House power from a 402 - Impala Tech
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post #1 of 5 (permalink) Old 06-26-2014, 08:44 AM Thread Starter
 
 
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House power from a 402

I'm a novice to mechanical Issues. I just purchased a 1971 Impala with a 402 big Block. I'm rebuild the engine and adding a few high performance parts like a bigger cam, roller rockers, bored over 40, original heads valve job shave down 20, high rise intake, 750 Harley carb, 3200 stall. Compression should be 9 To 1.my question is how mUch horsepower can I realistically expect
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post #2 of 5 (permalink) Old 06-26-2014, 09:42 AM
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With the stock heads you will be making about a bit more than stock power. It is the heads that makes the power. The cam tells you where you will make the power in the RPM band. With a bigger cam you can make more peak horsepower but it will be so high in the RPM range that you will not be able to use it on the street.

The only thing you have done to raise the horsepower over a stone stock motor is to make it marginally bigger. The greatest contribution to power was made by your raising the compression from the stock 7.8:1 compression ratio to 9.1:1. Bigger valves in your stock heads would also increase your power levels.

You have to keep in mind that it isn't horsepower that you want unless you are planning to take your car up on the high bank oval track. To accelerate your car you want to make gobs of torque. This is why the 406 SBC is so popular. It makes more horsepower than your BBC 410 (40 over 402), but the same torque because it has the better head found on the SBC, and does so for less money.

Torque is made by cubic inches. If you choose to make those cubes by relying upon a longer stroke you will make more torque than if you where to increase the bore spacing to maximize the bore to make the same cubes.

An example of this is the 496 which was the last production big block made to compete with light diesels in pick-up trucks. The 496 was a 454 engine with a quarter inch longer stroke crankshaft (it also used bigger, to fit on top of a Gen V big block, LS-1 heads with dedicated EFI). Chevy also offered a 502 which is about the same displacement, but makes it's power by boring out the motor to a 4.5 inch bore. The 496 pulls like a diesel. It makes lots of torque, but slow reving with all of it's power made below 4,200 RPM. The 502 made more horsepower, but less torque, and had to rev to 5,400 RPM to make that power.

This is why the 496 is so popular (it is the 383 of big blocks). The best big block that makes the most power for your money is the 540. It has the big bore and long stroke that can be assembled from off the shelf parts. Go bigger and the price doubles quickly, but bigger is better with a big block motor.

Big Dave
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post #3 of 5 (permalink) Old 06-26-2014, 11:58 AM
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On this one I am with Big Dave.
You will be $5-6k into the engine and still be at 400hp at the crank and not more than 300hp at the wheels. (Unless you can find a 427 crank and stroke it, which is damn near impossible).
Save your money, hit Craigslist, for the same amount that you are gonna spend on this engine you can find a monster that puts out upwards of 500hp. In my area there was couple of real nice roller 454's fresh rebuilt carb to pan for about $5-6k.
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post #4 of 5 (permalink) Old 06-26-2014, 12:12 PM
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Ah the 71' 402 nice, but from my post's I'm a little biased to them. Nice little engine, back it with a 4sp auto or manual and a 3.73 and it's a good blvd. cruiser.
X2 with Big Dave.
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post #5 of 5 (permalink) Old 06-26-2014, 11:30 PM
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3200rpm stall on those stock heads seems like an odd combo to me.
I'm not as experienced as others here on the Big Block so my opinion is worth about a nickel.
I didn't see any details on the cam, nor if you had headers, what size, or stock exhaust manifolds but I would be surprised if your 402 was making more than 350-380hp.
Like they said, you want torque on that big car anyway.

HOW A NOVICE REBUILDS A 66 IMPALA CONVERTIBLE:
http://www.impalas.net/forums/blog.php?u=1432
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