Tips & opinions for a possible 403 swap, good idea?

[71HolidayCutlass]

Is the '78 to '79 Olds 403ci engine a viable performance engine?

I currently have an Olds 350 and read up on the 403 as being a large bore short stroke engine and I assume it may have some "revy" characteristics.

Is the bottom end relatively strong?

What kind of power can I expect? (flat torque curve, higher rpm performer, poor low end, etc.)

I know Pontiac and Buick also used this engine. What is the availability in terms of after market performance parts?


Any information, tips, or opinions you may have would be most appreciated. Just a side note- I am most likely going to turbo the 403 (if I go that route) or the 350 I currently have.

Thank you

[joe_padavano]

Originally Posted by 71HolidayCutlass

Is the '78 to '79 Olds 403ci engine a viable performance engine?

I currently have an Olds 350 and read up on the 403 as being a large bore short stroke engine and I assume it may have some "revy" characteristics.

Is the bottom end relatively strong?

What kind of power can I expect? (flat torque curve, higher rpm performer, poor low end, etc.)

I know Pontiac and Buick also used this engine. What is the availability in terms of after market performance parts?


Any information, tips, or opinions you may have would be most appreciated. Just a side note- I am most likely going to turbo the 403 (if I go that route) or the 350 I currently have.

Thank you

The bottom end of the 403 (and any 77-later Olds motor) is relatively weak. Olds cast large windows in the main webs for weight savings. These have little effect on a street engine but you'll be asking for trouble with a turbo motor. My suggestion for a stout turbo motor would be an early (68-72) 350 with a forged crank from a 64-67 330. All Olds small blocks use the same stroke and (with the exception of the diesel) the same bearing sizes.

[71HolidayCutlass]

Thanx Joe. Like I said, I already have an original Olds 350ci (unfortunately #7 heads though) and will probably boost it if it is a better candidate.

What about in a N/A application? Hows the 403 compared to the 350?

Whats a better investment: sticking with my 350 (more than likely boosted) or buy and build up a 403?

[2blu442]

If your thinking of the investment of putting a turbo on a 350 engine have you considered swapping in a 425 or 455? That would be a simpler way to get more horsepower and torque. John

[joe_padavano]

Originally Posted by 71HolidayCutlass

Thanx Joe. Like I said, I already have an original Olds 350ci (unfortunately #7 heads though) and will probably boost it if it is a better candidate.

What about in a N/A application? Hows the 403 compared to the 350?

Whats a better investment: sticking with my 350 (more than likely boosted) or buy and build up a 403?

It depends on how hard you plan to beat on it. All 77-later Olds motors suffer from the weak windowed main webs. There HAVE been notable 403 buildups that put out tremendous performance, and you certainly can't beat cubic inches. The 403 has a certain appeal due to the large bore and shorter SBO stroke. Dick Miller and Mondello both sell full girdles that can help. I'd personally go for a 330 forged crank and big block heads with a Performer RPM. I've got a couple of 403 blocks and a 330 crank waiting for the right time...

[71HolidayCutlass]

Theres absolutely nothing wrong with big blocks like the 425 or the 455. BBO's are a dime a dozen and the build ups are just as numerous. I just want something different. I can't help it, I love turbos. I love seeing what they do to small displacement engines. That said, engines that have higher redlines are usually better candidates than low rev-ing BB.

Besides, BB are known for there low end & torque. The jetting and timing required for turbos, as well as the low compression, causes the low end to suffer which defeats the purpose... at least for a good streetable engine.

And another pro for boosting a 350 is MPG. With a 700r4 I bet I could still get 20+ miles to the gallon. I don't have any experience but I'm guessing a typical 425/455 averages 12 mpg (TH350/400 equiped)...?

[71HolidayCutlass]

Originally Posted by joe_padavano

It depends on how hard you plan to beat on it. All 77-later Olds motors suffer from the weak windowed main webs.

Aren't the pre-77 350's also windowed? And I thought 75 and later 350's were SBC? with four bolt mains? I could be wrong....

I do understand that the "Olds" 350's can have the bottom-end reinforced (girdle, alloy mains, etc.) and that the 2bolt mains can be as strong as a 4 bolt from other GM engines because the Olds foundry used higher nickle content in the steel during casting... per-75... I think.

