1966 400 vs. 425
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From: Nova Scotia, Canada
1966 400 vs. 425
Morning all,
Just curious about the big block options available at Olds in 1966. Looks like they had a 400 ci big block engine and a 425? I assume the 400 was the high performance option for the A body cars? Maybe others as well? Just wondering why they didn't have the 425 as an option for those cars as well or was the horsepower difference somewhat negligible? They both used the same Q-Jet carb, according to the spec sheets in the #9 Delco book.
I guess I'm just wondering what the differences are between these engines as the cubic inch difference is pretty small and they're both considered big blocks (I think).
Thanks!
Just curious about the big block options available at Olds in 1966. Looks like they had a 400 ci big block engine and a 425? I assume the 400 was the high performance option for the A body cars? Maybe others as well? Just wondering why they didn't have the 425 as an option for those cars as well or was the horsepower difference somewhat negligible? They both used the same Q-Jet carb, according to the spec sheets in the #9 Delco book.
I guess I'm just wondering what the differences are between these engines as the cubic inch difference is pretty small and they're both considered big blocks (I think).
Thanks!
Last edited by ourkid2000; Apr 2, 2026 at 05:00 AM.
Old(s) Fart
Joined: Mar 2007
Posts: 50,840
From: Northern VA
The only substantive difference between the 400 and 425 motors was the bore: 4.000" vs 4.125". Olds shared the expensive crank and rods across BBOs within a given model year. That's why the G-block 400 had the ridiculous 3.890" bore - so it could share the 4.25" stroke with the 455. At the time, GM also had a corporate mandate that 400 cu in was the limit in the A-body cars. That's how the Hurst/Olds was born (despite the fact that the 455s were actually installed on the assembly line).
Oldsdruid
Joined: Dec 2007
Posts: 10,635
From: Southside Vajenya
GM had an internal rule thru 1969 that prohibited engines larger than 400 cubic inches in the small and mid-size cars. The rule relaxed in 1970 when the 454/455 was allowed in the A-body cars.
Chevrolet's 396 actually measured out to 402 ci but they got around the rule by advertising it as 396. Which sounded better for marketing anyway.
Likewise the 65-66 Buick GS 400 was actually a 401 Nailhead, and Buick fudged the numbers too until their true 400 appeared in 1967.
I have wondered if Oldsmobile's warranty costs on the long-stroke 400 influenced the corporation to relax the 400 ci rule in favor of a less-stressed 455. Couldn't let one Division have it and not let the others follow suit. Plus Ford and Chrysler were upping the ante with allowing 428/429 and 440 in midsize cars and GM had to catch up.
I have also long believed the 400/425, 396/427, 400/428 and 400/430 engine families were superior to any GM Division's later 454/455. But that's just me.
Chevrolet's 396 actually measured out to 402 ci but they got around the rule by advertising it as 396. Which sounded better for marketing anyway.
Likewise the 65-66 Buick GS 400 was actually a 401 Nailhead, and Buick fudged the numbers too until their true 400 appeared in 1967.
I have wondered if Oldsmobile's warranty costs on the long-stroke 400 influenced the corporation to relax the 400 ci rule in favor of a less-stressed 455. Couldn't let one Division have it and not let the others follow suit. Plus Ford and Chrysler were upping the ante with allowing 428/429 and 440 in midsize cars and GM had to catch up.
I have also long believed the 400/425, 396/427, 400/428 and 400/430 engine families were superior to any GM Division's later 454/455. But that's just me.
Thread Starter
Registered User
Joined: Mar 2010
Posts: 1,324
From: Nova Scotia, Canada
The only substantive difference between the 400 and 425 motors was the bore: 4.000" vs 4.125". Olds shared the expensive crank and rods across BBOs within a given model year. That's why the G-block 400 had the ridiculous 3.890" bore - so it could share the 4.25" stroke with the 455. At the time, GM also had a corporate mandate that 400 cu in was the limit in the A-body cars. That's how the Hurst/Olds was born (despite the fact that the 455s were actually installed on the assembly line).
GM had an internal rule thru 1969 that prohibited engines larger than 400 cubic inches in the small and mid-size cars. The rule relaxed in 1970 when the 454/455 was allowed in the A-body cars.
Chevrolet's 396 actually measured out to 402 ci but they got around the rule by advertising it as 396. Which sounded better for marketing anyway.
