403 Olds engine

[PetChemMan]

Circa 1979, a 403 CID engine was available in the full sized Olds cars. What was the origin of that engine? Was it an enlarged 350 ?

[Oldsmaniac]

The 403 is based on the 350 or should I say on the small block Olds but has siameesed cylinder walls meaning there arent any cooling passages between cylinders.....cooling problems can be had with the slightest of overbore on the 403....the stroke is longer as well compared to a 350.....

[jdorour]

The 403 CI engine is a low deck (small block) introduced in 1977 to replace the 455 as the top of the line Olds engine. The 403 does not have siemesed cylinders and has the same 3.385 inch stroke as the 260, 307, 330 and 350 engines.
The 403 was used in the entire Olds line as well as Buick, Motorhomes and The Pontiac TransAm.
It has "windowed" main webs that cause the bottom end to flex when under extreme loads but the engine is fine for street use. You can really wake a 403 up with a set of big block heads and a good cam.

[Oldsmaniac]

Ok, I stand corrected on the stroke issue with the 403....On 442.com they state that the 403 does have siamesed cylinders a few times in their FAQ's....and I thought I indeed saw that with my own eyes on the disassembled 403 thats in my brother's garage....

[jdorour]

I could be wrong. I was going on memory. I have not had a 403 apart in a long time. It could have siamessed cylinders.

[texasred]

Yes, it has siamesed cylinder walls. But, as I've said before, I wouldn't trust the 442-dot-bomb FAQ for tech info without a second independant source.

C.J.

[Lutzifer]

I have a 403 in my 19ft. Sunkisst jetboat. I was thinking of changing it over to a 455 but need info on a few things first. Is the bell housing and mounting the same? Is the engine mounts the same? Exhaust manifold same? Anyone have info or a link to basic engine specs for horsepower and rpm?

[jdorour]

The bellhousing, flywheel, and engine mounts are the same.
The 403 uses small port small block heads and manifolds. The 455 uses the large port heads and manifolds. However, the bolt pattern is the same. If you install the manifolds from your engine on the 455 heads you will, at best, have a restriction. I think the restriction would be minimal. At worst you will burn the gaskets if you use the small block gasket set and the small block manifolds on the big block heads. This will cause an exhaust leak. The 455 is also a little heavier.

The factory output rating for a 455 will depend on the year and model you choose. They had a high of 400HP & 510LBS/FT (SAE Gross) to a low of something like 180HP & 200LBS/FT (SAE NET) by 1976. But if you paln to build the engine for use in a boat you can get all kinds of power from any 455.

If you build your engine make sure you tell your engine builder/Camshaft MFG that you are puting it in a JET BOAT as the performance requirements are a little different than for a Prop Boat. AND FOR YOUR SAFETY TRY TO USE AS MUCH MARINE SPEC COMPONENTS AS YOU CAN. I grew up on the water in Connecticut and saw a lot of people get hurt from bilge fires.

Have fun
John

[Lutzifer]

Thank you for the quick info. jdorour. The 403 installed now is still running, so I'm not changing anything yet. But maybe in the future a 455 or just a slightly pepped up 403. I basically want some more speed with little money and mod's.
As for marine equipment; yes that is important stuff! I was informed that boats must have special marine grade alternator, carb, fuel pump, and fuel lines inorder to help reduce the risk of a fire and/or kabooms!