I'm posting here to see if my ideas are absurd (don't bully me ). I would like a destroked 425 run on E85 for autocross and maybe road racing.

I would go for an .060 overbore and an offset ground diesel crank. I think this will leave the CI around 391, and allow use of light Chevy rods. I want it this way so it can be stable and pull well at 6000 rpm. If 391 is too small, a custom crank would probably exceed the price I'm willing to pay and negate the revving characteristics I'm looking for. All the normal performance goodies would be included for high RPM. E85 would make way higher compression more doable, plus it's available nearby, but I don't know what the future of flex fuel looks like. Procharger for the high end could be an option too.

I'm not really interested in an LS.
If I could build this for a reasonable price, would it even be worth it compared to other Olds options?

 

If you're not going to take advantage of the additional cubic inches there really is no reason to build a big block as opposed to a small block.

 

This sounds like you might already have some engine parts laying around and are trying to make work. What specifically do you have now?

 

Here's the thing - what I've got now is a 350 that satisfies my daily driving needs, and a 455 on a stand that seems to need rings.

I'm desperately trying to reinvent the wheel here, even if it takes a few years of gathering parts.

 

Whats your budget ? Race car or Street car ?

 

The goal is autocross with street driving around town sometimes. I'll be patient and gather parts for a few years, I need to finish school anyway. I'd budget 11,000 USD (inexperienced rough guess), but since it's a passion project (and I'm single) I could see myself spending what it takes.

 

OK, I get the need to tinker with stuff.
What about putting a 425 crank and aluminum rods in your existing 455? The 4 inch stroke can handle 7000 rpm easy. You could grind the crank down to reduce bearing sizes.

(The practical answer is Chadmans, but you said your were doing this for fun / to be different.)

 

For some reason I assumed the 425 and 455 couldn't swap cranks. The 455 block would be at lot easier with not having to deal with the funky cam angles and lifter sizes. Both the diesel and 425 crank are strong, but the diesel crank is probably cheaper.

If my math and olds knowledge works out, I could offset grind a 425 crank in my current 455 to have a 3.735 stroke and 393 CI. But I could also choose any other stroke between that and 3.975. It all works out as a question of going for CI or trying to rev to the moon. Mightaswell build it to handle 8000. What situation could I ever be in that requires 8000 RPM? I'm really trying to rationalize here.

 

You're focusing on the wrong things here. The bore and stroke of a 425 is almost the same as an LS7 - a factory 7000 RPM monster. That part isn't a problem and doesn't need to be destroked to run that RPM. The problem with the Olds is that you're going to run out of airflow unless you really pump some money into the heads. Then you'd need to focus on valvetrain stability. Then start beefing up the oiling system (especially if you're going to be road racing, but still important with autocross, assuming you're running a decent suspension and 200 treadwear tires).

You will need to make sure the bottom end is built properly, with the right tolerances and stiff components, but the RPM you're shooting for doesn't make sense unless you can make power up there. Mark (CutlassEFI on here) can hook you up with good parts and I believe has done some road race engines (I could be mistaken about that).

 

The big block cranks all interchange and the 4 inch 400/425 cranks are all forged cranks. Not all 425s are large lifter or 45 degrees bank angles. I have a .842 39degree 425 in my stash along with a McLeod aluminum flywheel for same. Never heard of anyone using diesel crank but BTR put destroked 425 cranks in diesel blocks. You might pick up his book on building Oldsmobile engines. Lots of good info.

 

The next iteration of the idea then is using a 425 block with 060 over pistons. The broader power range just makes more sense. I forgot I have some other parts sitting around too.

There are Ford Windsor roller rockers that I could use. Don't know whether I'd find shaft mount rockers or just get a girdle.
I also have Speedmaster heads laying around in stock form. The general consensus is that the hardware is bad out of the box and it needs to be shaved down/ported to be better than iron heads. I'll see if they are decent for use with a procharger. The future might call for CNC work or just all out replacement.
For oiling, I'll definitely get restricted pushrods. Part of my schooling will be welding and machine shop work, so I might be able to fabricate a dry sump pan.

Looking around online, there isn't a huge crank selection. I see used 425 cranks on Ebay for 1000 dollars, and 425 cores going for 700. Destroking the 455 crank wouldn't exactly be bulletproof for this setup, so I might find myself buying a full 425 core anyway.

Together, parts list is
Roller cam+lifters - 1000
Pistons - 500
Fittings+metal for modified oil pan, oil reservoir, pump - still a lot probably
Machine work - 2000 dollars?
Crank or core - 700
Procharger - 5000
Fuel system, EFI or blow through? - 2500?
Speedmaster port/deck - 1000?

It will take a lot of hot rodding and also around 15,000.


If I I win the lottery I'll pick up a rocket racing block and its 4.4 inch bore.

 

Don’t use a diesel crank, in any form. They’re brittle and won’t stand up to any real hp. Plus you’ll need a rod and piston so tall it won’t make sense.
If you’re going to autocross it, you need tq and throttle response. Put your money into lightening the rotating assembly and the right intake setup.
Better yet, just go with a healthy small block. Cheaper and should fulfill your goals if done right.

 

I agree with Chadman and Cutlassefi. Use a new forged 4.00 small block crank and a 350 block to build a 428ci SBO, (much lighter crank, rods, and pistons).

 

what’s wrong with a 403? with a 330 crank , good rods and lite piston , it’ll turn 8,000 no problem. it’ll need a bottom end supper system which isn’t a big deal.

there’s your short stroke.

it would be the most economical and simplest choice

 

Excellent idea, and a block that will breathe better at high rpm downstairs and up top in the bore also.