompCams XE256H, used with 1.6 ratio rockers

- Will also run a 160 thermostat and fab a cool-air intake system.
- Car is '73 Dodge Dart, A518 OD trans, 3.55 rearaxle.

All pistons are .015" below above deck on 1 side of the block and 0.010" below on the other side. I will equalize this with use of .028" and .039" headgaskets and a slight mill on 1 head.

- Calculated Scr is 11.6:1 / Dynamic cr is 9.96:1


Cam card



Screen pic of calculated Dcr below, taken from:
https://www.uempistons.com/index.php?main_page=calculators&type=comp

Why do you show the pistons .015" above the deck in the compression ratio calculator screen shot?

If it were mine and I wanted the quench that tight I'd mill the tall side of the block .025" and have the pistons out .010" on both sides.

Was a typo in the topic-text. I had it reversed. Changed it. The screenshot is correct.

In a perfect world the block would have made the pistons end up at 0-deck. but I was only able to check the parts until now.
I've decided I won't be taking it all apart again to just deck the block, the project has been taken too much time already and needs to be done fairly soon. The difference in deckheights can be 'fixed' with headgaskets and a slight cut on one head, don't see no issues there.
I measured the 4 pistons on all corners and show no slope in the decks.

My concern is mainly the DCR as a result of such a mild cam.

Correct your intake closing point and your DCR will be less, i noticed you didnt add the 15°

Aside from the fact that doing different HG on each side is a total "hack" way to fix this...

The .025" is pretty tight. I ran one with KB pistons's before between .030" and .035" and you could just see signs of "kissing" the head. This was at 12.2:1 that I would run most of the time on pump gas, but it had a 250@.050" solid roller in it, so I had that to help the DCR.

I would back the Piston to Head down a bit, and let it go. Worst case is that you watch the plugs close and take a couple degree's of timing out of it.

I think you may be OK with using the propane?
The auto-ignition temp of Propane is 878 degrees F.
Compared to Gasoline at 536 F, or Diesel at 410 F.

On the octane, 104 equivalent research octane would be about 99 or 100 octane with the (R+M)/2 method. So that's about the same as 100 octane for a US reference.

Not a SB expert but I think 11.6 CR will be close with the torque cam... with the cool T stat and cool air probably OK.

I think you would be completely safe with 1/2 or 3/4 point less. Many/most KB pistons can take quite a bit of a cut. You could cut a step dish to drop CR (and would then need to re-balance) or just put it together and give it a try.

Personally I would not cut the head... keep it matched. No big harm if there is a bit less compression on one side.