Likewise the 65-66 Buick GS 400 was actually a 401 Nailhead, and Buick fudged the numbers too until their true 400 appeared in 1967.
I have wondered if Oldsmobile's warranty costs on the long-stroke 400 influenced the corporation to relax the 400 ci rule in favor of a less-stressed 455. Couldn't let one Division have it and not let the others follow suit. Plus Ford and Chrysler were upping the ante with allowing 428/429 and 440 in midsize cars and GM had to catch up.
I have also long believed the 400/425, 396/427, 400/428 and 400/430 engine families were superior to any GM Division's later 454/455. But that's just me.
Chevrolet's 396 actually measured out to 402 ci but they got around the rule by advertising it as 396. Which sounded better for marketing anyway.
Likewise the 65-66 Buick GS 400 was actually a 401 Nailhead, and Buick fudged the numbers too until their true 400 appeared in 1967.
I have wondered if Oldsmobile's warranty costs on the long-stroke 400 influenced the corporation to relax the 400 ci rule in favor of a less-stressed 455. Couldn't let one Division have it and not let the others follow suit. Plus Ford and Chrysler were upping the ante with allowing 428/429 and 440 in midsize cars and GM had to catch up.
I have also long believed the 400/425, 396/427, 400/428 and 400/430 engine families were superior to any GM Division's later 454/455. But that's just me.
Old(s) Fart
Joined: Mar 2007
Posts: 50,840
From: Northern VA
Registered User
Joined: Sep 2017
Posts: 5,220
From: St. Paul Minnesota
GM had an internal rule thru 1969 that prohibited engines larger than 400 cubic inches in the small and mid-size cars. The rule relaxed in 1970 when the 454/455 was allowed in the A-body cars.
Chevrolet's 396 actually measured out to 402 ci but they got around the rule by advertising it as 396. Which sounded better for marketing anyway.
Likewise the 65-66 Buick GS 400 was actually a 401 Nailhead, and Buick fudged the numbers too until their true 400 appeared in 1967.
I have wondered if Oldsmobile's warranty costs on the long-stroke 400 influenced the corporation to relax the 400 ci rule in favor of a less-stressed 455. Couldn't let one Division have it and not let the others follow suit. Plus Ford and Chrysler were upping the ante with allowing 428/429 and 440 in midsize cars and GM had to catch up.
I have also long believed the 400/425, 396/427, 400/428 and 400/430 engine families were superior to any GM Division's later 454/455. But that's just me.
Chevrolet's 396 actually measured out to 402 ci but they got around the rule by advertising it as 396. Which sounded better for marketing anyway.
Likewise the 65-66 Buick GS 400 was actually a 401 Nailhead, and Buick fudged the numbers too until their true 400 appeared in 1967.
I have wondered if Oldsmobile's warranty costs on the long-stroke 400 influenced the corporation to relax the 400 ci rule in favor of a less-stressed 455. Couldn't let one Division have it and not let the others follow suit. Plus Ford and Chrysler were upping the ante with allowing 428/429 and 440 in midsize cars and GM had to catch up.
I have also long believed the 400/425, 396/427, 400/428 and 400/430 engine families were superior to any GM Division's later 454/455. But that's just me.
That's great information, thanks so much fellers! Was there much of a power difference in these engines? Seems like the 400 is known as a really powerful engine with possibly underrated ratings in the official numbers from the factory.......at least that's what the interwebs seem to say. When did they give up on the 400 anyway?
The factories measured horsepower on "in house" dyno's and were probably correct for an assembly line built engine and driven off the showroom floor. During these years Sunday drag racing sold cars on Monday morning. Stock cars in NHRA were classed, based on horsepower divided into shipping weight. It was ideal to have a horsepower to weight ratio near the low number for the class (example 9.00-9.50). The lower the, the likelihood the car would accelerate faster than a heavier or lower horsepower car. The first one to the other end of the 1/4 mile was the winner. Overstating horsepower would mean slower acceleration.
Drag racers tried to exceed factory horsepower ratings to win. This was possible when "blueprinting" came into vogue. All the factories supplied specs to NHRA. Cyllinder head and block deck were machined to specs that increased compression ratio (horsepower). Camshafts were machined to give the maximum duration and lift specified. What this did was eliminate all the manufacturing tolerances of assembly line engines. All these things and many others produced a "perfect engine" producing maximum horsepower.
