Posted by Alan Bowes on December 08, 1998 at 23:42:24:
In Reply to: Improvements to 230/251 flatheads during rebuild? posted by Chris on December 08, 1998 at 11:48:30:
Most important improvement during rebuild: Rework the connecting rods. Not necessarily in this order, magnaflux, match big and small end weights, resize, polish the beam, shot peen, add high-strength rod bolts (which may require some minor machining). Next most important thing is dynamic balancing of the crankshaft. Naturally, all of this should be accompanied by a full block rework (drydeck, align bore, cylinder bore/hone, guides, valves, hardened valve seats, cam, bearings, etc. etc. etc. etc.
Want more carb CFM capacity? You can still buy dual-carb manifolds, but, quite honestly, you'll need a taller or wider cam (lift/duration) to take advantage of this increased carb capacity. Otherwise, you're tossing your money down the toilet. If you add too much carburetor CFM capacity without increasing the engine's pumping ability, you're just going to lose velocity through the venturis, reduce throttle response, reduce atomization efficiency, lose low-end power, etc. An "RV" grind cam might help a bit, but...well...let's just say that you're going to have to do a lot of experimenting to get it right, unless you find a fully tested combination that you can copy. There are a lot of factors to consider when doing these modifications. It's one thing to build up something like a small-block Chev, where thousands of people have already done the research for you as to workable combinations, but as to an ancient flathead six, most of the engine-specific knowledge learned back in the '50s about hopping them up is either lost or hiding on some dusty used bookstore shelf, or is still trapped in the gray cells of an early hot rodder. There are a few web sites devoted to inline sixes, and there may be some valuable knowledge to be gained there, as more people "rediscover" how much fun it is to play with these engines.
There is another way to take a research shortcut: You can buy software programs that will "dyno test" your dream engine for you. Enter the specifications and let your PC crank away at it. You'll have to make sure that you find one that is not specifically set up for OHV engines and will allow for the less efficient flathead breathing characteristics. You can enter things like bore, stroke, valve diameter/lift/duration, intake/exhaust passage dimensions, ignition timing, advance curve, fuel system type/cfm/carb config, etc. etc. etc. Keep in mind that the results from even the best of these programs will only be approximate, but they may get you a lot closer to an optimized combination of modifications.
The safest/simplest power mods might involve a bit of head milling, minor port cleanup, manifold-to-block port matching, flow matching, tuliped valves, low-restriction muffler, electronic ignition, etc. Keep in mind that flow improvements in and of themselves may reduce pumping losses a bit, but they probably won't help this engine to a very large degree, since it's such a LOW-speed mill. The head milling and electronic ignition are the easiest and best bets for some minor power improvements. Be gentle with any head milling and make sure there is adequate room above the fully open valves to allow a little valve "float" (at a redline of 3400 RPM, there shouldn't be much, if any, float; however, boosting the redline has the potential of introducing a bit of float, depending on valve spring characteristics).
As to stroking...well, if you're going to get that exotic, especially with the 230, which will probably require submerged-arc welding on the crank and regrinding, I'd suggest starting by reading some good books on engine theory, because you're going to have to do a lot of computations and experiments to get it right. It will also be quite expensive.
I'm not saying you shouldn't do this stuff. Just be prepared for a lot of work and expense if you want to do a quality job. If that's fun for you, go for it.
Maybe the first question you should ask is: How will you be driving this? Do you want to keep your low-end power? Will it be used for sustained periods of highway speed use? Will it be primarily crawling along rough roads and trails? Will it be used for all-around use? Pulling a heavy trailer? Etc. This will determine what your horsepower and torque curves should look like. How about gearing? That's another major consideration.
Alan