Originally Posted by gasxd
this is a bit of an obscure question that i've been thinking about for a long time.
For an all out race motor after maximum power out of a small block, what is the optimum stroke?
We all know that increasing the stroke increases capacity of the engine and the torque exerted on the crank by the piston (T = Fr). Both of these factors increase torque. Increasing stroke with a given deck height also increases the rod angle which creates increased sidewall pressure (friction) and increases stress. Increasing stroke also increases acceleration of piston at a given rpm which means more torque is neaded to drive it (F = ma).
What is the general opinion on the optimum stroke, i'm pretty sure you can't squeeze as much power out of a 289 (no matter how high you rev it) as a 351. Do you get to a point where you have too much stroke in a given block? For instance can you get as much all out power out of a 420 as a 351 or 393?
I'm talking about hypothetical race motors where the power band doesn't matter because transmission and diff ratios can be tailored to suit. I know it doesn't have much to do with real life but i'm just wondering...
I wouldn't be so sure of how much power you can squeeze out of a smaller engine...
How does 1029 HP at 9500 RPM sound from a 330.8 CID BOSS 302 (based on a Dart block) with Yates heads? That's with a fairly big solid roller camshaft and 18:1 compression with a Hogan's "sheet metal" inlet and a pair of Ron's T2-Terminators running methanol. Peak torque of 622 at 7200 RPM...we're talking about 3.11 HP per cubic inch. That's fairly damn good. The bore is 4.155" and the stroke is 3.050". The bore to stroke ratio is: 1.3623:1.
Compared to a 393" stroker Cleveland (mine), which makes something like 630 HP at about 7500 RPM...1.60 HP per cubic inch or nearly half the power per cube of the smaller engine...with a bore to stroke ratio of: 1.0468:1.
More importantly is what you mention in your f=ma with regard to piston speed. For a given bore, we can't do much to reduce mass without grinding the skirts down to a point where they don't work very well. But we can easily change piston speed by shortening the stroke. Piston speed is represented by the following: fpm = (stroke*RPM)/6
For the shorter stroke at 9500 RPM we have the pistons moving at: 4829 fpm. The longer stroke engine at 7500 RPM we have pistons moving at: 4813 fpm. The two are nearly identical (16 fpm difference) and both well within the material limits. The BOSS piston is actually heavier due to the considerably larger bore, but still quite safely within limits at 4829...in fact, it can spin to 10,000 RPM if desired. It would be quite a bit of work getting the 393 to 8000.
Assuming that we were more closely comparing apples to apples with the two engines and both had the same compression ratio and fuel type (and induction system!), it is reasonable to suggest that the 393 would make somewhere around 870 HP at 7500 RPM. That makes for a better comparison, but it is still way behind the smaller engine in power.
To give you an idea of the cost differential for the two engines, the 393" stroker cost me about $15K US. The BOSS engine inlet alone cost $3800 USD. The heads bare were another $4800 for the pair...before porting. The Ron's Terminators were another $2900...and we're not talking about the cost of the Dart block, the ultra-light 4340 crank (the cost of the internal balancing alone was $800), titanium rods (another $2400), Ross custom pistons (you don't get an 18:1 Yates piston at 4.155" bore out of a catalog) another $950. The shaft-mounted rocker arms were $2300 and the valve springs were $900 for just the intakes! The titanium valves were $920 for intakes and another $810 for the exhausts.
All of this is to illustrate a point...it is a lot cheaper to go bigger and slower than smaller and faster...and, whether we're talking hypothetical engines or not, practicality will always play a role in every engine built. The "optimum stroke" for any engine is based on your budget!
Take that same 393" stroker, add a stouter block and a 400 HP dual stage nitrous kit and suddenly you're running 1000+ HP for not much more than about $20K US. That is some fairly serious grunt.