Dropped a valve!!!!!!!
 

I was currently running a 489 (454 stroker), messed up the head a little (merlin 320's) need to replace 2 pistons. I was thinking of using my 502 block and using my crank, rods and buying new pistons and building a 540. Is it worth all of the extra machine and money to do, or should I just put 2 new pistons in my engine and run it? obviously the 540 will make more power and torque, but real world numbers how much?

If you use the same cylinder head the 540 will not make anymore power than the 489. The difference is the 540 will make the power at a lower rpm. There is only so much power potential in a cylinder head doesn't matter if it's 400 ci or 800ci.

You're saying my merlin 225/1.88 320 cc runner heads will not support a 540?

IMO, the larger bore of a 4.500+, will help out those heads, over a 4.280 bore. You would need to swap cams more than likely.

Generally speaking, those Merlin heads kinda suck. Not horrible, but not exactly CNC AFR's, Darts, or Brodix's. The casting was a good quality casting, but they didn't kill it in the airflow department. Porting them can get expensive too being iron.

However, I built a 540ci using those heads, single plane dart, 9:1 static, crane 741 hyd roller, 1050 carb years ago. Made right around 600HP at 5700RPM. Not awesome, but still over 1hp per ci. Was a very mild mannered engine, and ran good.

I say build the 540 bottom end. .

My current cam is a Howard's 120265-12 237/243 dur. 634/640 lift Hyd roller, I believe it's a little big for the 489

I have the option of dart 345 heads!!!

The dart 345 is a good head. I'd prefer the 325cc on a 540, but , if you have them, they will work. And probably better than the 320 merlins.

That cam is a bit light for a 540, and better suited to the 489ci in my opinion.

Not what I said at all. What's I said is the 320 will only make so much horsepower. The cubic inch of the motor will dictate the rpm that it will make its peak power. The 2.25 valve doesn't do to bad with the smaller bore. The larger bore will help a little but not much considering you are using the same head with the same combustion chamber and the chamber is what really makes the difference when moving to larger bore motors regardless of what the gurus say.

The nice thing about moving to a 4.500 bore, is the chamber can be modified easily to accomodate the larger bore, if it needs to be opened up.

You really dont want a head with a chamber for a 4.250 bore, on a 4.600 bore, and you def dont want a chamber for a 4.600 bore on a 4.250 bore block.

Bolt the head on the bare block, turn it upside down and take a look.

the camshaft will also dictate the rpm the engine will make its peak power, at least on an average 6000 RPM or less marine big block chevy.

Quick mental picture.

AT 100% effeciency, numbers rounded, we have:

350cid at 7171 rpm = 1,255,000 cid per minute

383 at 6554 rpm = 1,255,000 cid per minute

400 at 6275 rpm = 1,255,000 cid per minute

454 at 5529 rpm = 1,255,000 cid per minute

502 at 5000 rpm = 1,255,000 cid per minute

540 at 4648 rpm = 1,255,000 cid per minute

598 at 4197 rpm = 1,255,000 cid per minute

632 at 3972 rpm = 1,255,000 cid per minute

705 at 3560 rpm = 1,255,000 cid per minute

1005cid at 2498 rpm = 1,255,000 cid per minute