[GoldOlds]

Originally Posted by 71HolidayCutlass

Aren't the pre-77 350's also windowed? And I thought 75 and later 350's were SBC? with four bolt mains?

I can't answer the first question above, but Olds kept making their own 350 up to about 1979 or so. True, the mid-70s was an era of GM mix and match, meaning every division was borrowing engines from the other three. Pontiac used a lot of Olds 403s in their '78 and '79 Trans Ams, which brings me to my next point. From what I've seen first hand, the 403 is a stronger engine than people give it credit for. A few friends of mine were HUGE fans of the 403; one had one in his '79 T/A, the other in an '80 T/A (originally a 301 car) and later an '84 Grand Prix. They kicked the living snot out of those motors and they never broke down. Putting a 330 crank in a 403 sounds like a good idea to me; you don't lose any cubic inches and you have yourself a forged steel crank. Just my 2 cents.

- GoldOlds

[GoldOlds]

Originally Posted by 71HolidayCutlass

I already have an original Olds 350ci (unfortunately #7 heads though)

What's wrong with the #7 heads? #5, 6 and 7 are all good heads. Add the slightly smaller upper-case "A" and that's a different story....

- GoldOlds

[71HolidayCutlass]

Originally Posted by GoldOlds

What's wrong with the #7 heads? #5, 6 and 7 are all good heads. Add the slightly smaller upper-case "A" and that's a different story....

- GoldOlds

Yep, I have #7A heads. Itty bitty intake ports. I don't know the cost but I wonder if hogging them out is cheaper and as effective as buying a new set of heads....

[71HolidayCutlass]

Originally Posted by GoldOlds

I can't answer the first question above, but Olds kept making their own 350 up to about 1979 or so. True, the mid-70s was an era of GM mix and match, meaning every division was borrowing engines from the other three. Pontiac used a lot of Olds 403s in their '78 and '79 Trans Ams, which brings me to my next point. From what I've seen first hand, the 403 is a stronger engine than people give it credit for. A few friends of mine were HUGE fans of the 403; one had one in his '79 T/A, the other in an '80 T/A (originally a 301 car) and later an '84 Grand Prix. They kicked the living snot out of those motors and they never broke down. Putting a 330 crank in a 403 sounds like a good idea to me; you don't lose any cubic inches and you have yourself a forged steel crank. Just my 2 cents.

- GoldOlds

GoldsOlds, were your friends running high compression (10:1+)? Performance cam? Timing?

In a bone stock 403 I am sure it would hold together even with a factory crank. Would it hold up to 500+ street horsepower? Boosted or not...?

[joe_padavano]

Originally Posted by 71HolidayCutlass

I could be wrong....

You are.

Originally Posted by 71HolidayCutlass

Aren't the pre-77 350's also windowed?

Windowed mains did not enter production until the 1977 model year.

Originally Posted by 71HolidayCutlass

And I thought 75 and later 350's were SBC? with four bolt mains?

As noted previously Olds made 350 gas engines until the 307 came out in 1980. Some SBCs were used in the downsized 77-up Oldsmobiles and were the subject of a famous lawsuit, but the Olds 350 was still in production.

[joe_padavano]

Originally Posted by 71HolidayCutlass

Yep, I have #7A heads. Itty bitty intake ports. I don't know the cost but I wonder if hogging them out is cheaper and as effective as buying a new set of heads....

Don't confuse the the 1972 7a (small upper case A) heads (casting number 409147) with the late 1980s 7A (large upper case A) heads (casting number 0142) used on the 307. The 7a heads are fine, the 7A heads are crap.

[71HolidayCutlass]

Originally Posted by joe_padavano

You are.



Windowed mains did not enter production until the 1977 model year.



As noted previously Olds made 350 gas engines until the 307 came out in 1980. Some SBCs were used in the downsized 77-up Oldsmobiles and were the subject of a famous lawsuit, but the Olds 350 was still in production.

So pre-77' 350s have no windows?

So 7a (small capital "A") are good heads (casting #409147)? Right?

[GoldOlds]

Originally Posted by 71HolidayCutlass

GoldsOlds, were your friends running high compression (10:1+)? Performance cam? Timing?

In a bone stock 403 I am sure it would hold together even with a factory crank. Would it hold up to 500+ street horsepower? Boosted or not...?

