Join Date: Jan 2019
Location: Sedgwick, Kansas
With a stock block, anything over 500 will eventually lead to it splitting down the middle. I would consider 450 kind of a 'grey area', where it is testing the block's limits.
Presuming that this is something you want to keep, you really need to start with an aftermarket block, which won't care a bit about 450.
If you have a reasonably good build on a 347, with decent heads, you can expect to get 400 horsepower very easily. Bolting up a supercharger will add another 40% to that, putting you around 560, not 450. I think it's safe to say that there's no way a stock block would take that.
I think the deciding factor would be what the car's intended purpose is. For a street car that you intend to live forever, just go with a mild cam, good heads, Weiand Stealth, and some good headers to get the burnt gases out. I'd stick with a 4.00 or 4.030 bore, even if it gives up a few cubic inches to what the aftermarket block can handle, for the simple reason that you'll have plenty of meat left over to bore it out later if you put a lot of miles on. Starting with a .125 overbore kind of limits your options when the engine's worn out.
The next thing you'd really need to worry about is strengthening the chassis and suspension in your 'vert. Not known for their tremendous rigidity anyway, 500+ horsepower would really pretzelize that car. I would also be concerned with survivability. Any car that can go 10.99 or faster in the 1/4 has to have a roll cage these days, and with that much power, you need to think about that. 450 horsepower will put a new 3800 lb Mustang into the high 11s. On a 2800 lb '68, you'd be in the tens.
These old 'Stangs aren't the great bloated computer-controlled beasts that cruise our highways now. If a wheezy, restricted 390 can make these cars a handful, putting out 320 factory horsepower, what do you think a much lighter smallblock Windsor would do, putting out another 100 or 200 horsepower?
It won't hold your hand and tee hee when you smash the go-pedal doing 70 MPH, 'because traction control'. It will just flat out go sideways, roll, and kill you. Keep this in mind. These days, horsepower is just a number. It's bragging rights. "My Hellcat has 750 horsepower!" "My Camaro's got 1150!" Just a value that tells other people how big your... ego... is.
We're talking about a convertible here. My advice? Build a wicked 347 with a stock block, but don't supercharge it. Use a mild cam, but get the best heads you can afford, so it pulls like a freight train at any RPM when you smash the skinny pedal. Don't worry about what the dyno sheet says, because this is probably not going to be a track car, or a dragster. It's too hard to make it strong enough to take the twist, or make it safe. Just go out, melt some tires, impress anyone that rides in it with how vicious that beautiful AFR or TW headed small-block is, and how well it handles! Enjoy the heck out of it, and when they say "HOLY GUACAMOLE! How much horsepower does this thing have!?" you can just laugh and say "Dunno, haven't had it on the dyno!"
Last edited by Grimbrand; 05-19-2019 at 06:12 AM.