Hello. I have a 68 sprint convertible and I want to take the motor and tranny out and replace them with some torque and a 4spd. I don't eant to do anything I can't undo though [future sale in mind]. I have my choice from a local guy here of a 428 or a 455, which do I go with??? I'm leaning on the 428 because she will rev a little more and still have lots of torque. What should I do??? Also have a line on a m20 4spd out of a 73 firebird with a 26 spline in and 32 out. Would this tranny work with my driveshaft from my 68 3spd or do I have to make one with a different yoke or length? Also will the bell housing from my 3spd work on this 4spd?

ok. you have several questions here so lets break it up. start with the engine. there are a few sprint cars here that have seen great results out of the engine you have in your car. as much as 450 hp and you save 200 pounds on the car by keeping it the way it is. if you want to go bigger its just your choice. both engines will fit with minor modifications to frame mounts. you'll need a set of 400 frame mounts and keep the 6 cyl. mounts for later use! that said lets move on to the next question. you can convert from a 3 spd to 4 spd no problem. some have used the 3 spd crossmember on the 4 spd but i have yet to do so. upon visual inspection there is isn't much difference! the only one i can see is the hole location in the crossmember being roughly 1/4" different. next question of using a 4 spd with that spline combo. the only thing you would need to change is the input yoke on your driveshaft. you should use the one that comes with the trans. just install it onto your drive shaft with new u-joints.
now for your bellhousing. it will work no problem! i have my 3 spd bellhousing bolted to my 4 spd at this very moment! bolted right up no problem! might want to spray a bit of wd40 in the holes before you thread the bolt in!

hope this helps you. if you need more help, others here have converted to 428"s and can give you more specific info on the exhaust issues and such but you should have no problem with whatever you choose!

Other than torque, what expectations do you have? How long do you plan on keeping the car? What is your budget? What are the head codes? Rear end ratio? Those are the questions you need answered.

Plan on an expensive overhaul especially with that motor. 428 pistons hard to come by unless $650 Ross or Wiseco. That car can handle a 428, but would look into upgrading the rear (axles/brakes), transmission and braking system.

You can have a lot of parts, but if they don't jive, you will not be going too far too fast.

I'm thinking of going to the 428 because of the rev and not just the grunt. If the 455 will rev and grunt then that would be the choice as that is the cheaper of the 2 motors, I don't know??? As for the head codes, I can get them within a day or 2. I think the 455 would run on pump gas and the 428 was a mixture. Both were supposed to be between 400 to 450 HP and 450 to 500 TQ.

My rear is the stock 10 bolt, not sure on the gear but I would say somewhere in the 355 range as its not bad on the freeway with the 3spd.

I don't plan on getting rid of the car anytime soon and budget isn't the largest due to the fact that the car is actually at the body shop now.

As for expectations, I don't plan on racing it. Sold the mustang and that was for racing [lost my ***]. I have a friend with a 68 chevelle convert w/428 and I love riding with him. It cruises great and it will smoke the tires at every gear shift without problem and I like that. Basically to cruise and have a little fun.

The 428 will rev more than the 455. The heads would be opposite if within the specs for a pump gas 428 (heads). You can have a pump gas 428 head which will not be compatible with a 455 for pump gas.

A 400 would give you rpm plus cost benefit, but not too much cheaper to build than a 428.

A 428 would be my choice..

Bit more revs, as pointed out before..

Bit less torque -but perhaps best if still using stock rear end.(I would install a posi, if not already there).

The components of the 455 and 428 will be more important than which motor you choose. Look at the heads--which heads, large valves heads for the 455, valve guide wear, number of valve jobs, compression. Which intake and carb do they have? Have the motors been bored previously? If so, is one of them already at 60 over?--which may be the limit. You can make either of them do what you wish. Look for cracks in the block and surface of the heads.

seconded! what the engines are as far as combination will determine your choice. also if they are rebuilt or rebuildable!

also most all pontiac and olds motors are in the 400 ft# range. an olds 455 rocket comes from the showroom with 475 ft# from the factory!

also note a basic principle of torque moves mass!

