I am in the process of converting my 360 into a 390. The truck is a '75 F150. A buddy of mine gave me a 4 barrel intake and matching heads. The head numbers are C6AE-R. I am wondering what my compression ratio will be? How do you determine compression ratio? I am planning on using stock '75 390 truck pistons, rods, and crank. Will those be okay with the older heads? Will the compression go up or down? I want to use 87 octane if possible. I am planning on putting in a Crane 901 cam. I want a lot of torque. Should I use a different piston or will stock pistons be okay?

 

goin by what im told for the 360 optimally i want a 9.5:1

 

Just an educated guess here because I normally run this stuff by my machine shop guy, but... You should be OK usung the stock pistons with your combination. I can't recall if the C6 heads had the small combustion chambers, but all else considered stock, you should be in the 9.5:1 ballpark and the 901 will work pretty good for you.

--Mike

 

Cool. I was wondering if the heads would raise or lower the compression ratio. I guess being that it's a '75 the compression ratio is pretty low to start with. 9.5:1 would be great.
Does anyone know what these C6AE-R heads may have come off of? The intake ports are much taller than my stock heads.

 

http://www.mainstaycomm.net/shady/cu...ace/infrm.html

This might help.

 

The stock 390 pistons, for 75 or 76, will NOT be okay. The factory low compression piston for the later truck engines has only a 1.66 pin height. That, coupled with the slightly lager chambers in the C6 heads, and the .041 thick head gaskets most rebuild sets contain, would give you 7 to 1 compression. Not nearly enough. If you choose a flattop piston, and the thinner steel shim gaskets ( .017 thick ) you can have 8.7 to 1. That would seem quite nice for 87 octane and truck use. That compression calculator is wonderfull. It doesn't have an entry for deck height, but you can compensate by adding deck clearance to the head gasket thickness. It would also seem that the published compression ratios for a lot of pistons is optimistic, to say the least. I'll post you some piston part #s when I get home. DF, from over at his dad's house

 

Hence my disclaimer on the C6 head volume.. Sorry, I was thinking the flat tops as far as the factory piston was concerned, should have asked for a discription in there. Learned something new today, thanks for the calculator.
--Mike

 

You gotta be careful and check out the pin height on replacement pistons, they can actually sit in the hole farther than the originals because the piston manufacturers assume the block's been decked sometime and they want their pistons to fit all rebuilds without sticking out of the hole on some.

 

Question?

I m putting together parts right now for My 390+.030 forged Speedpro..And Moly rings..The block right now is stock but the bore..Going to run RPM Intake and Heads.

I think I might go with the cam Like Sean is going to run..Just a smaller one ..
Its the Lazer Hyd.. #2700140 with the standard .041 head gaskets.. The speedpros are I think 9.63 to 1... I wonder what my actual compression would be?

 

Thanks! That calculator is great. What is the combustion chamber volume on the C6 heads? So this throws a kink in things. I was going to order a master rebuild kit for a 75 390 truck engine. Including pistons. So could I order a master kit for say a 66 Ford 390 truck engine? I am trying to get a kit because they are cheaper. Anybody know what these C6 heads came off of? If you can get me those piston numbers I would appreciate it.

I found a thread on reusing my current 360 pistons to boost compression. Will the 360 pistons I have now raise my compression enough with the 390 rods and crank? They are in good shape.

 

360 pistons and a 390 crank should get you the GT CR of about 10 to 1