It's something I've been wondering about since horsing around with Desktop Dyno and running across a few configurations that the computer thought would make more power with a 198 cubes than 225. Of course, it had less torque, but somehow it revved higher and that seemed to compensate for the lack of cubes. Since I'm not sure just how accurate Desktop Dyno's predictions are, I wanted to see if anyone's run such a comparison in the real world. So:

Has anyone done a dyno comparison between two modified slant sixes that used different strokes but were otherwise identical? If so, what was the result?

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"Mad Scientist" Matt Cramer
'66 Dart - turbocharged 225
My blog - Mad Scientist Matt's Lair

There are a few reasons to run a smaller slant 6. The main one is the head. Since the heads are all (basically) the same, then reducing the engine size is the same as changing to a higher flowing head. That means it's easier to get a 170 to rev higher than a 225 with the same head (based just on head flow). With the right parts to take advantage of the rpm range, then you can make more hp per cubic inch - but as you noticed, the torque is not as high.

A 170 has one inch less stroke than a 225 with the same bore, so the 170 has a much better bore/stroke ratio. The 170 also has a 5.7" rod and has a rod ratio of (I recall) 1.85 - the same as a 440. Please keep in mind that I'm at work and don't have completely accurate numbers in front of me and am also doing this off memory from YEARS ago.

Yes, the 225 has a 55 ci advantage in size, but it has a very poor rod ratio, too long a stroke for the bore size and is "strangled" by the poor flowing head. The 170 with it's shorter stroke and lower piston speed is less prone to engine knock and can therefore run more compression for more gains. Doing this in a 225 will result in piston shattering "ping" - or a need for race gas. The extra compression will let you run more cam, which will also help upper end (max) power as well as giving better vacuum and throttle response.

A 225 is 37.5 cubic inches per cylinder, and a 170 is only 28.3. That's a HUGE difference to the engine. If these were V8s' the 225 would be 300 cubes (37.5x8=300) and the 170 would translate to only a 226. I find that (for me) translating to normal V8 sizing helps figure out flows easier.

So, with an engine that small (170) the stock size ports and valves start to make more sense. Obviously porting the head and increasing the valve size will help either engine, but remember that the COMBINATION OF PARTS USED will dictate where in the powerband the engine runs best, how much max power it will produce and also how much USEABLE power it will make.

I stress useable because if it makes 300 hp at 8000 rpm and you never have a chance to rev it that high, then you're stuck down in the wrong rpm range where you may never have more than 100 hp.

As with anything, picking the right combination of parts for the application will make or break your performance goals.

In a really light car, I think the 170 would be the way to go. For a daily driver, the 225 will do better because of the extra torque to move a heavier street car.

Anyways, that's some of my 2 cents worth.......
Marc