Been trying to get my 408 fine tuned and we've run into some issues. It put down a best of 505 RWHP with no issues and then 1 day on wya back form dyno I got on it and had a HUGE backfire, thought I blew the motor. We've been messing with a few things here and there to see if it goes away but seems ot be backfiring almost all the time on the dyno after about 5000 rpm and the few complete runs I am able to make show me putting down about 50 less HP.

We're thinking it may be an issue of valve float. Motor only has about 5000 miles on it if that much with a 253/261 644/624 110+4 cam. Not exactly sure which springs I'm running as I bought the motor assembled unused. Would valve float account for the loss in HP and the backfiring?? Thinking of maybe picking up a set of TSP's new springs to see if that solves the problem. Comments / suggestions??

 

Could be fuel system issue or bad coil.

 

was the backfire during the pull or just after the gas pedal was lifted

 

during pull

 

This is what I'm thinking.

If your using MSD coils, I'd put $50 down that one or more of them has failed. If they are stock, one could still be faulty, but not so likely.

 

Using stock coils but MSD wires with a coil relocation kit. What would be the proper method to check my coils to make sure that's not the issue?