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Picking the Best Big Block

Picking the Best Big Block :: -

Copyright: Olds FAQ
Submitter: Ed


What Should I Look For in a BB For My Performance Olds?

That depends a lot on whether you are going to fully rebuild your engine-buy new pistons, have it bored, all that. If you are buying new pistons, then just about any serviceable block will do, and what you are really looking for is the best serviceable heads and crankshaft for your money, with the crank and rods compatible with your desires: 425 CID vs. 455. While there's "no replacement for displacement," you should be aware that all 425's have forged cranks, whereas 99.9% of all 455's have cast cranks. Both have the same bore; the main difference is the internals: the stroke, rod length, piston compression height combination.

Some folks prefer the stronger crank and shorter stroke of the 425 engine, even if it is 30 CID smaller. You can swap the internal reciprocating assembly, to make either a 425 or a 455 out of either block. However, most 425 blocks have the older style cam bank angle, which makes cam selection a bit more difficult, and the other 425 blocks use the larger lifters, which cost more.

Some see the 425 as definetely the better choice for high RPM applications. The engine is over-square, meaning the bore is bigger than the stroke, which usually always means higher revving than an engine which is (bet you guessed what's coming next) under-square as is the 455 - bore smaller than stroke. Not to mention the forged crank in the 425.

If you plan to use the engine 'as is', or just do a simple cylinder hone, ring/bearing overhaul, then you'll be looking for a pre-71 engine, as they had compression of about either 10.2 or 9.0. Quick ID tip: pre-71 engines had heads with casting ID "F" or earlier at the lower left corner of the head. Engines with G, Ga, or J heads were low compression- about 8.2:1 at best. Generally, C-head engines are best.

A Toro oil pan is a cheap way to increase your oil capacity. It will hang an inch or so below the A-body frame so it is a little more exposed to danger. If you have headers they will usually be sacrificed to the pavement/parking stop gods first.

If it were my engine I would buy a new oil pump. The key thing to remember is that the Toro pan is deeper so you need a Toro pickup. The Toro oiling system also includes a crank scraper on the side of the pan, a baffle/crank scraper above the oil pump and a skimmer for the timing chain. All this keeps the oil where it belongs and even adds a few hp by making everything lighter (since it isn't slinging oil around). You need the whole setup which includes all these goodies and the main bearing cap bolts with the studs incorporated so it can all be put together.

Most of the 425's have the 45 degree lifter angle with .842" diameter lifters. The Toro blocks have a 39° lifter angle and .921" diameter lifters. The 1967 425's have the 39 ° lifter angle and .842" diameter lifters.

FYI, a bigger lifter diameter lifter acts more like a roller lifter. This is what led to mushroom cams in NASCAR. Just a note, the .921" lifters are much more expensive than the more common .842" diameter lifters.

For pistons, TRW does not make a forged piston for the 425. However, Speed-Pro lists one. I ordered a set of the Speed Pros, and after being on back order for 3 months I had to cancel my order. I found a guy with a set of JE piston for a 425 for at a very reasonable price.

I called ARP for a set of rod bolts and they informed me there are two different bolts used in the 425 rods. There suggestion was that I bring one of the rods in so they could match it to my connecting rod. ( I live about 15 miles from ARP) I also picked up main studs with windage tray standoffs for a 460 Ford for my Olds. According to the guys at ARP they feel it fits better than the stud they sell to Mondello for the Olds motors.

The flywheel bolt pattern is different than the 1968 and later motors. My car is a manual trans and it took a bit of digging to locate a manual trans flywheel.

In general, for a given displacment,

  1. A larger bore x shorter stroke will make an engine that is easier to operate at higher RPM, and produces greater HP.
  2. A smaller bore x longer stroke will make an engine that runs at lower RPM, and produces greater torque.

So, as an over simplification,

bore = hp
stroke = torque

HP is the mechanism that lifts a 550 pound weight at the rate of 1 foot/sec. Torque is the mechanism that keeps the engine turning when a load is applied.

They both have value. In engines as close as the Olds 425 & 455 the size numbers are fairly similar, so the results of building either will be similar. The 455 is about 10% greater displacment, so theoretically it should yield about 10% greater HP, everything else remaining constant. You gain the displacment by increasing stroke, so the torque aspects of the 455 should be superior. On the other hand, you could get equal HP out of the 425 by turning it faster (part of the value of a shorter stroke), but shorter stroke yields less torque, everything else remaining the same.

455's have been used extensively for high output engine building. They are certainly more obtainable than 425s, and they seem to live fairly well, so that the benefits of a steel crank may, in fact, be fleeting. The greater displacment has value, and obtainability does too, in terms of initial cost. If you've got a 425 build it, it should do very nicely, if you've got to buy the core motor then buy a 455 as it will cost less as a core, and should not cost more to build.

[ Thanks to Chris Witt, Bob Handren, Ron Forsee, Cliff Feiler for this information ]
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