455 Sd?

[soxs]

Anyone have info on this motor? A buddy was telling me this was a very rare and second only to the hemi....put out more hp and torque than the ls6...?

[MrPbody]

soxs, your buddy is both right and wrong. It IS a very rare and rather "exotic" American V8. It was the last gasp or brute force from Detroit in the era when the muscle car was quickly becoming extinct.
Does it make more horsepower than a Hemi or LS-6? No. Does it make more low-end torque than Hemi or LS-6? You bet! Is it a very potent street performance engine? Well, consider that it has 8:1 compression, versus the 11.5:1 in LS-6 and 12:1 in Hemi. It makes 310 "NET" horsepower (roughly 400 in "brake" horsepower). Add 3 points of compresson, would give it about a 25% increase in horsepower with no other changes. When the playing field is "leveled", yse, SD is THE engine from Detroit. Problem. Raising the compression in the Pontiac is no small task. Yes, bump-top pistons would raise the compression, but... Pontiacs, by design, have a very efficient chamber, mixing the best elements from hemi and wedge. The problem comes in when you put a dome on the piston. It significantly interferes with flame propegation (the way the fire spreads out across the piston). Unfortunately, Pontiac didn't see fit to produce those beautiful heads with smaller chambers for racing. The stock size is 114 CCs. A 72 CC version would REALLY change the muscle car and racing scene, as the Edelbrock and Kaufmann heads are now showing the Chevy ad Ford crows, Pontiac is NOT to be taken lightly in today's drag racing community.
For information purposes, 455 SD was made only in model years '73 and '74. It was factory installed ONLY in Firebird Formula and Trans Am. Zero GTOs of GrandAms were equipped, yet there are some publicity shots of a '73 GrandAm with 455 SD badges on it. There were very few made. I don't have production numbers handy, but less than 1,500 cars through both production years.
It is no longer condsidered practical, to try to build one , except for restoration purposes. There are better pieces available through the aftermarket today, to make more power and better longevity, in the Pontiac. And no, I DON'T mean a Chevy engine...

[soxs]

Thanks Pbody...good info, So what is the diff between the 455HO and the SD?....I'll also ask very probable dumb question...why not mill the heads?....114 ccs is huge!...thanks

[MrPbody]

SD heads have the first "wide port" design, where the pushrod bulge is removed, and a tube is pressed into place to seal the hole, for the pushrod. HO heads didn't have that. The exhaust flange is the same on SD as HO and Ram Air IV, but the port is a little better shaped.
With the rule of thumb used of "1 CC for every .005" removed", it would take a milling of .125" just to get it to 9:1. There isn't enough meat in the deck surface to do that without making it too thin. Then, you have a bunch of valve train geometry issues to correct.
With the availability and price of Edelbrock and KRE heads, it makes little sense to carve up classic parts, and end up with less power. Those SD heads are collector's dreams, and a racer's nightmare...