My doomed 408 build

It’s a shame really, overall....... the dyno data for the first test doesn’t look too bad, other than the ridiculous water temps.
Had it been tested at 175* or so, it would have likely made 10-15 less hp.

I have some springs here off a set of RPM heads for flat tappet cams.
On the SB RPM’s I’ve had here the spring heights were set up at a nominal 1.800.
At that height, the springs test at about 115-120lbs closed, and [email protected] lift.
This would be adequate for some old school style hyd cam, but is going to be a little light with a fast rate cam using heavier than stock roller rockers.
I usually shim the springs to 1.750(130-135lbs) in that type of application(and I wouldn’t expect that XE284 to make it to 6000 without issues using that spring set up that way).

There is no way those springs can provide adequate load for the hyd roller cam you had, and still maintain control in the upper rpms.

I’m not really a big fan of the Thumper cams, but in that combo, with that cr, it should have made more power than the hyd flat tappet cam.
The fact that it didn’t, along with the poor bsfc numbers, tells me there was “something” going on right from the very beginning.