best gears for 67 gto 3 speed - Pontiac GTO Forum
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post #1 of 5 (permalink) Old 02-09-2016, 09:43 AM Thread Starter
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best gears for 67 gto 3 speed

I just bought a 67 GTO...its a numbers matching 400..335 hp 3 speed...she seems a lil sluggish. My question is.. if I wanted to upgrade transmission gears and rear...what would be the best suggestion? I want to get off line so to speak a lot quicker,and be "more GTO-ish" and yet still be able to drive 75 miles to a car show with no worries. Im a roofer who loves hotrods,not a Mechanic, so pardon my ignorance
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post #2 of 5 (permalink) Old 02-09-2016, 11:37 AM
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An old tired 335 isn't going to be more goatish and you need to decide if you want an around towner with a lower gear ratio or a freeway flyer with taller gears.
Mine has a 2.93 pegleg rear end and a TH-400.
Since it is also a parts matching car I didn't want to change the tranny or rear end so I had the engine rebuilt with a few changes to the internals but kept it looking stock.
It isn't a rocket off the line but when it goes into second it is very goatish.
I don't drag race it and do a lot of mountain driving so I really don't want posi, I don't need it to swap ends when I get on it coming out of a corner.
I would start with the engine, mine was a very tired 335 now it is a lot peppier and runs on pump gas.
At idle you can tell it doesn't have a stock cam and it easily hits 100 mph in second gear.
I would rebuild the engine first and run it then decide if you want to change the tranny and/or rear end.
I am a carpenter and a shadetree mechanic, there are others on here who can tell you a lot more about the different trannys etc.
You have to decide if you want a 1/4 miler, a freeway runner of something in between the options are only limited by your budget.
Welcome to the group and you also posted in the '04-'06 GTO section, you may get more responses in the '64-'74 section.
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post #3 of 5 (permalink) Old 02-09-2016, 12:21 PM
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I'll move it to the right section.
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post #4 of 5 (permalink) Old 02-09-2016, 03:54 PM
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Welcome to the forum! Some history on your hd 3spd transmission...very late '65 model through '69 model GTO's came std with the Dearborn (ford top loader) 3spd. They were also used in late '65 Olds 442's, also used through the '69 model year. The Dearborn 3spd was also used in Buick GS's through '69. The latest, and oddest vehicle I have ever pulled an originally equipped Dearborn 3spd out of, was a '70 Catalina factory 400 4bbl car, so they were used for a few years in big cars as well. The gear ratio of the Dearborn 3spd is a wide ratio spread with a 2.46 first gear. Swapping to a Muncie 4 spd will only tighten up the gear spread some. I've had Pontiac buddy's who went from the HD 3spd to an M20 Muncie, while keeping the same rear gear ratio, and their cars actually slowed down a little at the strip, so it's not like there is much to gain swapping to a 2.52 1st gear ratio Muncie.

In your '67 GTO's rearend, if the car has factory r/e, and factory ratio, without factory AC, it should have a 3.55 Pontiac 8.2 10 bolt. With a healthy stock '67 WT 400, and the Dearborn, the 3.55 is the optimal ratio, not a 2.93-3.08, the engineers in the '60's also had to be concerned by premature clutch wear, so with a manual transmission, highway gears were out.

Today, if the plan is to cruise out on the highway, and not get into the stupid expense of swapping in a strong manual OD transmission, the easiest way to stay with a factory manual trans is to build the engine up for considerably more usable torque. Utilizing cast iron heads, you want higher port flow while maintaining velocity, while lowering the CR so the engine won't suffer detonation and a short life. Then with all the midrange torque, you can run a taller rear tire/wheel combo and have the rear (or a much stronger rear) built with a higher ratio ring and pinion, like a 3.23. There are numerous different "recipes" to get one there utilizing the Pontiac 400, whether it be the original or with a replacement 400 block, its all been done before, it all comes down to choices... One can build a healthy pump gas 406-412-416 or build a stroker 434-440-442-451, or even a 455-461-462-465 stroker. Last, such a pump gas engine build can be done and done right w/o spending 9-12k just on a boutique engine builder flavor of the year engine build.
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post #5 of 5 (permalink) Old 02-09-2016, 07:25 PM
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Gotta assume a 3-speed manual and not a 3-speed TH-400, right?

The 335HP engine appears to be the same as the 360 HP engine except the slightly smaller "067" cam versus the "068" cam. It should run pretty good.

Rather than consider changing any of the major driveline components, I would first do a little work on the engine to make it more "Goat-ish."

The 2 best areas to focus on are the exhaust & carb. Pontiacs respond well to better flowing exhausts as they have poor exhaust port flow for a high performance head - that's why Pontiac built in additional duration in their cams. So, Ram Air cast iron exhaust manifolds, 2.5" pipes, and free flowing mufflers, would be my first investment. Pontiacs gotta breathe.

Next would be a rebuild of the factory Q-jet. If you have anything less, this could hurt that Goat-ish feel. Small primaries for cruisin' and good gas mileage, big secondaries for a kick in the pants feel and big power. However, the Q-jet needs to be dialed in to be optimum for your application. It can be snappy and peppy, or bog down and be sluggish.

Then I would look at the engine tune-up and engine timing. Timing is very important. Having the correct distributor advance for your engine can wake it up. The factory compression is 10.75 but it more like 10.5 so I assume you are running high test/premium gas. I don't suspect your engine will run on 87 regular without it pinging. If it does, then I would suspect that someone has retarded the timing a bunch and this will kill power and make the engine a little sluggish. But better to be sluggish rather than have engine knock and destroy a piston or engine.

If you wanted more power, then I would recommend a cam change as long as the engine is in good shape, doesn't use excessive oil or smoke. Pontiacs are torque engines and you build your engine and drivetrain for torque, not RPM. If you change gearing in the trans or rear, you raise the RPM band that you will be cruising. My '68 Lemans has the HD Dearborn 3-speed manual trans, with 3.23 gears. Had a 350CI originally and it ran OK. Rebuilt a mild/streetable 8.2 compression 400CI to replace it. Used a 110 LSA Competition Cams hydraulic cam to match the low compression. The Lemans was very Goat-ish to say the least. I even went with taller rear tires which probably dropped my rear end ratio down to more like 3.08's. The engine's torque/HP was more than enough to accomplish nice burnouts, nifty redlight accelerations, several street races, and got 16 MPG on the highway @ 65-70 MPH.
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