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OK, I posted before. To summarize, I'm an old Pontiac guy. I want to put an LS motor in an 81 to 87 Grand Prix. For nostalgic reasons, I want the displacement to match something that was in an old Grand Prix and I want to put old badges on it for the motor, similar to the one attached. I'm down to two choices at this time. I can take and LS3, bore it .010", and use a 4.1" stroke crank to give me a 428. That was the big engine used in the '68 and '69 Grand Prix. The second option is using a stock bore LS3 and a Molnar 3.75" crank to get to 389. The 389 was the first motor put into the Grand Prix. These are both good options for me. There are pros and cons to each. With the 4.1" stroke crank you get more power, apples to apples. But there is the potential for piston slap at the bottom of the stroke. I read where this problem has been corrected, but... With the Molnar crank and a 389, I would need a good running motor that only needs a hone for the new pistons. New pistons will be needed because I would need to lower the compression height to avoid piston to valve contact. I guess my biggest question is what kind of power increase can I expect out of the additional 13 cubes with an otherwise equal motor, ie stock heads, cam, TB, etc? The second question is, how much cam would I need to get to 500 RWHP? I want my cake and I want to eat it as well. I want a very fast, well mannered car that I, or my wife, can drive anywhere anytime. Thanks in advance.
Is it an auto or manual? Just build something common like a 416 and slap a 389 badge on the side, some of those old Pontiacs were great sleepers and that would fit right in with that theme. A 416 or 428 would get to 500 RWHP with a considerably less rowdy combination.
Last edited by speedfreak440; Aug 13, 2023 at 06:22 AM.
Do a 416. If its an old pontiac I feel like its a sin to have less than 389 cubes. My dad's build thread for his 77 T/A is in here somewhere for reference. Car made 530 RWHP and drives like a dream with a smaller cam and a single plane intake with a Holley Terminator LS swap package. Those older cars with 500 horse+ are terrifying but also insanely fun.
The 428 sounds like a killer combo for your ride imo. I'd much rather have the extra cubes for a street cruiser. LS3 top end, schmedium cam with a nice idle and peak power around 6000 rpm.
You can get to 500whp with stock LS3 heads and LS3 intake assembled as-is from GM with a 428.
The 428 sounds like a killer combo for your ride imo. I'd much rather have the extra cubes for a street cruiser. LS3 top end, schmedium cam with a nice idle and peak power around 6000 rpm.
You can get to 500whp with stock LS3 heads and LS3 intake assembled as-is from GM with a 428.
I would build the 428. I'm actually thinking about building a 426 with an LS3 top end for my Camaro, after I finish the 6L80e swap.Gotta finish one project before starting another.
I came up with another option. If I start with an LQ4, bore it +.030", and use a 4.125" stroke crank, it will yield 421 cubic inches. This is another great Pontiac motor. For those that have suggested 416 or other builds that could probably be done a whole lot less expensive and similar power, you missed my point. The most important part of this build, to me, is that whatever I end up with, it must have been a CID that pontiac offered in the Grand Prix, plain and simple. I know this will cost me some extra money, but this will be my last muscle car. Technically not a muscle car, but that's another story. It will be a very fast car. Anyway, starting with an LQ4 and making a 421 will save money over starting with an LS3 and making a 428. Yes, I have given up power with the heads and intake, but I believe it to be a good sacrifice. I know with the longer stroke, it will not rev as high sacrificing upper end HP, but I believe it will be made up with low end torque. I still believe in the old adage, "HP sells cars, torque wins races." I think I will still be able to attain my goal with the added cubes and a cam in the 228/228-600/600 range. This should be a fairly mild mannered ride that's ready to kick some ***. Thoughts?
That cam is way too small. Even for daily street use, I would use something in the 238/244 range. The big cubes will like the bigger cam, and still be tame on the streets. The 228 cam you mentioned, would be like a stock LS3 cam in a 5.3L motor. The .600 lift is fine. I would just give it more duration.
Did they ever put a 409 CID in a Pontiac? That would also be pretty cool. The pistons would be expensive, but it's just a 4" crank. The 6.0L iron block gives you lots of options.
Ls3 with 3.75" stroke 
