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Discussion Starter · #1 ·
Just wondering how big of a carb I really need for my upcoming 521 engine build. It's a low-RPM, modest compression street engine with the emphasis on torque on dirivability. Going to be using an Edel RPM intake, probably the big-chamber RPM heads, 1 7/8" hooker headers, and I plan on getting a custom cam from Scott at RHP. I'm also going to be keeping the high gear ratio (3.0:1) until I get an overdrive unit, then probably go with 3.27 or 3.55. This is going to be in my Elite, which is of course on the heavy side.

According to the calculator, a 521-inch engine at 90% efficiency only requires about 750 cfm up to 5500 RPM. Is this accurate? What is the average efficiency these engines operate at? Is an 850 really needed, or will that just help out on the top end at the cost of some throttle response and gas mileage?
 

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It's always a good idea to carb up by around 10%. This seems to be the best comprimise to keep good low rpm intake velocity and still have as little restriction as possible at full boogie. Going to an even larger carb will probably free up a HP or two up top, but at the expense of a good bit of off-idle throttle response and mid-range driveability.

I'd stick with the 850.

Brad
 

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I have done builds exactly like the one...

...you are contemplating.

With a 5,000 to 5,500 rpm hp peak a 750 will work but is on the small side once you get above 4,700 RPM.

An 850 will not cost any driveability on an engine this large and I highly recommend a 950 HP. It has the smaller venturis of the 750 with the radiused air entries for good overall flow and excellent throttle response.

The truck build I did and posted on my site would be ideal with the rpm heads and a bit more cam than I used.

The link is in the engine build section under known engine combos. I made 500 HP with a 750 and a 226/234 @ .050 cam peaking at 4,600 RPM and carrying through to 5,300 RPM. This was with large valve D3's...



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Discussion Starter · #4 ·
Thanks for the insight, Scott. That truck build seems to be exactly what I'm looking for, ~500hp and way more torque than I should be trusted with.
 
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