Oldsdruid
Joined: Dec 2007
Posts: 10,635
From: Southside Vajenya
69 was the last one. Available in 442 and Vista Cruiser. The long stroke 68-9 G-block 400 was a great engine for Vista Cruiser since it didn't get beat on in those like it did in 442. The G-block with its long stroke didn't really like revving.
1965 400 is B block, 1966-67 is E block. Both are oversquare designs with large diameter pistons and short strokes. 1968-69 is G block which is horribly undersquare with small pistons and long crankshaft stroke. Good for torque, not for high RPM. Ergo a great station wagon engine.
Some reviews of the G engine claim the undersquare design was done for emissions. Generally accepted story is that Oldsmobile introduced the 455 in 1968 to haul around the Ninety Eight's ever increasing weight and used the 455 crank and rods in the G 400 so the engines could use the same parts and be built on the same engine lines. They had done the same thing with 65-67 400 and 425. But, to stay under the 400ci limit they had to reduce piston bore.
Oldsmobile had the first 455 in 1968 which, as you might guess, caused a whole lot of angst at Buick and Pontiac. Their 430 and 428 were good engines (better than either's 455 IMO) but their cars were also gaining weight and needed more engine, so they had 455s starting 1970.
428 was realistically as far as Pontiac should have gone with their block. Their bored-out 455 tended to run hot, which benefited emissions, but created starter heat soak problems along with cooling system problems. Ever seen the fan shroud on a 70s 455 Pontiac?
1965 400 is B block, 1966-67 is E block. Both are oversquare designs with large diameter pistons and short strokes. 1968-69 is G block which is horribly undersquare with small pistons and long crankshaft stroke. Good for torque, not for high RPM. Ergo a great station wagon engine.
Some reviews of the G engine claim the undersquare design was done for emissions. Generally accepted story is that Oldsmobile introduced the 455 in 1968 to haul around the Ninety Eight's ever increasing weight and used the 455 crank and rods in the G 400 so the engines could use the same parts and be built on the same engine lines. They had done the same thing with 65-67 400 and 425. But, to stay under the 400ci limit they had to reduce piston bore.
Oldsmobile had the first 455 in 1968 which, as you might guess, caused a whole lot of angst at Buick and Pontiac. Their 430 and 428 were good engines (better than either's 455 IMO) but their cars were also gaining weight and needed more engine, so they had 455s starting 1970.
428 was realistically as far as Pontiac should have gone with their block. Their bored-out 455 tended to run hot, which benefited emissions, but created starter heat soak problems along with cooling system problems. Ever seen the fan shroud on a 70s 455 Pontiac?
Old(s) Fart
Joined: Mar 2007
Posts: 50,840
From: Northern VA
I was going to mention that. There's a claim that the increased surface area to combustion chamber volume was beneficial for reducing NOx. If that were true, why limit this to the G-block, which was the lowest production volume engine Olds built in those years? Again, a marketing BS story to justify sharing the expensive crank and rods with the 455 to save cost.
Old(s) Fart
Joined: Mar 2007
Posts: 50,840
From: Northern VA
I'm pretty sure that the factory HP ratings (at least on the A-body cars) was driven more by the corporate mandate of no less than 10 lbs per HP. Am I the only one who finds it curious that the 1970 W30 was rated at exactly 370 HP in a 3,700 lb car? Or that Olds would spend the money to develop unique heads, intake, carb, cam, and distributor for these motors to make LESS horsepower than the run-of-the-mill Toro motor? Or that the AT motor with a milder cam and different carb and distributor made the same HP and torque as the ST motor? And once you cap the W30 at 370, you naturally have to ensure that the lesser motors are rated at lower HP.
Registered User
Joined: Sep 2017
Posts: 5,220
From: St. Paul Minnesota
I'm pretty sure that the factory HP ratings (at least on the A-body cars) was driven more by the corporate mandate of no less than 10 lbs per HP. Am I the only one who finds it curious that the 1970 W30 was rated at exactly 370 HP in a 3,700 lb car? Or that Olds would spend the money to develop unique heads, intake, carb, cam, and distributor for these motors to make LESS horsepower than the run-of-the-mill Toro motor? Or that the AT motor with a milder cam and different carb and distributor made the same HP and torque as the ST motor? And once you cap the W30 at 370, you naturally have to ensure that the lesser motors are rated at lower HP.
