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Old 03-16-2006, 09:30 AM
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250 troubles

ok, so I bought a built 250 from a guy who basically had all of the work done at a machine shop, he found an article in hot rod magazine with a build up for a 250, and the shop built it to their specs with a couple of upgrades. Unfortunately, I have had no luck locating the write up for specifics. I was told that they used 327 pistons (thinking it may actually be 307s?), a 230 head to up compression, roller rockers, z28 springs, & bored 0.060 over. The owner claimed that the engine was setup for 12.0 : 1 compression.....this seems.....umm....a little high to me, he was planning on running premium, after reading a couple of posts, I think with 12:1 I am gonna need race fuel.

My question is this, is it likely that the combo listed above would result in 12:1 compression? If so, what is the easiest way to bump it down a bit to a point where i can run premium? I would like to keep the head, as it has already been rebuilt, as well as ported and polished, but I would like to get to a safe cr and be able to run gas that I can actually buy around here!

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Old 03-16-2006, 11:37 AM
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If you are not truly sure of the CR and there are no assembly details, I would imagine you would have to CC the combustion chamber (both block w/piston at TDC and head) to come to a true CR.

Cheapest way to lower CR then would be with thicker head gaskets or doubling a set.
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Old 03-16-2006, 07:44 PM
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307 pistons have the same diameter & almost the same height as 250 pistons, but they're flat-tops. 230 & 250 used the same head, but using a 194 head was a common way of upping compression on those motors. Problem was, the intake valves were shrouded, so flow was not as good.

Compression could very well be too high for street use, but it really depends on what cam you use. A flat-top 250 has about the same swept volume as a 334 (305 block, 400 crank), so here's a chart of chamber size vs. compression ratio for a flat-top 334 from the KB piston catalog.

55cc -11.1
58cc - 10.7
64cc - 9.9
76cc - 8.7
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Old 03-16-2006, 10:48 PM
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Even with a 194 head on a 250 You will not Hit 12-1 Unless They Milled the dog off it. close to .100. . But the 194 head with a True Flat top No Valve releaf and NOT been Milled you would be right around 10-1. The 250 pistons are Not Flat toped to start with they are Dished. a 230 Piston is flat topped.
the 307 piston Is common to be used in the 250s. a 283 Piston Is used for the 230 blocks. If they HAD Used a 327 You're talking Punching that Block to a 4in bore This would leave Very thin walls for a daily driver and could very well Over Heat.If the motor does have Pop-up pistons You may think about having the domes Milled down some.
So you best beat would be to spend the Bucks for a new head gasket and take a Look at the Pistons and Also Look to see if they have Unshrouded The valves. If they have Not You can have them Unshrouded to lower the compression. Doubleing up the Head gasket IS NOT a very good idea to do with OUR L6s Because of the Head bolt layout.
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Old 03-16-2006, 10:52 PM
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Quote:
Originally Posted by Twisted6
  • If the motor does have Pop-up pistons You may think about having the domes Milled down some.

  • So you best beat would be to spend the Bucks for a new head gasket and take a Look at the Pistons and Also Look to see if they have Unshrouded The valves. If they have Not You can have them Unshrouded to lower the compression.

  • Doubleing up the Head gasket IS NOT a very good idea to do with OUR L6s Because of the Head bolt layout.
ALL very good advice here...
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Old 03-17-2006, 07:32 AM
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I'd do two things: get the casting number from the head and see if you have a 194 or 230/250 head, and pull the head to get a gander at the pistons. If you have a 230/250 head and flat top pistons you should be good to go. Premium will keep you going at around 10:1.

If the engine is already in the vehicle, you might want to drop some premium in and see how it runs. Set the timing by ear -- drive it and if it pings on a uphill run get out and retard a little, hit the same hill, and retard until it stops pinging but still runs good at idle. If you can't retard it enough to run good and not ping, you're going to have to do something with the compression. 230/250 heads are easy enough to find if you have the 194 now.

Another option is to change the cam for one with a longer duration. That will bleed some cylinder pressure and effectively reduce compression. Jeepers do that when building a "budget" stroker (4.0L w/258 crank and rods) to keep compression down without custom pistons and a lot of machine work. It's not the most efficient way to go since you lose a little power potential due to restrictions of the cam, but I could afford to lose 10 hp vs. another $600 in work.
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Old 03-17-2006, 11:57 AM
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Why don't you run a compression test to give you a general idea of what cr you're running?
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