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My son and I are in the early stages of restoring my 71 Cutlass convertible. It's currently a 350 automatic open diff. I'm leaning towards a 5.3 or 6.0 with possibly a 6 speed manual and 373 lsd gears. So far I've got the interior, side glass, engine and trans, front end, not including suspension out. I've got a OAI hood for it. I am not concerned with keeping it original as when I can no longer drive I'll give it to my son, and I'll keep all original parts. If I go LS I'll make a build thread on it.
Should I be concerned with the stresses of a 400-500 hp ls in the old convertible with frame flex and a 40+ year old car?
I may have the opportunity to buy a buddies 2001 silverado with a 5.3 that insurance is writing off after a lightning strike if it's cheap enough. So I could put that in there. I'm no mechanic, but my son can do about anything with cars.
Well it's really just a matter of opinion, but since you asked for opinions....
I'd pass on the LS power and instead do an Olds 455, possibly with a stroker crank. Even though I love the LS1s in modern vehicles, I'm just not a fan of modern tech in old cars. Just my preference, though plenty of people feel differently and we have a busy section dedicated to just this sort of practice (Conversions and Hybrids section).
If you are looking for power in the range of 400-500hp, this is very doable with a 455ci while still being reasonably streetable. Getting 400+ hp from such a big engine is really no problem at all. Getting numbers like that from an Olds 350 would be a different story though. If you prefer to stick with a smaller displacement engine, then the LS would be the way to go for your power goals with a street car.
Thanks RPM WS6, every opinion helps. One of the disadvantages to me for the 455 is the mileage. I plan to drive this car a lot and on some long trips. But the torque of the Olds is a definite win.
if you can get a motor and trans ,and a complete wiring harness from 1 car ,that would make the transplant alot easier .check the conversion section for more info .see if you can get the complete fuel system .
Considering that it's a convertible I'd go for a LQ4 4L80E combo with a 28-3200rpm stall converter, 3.23 gears, and a small cam. It would make the car a reliable and economical cruiser that runs on 87 octane and still has plenty of power for having fun. A different plan would be to build the car with a 403 Olds motor with a well built 200R4 and either a q-jet carb or one of the self tuning 4 barrel efi systems, the car would be just as fun and reliable but the gas mileage would probably be in the high teens instead of the mid twenties. Building it with the 403 and a carb would probably be quite a bit cheaper but if you were to drive it 3000 miles a year it would cost $150-200 more in fuel cost.
are 403s easy to get parts for? i always thought they werent the best compared to other olds motors.
The 403 has decent potential, but is already pretty maxed out displacement-wise since it's a small block, and the small block Olds engines don't have the same sort of aftermarket support that SBC engines do. I would skip that and go with the 455 instead, which can still be stroked to even bigger cubes if desired, or just built with stock displacement if max effort is not the goal. The only advantage I could see to the 403 is the lower deck height and thus easier fitment into some of the more modern platforms (such as a G-body).
But in a '71 A-body car, 455 fitment is a non-issue (since they could be had that way stock), so might as well benefit from the bigger cubes and greater potential. A 455ci with aftermarket heads and a mildish cam will make gobs of torque and over 400hp with excellent streetablility. But, I continue to agree with the fact that the LS engines will offer better MPG if that is a big concern.
Sourcing a decent 455 might be harder than finding an LS engine these days. But the 455 will be a direct bolt-in, rather than dealing with the conversion aspects of the LS. Either engine will provide tons of fun on the street and have potential for more power down the road.
If MPG is a concern, the LS is probably a better choice, though even a big 455 can get decent mileage with the above recommended OD trans and reasonable gear ratio. I also agree with the recommendation of a Q-jet rather than aftermarket carb, for applications where MPG is a significant concern. The small primaries of the Q-jet are great for tip-in/part throttle response and MPG, and the big secondaries allow the engine to breath when needed.
are 403s easy to get parts for? i always thought they werent the best compared to other olds motors.
