Should I modify my original 389?


Asked by Nov 26, 2013 at 03:59 AM about the 1966 Pontiac GTO Coupe

Question type: Car Customization

I have a numbers matching 66 GTO; I have been thinking about boring 30 over, roller rockers, dish pistons, and a cam.  Is this a mistake?  Should I keep it original?

18 Answers

Ah, the TriPower. Had one myself a million years ago. I can't imagine why it would be a mistake, you are asking for opinions so I guess you are hesitant about doing the performance project. I say go for it. But do you mean 0.30" over? The stock 389 is 4.062 bore

6 people found this helpful.

Just coming back for another look. If that pic is what you have now it seems a shame to tear that apart, it's far from original there. Having second thoughts...if that's what (the pic) you want, cool... but if you have original factory engine and no problems maybe the performance project is not such a good idea. It's a classic.

5 people found this helpful.

I have done a number of these. If originality is your main concern, go ahead and rebuild the 389. However, you have a major decision to make: 1. The stock heads are high compression. You will not find pump gas that this engine can live with, without additives. Boring the engine .030 will make it worse. 2. The valves are steel and will wipe out in no time without more additives in the oil and gas. Swapping them to Stainless will solve that, but at that point you're better off starting with a later model head. Which leads to my answer: 1. Take the original engine out, dismantle it, and put it away. It's valuable as a numbers-matched piece. Find a clean 400 block. 2. If you like the tripower and 8 bolt water pump "look" you'll need a timing cover from a 66 or 67. They are out there - or re-use your's. 3. Go ahead and bore the block, do whatever you want. It's not original, who cares. It will LOOK original if you want it to, and that's up to you. I like the original "look". 4. You want a set of 6x or similar "open chamber" heads. Late 70's. These are low compression (bear with me here) and will run on pump gas. Get the valves changed to Stainless, with bronze guides. Get the heads ported and polished. (Important!). 5. Get a set of aftermarket RA3 exhaust manifolds, or headers if you're a masochist. Doug's are the only ones I would use, and it's still a nightmare on the driver's side. 6. Go with a fairly stout HO cam and HEI ignition. If you're re-using your trips, convert them to mechanical linkage. If you do this, you will have a pump-gas engine that uses regular oil, runs great, and puts out about 375 HP which is a slight improvement over your 389. And it will be reliable, serviceable, and if you paint it '66 blue, only the purists will ever know what's inside. Good luck!

5 people found this helpful.

Keep it to stock spec's even with after market parts, cam, pistons, etc. OR buy a 421" short block and add all your compatible stuff. Newer 400 & 455 engines where a hyd cam versions of the meaner 389 & 421's.

2 people found this helpful.

421 is a great alternate if you can find one. The 428 is a better starting point, or the 455 of course. All are virtually identical in appearance. S+J Engines in Washington can get you a strong short block setup, with port and polish, cam, SS valves, for around $2500. 8.75 CR runs on pump gas if you're planning to keep it as a driver.

1 people found this helpful.

Saving the stock motor for "later" and creating a look-alike replacement is likely the best plan... you do not want to depreciate the future value of your vehicle.

1 people found this helpful.

go for building the oringinal 389 its always better when the numbers match .it depends on how much hp you want.if you want a race car i would look for a 421.but for the street your tri power has plenty of hps.have fun buddy a 66 gto is my favorite pontiac.i gotta 62 cat with a 389.

1 people found this helpful.

So if I have a 64 389 and I find a 421 short block same year. I could literally swap blocks using the original 389 parts for the 421? I am curious James.

1 people found this helpful.

Yes, but depending the 421 heads might be better? With bigger valves & ports. If your 389 heads are special hi-perf use them. & or get the 421 heads rebuilt for spares. Measure the valve diameters with a caliper to be sure. The only advantage from 389 heads might be higher compression from the smaller cc combustion chambers & or stiffer valve springs. 421's have a lower rpm but higher torque.

1 people found this helpful.