Thread Starter
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Joined: Mar 2010
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From: Nova Scotia, Canada
69 was the last one. Available in 442 and Vista Cruiser. The long stroke 68-9 G-block 400 was a great engine for Vista Cruiser since it didn't get beat on in those like it did in 442. The G-block with its long stroke didn't really like revving.
1965 400 is B block, 1966-67 is E block. Both are oversquare designs with large diameter pistons and short strokes. 1968-69 is G block which is horribly undersquare with small pistons and long crankshaft stroke. Good for torque, not for high RPM. Ergo a great station wagon engine.
Some reviews of the G engine claim the undersquare design was done for emissions. Generally accepted story is that Oldsmobile introduced the 455 in 1968 to haul around the Ninety Eight's ever increasing weight and used the 455 crank and rods in the G 400 so the engines could use the same parts and be built on the same engine lines. They had done the same thing with 65-67 400 and 425. But, to stay under the 400ci limit they had to reduce piston bore.
Oldsmobile had the first 455 in 1968 which, as you might guess, caused a whole lot of angst at Buick and Pontiac. Their 430 and 428 were good engines (better than either's 455 IMO) but their cars were also gaining weight and needed more engine, so they had 455s starting 1970.
428 was realistically as far as Pontiac should have gone with their block. Their bored-out 455 tended to run hot, which benefited emissions, but created starter heat soak problems along with cooling system problems. Ever seen the fan shroud on a 70s 455 Pontiac?
1965 400 is B block, 1966-67 is E block. Both are oversquare designs with large diameter pistons and short strokes. 1968-69 is G block which is horribly undersquare with small pistons and long crankshaft stroke. Good for torque, not for high RPM. Ergo a great station wagon engine.
Some reviews of the G engine claim the undersquare design was done for emissions. Generally accepted story is that Oldsmobile introduced the 455 in 1968 to haul around the Ninety Eight's ever increasing weight and used the 455 crank and rods in the G 400 so the engines could use the same parts and be built on the same engine lines. They had done the same thing with 65-67 400 and 425. But, to stay under the 400ci limit they had to reduce piston bore.
Oldsmobile had the first 455 in 1968 which, as you might guess, caused a whole lot of angst at Buick and Pontiac. Their 430 and 428 were good engines (better than either's 455 IMO) but their cars were also gaining weight and needed more engine, so they had 455s starting 1970.
428 was realistically as far as Pontiac should have gone with their block. Their bored-out 455 tended to run hot, which benefited emissions, but created starter heat soak problems along with cooling system problems. Ever seen the fan shroud on a 70s 455 Pontiac?
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You're close but not quite there. The B-block 400 was rated at 345 HP and was '65 only. The E-block was '66-7, was rated at 350HP (360 in the W-30)was little more than a minor evolution of the B. The G-block was '68-9 and was a new long-stroke design although basic architecture was still the same. Each one of these 400s shared crank and rods with its larger stable mates.
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You're close but not quite there. The B-block 400 was rated at 345 HP and was '65 only. The E-block was '66-7, was rated at 350HP (360 in the W-30)was little more than a minor evolution of the B. The G-block was '68-9 and was a new long-stroke design although basic architecture was still the same. Each one of these 400s shared crank and rods with its larger stable mates.
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You guys are all leagues beyond me in history, but simplistically I've always thought of the '66 400 as an underbored 425.
As a big car guy (with increasing weight!
), the 400/425 external accessory commonality just meant I could scour boneyard A bodies for some parts I could use on my aftermarket-underserved (well, really, un-served) big cars. Convenient, but that's 40 years ago now too.
Sure is fun to see the games GM/Olds played rather fast & loose with corporate mandates, regulation, and insurance before anyone really bothered to measure with digital precision.
Glad to see you all combatting truth decay. Carry on.
Cheers
Chris
As a big car guy (with increasing weight!
), the 400/425 external accessory commonality just meant I could scour boneyard A bodies for some parts I could use on my aftermarket-underserved (well, really, un-served) big cars. Convenient, but that's 40 years ago now too.Sure is fun to see the games GM/Olds played rather fast & loose with corporate mandates, regulation, and insurance before anyone really bothered to measure with digital precision.
Glad to see you all combatting truth decay. Carry on.
Cheers
Chris
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