Howdy yall. I got a new motor im building to throw into my 73 F250. Specs are as follows. 390 crank and rods block will proably be 30 over to clean up but hasant been bored yet. Heads are 71 mid risers. Im debating on trying to throw in larger valves or not. Got to have hardened seats thrown in figured while im their ya know? im debating on running flat tops and a thiner head gasket or milling the block or heads to get to bout 10.1. Is this too high for just 91 pump gas? I know it depends on cam and timing but just wanting some of yalls advice. Thanks in advance

 

With the right cam and timing 10:1 would be fine. Stock 428 cj's were 10.6:1

 

Not a simple question.

Depends on:

-Cam selected
-fuel desired
-weight of vehicle, weight of potential load
-gearing
-altitude
-engine cooling ability
-quench
-efficiency of combustion chamber
-intended usage. Big difference between a daily driver, and a fun weekend vehicle.

etc.
Although as 69cj mentioned, the 428cj's were higher compression and ran fine. They also weren't installed in trucks running 2.73 gears, nor hauling 10,000lbs trailers :P It's a whole package thing.

 

Thats why I ask ya fokes. Its my daily driver and its an old 3 on the tree that i still do use to haul. Im in the navy going port to port so ill usually stay at sea level but home is higher up. Cam is still undecided waiting on luniti to reply.

 

simple answer 10:1 is fine , A set of flat tops with valve reliefs will get you pretty close .
I run mid grade gas in my 390 with no issues

 

+1 on Lunati cam !!

 

Great thanks yall

 

You can run 10:1 with the proper cam and tuning, if you do much hauling 9:1 gives you more flexibility and you won't notice much difference in power output.

 

If he does much towing he needs a lower rpm higher torque type cam. I hate to use the term RV cam since it's so generic.

 

I've ran (RV style) low rpm high torque cams in some 4x4's that worked pretty good.
The old stock performance engines normally had 8.5:1 and 9:1 compression, for a daily driver and hauler I don't see the need or enough gain to go much higher.

 

8.5 or 9 compression was the low compression regular fuel motors in the '60s.

10 to 12 compression was performance territory.

FE Series Engine Specification Chart

 

8.5 - 9:1 compression, stock performance
10-11:1 compression, high performance
12:1 and higher, race engine

With todays gas many 9:1 engines need premium fuel and 10.5 to 11:1 is getting real close to needing race fuel.

 

I don't agree, I ran two 390's with 10 to 1 and mild cams (Cam Dynamics and Crane 510's lift and one had a 272* duration, the other a 296*) both ran great with pump gas towing, if anything he could dial back the timing a few degrees to tow. The Cam Dynamics motor was in a 70 F100 and ran fine on regular unleaded. There wasn't anything you could hook to the bumper it wouldn't pull with that motor.

 

I ran a Cam Dynamics 272 in my 78 F-150 4x4 400 ci engine they only have 216 our at .050 and where considered good 4x4 - towing cams designed for low rpm torque (similar to what some call RV cams).
Years ago several companies made cams that closely copied the 428 CJ cam, these where great DD and towing cams with just a little extra ump.

 

Ive been doing some homework