Posted by Alan Bowes on October 03, 1998 at 08:38:25:
In Reply to: Re: The Great CDN M-37 Engine Displacement Controversy posted by Bob C. on October 01, 1998 at 08:31:04:
To answer your question Bob, Stroke is determined by the crankshaft.
Rod length affects "rod ratio," but not stroke length. Rod ratio is the ratio of rod length to stroke. When you change the length of connecting rods, you must also use different pistons, with the wrist pin location either further up or down the piston, depending on whether you use a longer or shorter rod.
Note: When you start experimenting with rod ratio, you are getting into rather deep water from a technical standpoint, since rod ratio affects the acceleration and velocity plots of the piston, the inertial moments that the reciprocating parts will encounter, the breathing characteristics of the engine, the angles at which power is applied to the rod journals at different points in the piston stroke, the relative dwell time at the lower and upper reaches of the piston stroke, etc. etc. etc. All of this has a huge effect on the horsepower and torque curves. Racers tend to like longer connecting rods (higher rod ratios), but they are also dealing with torque/HP curves that have peaks at a MUCH higher RPM than you'll ever see in a 230 or 251 or any variant of it. Usually, manufacturers try to come up with the best compromise for the intended use of the vehicle.
One might be able to other engines that use the same block and crank, but different rods and pistons, which may provide a different rod ratio. I can't recommend anything just offhand, without doing some research on it.
My advice, unless one really wants to delve deep into this technical area, is to stay with the stock components, and concentrate your efforts and budget on doing some conservative things that will improve the engine's toughness and provide a small power boost.
Since I just posted something along this line on another forum, I'll go ahead and include that here:
The rods tend to be the weak link in the chain. Stroke length and rod ratio are factors that affect the inertial forces that the rods will see. There isn't much
you can do about that without some advanced parts swapping and crank work. However, having the rods reworked is not all that expensive and it can
probably double (a guestimate) their fatigue strength and longevity. Having them sanded a bit here and there and shot peened is a good way to improve
the fatigue resistance of the rod itself. High-strength rod bolts, such as from SPS, are highly recommended. Torque the SPS bolts according to the
specs for the new bolts, not to the original specs in the manual. Having the rods resized (re-rounded) is pretty much standard practice for a good engine
rebuild.
Balancing the rods (all big ends same weight and all small ends same weight), and having the crank balanced with weights in place to simulate the
reciprocating parts will also help reduce the chance of a higher-RPM failure, and it will make your engine run a bit more smoothly.
Naturally, you need good-quality machine work done on the block (lap and align bore, drydeck, cylinder boring with torque plates, good-quality
matched-weight pistons, new wrist pins, all clearances correct, etc. to do a total rebuild that make the engine a lot tougher at higher RPM and which will
also result in an engine that is better and tougher than any 230 that ever rolled off the assembly line by a substantial margin. Besides the higher-strength
rods and balancing, a properly rebuilt engine will also have a seasoned block that is less prone to warpage, and will be machined to closer tolerances
than the stock engine, etc. etc. etc.
The stock oiling system should be fine, but make sure you've got a good pump and the proper relief (bypass) spring/valve. I'd suggest installing a new oil
pump.
As to how much higher you can push the practical RPM limit, I won't speculate. There is no doubt, however, that a top-quality engine rebuild will improve
the engine's reliability at a higher RPM. You should certainly be able to raise the redline by a couple hundred RPM, but I won't go out on a limb beyond that
point, or someone will be blaming me for a rod going through a cylinder wall. Every engine rebuild is slightly different and there is no such thing as two
identical parts. One engine might be able to handle a thousand extra RPM (scary) and another might blow with only 500 additional RPM. In other words,
no guarantee.
Don't expect to turn the 230 (or 251) into a racing engine, although there are those folks who did, and still do, soup up their flathead sixes and take them to the
strip. What appeals to me about a really good rebuild is that it provides a greater margin of safety and longer engine life, even though I would personally
not push the new, higher RPM envelope too hard.
Having the ports flow matched would be a nice touch and could add a hair more power. However, to take maximum advantage of flow matching, you
might want to experiment around with carb jetting (this requires some theoretical knowledge and a few good sensors/instruments).
You could probably also shave the heads just a bit, but don't get carried away. At least have them milled enough to clean up any warpage and minor
scratches. If you have any deep gouges, you might want to pick up an NOS cylinder head. They're still readily available.
I won't go into cam and valve work here, but there are various possibilities there. Carburetion changes aren't recommended unless you like to experiment
a lot.
Shop around for a good engine rebuilder with a solid reputation. The good shops are not necessarily much more expensive than the discount outfits, and
might be cheaper in the long run.
Naturally, budget can be a factor. Prioritize by starting with a good basic rebuild, then taking care of the higher-priority "extras" first. The rods are at the
top of the list of "extras" beyond the basic rebuild, followed by dynamic balancing, followed by flow matching, etc.
Have fun,
Alan