Stroked LS3 '81 to '87 Grand Prix

Tony1968

Royal Smart Person
Supporting Member
Jul 1, 2018
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Tony, yea, I was thinking my goals should be fairly easy to hit. Do you think I can do it (500 hp) with a mild, non lopey, cam without doing any port work?
I have a mild cam in my 370 LS and I expect to be over 500 so yes I firmly believe you can easily crest that number. Plus a lower duration cam should give you more torque lower in the curve plus be flat and long. Excellent for spirited driving.
 
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Texas82GP

Just-a-worm
Apr 3, 2015
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Thanks, Tex. That's the information I'm looking for. I'll stick with the 225's if it's even going to be close.
I should have mentioned that my setup was totally stock down to the diesel springs. I have the S10 ZQ8 wheels on the car. You might want to shy away from 16" wheels. Tire selection for 16" is limited. Unless if I'm wrong, the ZQ8 wheels are 8" wide with 4.380" backspacing. I would avoid buying tires until you have any suspension modifications done and you have the new power team in so you can dial in ride height and then measure for clearance.
 
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GR8ONE68

Not-quite-so-new-guy
Jan 9, 2015
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What would the max safe compression ratio be? I have some more research to do with the stroke and compression height of the pistons to determine the CR on this build. My initial calculations give me 11.9:1, which scares the hell of of this old cast iron thinking guy. I know aluminum and the computers make it safe to run a lot higher, but how high is high?
 
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mclellan83

Comic Book Super Hero
Jun 27, 2017
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Sounds like a good plan, looking forward to seeing it come together
 
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Tony1968

Royal Smart Person
Supporting Member
Jul 1, 2018
2,148
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NW Indiana
What would the max safe compression ratio be? I have some more research to do with the stroke and compression height of the pistons to determine the CR on this build. My initial calculations give me 11.9:1, which scares the hell of of this old cast iron thinking guy. I know aluminum and the computers make it safe to run a lot higher, but how high is high?
This also depends on cam selection. I am a touch under 11.4 to 1 and I have good 93 octane to keep detonation at bay. There is a calculator out there to see if you will be ok but that uses cam information to determine.
 
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RabbitHoleSS

G-Body Guru
Dec 8, 2019
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OK, I've been lurking, researching, and learning. First of all, I'm a Pontiac guy through and through. I had a '68 GTO for 25 years. I traded it back in 2017 with no regrets. I had run my course with that car and had a lot of fun along the way. 1967 was my favorite year for the GTO and all GM cars for that matter. My plan was to get a '67 to retire with, but the way the old car market has gone in the last 5 years, I'm out on that idea. I refuse to pay $50k for an average condition car. The first new car I ever bought was a 1983 Pontiac Grand Prix LJ Brougham. I have loved that body style ever since. With the '67 GTO out of the picture, I've decided to pick up a 1981 to 1987 Grand Prix and do an LS swap. I won't go into details of what I want, but I'll be looking for a while before I get one. I have been researching and putting together a plan and wanted to throw it out there for comments to make sure my logic is right and recommendations on some areas that I haven't settled on yet. I plan to go with an LS3. I will bore it out .010" and use a 4.1" stroke crank. Why, you ask? Nostalgia! If you do the math on that combination, it comes out to 427.8 cubic inches. Most would call that a 427, to me, it's a 428. This wasn't the way I really wanted to get there, I wanted to bore an LS3 .060" (4.125") and use a 4" crank. That is almost exactly what the Pontiac 428 bore and stroke was, 4.12" bore x 4" stroke. But that was the old cast iron block thinking, you can't bore an aluminum block that much. Oh well, the .010" overbore and 4.1" stroke gets me there. I will put old '67 to '69 Grand Prix 428 badges on it. Below is a breakdown on what I plan so far:

Motor:
LS3, +.010", 4.1" crank
Cam, TBD based on recommendations with my goals in mind.
Heads, stock LS3 heads, upgrade the valve train if my cam choice dictates the need.
Intake/throttle body, Stock LS3, but willing to change if it's necessary to get me to my goals.
Rotating assembly will be forged and balanced.
I will want/need an alternator, P/S pump, and A/C, I read something about an F body pulley for one of them to clear, help? Will a stock serpentine system work? Which one?

Transmission:
4L80e, will I need to upgrade it to handle my goals?

Controllers:
Holley Terminator X Max to control the motor and trans. Is there any reason to go with the Dominator other than "It's better"? Does it have any features that I may want? What are some of the inputs/outputs available that may be nice and I'm not thinking of?
Dakota Digital ECD-200 to keep the stock speedometer with the electronic trans.

Rear:
12 bolt Posi with disc brakes. If I get it going, I want to make sure it stops...
Gears: TBD based on the combination. I'm thinking 3.55's, but I want it to match the cam choice so it gets me into the right RPM range.

Wheels/Tires:

I want a stock looking car. I plan on using Wheel Vintiques Pontiac Rallye II's 17" x 9" with 275-40-17 tires in back and Wheel Vintiques Pontiac Rallye II's 16" x 8" with 225-50-16 tires up front. Will 235 50 16's fit up front without rubbing in turns?

Goals:
I want a stock appearing, very fast car. A sleeper. I would like it to hit 500 hp. I think this is a very reasonable goal, the LS3 is 430 HP stock. I'm thinking that with the added 52 cubic inches and a mild cam upgrade, I should be there. I'm not trying to squeak out every HP I can, I don't want a lopey cam. I had that in my GTO and loved it. But, at my age, I'm past that.

Thanks in advance!

Thoughts and recommendations?

If your wanting to go bigger then a 4" crank, you might look into aftermarket Darton sleeves.
It would also allow you to hit your goal a of 4.125" bore too. Call darton if your serious though. I just copied the first link on Google.

An ls3 dropout is gonna be like 5k. And if you put a forged rotating assembly that's another 3k. Imo, below is your best route(well a junkyard lq4 is the cheapest, and is how id build a stroker) This block comes with your 4.125" bore.

They make an aluminum block too but it's double the money. Then buy your rotating assembly and take it to the machine shop. Go to picapart and find some gen 4 recport heads off a truck/van/box trucks. Take/send those to a known head porter.
You'll need springs , pac 1218(.600)or 1219(.625) are what most people use.
I know fbody and truck serpentine setups will fit in a gbody. I'm not sure about the corvette setup(I assume it does).
Are you going to tune it?
The stock pcm can control everything you want but the holley is way more user friendly.
Terminator x MAX is more then enough for what you're doing, no need to buy a dominator here imo.
Lol as im sitting on lunch, proofreading my post, it hits me....... if you break down the money, this is assuming you do all the work except machining.
Engine:10k
Transmission/converter:3k
clean car:10k
Rear end/new brakes:5k
wheels:2k
Electronics:2k
+ atleast 50%(being generous)per the make a budget and double it rule.
I think you'll be damn close to the car you want(67' gto)
I'm not trying to talk you out of anything, this would be an awesome build. I'm just trying to add some perspective.
 
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