All Small Six 250 CR errrr.... mistake.

[TucsonHooligan]

So I got a 250ci from a guy who pulled it from a 74 Mustang. I started to build it (it had evidence of recently being machined) and went with the pistons out of that oddball V8 Ford made for like two years in the early 80s. Forget the displacement. Anyway, they're essentially flat-top pistons, with like a 1 or 2cc dish where the valves come down. Originally I was calculating everything out for the compression ratio using the stock head with some porting and valve train work. The V8 pistons were the quicker and cheaper way to boost the compression since I would have purchased new stock ones anyway. That way i didn't have to have the head machined since it was all fresh anyway. With all that, it would put me around 9.1:1, which I thought would be great for a daily with a little more oomph. Then, Vintage Inlines came calling. 20 years after I put my name on the damn list for the aluminum head, he suddenly does a run of 30 or so and I was eligible. I hadn't even thought about it since the guy from phx passed away and it seemed like the head went with him. Anyway, I had just got my bonus from work and decided to blow it on the aluminum head. Problem is, he says the chamber is 55cc. With that, my CR jumps to roughly 9.9:1. That seems super high to me for a daily driver on pump gas. I plan to run an EFI system on it, but my question is, do I need to scale that CR back a bit? I'd hate to go through the hassle of swapping pistons again (they're already on the rods, ringed, and installed in the block with ARP rod bolts locking them down), but I'd rather disassemble it all and start over if that high of a ratio is gonna grenade my block or my brand new (expensive) aluminum head. Anyone have any tips here? Is that CR as outlandish as I think it is, or can I get away with it as a daily if I run 91 octane and don't hot rod it everywhere? Thanks!

 

[DON]

I’m not a pro like some here, but they are going to want to know two thing , are you talking static compression or dynamic compression and what cam are you running. Ok, maybe 3 things. How Is the cam timed?

 

[aussie7mains]

10:1 with the alloy head should be OK, alloy dissipates heat better than iron. Forget all the "dynamic compression" stuff, its mostly snake oil. Using a bigger cam to get the DCR down is basicaly throwing the volumetric efficiency away to get away from knock, you want the VE as high as possible at ALL rpms, get those cylinders filled! Our octane ratings are slightly different to yours, you might find 91 octane (87 for us) is a bit low for you, so use something around 95. You will need to have the correct ignition curve, if your EFI can map a curve that would be really good. How far down the bores are the pistons you have?

 

[pmuller9]

Are you using the Crane cam that you previously posted?
Which EFI system are you going to use?

 

[drag-200stang]

I would not worry about it, until you can CC it, and know 100% what it is.