I don't believe compression was as high as 10.0:1, but I know the 403 in the '80 Trans Am/'84 Grand Prix had a pretty decent cam - even with 3.08 gears, he embarassed more than one mid-80s 5.0L Mustang (which were significantly lighter than either of his cars). As far as timing goes, I honestly have no idea. I'm sure if he had shaved/replaced the heads and/or pistons and thrown in enough cam for 500+ horsepower without doing ANYTHING else to the engine, yes, it probably would have quickly become a paperweight... but the same could be said for a lot of different engines.

- GoldOlds

[71HolidayCutlass]

Thanx Gold Olds. I appreciate your input.

My main goal is determine the strength of the 403. People have said good and bad things about the block and I think I'm just gonna stick with the 350ci.

What helped me make this decision was the notion that the windowed block reduces the maximum redline to approx 4000-5000 rpm. With the bore to stroke geometry as it is the 403 should love to wind-out. While 5000 isn't bad, I'd like to see a 6000 rpm redline or even in the neigborhood 6800 (dreaming maybe). I'd bet I could achieve a 6000+ redline with a 350 after balancing and blueprinting, roller rockers, knife edged crank, etc.

[71HolidayCutlass]

Originally Posted by joe_padavano

Don't confuse the the 1972 7a (small upper case A) heads (casting number 409147) with the late 1980s 7A (large upper case A) heads (casting number 0142) used on the 307. The 7a heads are fine, the 7A heads are crap.

So are the plain #7 heads any good? Just the number "7" with no other letter. Casting number is 409147 near the exhaust ports. The heads are a bit dirty and caked with grime/oil but I see no letter.

[marcar1993]

If I'm correct, maybe not, they are the same as 7a's with smaller valves.

[joe_padavano]

Originally Posted by marcar1993

If I'm correct, maybe not, they are the same as 7a's with smaller valves.

I'm not convinced the valve sizes in the FAQ are correct. I was sure all 350s except W-31s used 1.880 intakes and 1.567 exhausts, but you'll need to mike your valves to be sure. The main difference between the 1971 #7 heads and the 1972 #7a heads is the valve spring pockets. Olds used rotators on the valve springs on some heads. The rotators were thicker than normal retainers so the spring pockets in the heads were cut deeper to provide the same spring installed height. #7 heads should have rotators (and thus deeper spring pockets) on the exhaust valves only, #7a heads should have deeper spring pockets on all valves. Not a big deal since you want to ditch the rotators for high perf use anyway (the added weight isn't conducive to high RPM performance). Depending on the spring pockets, you can add shim washers to get them where they need to be. Except for this spring pocket difference, the 7 and 7a heads are equivalent and are also equivalent to #5 and #6 heads as well.

[71HolidayCutlass]

Originally Posted by joe_padavano

... Except for this spring pocket difference, the 7 and 7a heads are equivalent and are also equivalent to #5 and #6 heads as well.

Thanks for the info Joe, but if I'm not mistaken don't the #5 heads have the larger intake ports? taller ports? #7 are of a small square type and the #5 are a tall rectangular shape I believe.

[joe_padavano]

Originally Posted by 71HolidayCutlass

Thanks for the info Joe, but if I'm not mistaken don't the #5 heads have the larger intake ports? taller ports? #7 are of a small square type and the #5 are a tall rectangular shape I believe.

Don't confuse the late 1980s 7A heads (large upper case "A") with the 7 and 7A (small upper case "A") heads from 1971 and 1972. The 80s heads used on roller 307s have the small ports. The 1960s and early 70s heads all have basically the same larger ports. Another tip is that the 307 heads should have the A.I.R. ports drilled, the early heads should not (except for the 66-67 Calif-only cars).

[71HolidayCutlass]

Thanks again joe.

I have yet to pull and tear down my 350. Its running so I figure I'll wait to I'm good and ready before I pull it apart. until then I have been trying to gather information on engine components in order to identify what I have and what i want to replace. If the #7 heads (no "a" or "A") are as good as you say they are then I'll clean them up, do some porting, maybe oversize the valves, and polishing as opposed to dumping $2500+ into edlebrock units. Most of the information online has been extremely helpful but I keep running into "sometimes" and "some engines were different because", you know, just like the way you said some have deep spring pockets and some had just half within the same year. Just gettin as prepared as I can get.