The Stovebolt Site Forums The Shop Area Engine Shop 250 rebuild, increased compression

I'll be beginning to overhaul my 250 soon and I understand that I have a couple of easy options to increase the compression ratio; use 307 pistons or use a 194 head. Which of these raises the comp ratio the most? If I do both, will it still run on regular gas okay? I plan to use a small 4 bbl carb on an offy intake and stock cam if that makes any difference. Any info or suggestions will be appreciated.

Thew last one I built that way got 307 pistons, and a .050" mill job on the 250 head. It runs on regular gas without any problems.
Jerry

The last one I built also got the 307 pistons, however it received a decent cam to make use of the new found squeeze to be able to ingest a little more air. The 194 head would have reduced the combustion chamber which would have helped my goal. I found the limitations being the intake ports on the head.

Scott / Jerry
Was the HP increase noticeable? I've got a 250 with the original pistons and you've got me thinkin'........
Ricky
"BlueDawg"

I didnt have the car dyno'd like Jerry would, however I would estimate close to 250hp, however it was a little finicky as I over-camed it for general cruising. http://store.summitracing.com/partdetail.asp?part=CCA-SK61-246-4&autoview=sku It did spin up to 6000+ RPM easily though where I had to use a rev-limiter to keep it below 5500. Power dropped off after 5000 rather quickly as I was running out of air due to intake flow. Cylinders 3 and 4 ran much richer than the other 4, however I think that was because of the intake I was using.

However if you keep compression to under 10:1, port the head, use a moderate cam, a set of headers and a decent intake you will be happy with it. However once you add up the cost of building the I6 including machine shop work, etc and compare it to building a V8, you will soon realize why there is an SBC put into so many things. My buddies Mid Am race engine was built for less in 2007 than my 250 was in 1997.

Yeh, as a lover of unusual engines (read: Ford flathead, 351 Windsor, Corvair pancake six, Pontiac 400, Chevy 250 I-6), I feel your pain... $$$$$$$$$$
250 HP is pretty awesome for an I-6. I'm certainly below 150 currently. My Mustang 4.6L came from the factory rated at only 260HP ! [and the 5.0 that everyone flipped over in the 80's was only rated at 235 HP from the factory]
Ricky

I don't plan to race or run at high rev's. I just want a cool engine with a little extra pep at normal driving rpm's. I've read in other posts that each option (307 pistons or 194 head) will raise the compression ratio to about 9:1. If that's correct, I believe that the 194 head would be the most cost effective way to go. I've never seen any info on the compression ratio if both 307 pistons and a 194 head are used together. If I knew the combustion chamber volumes for each head and the volume of the dish on the standard pistons I could calculate the compression ratio for any option but I haven't been able to find that info. If anyone can provide this info or tell me where to find it, I'd appreciate it.

Since the 307 piston can be obtained in any CR from 5.5 to 14:1, you can have your cake and eat it too.

a simple google search will get you combustion chamber volumes, etc.

I'd want to do some very detailed checking on combustion chamber shape, gas flow, and piston-to-valve clearance before I'd try to use anything taller than a flattop piston. The main advantage to the 307 stock piston is that it's a drop-in replacement for the dished 250 piston, same weight, bore size, wrist pin diameter, and compression height. I believe it's also available as a hypereutectic alloy which rivals a forged piston for strength, and it has virtually zero thermal expansion.

The first high-compression 250 I built went into a Nova 2-door, and it was a real hotrod. My student did a very precise weight-matching job, down to 1/10 gram on everything that moved, plus a good port & polish and a 3-angle valve job with stock-size valves. We didn't dyno it, but he and another student with a Firebird with a Pontiac 350 in it used to race to school frequently, 20 miles of 2-lane highway, and neither one could outrun the other. I think they both got a very expensive speeding ticket one morning!
Jerry

Here is what you are looking for,
http://www.inliners.org/tech/heads/headindex.htm

As you can see there are a few 194 heads, so be sure you get the right one. Also, the 307 pistons are now universal and have larger dishes in them. Check them close, I bought a set and sent them back because the dish was as big as the 250 piston was!

Joe