Yes, I have heard of this and it's a common myth. A shorter stroke will not allow the engine to spin any faster than a longer stroke engine. Max engine speed has more to do with cam profile, intake runner length, spring strength, etc, etc. It has very little to do with stroke or displacement. What perpetuates this is the thought that since the piston doesn't have to travel as far, the crank should be able to spin faster. This is not even remotely true. Piston velocity and acceleration, while a function of stroke, is not limited by the stroke. In the case of the 327 vs. 400 chevy we're talking about a 3.25 stroke vs. a 3.75 stroke. If we do the math (at 6,000 RPM), we determine that by increasing the stroke .5 inches and still using a 5.7 inch standard chevy rod the maximum piston acceleration only increases 3.2%. Since the 400 slug is also lighter than the 327 piston we can probably assume that the rod stress is a wash or might even be a decrease if the mass decrease from a 327 to 400 piston is greater than 3.2%. If you increase the rod on the 400 to a 6" rod the piston acceleration increases by only 2.1% over a 327. Since velocity is the derivative of acceleration we can also conclude that maximum piston velocity will also increase with the 400 and this should fill the cylinders better. So, there is no scientific reason why a 400 would not spin as fast or faster than a 327 unless the air velocity was hitting Mach 1 speeds and choking the flow (which I kind of doubt). At which point you should be hitting the MAX HP potential of the induction system. The only reason to ever ever destroke an engine is if you have a cubic inch limit or if you absolutley can't stand that extra 4 or 5% increase in acceleration. Then you get on a trade off curve over bore vs. stroke. Bigger bore means more force per the same cylinder pressure, longer stroke means more torque for the same piston force. As long as I'm on my rant... Smaller engines are FORCED to spin faster to produce the same HP as a larger engine spinning slower. HP is a measure of work done during a period of time, such as moving a car down the 1/4 mile in X number of seconds, or creating a certain torque at a specific RPM. Again, it's all a trade off.
Destroking is common in racing, even without rules limiting CID. It may be true that an engine could spin just as fast with a longer stroke, however it is also true that a shorter stroke is easier to spin up. It is not really a 'myth', it is just common sense that if you want to run an engine to 9k rpms, you have to make it bulletproof. It is MUCH harder to make a 4" stroke bulletproof, or durable if you will, than a 3" stroke. Less rotating mass means parts don't need to be as heavy and take less to spin. Dave
Ok. Good Point/Counterpoint going on there. But it seems that they both make sence, but Ratio 411 seems to make more sence.
Let me address these points one at a time. 1st It's easier to spin up a short crank. Yes, it will take more energy to spin up a taller crank, but the extra cubes offset that quite a bit. We're developing more mean cylinder pressure due to the extra stroke and have a larger mechanical advantage. 2nd. It's easier to make a 3" rotating assembly hold together than a 4" assembly due to weight. I've never seen a forged crank fail simply from overspeed based on the weight of the throws and balances. Maybe if it was a cast chinese piece of junk it could possibly fly apart, but even then I'd be willing to put it on a maching and buzz it up to 9k. What generally fails is the connecting rods, wrist pins, or pistons. Sometimes you can twist cranks apart, but man that's a lot of power. What get's heavier though? The rod will get a little heavier if it's longer, but the piston will invariably be lighter. A short skirt piston with the pin pushed way up into the ring lands is much lighter than a comparable piston that would be used in a short stroke engine with the same rod. Piston weight is your biggest concern regarding rotating mass (which is actually reciprocating mass, but translates to rotational). The mass of the rods are also a concern. However, using the same piston in a 3" and 4" assembly, you'll need a rod that is 1" longer to get the piston high enough in the hole on the 3" assembly. That increases the reciprocating weight again, but the larger rod ratio will hopefully offset that by reducing stress. The crank will get heavier, and take more energy to accelerate, but that has nothing to do with its ability to hold a constant velocity of 9k RPMs. Like I addressed above, the increased ability to do more work should offset the energy the bigger crank requires. You also get higher parasitic losses from a longer stroke due to ring drag. In a drag-race engine, you can run low tension rings to reduce the drag, but burn some oil. In a street engine again though, I'm going to say that the extra cubes overcome the ring drag. The problems generally associated with tall cranks have to do with oiling and clearance issues. Taller cranks tend to create more windage and don't oil the rod bearings as well as they should. This is easily overcome with a better oil pump though. Sometimes you have to clearance the block and index the cam so it doesn't hit the crank and con rods. Rod/wrist pin/piston stress and therefore reliability is based on piston acceleration, NOT velocity. Velocity has very little to do with reliability. The only thing I'd be worried about is extra heat in the piston skirts, but even then, that only an after thought when brainstorming about what could possibly go wrong. I'd be more worried about keeping the valve train together at those speeds (9k rpms) rather than the bottom end. Also consider that a big bore short stroke engine that has the same rod ratio and CID as a small bore long stoke engine will produce MORE stress on the con rod due to the increased weight of the piston, and as such will fail long before the long stroke engine at 9k RPMs. One thing I've learned through engineering school and working in the field is that a lot of times what seems intuitive and makes good common sense is actually backwards from what is happening. I've learned many hard lessons and been embarrassed when we actually ran the numbers and I was way out in left field.
Wanna hear somthing Funny. A couple of guys i know that are complete gear heads. They bought an RX7 and tried to get my 302 out of my Maverick. Instead they found a stroked 302 Fox body and dropped that into it, He also plans on getting some GT 40 heds. Project name Littleboy racing. Funny stuff
