I've owned and driven many of each, but opinions vary.

My first three cars were Plymouth flathead sixes. Being young and stupid, I did everything I could to destroy them, but they were tough. All would peg the 100 mph speedometer, but I have no idea what the actual speed was. While drag racing friends with their OHV sixes, I would always get beat off the line. but the way the transmissions were geared, I could run with or ahead of any of them once I started to wind out in second gear, so performance was comparable, as was fuel efficiency. The feature that outclassed the Ford and GM sixes was the smooth, quiet performance. They cruised easily at the speed limit and never felt like the engine was overworking. They were by far the smoothest running cars I ever owned.

FWIW, I sold my '48 Plymouth to a friend who lost it when a tornado dropped a garage on the back half. The body couldn't be repaired, and he decided to see how much abuse the engine would take. He put it in neutral and placed a rock on the accelerator. We watched it for a while before we got bored and walked away. It was over an hour later that we heard a noise from the garage rubble that told us it finally blew. Kids are indeed stupid.

My first slant six was a '63 Signet hardtop with a 225 and automatic. I drove it hard and street raced all kinds of cars. I could run away from any similarly equipped OHV Ford or GM of similar age. I never broke a slant from abuse, but I have seen a few with vent holes in the side of the block when over-revved. When I was selling Plymouths in '64, we had a V200 four-door demo with a 225 and a four-speed that our sales manager liked to race against small block V-8s of any make...and he beat a lot of them.

The slant six car in totally stock form would probably perform better than the flathead in the same car, so I agree that the transmission has much to do with perceived performance superiority. In my opinion, both were better than their competition in every way, and I would love to have a car with either engine and a better geared transmission than what they had from the factory.

Any stock slant 6 that I have driven tuned correctly / carb / valves / points / timing is seat of the pants faster than the OE 1936's that I have driven of my dad's..
He has 2 Bone stock 1936's one s a truck and one is the D2 Sedan. Of course by 1959 flathead's could be another matter.

I'd leave it a Flathead. If it's tired, rebuild it. It's not as 'good' of an engine but the world doesn't need yet another old vehicle getting its engine swapped.

I'd leave it a Flathead. If it's tired, rebuild it. It's not as 'good' of an engine but the world doesn't need yet another old vehicle getting its engine swapped.

Can you get to 9 to 1 with a flat-head?

Doesn't airflow reduce thru the flat portion of the combustion chamber if you go to high on compression?

Can you get to 9 to 1 with a flat-head?

Doesn't airflow reduce thru the flat portion of the combustion chamber if you go to high on compression?

The beauty of a Flathead, to me, lies in it's simplicity and lack of fanfare.

The simplicity part needs no explanation.

The lack of fanfare...means they run without a lot of noise or commotion and are very docile engines. That's what you get when you have a fairly heavy, cast iron engine with low output. So hot rodding one with high compression, cam, carb, etc. is kinda counter to its nature. It's like when you see the guy who's slammed his dually on the ground or added a snow plow to his Ferrari.

One time, one of my kids asked, "What is trail mix made out of....? M&M's and whole lot of disappointment!"

That's a flathead to me....the more 'stuff' you do, the more disappointment you get. Just enjoy the M&M's.