Katanna

iTrader: (1)

Anyone know the cam specs of a 1976 merc 280? Would like to freshen the motors this winter and get a little more sound out of my idle and need to have a base to start with.

mcollinstn

That's a fairly uncommon motor. The Merc 280 was essentially a Merc 260 longblock mated to a transmission and TRS outdrive. Standard heads, standard rotating assembly, etc.

The cam, however, was a little hotter than the 260 cam. The cam that came in the 280TR package was the same part number used in the later Mercruiser 300 Tempest, and also the Mercruiser 320EFI. Both of those motors (300/320) had improved heads and intake hardware, but the cam is definitely a little stouter than the 260 cam. "Rumor has it" that the Merc cam was the Chevrolet L-82 Corvette cam (224/[email protected] .450/.460 on 114LC). I've heard guys who "upgraded" to the Comp XM262H cam (218/[email protected] .462/.477 on 112LC) and found zero difference in performance. There are, however, guys who replaced their 300 Tempest cams with the Comp XM268H and found added power throughout the rev range, a little more top end, and a noticeably more aggressive "lope" at idle. Keep in mind, though, that the Tempest 300 and the 320EFI run a bit more compression than the 280TR.

But then there are guys running much larger cams than that in smallblocks.....

But that should get you familiar with your "baseline".

MC

Katanna

iTrader: (1)

Looks like their may be some other differences also. Here is a mercruiser service bulliten I found. Looks like they were built by Mercury racing and had 4 bolt mains.

Model GM Code Comments
Flywheel
Diameter
in. (mm)
GM V8 350 cid (5.7L) with Flat Lifters continued.
Inboard Models (rotations shown).
255 WH (WJ) 1975, LH (RH). 14 (356)
255 WN (WR) 1976-1980, LH (RH). 14 (356)
260 WN (WR) 1981-1985, LH (RH). 14 (356)
260 AN (AR) 1986-1987, LH (RH), start full circle rear
seal, 7AN (AR), LH (RH), start new cylinder
heads.
14 (356)
5.7L AN (AR) 1988-1990, LH (RH). 14 (356)
5.7L MN (MR) 1991-1993, LH (RH). 14 (356)
5.7L NN (NR) 1993-1995, LH (RH). 14 (356)
Mercury Racing built Sterndrive Models (LH rotation), 4 bolt mains.
280 WK 1975-1976, GM hi-rise intake. 12-3/4 (324)
300 Tempest WK 1984-1985. 12-3/4 (324)
300 Tempest AK 1986, start 14 in. flywheel, full circle rear
seal.
14 (356)
HP320 EFI AK 1987-1989, Mercury Marine MPI intake. 14 (356)

rmbuilder

Katanna,

The 280 HP/ 300HP Tempest cams were originally produced over 40 yrs ago by Crane for GM. Part # 12353918.
They rated:
214º / 224º @ .050”
.295” / .310” lobe
.442” / .465” Valve
112º Lobe Sep
These engines yielded ~ .82-.85 HP per CID
Bob

Katanna

iTrader: (1)

Thanks Bob, that is exactly what I was looking for. I will be contacting you around the first of Dec for my parts.
Steve

dereknkathy

i was gonna say see bob madera. i guess he saw you first...how about bumpstick bob for a nickname?