Zennex

I'm new to Oldsmobile engines, my main expertise is Chevy small blocks. I got a '79 403 Oldsmobile smogger that I want to turn into a decent screamer with lots of low end torque. I wanted to put a rather hot cam in it, with 224/234 duration and .496/.520 lift for the idle and sound but I was wondering if it's even possible to run the engine with something that big.

I wanted to run the engine with this cam with the stock heads and rear end gears first to get a feel for the car, but I have a set of aluminum BBO heads and 3.73 rear end gears if it wouldn't work.
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Is it possible to run this cam or should I go with something a bit more tame?

olds 307 and 403

The problem, even with the aluminum heads is the 8 to 1 compression you would have with stock pistons. The 3.73 would help and a 2500 stall converter minimum is also necessary.

Zennex

Would it be worth it to mill the aluminum heads down from a 77cc chamber down to like 70cc or even 64cc like the old 350 heads? Or would that create a problem with the bigger cam causing the valves to hit the pistons?

olds 307 and 403

The pistons won't hit the valves. You will need either the Performer RPM intake cut, the only intake that fits other than the Victor without welding. or the intake face needs milled at least the same amount as the head are cut. You will probably be using composite intake gaskets in the BBO port size, so a bit more might not be a bad idea, they are much thicker than the factory turkey tray, which won't work. Your stock pistons have around a 21cc dish and probably sit .020" in the hole. The 70cc will put you around 8.9 to 1, 64cc around 9.3 to 1, I would not go any less.

Fun71

My 350 engine has a 217/221 cam and it idles very nicely with 16" vacuum. I suspect that 224 cam in a 403 would be similar due to the 50 extra cubic inches.

As said above, the issue would be the pathetically low compression ratio due to the huge dishes in the factory pistons.

cutlassefi

Stay with a single pattern to retain some cyl pressure while using otherwise stock components.

oddball

Building a 403 is a sacred art, only performed by those of us with the right dedication (and mental health issues). Tons of good info here and various places. The only real thing to note is the bullshit about "solid main web 403" is just that - bullshit. Don't exist. So need to just manage the weakness of the windowed main webs.

The stock 4A heads are pretty terrible. You won't get far with them.
The Edelbrock heads are vastly better. The deck is very thick on them so they can be cut down. The problem with an Olds engine is getting the intake to seal. The intake is really wide, so any movement of the head (such as milling the head or decking the block or changing head gaskets) can change the intake alignment and you get leaks from the top (Easy to spot) or bottom (pulling oil from the valley, less easy). Lots of info and opinions on here about how to deal with that. Also remember to open the holes for the 1/2" head bolts. Edelbrock helpfully started the job for you - just drill from the bottom.

Note that nearly every Edelbrock intake, straight out of the box, isn't machined flat. They all need 10 thou or more cut from each intake surface to square them up.


224/234 is NOT very big for a 403 cubic inch motor. Built right that can be a very nice street machine. You'd need much more compression. Vacuum might be a touch low for the brakes.

Listen to Mark (cutlassefi) on cam choice. I'd say you need to pick how much stuff you want to replace, then get a cam that will work with that setup.

My current 403 hit 450 hp/480 ftlb on the dyno. Full build specs are on realoldspower.com in the engine builds forum. I went overboard in building the bottom end (330 crank (the early Olds 330 had a forged crankshaft. Dimensions on all 330/350/403 cranks are identical except the 330 uses the early flywheel bolt pattern, so they all swap around. Cranks got lighter and weaker over the years), 5 main halo girdle) for what it is and just street driving - it could have been done cheaper and survived just fine. Just depends on how much you want out of it, what you'll do with it, and how much you want to spend.

P.S. I have a spare std/std 330 crank sitting in the corner.

Zennex

I found someone willing to custom fab a nice set of heads and matching intake for me. With the new heads at 65cc I wasn't expecting much more than a 9.5:1 comp ratio. I was looking for just over 400 hp, maybe, from this engine. Would the girdle be recommended for that? I'm really never going to run the engine very high, it's just a street car for me.

oddball

Frankly I don't think even a halo girdle is worth the cost at that level. The problem is the first time you say "F* it!", put slicks on and hit the strip, the 403 might spit out the rotating assembly.
What I often heard is the 400-ish hp range is where you need to start adding things to the bottom end. The big condition is how the engine is using that power. Street tires and street surfaces can't hold that much power, so you'll just spin the wheels most of the time.
Installing a 5 main halo is somewhat involved. Not every machinist has the machines, and not every machinist with the equipment can actually use them properly. e.g., on my 403, the crank ended up sinking into the block by ~0.010". Thankfully the Cloyes -10 timing sets were still available.
All-in you'll be looking at $500-$700 to add a halo.

SO, if you want to just feel secure that you did everything you could and don't want to touch the bottom end ever again, then go ahead and put on a halo.
If you're going to stick with your plan (mild build, street), and you're OK possibly having to replace the block if you get too excited, then skip it.

Zennex

I know that the 403 is not a high revving motor to begin with. If I wanted a drag car I'd drop my Pontiac 400ci in the car instead. But I know that the 403 is nearly bullet proof. Everything I saw was that as long as I keep it under 6,000 RPM I should be fine with a stock bottom end. Maybe I'll tone it down to around 375hp or so just to be sure.