Factory 402 cam specs - desktop dyno

[davepl]

I have a 1970 GMC 402, the L-47 310hp edition. It's a T042TTBG block and 3975950 heads.

I'm about to rebuild it (stock) and was going to play with the cam a little in Desktop Dyno, but can't for the life of me find the original cam specs.

Anyone know (or know where I can find) the numbers for the stock cam?

Thanks!
Dave

[davepl]

In case anyone else finds it useful, I found this as a starting point, but it's the LS3 (330hp), and while we may use the same cam, it's possible that ours is even tamer.

Camshaft number = #3874872
Camshaft Intake duration @ .050" == 196°
Camshaft Exhaust duration @ .050" = 196°
Camshaft lift - Intake == 0.398"
Camshaft lift - Exhaust = 0.398"
@ 114° LSA

Still though, if you know the actual numbers for the truck version, please chime in!

[4xshort]

The chevrolet pickup parts interchange manual by Paul Herd says the
1965-1970 396 is part number 3874872
1971-1972 396/402 is part number 3963544
Hope that helps some it doesn't list the specs, I have a big block
interchange manual but its at work,I will look up the part numbers
on monday for you.

[davepl]

Thanks! Using the 3963544 numbers it "dynos" out to 320hp with open headers (vs 310 rated) which is pretty close.

I'm assuming 2.060/1.7 valves, canted oval ports, 9.00:1 compression, 750 cfm, single plane intake.

[4xshort]

The manual also says the shortblock interchanges with 1970 Camaro
Chevelle and Monte Carlo and it rates it at 310hp, it doesnt have any info
on the heads. The torque curve looks pretty flat will the desktop dyno go down to 1000rpm?

[davepl]

No, because of the way flow/mass modelling works it can't go below 2000, and from what I understand is more accurate from about 3000 on up. So not the best for trying to tweak truck motors!

[68Gold/white]

Originally Posted by 4xshort

The manual also says the shortblock interchanges with 1970 Camaro
Chevelle and Monte Carlo and it rates it at 310hp, it doesnt have any info
on the heads. The torque curve looks pretty flat will the desktop dyno go down to 1000rpm?

I don't know (or care) the spec's of the specific cam...BUT,

I tore the 396 down (from my 68 pickup) it had a groove fully machined in the rear most journal. THis groove IS needed in the 65-66 big blocks, because of oiling differences...

The 396's in pickups were rated w/ differnt HP's than cars. I've tore down car AND pickup engines (396's) and the ones I tore down were identical...who knows...

I asked my engine builder friend, who has built hundreds of big blocks about this... he told me the the cam in my 396 was the the lowest H. P. stock type cam the Chevy put in essentially ALL of the base model big blocks. He guessed that they made a monster batch of cam's in 65 and 66 and kept using them until they ran out, and that the early cams CAN be used in later engines but late cams CANNOT be used in early engines without the journal being machined. He said he didn't usually see many of the grooved cams much past 1969 stuff...

My point...
The 396 stock camshaft in my 68 pickup, even though it wasn't a fresh build, I didn't think was worth a crap!!!, It had no particular good power in ANY RPM range, just didn't perform very well, my opinion.
Run it on your computer dyno, but DON"T look for one to use in a real engine...

AND don't use the closed chamber 396 heads. THe smallest open chamberd 454 heads are good heads for a 396.

Just trying to be helpful...

[davepl]

Why wouldn't I use closed chamber heads? I assume mine are open anyway, but when I hear "don't do what the factory made millions of for almost a decade", my ears perk up.

I probably will select a different cam, not because I think I'm smarter than the factory engineers, but because cam technology has actually improved in the last 40 years (though it might take a hydraulic roller to really benefit).

There's pretty much no amount of cam that's going to make one of these trucks fast, so I prefer that it's got a good idle, good vacuum, good cold-start driveability, and so on.

[davepl]

Just as an update, it dyno'd (engine dyno cell) at 326@4500 and Desktop Dyno predicted 320@4500 so actually pretty darned close. It overestimated torque by about 10%, but that probably means the torque curve got shifted in the RPM band.

In any event, it's a sample size of one but it was close.