whats the power difference between a 500 and 440 inches assuming all is the same...

bbc 8 71 hh retro blower bird catcher alcohol injection, 25% over driven, same heads 340 runner merlins, i have 2 options on short blocks. one would be a 4.625 bore with a 3.750 crank the other is a 4.310 bore with the same crank.im alittle scared of the large bore block only cause the bore size is so big im not sure how well the head gaskets will seal. both combos will be the same parts just different blocks. both blocks are filled and o ringed.

 

Well if you do the math and assume 15 psi boost turning 8000 rpm I get 1795 HP for the 504 in3 and 1551 HP for the 438 in3 engine.

Joe

 

what block is the 4.625?
And the amount of increased inches is only half of the tale. Merlins love unshrouded intake valves. That is worth probably 100 hp right there. And with the amount you are running, head gaskets should not be that much of a deal. I always go with the larger bore in BBC's. Pays off every time!

 

the block is a bowtie block that is filled to the top.this is my first blower and first alcohol engine in coming from years of nitrous use.

 

I have run lots of the bowties and cnc bowties with lots of cylinder pressure. I would give it a try.

 

go with the thicker bore

The air into the engine is a function of the blower and overdrive. So the power made by the air and fuel will be about the same for both engines.

440: For the same overdrive, the smaller engine will have a higher boost. And more power loss at the crank to drive the blower. But also less frictional loss (est. 380 HP of friction at 9,000 RPM).

500: The larger engine will have three or four psi lower boost from less power loss at the crank to drive the blower. But more frictional loss (est. 430 HP at 9,000 RPM). Valve unshrouding from the larger bore around the intake valve may boost power in an NA engine, but it is less of an issue in a blown engine.

440: Block stifness is more of an issue. The 8-71 at 1 to 1 is the equivalent to a 900 ci engine regardless of the long block size. A lot of block strength helps that much air.

We built a 438 and a 478 that both ran about the same when the tuneups were the same. 440 is close to what most are running in AA/GS making 1,600 to 1,900 HP with a 6-71. My choice would be the smaller engine with less blow-by and more reliability.

Bob Szabo
bob@racecarbook.com

 

Well, I assumed that he was speaking of a small bore, because it was a stock 'truck' style block. So the bowtie was thicker in the walls at current bore and more stout on the bottom end in every aspect. But, if the small bore is an old merlin pro, or something like that, that would change everything.
Again, making decisions without all of the facts

 

the large bore block is a bowtie but the tall deck small bore block is just a standard tall deck block,just found out its actually standard bore....at 4.250. i already have a callies crank with the 3.750 stroke