The heads interchange between "big block" and "Small block" Olds motors, tall deck and short deck would be a more accurate designation. And although it had a shorter production run it was a corporate motor so it was put in several different BOP cars. I would expect it to be cheaper and easier to find than a 400 or 455, and I think a 400 or 403 would be a good size compromise between HP and MPG.
The heads interchange between "big block" and "Small block" Olds motors
This is true to a point (excluding the really small Olds V8s, such as the 260), but not without some port work done to the intake due to the deck height difference (and not all intakes are a candidate for this). Keep in mind that Olds small block V8s go all the way down to 260ci displacement, and you would be almost guaranteed to have valve-to-cylinder wall interference issues were you to put big block heads on a 260 (or even some of the larger displacement small block heads).
Depending on the year and head casting, there are also combustion chamber/compression concerns with such a swap.
Considering that it's a convertible I'd go for a LQ4 4L80E combo with a 28-3200rpm stall converter, 3.23 gears, and a small cam. It would make the car a reliable and economical cruiser that runs on 87 octane and still has plenty of power for having fun. A different plan would be to build the car with a 403 Olds motor with a well built 200R4 and either a q-jet carb or one of the self tuning 4 barrel efi systems, the car would be just as fun and reliable but the gas mileage would probably be in the high teens instead of the mid twenties. Building it with the 403 and a carb would probably be quite a bit cheaper but if you were to drive it 3000 miles a year it would cost $150-200 more in fuel cost.
I like the above option very much OP. I have a '70 Skylark convertible and that is the plan. Apart from a decent cam, I am keeping the iron heads and all truck accessories as well as a/c from my truck. If you are partial to keeping every thing old school, you would not be on the fence. Allow me to kick you off into the ls realm...lol.
Seriously though, I have a ton of irons in the fire: Skylark LQ4/4L80e swap, Chevelle LY6/4l80e, Lemans 5.3/4l60e and 84 C10 LQ4/TH350. I fancy fuel injection and I like mixing the old with the new. I get the classic looks without the key things that made the cars a little less pleasant: economy, controlled fuel leaks (carb) and hit or miss performance influenced by weather. I have my eyes on a 68 Charger and the first thing I am tossing out is the 440. It is important to others, just not me.
Good luck bro.
This is true to a point (excluding the really small Olds V8s, such as the 260), but not without some port work done to the intake due to the deck height difference (and not all intakes are a candidate for this). Keep in mind that Olds small block V8s go all the way down to 260ci displacement, and you would be almost guaranteed to have valve-to-cylinder wall interference issues were you to put big block heads on a 260 (or even some of the larger displacement small block heads).
Depending on the year and head casting, there are also combustion chamber/compression concerns with such a swap.
True but with a 403 the opposite is true and you need to put bigger valves in to gain flow. It has something like a 4.35 inch bore, thats 0.10 bigger that a 454 bbc. It looks like before aftermarket heads were available putting bigger valves in Olds 350 was a good performance option, the Olds 350 crank can also be used as it is stronger than the 403 crank. The intake manifold shouldn't need much port matching as there are specific tall deck and short deck manifolds. But lets face it with almost any older 400+cu motor if you can make 350hp your probably going to have very flat torque curve the a peak around 400 ft lbs. I personally don't see the need for more than that in a convertible, I just think they're for cruising not going fast and that's plenty of power for burning tires and having fun.
WOW, lots of great info. I'm leaning to a mild 5.3 or 6.0. I'll probably try to find a 6 liter. Would like a T56, but that may not fit the budget since I have to do a full resto. I may know about my buddies truck tomorrow, but it's probably going to be to much to make worthwhile.
I just had an LS1/T56 swap completed on my '68 Camaro ragtop. Highly recommend this swap. I have about 400 whp and she takes it just fine. I do have subframe connectors but you shouldn't need them on a full frame car.
09-19-2014, 09:53 PM
71 Cutlass Conv. Should I LS it? 
