I scored a local craigslist 390 for a great price, I've been wanting to replace the 360 in my truck (1969 F-250 C/S) with a 390 for ages. It seemed stock from the exterior, no intake but it had either C8 or D2 heads on it (can't remember off the top of my head). I've run into a slight snag while tearing the motor down and checking everything out though, it looks like it's been rebuilt .030 over with Speed Pro/Sealed Power H395P flat top pistons. The pistons sit .030 below deck, but even so according to my calculations I'm looking at a 10-10.5 static CR depending on what gasket I use .
Is there any cam I could use to make this motor streetable on pump gas or am I stuck replacing pistons? My truck doesn't get raced, but gets used for hauling things around town and occasional heavier towing. It looked like I could run a comp XE284 to get my dynamic compression down around 8:1, or even a comp 292. I'm concerned that these will be too much cam for a mild street truck. The end plan is to do some mid length headers, stock ported heads, and an Edelbrock Streetmaster 390 intake. Any suggestions on what I can do with this motor?
What combustion chamber volume are you using to get the 10.5 SCR number?
My 390 build is a 60 over block with H395P pistons, 7 thou below deck, using C6AE-R heads with a combustion chamber volume that CC'd right about 75cc. Using a head gasket with a 41 thou compressed thickness (the Fel Pro performance ones), my SCR is right about 9.75. I have a Howards hydraulic roller (CL250705-12) and the DCR is right at 8.00. Runs well.
sparky72 wrote:What combustion chamber volume are you using to get the 10.5 SCR number?
My 390 build is a 60 over block with H395P pistons, 7 thou below deck, using C6AE-R heads with a combustion chamber volume that CC'd right about 75cc. Using a head gasket with a 41 thou compressed thickness (the Fel Pro performance ones), my SCR is right about 9.75. I have a Howards hydraulic roller (CL250705-12) and the DCR is right at 8.00. Runs well.
I crunched the numbers with a combustion chamber size of 70 CCs. I am using either C8AE or D2TE heads, can't remember exactly which the motor has but I know they use the same chamber. I also punched in a head gasket thickness of .015, I was thinking of using the old style steel shim gaskets to give myself better quench since I'm .030 in the hole. I dialed in my numbers a little better and I'm now getting a 10:1 SCR. How do you like your Howards cam? I was actually looking at their American Muscle Cam that's a clone of the 390 GT cam. Link here: https://www.howardscams.com/hydraulic-f ... ams-257711. I've haven't really been able to find anyone online using this grind to see what they think though. According to a DCR calculator it would give me a DCR of about 7.35 which would be great for pump gas.
I like it. It’s a little bigger than I originally planned but then again I was planning on a lower CR than I ended up with. It actually behaved fairly mildly. Howards makes good cams, not the most modern grinds but they work well. 390s in general seem to tolerate bigger cams than you might instinctively think.
7.35 DCR would be great for a truck on pump gas, especially with good quench. 70cc head volume sounds about right; the C8 heads I started with cc’d at around 68. Since you’re kind of on the edge I would cc all 8 chambers, or have a machine shop do it, to verify your calculations.
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