My 67 has a new 400 engine and I noticed a considerable HP gain yesterday as we cross the 200-300 break in threashold. Maybe as much as 100 HP. Which got me thinking about people who dyno their new engines. Maybe a second dyno would show a marked increase after break in.

Speaking as one who dynos street and racing engines on a regular basis, no. A properly built engine will produce very close to its best after about twenty pulls on the dyno. Even then, the difference between first and last pulls is less than about 2-3%. That doesn't include changes from optimizing timing and jetting.

I've dynoed the same engine package year after year in a very competitive class and other than the incremental changes that I've made during the development, there has been very little change.

If there were large changes after break-in, engines would be run longer during break-in before tuning. The engine has to be tuned when producing its maximum power.

If you got another 100 hp without changing anything except run'n it, then there was/is something wrong.

Does the foot on the accelerator count as added HP. More gas more power!!!

LOL Maybe 'WAS' but I can assure you it's not 'IS'. And if it fixed itself, 'WAS' wasn't very bad.... It could be that I didn't get all the 'three year old' gas out the tank when I siphoned it. It runs like a top now. Over 300 miles yesterday without a miss.

Three year old gas? I'm burning off the remnants of 13 year old gas in one car, 22 year old gas in the other. Yuck.

Is that what they mean when they say "FLEX FUEL"?

I was siphoning it out of the cars and pouring it into the tank in the lawn tractor, but the tractor didn't like it. It's not very volatile any more, when it runs down your arm when you pull the fuel line it doesn't burn your skin, it just smells. Even worse, it was 22 year old California gasoline...was it leaded in '86?

Back in my empty wallet days where everything went into the cars, I'd take a 5 gallon gas can and siphon pump to the junkyard with me, drain new incoming cars of whatever they had left. Sometimes those cars had been sitting a long, long time...sometimes a shot of starter fluid was needed to allow the initial combustion to get things going. Then it would burn like...E85. LOL. Low power fuel.

I wonder if I called U-Wrench-It if they'd let me drain tanks and for how much a gallon?

Personally, I'd be trying to find out what the problem was to make sure it didn't happen again, or happen on the next engine, or cause another problem, and also to see if the problem that was limiting the hp is gone completely, and the engine is now working at its full potential.

Given the change, I'd have a look at the mechanical advance to make sure it is moving freely. I've seen them stick at both extremes of the range, and with three year old gas or even fresh gas, a mechanical advance that tends to stick anywhere but especially at full advance is opportunity knocking - for an engine builder.

Nothing to be concerned about. It's all new, all high quality parts, and running perfectly. I don't mess with a good thing, I drive it.

Maybe I should rebuild my 326 and hope for that!

Yah! It's a good thing. I've had a chance to mention it to some savvy race car builders and they say it's very common to get more power/performance out of a car that is freshly built after a few hundred mile shake out. Man, it's easy to get these guys talking. A few liked to mention how even their factory stock performance cars gradually increased in performance as they got some miles on them. My Uncle was a foreman at GM through the late 60's and early 70's and got a new 'hot car' every year. He said his cars always ran harder and faster before he turned them in at 20K then they did new. So this is common and can be expected and enjoyed by all.

thats why "running in" a car in them olden days was importatnt...
when I was in the service..we among other things rode motorcycles...my pluton 'ran in' 4 sets of bikes for others...it was fun!(= more fun than other military things! lol) just some "cruising" on high ways on dirt bikes....

They do run better after break in.
Mine gained .5 and 5 mph after 100 miles.