Someone please correct me if I'm wrong but one of the advantages to an aftermarket or Bowtie block is the "stiffness" of the block as a whole. Where a stock block might start to distort the cylinders at higher power levels an aftermarket block would flex less, and thereby allow greater power production.

Again, I am not 100% sure on this matter. I think Rich would know more than I do, having actually built motors to withstand that kind of power.

 

When you get to very high cylinder pressures, having thicker/stiffer cylinder barrels will distort less and make more power. If cylinders walls get too thin, not only can they flex but they can crack. That's why it's generally recommended for production small blocks to not go more than 30 over on the bore and 60 for big blocks. None the less, there are lots of small blocks out there more than 30 over and big blocks more than 60. The BB in my race car is 70 over. Would it make more power at 60 under? Maybe a couple of hp. Significant differences in cylinder distortion or splitting is much more likely with high boost or even more so with nitorus due to the high cylinder pressures.

How thin is too thin? And how much can you overbore? Well, different people have different ideas on cylinder wall thickness. A decent rule of thumb for a SBC is 0.140" for NA and 0.180" for boost/nitrous. If high boost/big nitrous is contemplated, 0.200" is better. There is a lot of variability in stock castings. That's where the 30 over number comes from - it ought to be safe on any block without sonic checking because the nominal wall thickness at stock bore is 0.225", or so I have been told. I bet this changes from time to time and the only way to know for sure is to have the block sonic checked if it si going to have a big overbore.

A big advantage of the aftermaket and Bowtie blocks is cylinder wall thickness and quality control. They are thicker and have less variability. The Dart is a minimum of 0.275" thick at a 4.185" bore. So, lots of them are built to a 4.25" bore. BTW: Dart makes the GM Rocket block. I am not sure if they also cast the Bowtie? One way to help compensate for thin walls is a partial block fill. I regard this as SOP for a high po motor bored more than 30 over. Doesn't hurt even on a stock bore though. Keep in mind that you can't just fill the block and assemble the motor. It need boring and honing AFTER the fill.

Rich

 

The block I have now will clean up at .020 so I should be good, and we are doing a half fill.

Both of the local guys I talked to said it would be OK with what I am going to do.

 

Agree - should be ok for almost anything. Don't worry, be happy Or, is that "what, me worry?"



Rich

 

Ok Rich I will quit worrying and just put this thing together