347 engine ideas? - Vintage Mustang Forums
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post #1 of 10 (permalink) Old 05-19-2019, 12:28 AM Thread Starter
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347 engine ideas?

Planning a 347 for my 68 convertible. Would like a strong base for eventual supercharger. Thinking Holley Sniper EFI for now. Can I get some ideas/recommendations from the group on how you would build it. Thanks for all opinions.
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post #2 of 10 (permalink) Old 05-19-2019, 12:30 AM Thread Starter
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I should say that initial HP goal is 420-450.
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post #3 of 10 (permalink) Old 05-19-2019, 01:38 AM
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331 for extended life.

floovb, floovb, vwomp, vwomp (sound of a bad tire driving)
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post #4 of 10 (permalink) Old 05-19-2019, 04:30 AM
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Get a blower cam to start off with if you are serious about putting on on there. Need good heads and forged internals for this.

If this is going to be a blow through set up, I don't know how the Sniper will deal with that. You probably need to call and ask Holley what, if anything, would be needed or is all that just in the tables. I have no idea.

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post #5 of 10 (permalink) Old 05-19-2019, 05:07 AM
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Dart block to start with and build a 363. you wont need a supercharger
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Last edited by cmefly; 05-19-2019 at 05:09 AM.
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post #6 of 10 (permalink) Old 05-19-2019, 05:56 AM
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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!"
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Last edited by Grimbrand; 05-19-2019 at 06:12 AM.
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post #7 of 10 (permalink) Old 05-19-2019, 06:33 AM
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You could build a reliable, naturally aspirated, 400HP, carbureted 351W for much less.

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post #8 of 10 (permalink) Old 05-19-2019, 07:41 AM
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Originally Posted by EvenFlow View Post
331 for extended life.
Ahhhhh geeeeez. I thought this had about been squashed by now......... a 331 isn't going to last any longer than a 347. The rod to stroke ratio is just fine and the pin placement on the 347 kits won't cause it to burn oil.

If you're going to supercharge a 347, don't waste your time with a stock block.
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post #9 of 10 (permalink) Old 05-19-2019, 08:38 AM
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A blown 347 for street driving in a '68, built from scratch? A stroked 351W will probably make the same usable power, easier and cheaper. The only tech reason for keeping it 289/302 is low weight and then an aftermarket block is a move in the wrong direction, except if you buy an expensive aluminium block. If the blower are the importent part, I would probably do it on a stock stroke 289/302 or move to a stronger 351W.

I got an old magazine were they got 400hp by adding a little cam and a blower to stock 302Ho. Then they ported the stock intake and added more cam and modern heads to the same engine and got more than 500 tq and I think 520hp at 5300 rpm still at a low boost level.
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post #10 of 10 (permalink) Old 05-19-2019, 09:32 AM
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Originally Posted by FastLouie View Post
Planning a 347 for my 68 convertible. Would like a strong base for eventual supercharger. Thinking Holley Sniper EFI for now. Can I get some ideas/recommendations from the group on how you would build it. Thanks for all opinions.
I just built what you are considering for my 64.5 vert, except I built a "big bore" 347. 3.25 stroke x 4.125 bore. Dart block, forged internals, internally balanced crank, 58cc AFR 185 heads, Air Gap manifold, 680 VS carb and a very mild custom cam (218/224 with lift under 600) that I got from Coast High Performance and compression of 10.25 On the engine dyno, it made 445hp at 5900 and peak torque of 430/ft lbs at 4600 and had a very flat torque curve. There was more than 400 ft/lbs of torque from 3000 rpm all the way up to 6000 and it idles well and should make plenty of vacuum for my power brakes. On the dyno, the engine was turning the water pump and the alternator.

I also have thoughts of a blower in the future and now, with a simple head swap to drop the compression, I can add a blower and go.

However, I went with a 302 based motor because I didn't want to deal with the limited room of a 351 in a smaller engine bay of the first gen cars. If I had a '67, I think I would have gone with a 351 based motor and stroked it to 408.
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