I have to ask, this may have been answered and I may have overlooked it.

Suppose I wanted to attempt what you've done on the LIM for Don. Could I just smooth the walls and leave the "port matching" out, will I still see gains or the port matching is where most of the gains come from?

Both, really. The rough cast causes a fair amount of turbulence. To answer your question: Yes, you will notice it if you simply smooth the walls. You will notice more if you match the ports.

I disagree. Smoothing the walls alone will not gain you anything you'll notice. The problem is a mismatch in port size between the LIM and the Cylinder Head.

Don't bother only smoothing the walls.

Thanks, just wanted to see, looks like port matching is the only way to go plus smoothing too.

Way to go, Bill and Don.

That old grampa car seems to pull quite nicely

Ok. I just mentioned that because in other vehicles, there was always a gain. It reduced the turbulence. Even in a '71 vehicle. Mathematically, there is a gain as well. Sometimes the gain wasn't noticeable in the butt, but it'* there.

Why wouldn't it here?

IMO, from what I've seen on the stock and ported + smoothed LIM, you MAY get some improvement by smoothing alone, but it'* almost a waste of time to do the smoothing without cleaning up the ports. Cleaning up the ports actually takes less time than smoothing all the runners, from my experience. It'* just more critical to get right and not overdo.

I also wanted to take this chance to reiterate, for those reading this that haven't followed all the previous discussions and cautions, that this is port matching and NOT gasket matching!

That'* very true. If you're going to smooth it, may as well port-match it. The only time-consuming thing would be the constant measuring, but it'* worth it.