318 build problems & new direction

cudaracer said

Here is an interesting output,

....as I played around with bore, going from 3.97" to a 4.07", hp only went up 1hp, but torque went up 20. If that is even remotely accurate, then there is your 318 to 340 difference, just torque. At least with the XR268 roller cam, and changing nothing else.

Click to expand...

Like I said torque is kind of easy to predict.
It don't very a lot like hp. In a N/A engine cubic inch is the main variable. Torque basically come from a per power stroke basis how full the cylinder gets (VE%) and how efficient the burn (CR, Chamber design etc..) HP is all the power strokes added up over time (RPM). Torque and HP are built by the same thing basically from Air flow and efficiency. So in a stock low power engine with low air flow and CR torque is around 1:1 to 1.15:1 lbs-ft per cubic inch. Once you started adding headers exhaust cams intake carbs decent heads etc... you start to get into 1.15:1 to 1.25:1. Once you get into professional built race engines above 1.4:1 is possible. So for us average guy 1.2 is a good guesstimate. Thats why people say build 360 or 408 over 318 cause that's the only really way to build torque in a N/A engine. So those cubic 42 and 90 cubic inch difference gives you about 50 to 110 lbs-ft more.

But not all is lost for a 318. It's not always doomed to loss against bigger engines since one major component is always left out of this conversation which is gearing.
If each engine has the same HP 318 vs 360 vs 408 if you gear basically a 318 .5 more than a 360 and 1full ratio more than a 408 you'll be putting similar power to the ground per mph than the other two. Eg. since 3.55 gear seems most popular so 4.10 to equal 360 and 4.56 to equal 408 and thats the real reason to go bigger most won't want to run deep gears. To me minimum gear for a 318 is 3.91 if you ain't willing to gear it go bigger.