Found out some of the specs for the 428. 69 block with 69 GTO C heads [d-port]62 code, 2.11 and 1.77, 72cc, ram air 4 camshaft, GTO oil pan and melling 60lb pump, screw in studs, stock or street dominator intake. Should be about 10.25 to 1 compresion, so I should be able to get by with mostly pump and a little race fuel. Hoping to be around 425 hp 500 tq. Sound good or am I dreamin???

I think you are grossly underestimating your need for high octane.

Otherwise sounds like a stout build.

you'll be at least a 1/2 and 1/2 mixture with that 1. going over 10.5 will put you @ 3/4 race and 11.0 will put you in the full race range! id back the timing off on that one to keep it in the drivable range! you need a close 9.5 to 9.75 ratio to run the lowest high octane pump gas available! ethanol will help but it requires closer to 11.0 to run well enough to see the benefits!

How will the ethanol help, and why won't it help at 10.25 ratio? I have my choice with of motor, the 1st is the one stated and the second is similar with 11.3 ratio. I thought it would be more cost efficient to have the stated motor. Am I wrong?

I have a pump gas 10.4:1 -drives with 96n octane (39 degrees total timing) -but works fine with 91 octane (with 34 degrees timing)


I used to have 11.3:1 but that was only working with 96 octane, and 29 degrees total.

Claus, what is the alcohol type and content of your pump gas? Does your fuel contain oxygenates, and if so, what kind and how much?

Fuel quality makes a big difference in performance, and the gas sold here in the U.S. is pretty much garbage. New cars will deal with it; old cars don't.

I'd go with the 428, fill the first tank with 110 octane (leaded if no hardened seats) and drive it and tune it. When the tank is empty, put in half a tank of pump gas. See if there is a difference. If you've lost your power or have detonation, top the tank with 110. Adjust from there. It all depends on the accuracy of the builder's claims as to compression ratio. If the 455 is 11.3 there is no way it will run on pump gas.

Either engine can twist your convertible like a pretzel if you get on it too hard. That's why 428s were not installed in first gens.

I have no idea I have to admit.

-However I could easily tell good gas from bad when I was driving the 11.3 compression engine.

But there is difference in octane rating in europe vs. the u.s. ..

-Check it out:
In most countries (including all of Europe and Australia) the "headline" octane that would be shown on the pump is the RON, but in the United States, Canada and some other countries the headline number is the average of the RON and the MON, sometimes called the Anti-Knock Index (AKI), Road Octane Number (RdON), Pump Octane Number (PON), or (R+M)/2. Because of the 8 to 10 point difference noted above, this means that the octane in the United States will be about 4 to 5 points lower than the same fuel elsewhere: 87 octane fuel, the "regular" gasoline in the US and Canada, would be 91-92 in Europe. However most European pumps deliver 95 (RON) as "regular", equivalent to 90-91 US (R+M)/2, and even deliver 98 (RON) or 100 (RON).

http://en.wikipedia.org/wiki/Octane_rating

normal for northern germany was 103. that was the base fuel and went up from there! if you have a vert, your going to need soem serious chassis mods or your going to twist that car in half like a pretzel! if your looking to have something like this, i'd go with a coupe body and at minumum frame connectors, posi rear end, GOOD AXLES and a STOUT TRANS!

Thanks for the respoces. What affect does ethanol have on my delima? I did buy the best frame connectors possible and the bodyman says he is strengthing every point possible {he knows the plan].

the ethanol a 110 octane rating. it requires higher compression ratios to run it effectively and needs your engine to run salightly hotter which pontiacs already do. might help a bit with the octane needs but not by much. check out an article in the CHP MAG. forget the month but it was winter of last year i beleive. they did a whole work up on the benefits and charachteristics of ethanol(e85) based fuel. have a 79 ford 4x4 460 that runs ok on it but requires at least a 1/2 tank of 94 to run right!