[ponyakr]

"...Their were some 481988 blocks for W72 Transam untill 1978..."


Never read that before. Everything I've read says that the '77 W72 used the 500557 block, but the '78 & '79 W72 engines all used the XX481988 block which was VERY similar to the earlier 488981 blocks, but had holes drilled & tapped only for the 3-bolt mounts.

[ponyakr]

"...You will also need to replace the 8.5 BOP rear end after you bust the case open with 450 HP..."

Actually, the 8.5 rear end can be beefed up to easily withstand that much power, and more. Just an aftermarket carrier and 30 spline axles will probably be plenty for 400hp. Or you can make a nice budget drag racing rear, with a few more mods.

http://www.superchevy.com/how-to/485...grade-install/

http://www.hotrod.com/articles/build...ip-eliminator/

https://www.google.com/search?rlz=1C....0.bJw9qPLABMg

[ponyakr]

"...Then the Turbo 400 tranny will get smoked 'cause it is in the middle of your latest upgrades..."

"...I also had the TH 400 taken out and rebuilt and reinforced to handle the 400 plus HP as well. Again this was done by a reputable local guy, that also said he would stand behind it as well. I trust both of these guys..."


If he installed a 34 or 36 element intermediate sprag, it will probably hold up. If he didn't, it probably won't, if your engine will make 500ft lbs of torque and you get good traction.

My 455 bracket engines broke 2 of the 16 element sprags, one in a TCI TH400, which was supposed to be a "full competition" model with a manual reverse pattern valve body. These engines probably didn't have much over 400hp, if that much, but they probably did have at least 500 torque at just over 3000 rpm. Replaced the broken sprags with the 34 element sprags, which, at that time I think my trans guy said was a GM part which originally came in some HD TH400's, like maybe the 500ci Caddy models. Never broke one of those in our low 12 & high 11 sec bracket cars. Hundreds of runs.

So, if it was me, I'd ask the trans guy if he used the 34 element sprag or not. If not, I'd have him go back in and do it. If it has the later model drum which had the weak roller clutch instead of the 16 element sprag, he'll have to change out the entire drum/sprag assembly. But, they are readily available. Lots of 'em on Ebay. Either the early TH400 or the later 4L80E drums will work. Ebay always has quite a few for sale, bare & complete.

https://www.ebay.com/sch/i.html?_fro...ag&_sacat=6028

[1977GPX]

Thanks ponyakr,

Appreciate all the information and advice. I'm going to take it all in. I've been researching so much information. Kinda taking the fun out of it.....LOL. Anyway will update you all when things start to take place with rebuild Hoping to start in next couple weeks. I have a lot to consider. I'm finding as much information supporting the use of 557 block as I am opposed to not using.
Guess its a crap shoot.........

[north]

The way to think of it is that by the 75 model year Pontiac knew that going forward their engines would top out at about 200 horse (net) but that the bottom end of these engine blocks had been designed for 300+ (net) horses. combine that situation with a then desperate (thanks to the 73-74 oil crisis and new government economy rules) need to lower weight and fuel consumption. The bottom end of those 557 blocks were thinned out to save a few pounds (and dollars) for use and gutless catalytic converter equipped power plants. Building up one of those blocks is fundamentally unsound, like building a house in a swamp.

Your GP is a big heavy car so why not put your original engine on a pallet and instead install an earlier 455 (or a 455 crank in an early 400 block). A set of 71-72 big valve low compression heads are cheap and will produce more power than later EGR heads. The extra cost to do this in the overall cost of setting up a 450 horse drivetrain is insignificant.

[1977GPX]

North,

Thanks for your help. I have a friend that has an early 1968 block.....YS stamp so I’m not going to take the chance with my 557 block. Going to store it and push forward with the better block. It’s just not worth the risk when I have the option now.

Gregg

[Greg Reid]

Originally Posted by 1977GPX

North,
I have a friend that has an early 1968 block.....YS stamp.

Gregg

That's a great one to build.

[Sprocket]

If you want to use your block, use it and run it until it blows, if it blows. Personally, I would get another shortblock, but that's just me.

Here is what I would do given the chassis, usage, and intended performance goal parameters that you have provided.

400 factory shortblock (the closer to zero deck the better)
Box stock 72cc Edelbrock d-ports
Edelbrock RPM intake
800 cfm carb
SDPerformance Stump Puller II hydraulic roller cam
RARE 68-72 GTO-style Ram Air exhaust manifolds 2.5" collector
2.5" mandrel bent exhaust front to back
TH400
3.55:1 ring and pinion at a minimum

That should get you damn close to 450 flywheel hp without giving up much low-end torque.

If you just want to do a lot of street sweeping without worrying about power much beyond 5000 rpm, look for a hydraulic flat tappet cam in the mid 220's/low 230's range, e.g. the Lunati 307A3 (which is the old Ultradyne 280/288) or the Lunati 60903.