My cousin recently completed building a 1965 4 bolt main 421 "157" engine for my 65 GTO convertible. Oddly enough it would be date code correct for my car. I never had the original WT code 389 manual transmission four barrel motor. I set out to have the 421 built to match a Super Duty or at least Bobcat performance level of the 421 ringer cars that were sent out by Pontiac to performance mags in 1964 for road testing ( Car & Driver infamously broadcasting 12.8 second quarter mile times on the 250 Gto Ferrari comparison issue cover ) trying to keep it as incognito as I could I used 2 valve cover gaskets & aftermarket spacers on the 389 valve covers to accommodate the roller tips on the rockers. I also went with a Hydraulic roller cam with .510 lift with .296 intake & .310 exhaust duration. The virgin motor was bored 0.30 over and the original Armasteel crank turned "0.10 . Lightweight aluminum dished Pistons were used with #12 1970 Ram Air III D-Port heads ported, polished with stainless valves & bronze valve guides. The compression level Is 9.69 to 1 with hardened valve seats to run on today's 93 Octane gas. To keep up the appearance as a 389 ( the transfer lug was removed many years ago by someone who also stamped the front passenger side of the motor as an 389 XS code Gto engine. The block casting # is 9778791. I just finished breaking in the engine. The cam is kind of lumpy and doesn't have the best street manners but man! Is it fast after 2500 rpm. I did my first and only burnout today to see what the car could do. I brought the car up to 3200 rpm dumped the clutch and broke the 1966 tri-power open power shifting into 2nd & 3rd gear. The car is a beast leaving tunnels of smoke and that sweet acrid aroma of spent rubber. After laying done about 60 feet of rubber in 3rd I let off the gas to save my red line tires and new engine when the car was still pulling hard. So in conclusion build another engine or keep the 389 street able. I'm not so sure about how I feel about my car now but I'm not afraid of any new Corvettes , Mustangs or Camaros other than the super cars. Nothing like that power for me but if you want smooth normal driving don't do what I did. I will have my cousin attempt to make the car a little more low idle friendly with some alteration to the center carb. Hopefully you or someone you know is as mechanically gifted as my cousin is. Good Luck and choose wisely


Let me rephrase that.. Say I swap the 64 389 for the 421 of the same year long block. I'm thinking the rest would be interchangeable?


Yes it would be interchangeable. I also have a 326 engine that I took some parts from. If it's an original 421 engine you would have to do some modifications to the valve train to run on today's gas. You would of course have to lower the compression ratio. My 421 has a compression ratio of 9.7 to one and still depending on the gas I purchase I have to add a small bottle of octane booster to it about 50 percent of the time to get it to run smoothly and eliminate pinging under heavy throttle, but I think it is more a case of unscrupulous station owners selling regular as supreme or high test as we called it so many years ago

2 people found this helpful.

I know what you mean about the gas. I had priced a 421 long block but unsure as to the vendor. I read bad reviews on him. I don't have the time for a $3000-4000 mistake. I may just stay/get a fresh long block 389 but make it a roller. Butler perf Has the fuel injection kits. I just need to locate a 4 barrel intake for a 64. Im thinking thatll make a fairly strong setup.


Additionally the slim Jim would def need to be swapped. Its OK for now..


I definitely want to keep itl sleeper looking setup


Does anyone know how much a 1961 Pontiac 389 4 bolt main 425A engine block is worth? Thank you.

1 people found this helpful.

Its hard to say unless its been taken apart & tested for cracks (magna-fluxed) & a 1961 389 isn't a high performance Pontiac anyhow, so not much. If you use the bare block & crank you would have to use aftermarket parts on everything else. And to upgrade it to a GTO quality engine will be costly. (heads, cam, carburation, etc.) Better to find another for sure horse power cheaper engine & swap it out.

Actually, the 1961 425a WAS a fairly high performance Pontiac. They were either tripower, or high output 4 barrel. I would LOVE to have a block like that.

Your Answer:


Looking for a Used GTO in your area?

CarGurus has 401 nationwide GTO listings starting at $7,000.


Pontiac GTO Experts

  • #1
  • #2
    Norm Davis
  • #3
View All

Related Models For Sale

Used Ford Mustang
303 Great Deals out of 40,193 listings starting at $1,312
Used Chevrolet Camaro
269 Great Deals out of 27,309 listings starting at $2,900
Used Chevrolet Corvette
81 Great Deals out of 15,209 listings starting at $4,499
Used Pontiac Firebird
5 Great Deals out of 686 listings starting at $3,995
Used Dodge Charger
332 Great Deals out of 30,194 listings starting at $2,000
Used Pontiac Grand Prix
15 Great Deals out of 1,695 listings starting at $1,500
Used Chevrolet Impala
518 Great Deals out of 26,213 listings starting at $995

Used Cars For Sale

2006 Pontiac GTO Coupe For Sale
3 Great Deals out of 86 listings starting at $7,000
2005 Pontiac GTO Coupe For Sale
61 listings starting at $8,999
2004 Pontiac GTO Coupe For Sale
102 listings starting at $7,900

Content submitted by Users is not endorsed by CarGurus, does not express the opinions of CarGurus, and should not be considered reviewed, screened, or approved by CarGurus. Please refer to CarGurus Terms of Use.